Author name code: mullan
ADS astronomy entries on 2022-09-14
author:"Mullan, Dermott J."
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Title: Magnetic Fields in Low-mass Stars Increase Photometric Masses,
Thereby Reducing Inferred Radius Inflation
Authors: MacDonald, J.; Mullan, D. J.
Bibcode: 2022RNAAS...6..146M
Altcode:
Standard evolution models of a given age provide a relationship between
stellar mass and T eff. Based on this, Cruz et al. have
derived stellar masses and radii for a sample of Kepler stars using
photometry alone: their radii are found to be inflated by up to 100%
relative to standard models. However, magnetic fields distort the M-T
eff relationship in a systematic way: here, we quantify this
distortion, and show that radius inflations in the Cruz et al. stars
are likely to be significantly smaller than 100%.
Title: Rotationally Constrained Convection in the Sun: Applicable
to Planetary Atmospheres?
Authors: Mullan, D. J.
Bibcode: 2022RNAAS...6...83M
Altcode:
Rotational suppression of convection can occur on length scales which
are larger than a critical length. Featherstone & Hindman suggest
that this can account for a second preferred length-scale in solar
convection on supergranule scales. Here, I explore if rotational
suppression of convection also occurs in the atmospheres of Earth
and Venus.
Title: Mechanical Energy Deposition in Cool Star Atmospheres:
Resonant Coupling to Coronal Loops in M Dwarfs
Authors: Mullan, D. J.
Bibcode: 2021ApJ...922..164M
Altcode:
The efficiency of coronal heating ɛ(cor) in a star can be quantified by
L X/L bol, i.e., the ratio of X-ray luminosity to
bolometric luminosity. The efficiency of chromospheric heating in the
same star ɛ(chr) is typically assumed to be proportional to L(H α)/L
bol or L(Ca K)/L bol where the lines H α and Ca
K are often the two strongest emission lines in the visible spectrum:
the constant of proportionality (η = ɛ(chr)/[L(Hα)/L bol]
> 1) includes contributions from many other lines emitted by the
chromosphere. In the case of the quiet Sun, it has been known for
decades that, in the Sun, the efficiency of chromospheric heating is
larger by a factor of ɛ(chr)/ɛ(cor) > 10 than the efficiency of
coronal heating. Over the intervening years, data pertaining to ɛ(cor)
and ɛ(chr) have been estimated for an increasingly large sample of
main-sequence stars with spectral types later than the Sun. These data
suggest that among M dwarfs, the efficiency ratio ɛ(chr)/ɛ(cor) may
in some stars become smaller than in the solar case. The effect of
this is such that the value of ɛ(cor) may become comparable to the
value of ɛ(chr). Here, we seek to understand why coronal heating may
be >10 times more efficient (relative to chromospheric heating) in
certain M dwarfs than in the Sun. Using data on coronal loop properties
in flaring stars, we examine the hypothesis that in M dwarfs, the
enhanced efficiency of coronal heating may be related to resonant
coupling between coronal loops and the source of mechanical energy in
the convection zone.
Title: Simultaneous Multiwavelength Flare Observations of EV Lacertae
Authors: Paudel, Rishi R.; Barclay, Thomas; Schlieder, Joshua E.;
Quintana, Elisa V.; Gilbert, Emily A.; Vega, Laura D.; Youngblood,
Allison; Silverstein, Michele; Osten, Rachel A.; Tucker, Michael A.;
Huber, Daniel; Do, Aaron; Hamaguchi, Kenji; Mullan, D. J.; Gizis,
John E.; Monsue, Teresa A.; Colón, Knicole D.; Boyd, Patricia T.;
Davenport, James R. A.; Walkowicz, Lucianne
Bibcode: 2021ApJ...922...31P
Altcode: 2021arXiv210804753P
We present the first results of our ongoing project conducting
simultaneous multiwavelength observations of flares on nearby active
M dwarfs. We acquired data of the nearby dM3.5e star EV Lac using five
different observatories: NASA's Transiting Exoplanet Survey Satellite
(TESS), NASA's Neil Gehrels Swift Observatory (Swift), NASA's Neutron
Interior Composition Explorer (NICER), the University of Hawaii
2.2-meter telescope (UH88), and the Las Cumbres Observatory Global
Telescope (LCOGT) Network. During the ~25 days of TESS observations,
we acquired three simultaneous UV/X-ray observations using Swift that
total ~18 ks, 21 simultaneous epochs totaling ~98 ks of X-ray data
using NICER, one observation (~3 hr) with UH88, and one observation
(~3 hr) with LCOGT. We identified 56 flares in the TESS light curve with
estimated energies in the range log ET (erg) = (30.5-33.2),
nine flares in the Swift UVM2 light curve with estimated energies in
the range log EUV (erg) = (29.3-31.1), 14 flares in the
NICER light curve with estimated minimum energies in the range log
EN (erg) = (30.5-32.3), and 1 flare in the LCOGT light curve
with log EL (erg) = 31.6. We find that the flare frequency
distributions (FFDs) of TESS and NICER flares have comparable slopes,
βT = -0.67 ± 0.09 and βN = - 0.65 ± 0.19,
and the FFD of UVOT flares has a shallower slope (βU =
-0.38 ± 0.13). Furthermore, we do not find conclusive evidence for
either the first ionization potential (FIP) or the inverse FIP effect
during coronal flares on EV Lac.
Title: THOR 42: A Test of Magnetic Models for Pre-main-sequence Stars
Authors: MacDonald, J.; Mullan, D. J.
Bibcode: 2021ApJ...907...27M
Altcode:
Precise masses and radii have been reported by Murphy et al. for the
components of the THOR 42 binary. Murphy et al. provide evidence that,
in their opinion, supports an earlier suggestion by Bell et al. that
THOR 42 is a member of the 32 Ori moving group, which, according to
Bell et al., has an age of about 24 Myr. From the positions of the THOR
2 components in the mass-radius diagram (MRD), Murphy et al. derive
ages of 22-26 Myr using standard (nonmagnetic) models. However,
they also find that these models cannot simultaneously reproduce
the observed masses, radii, effective temperatures, and luminosities
of the assumed coeval components. Their Hertzsprung-Russell diagram
(HRD) ages are found to be 2-4 times younger than the MRD ages, and
the masses determined from the HRD are found to be ∼50% smaller than
the dynamical masses. Here, we attempt to replicate the precise global
properties of THOR 24 by applying a model of magnetoconvection. We find
that MRD isochrones and HRD isochrones lead to consistent ages provided
that the surface magnetic fields have strengths of 300-400 G. However,
the ages that our models yield for THOR 24 are in the vicinity of 40
Myr. If this age estimate is correct, it could call into question the
membership of THOR 42 in the 32 Ori moving group.
Title: Pre-main-sequence Stars in Taurus: Comparison of Magnetic
and Nonmagnetic Model Fits to the Low-mass Stars
Authors: Mullan, D. J.; MacDonald, J.
Bibcode: 2020ApJ...904..108M
Altcode:
Stellar associations should be co-eval, but some associations exhibit
apparently discrepant ages: M stars appear younger than FG stars. Among
M stars in the Upper Sco association (age ≍ 11 Myr), magnetic effects
have been shown to slow down evolution of the lowest mass stars, thereby
alleviating the age discrepancy. A recent study of pre-main-sequence
(PMS) stars in the much younger Taurus star-forming region (SFR) suggest
that an age discrepancy may also exist there. Our goal is to compare
two approaches to magnetic/nonmagnetic modeling of the Taurus stars. In
one approach, Simon et al. concluded that an age discrepancy exists, but
it can be resolved by magnetic models if one imposes an "equipartition
condition" on the photospheric field of each star. In this paper, we
examine an independent sample of seven Taurus PMS stars reported by
Rizzuto et al. Our approach to modeling magneto-convection imposes no
condition on the photospheric field strength. We find that nonmagnetic
isochrones can successfully fit the stars in the Rizzuto et al. sample
with ages 1.6-2.4 Myr, without the need to invoke magnetic effects. An
upper limit on the vertical field strength on the surfaces of the
Rizzuto stars is found to be ∼300 G for the low-mass secondaries of FF
Tau and HP Tau/G2 and ∼100 G for the other, more massive stars. The
Taurus SFR is so large (extending over many square degrees in the sky)
that magnetic conditions may plausibly differ from star to star.
Title: K2 Ultracool Dwarfs Survey - VI. White light superflares
observed on an L5 dwarf and flare rates of L dwarfs
Authors: Paudel, R. R.; Gizis, J. E.; Mullan, D. J.; Schmidt, S. J.;
Burgasser, A. J.; Williams, P. K. G.
Bibcode: 2020MNRAS.494.5751P
Altcode: 2020MNRAS.tmp.1373P; 2020arXiv200410579P
Kepler K2 long cadence data are used to study white light flares
in a sample of 45 L dwarfs. We identified 11 flares on 9 L dwarfs
with equivalent durations of (1.3-198) h and total (UV/optical/IR)
energies of ≥0.9 × 1032 erg. Two superflares with
energies of >1033 erg were detected on an L5 dwarf
(VVV BD001): this is the coolest object so far on which flares have
been identified. The larger superflare on this L5 dwarf has an energy
of 4.6 × 1034 erg and an amplitude of >300 times the
photospheric level: so far, this is the largest amplitude flare detected
by the Kepler/K2 mission. The next coolest star on which we identified
a flare was an L2 dwarf: 2MASS J08585891+1804463. Combining the energies
of all the flares which we have identified on 9 L dwarfs with the total
observation time which was dedicated by Kepler to all 45 L dwarfs,
we construct a composite flare frequency distribution (FFD). The FFD
slope is quite shallow (-0.51 ± 0.17), consistent with earlier results
reported by Paudel et al. for one particular L0 dwarf, for which the FFD
slope was found to be -0.34. Using the composite FFD, we predict that,
in early- and mid-L dwarfs, a superflare of energy 1033
erg occurs every 2.4 yr and a superflare of energy 1034
erg occurs every 7.9 yr. Analysis of our L dwarf flares suggests that
magnetic fields of ≥0.13-1.3 kG are present on the stellar surface:
such fields could suppress Type II radio bursts.
Title: A Transition of Dynamo Modes in M Dwarfs: Narrowing Down the
Spectral Range Where the Transition Occurs
Authors: Mullan, D. J.; Houdebine, E. R.
Bibcode: 2020ApJ...891..128M
Altcode: 2020arXiv200108721M
Houdebine et al. combined Ca II data with projected rotational
velocities (V sin I) to construct rotation-activity correlations (RAC)
in K-M dwarfs. The RAC slopes were used to argue that a transition
between dynamo modes occurs at a spectral type between M2 and M3. H17
suggested that the dynamo transition corresponds to a transition to
complete convection (TTCC). An independent study of GAIA data led Jao
et al. to suggest that the TTCC sets in "near M3.0V," close to the H17
result. However, the changes in a star that cause TTCC signatures in
GAIA data might not lead to changes in Ca II emission at an identical
spectral type: the latter are also affected by magnetic effects, which
depend on certain properties of convection in the core. Here, we use
Ca II emission fluxes in a sample of ∼600 M dwarfs, and attempt to
narrow down the transition from one dynamo mode to another: rather
than relying on RAC slopes, we quantify how the Ca II emission flux
varies with spectral type to identify "steps" where the flux decreases
significantly across a narrow range of spectral types. We suggest that
the dynamo mode transition may be narrowed down to between M2.1 and
M2.3. This is close to, but earlier than, the TTCC location identified
by Jao et al. We suggest that the transition in dynamo mode may be
related to the existence of a small convective core, which occurs for
a finite time interval in certain low-mass stars.
Title: Mass Loss on the Red Giant Branch: Plasmoid-driven Winds
above the RGB Bump
Authors: Mullan, D. J.; MacDonald, J.
Bibcode: 2019ApJ...885..113M
Altcode: 2019arXiv190909204M
The onset of cool massive winds in evolved giants is correlated
with an evolutionary feature on the red giant branch (RGB) known
as the “bump.” Also at the bump, shear instability in the
star leads to magnetic fields that occur preferentially on small
length-scales. Pneuman has suggested that the emergence of small-scale
flux tubes in the Sun can give rise to enhanced acceleration of
the solar wind as a result of plasmoid acceleration (the so-called
“melon-seed mechanism”). In this paper, we examine Pneuman’s
formalism to determine if it may shed some light on the process
that drives mass loss in stars above the RGB bump. Because we do not
currently have detailed information for some of the relevant physical
parameters, we are not yet able to derive a detailed model: instead,
our goal in this paper is to explore a “proof of concept.” Using
parameters that are known to be plausible in cool giants, we find that
the total mass-loss rate from such stars can be replicated. Moreover,
we find that the radial profile of the wind speed in such stars can be
steep or shallow depending on the fraction of the mass-loss rate that
is contained in the plasmoids: this is consistent with empirical data
that indicate that the velocity profiles of winds from cool giants
span a range of steepnesses.
Title: VizieR Online Data Catalog: Stellar parameters of M and K
dwarfs (Houdebine+, 2019)
Authors: Houdebine, E. R.; Mullan, D. J.; Doyle, J. G.; de La
Vieuville, G.; Butler, C. J.; Paletou, F.
Bibcode: 2019yCat..51580056H
Altcode:
Observations of (R-I)C (Cousin's photometric system) or
(R-I)<SU photometric system) for our samples of K and M dwarfs
were taken from several papers. We selected eight samples of K and M
dwarfs according to their (R-I)C colors. This represents
a total sample of 2765 K and M dwarfs. Most stars in this sample
are nearby or large proper motion stars. We also completed these
samples of stars with a ninth sample of stars: the M0-M1 sample,
which includes some stars from the samples of stars from B12 and M15
(see Table 1), as well as several stars initially from the M2 sample,
which were found to have higher temperatures (we included in the M0-M1
sample stars down to the spectral type dM1.5). (3 data files).
Title: The Mass-Activity Relationships in M and K Dwarfs. I. Stellar
Parameters of Our Sample of M and K Dwarfs
Authors: Houdebine, Éric R.; Mullan, D. J.; Doyle, J. G.; de La
Vieuville, Geoffroy; Butler, C. J.; Paletou, F.
Bibcode: 2019AJ....158...56H
Altcode: 2019arXiv190507921H
Empirical correlations between stellar parameters such as rotation
or radius and magnetic activity diagnostics require estimates
of the effective temperatures and the stellar radii. The aim of
this study is to propose simple methods that can be applied to
large samples of stars in order to derive estimates of the stellar
parameters. Good empirical correlations between red/infrared colors
(e.g., (R - I) C ) and effective temperatures have been
well established for a long time. The more recent (R - I) C
color-T eff correlation using the data of Mann et
al. (hereafter M15) and Boyajian et al. (hereafter B12) shows that
this color can be applied as a temperature estimate for large samples
of stars. We find that the mean scatter in T eff relative
to the (R - I) C -T eff relationship of B12 and
M15 data is only ±3σ = 44.6 K for K dwarfs and ±3σ = 39.4 K for M
dwarfs. These figures are small and show that the (R - I) C
color can be used as a first-guess effective temperature estimator
for K and M dwarfs. We derive effective temperatures for about 1910
K and M dwarfs using the calibration of (R - I) C color-T
eff from B12 and M15 data. We also compiled T eff
and metallicity measurements available in the literature using the
VizieR database. We determine T eff for 441 stars with
previously unknown effective temperatures. We also identified 21 new
spectroscopic binaries and one triple system from our high-resolution
spectra. Based on Gaia DR2 and Hipparcos parallax measurements.
Title: K2 Ultracool Dwarfs Survey - V. High superflare rates on
rapidly rotating late-M dwarfs
Authors: Paudel, R. R.; Gizis, J. E.; Mullan, D. J.; Schmidt, S. J.;
Burgasser, A. J.; Williams, P. K. G.; Youngblood, A.; Stassun, K. G.
Bibcode: 2019MNRAS.486.1438P
Altcode: 2019MNRAS.tmp..860P; 2018arXiv181207631P
We observed strong superflares (defined as flares with energy in excess
of 1033 erg) on three late-M dwarfs: 2MASS J08315742+2042213
(hereafter 2M0831+2042; M7 V), 2MASS J08371832+2050349 (hereafter
2M0837+2050; M8 V), and 2MASS J08312608+2244586 (hereafter 2M0831+2244;
M9 V). 2M0831+2042 and 2M0837+2050 are members of the young (∼700
Myr) open cluster Praesepe. The strong superflare on 2M0831+2042 has
an equivalent duration (ED) of 13.7 h and an estimated energy of 1.3
× 1035 erg. We observed five superflares on 2M0837+2050,
on which the strongest superflare has an ED of 46.4 h and an estimated
energy of 3.5 × 1035 erg. This energy is larger by 2.7
orders of magnitude than the largest flare observed on the older (7.6
Gyr) planet-hosting M8 dwarf TRAPPIST-1. Furthermore, we also observed
five superflares on 2M0831+2244 which is probably a field star. The
estimated energy of the strongest superflare on 2M0831+2244 is 6.1 ×
1034 erg. 2M0831+2042, 2M0837+2050, and 2M0831+2244 have
rotation periods of 0.556 ± 0.002, 0.193 ± 0.000, and 0.292 ± 0.001
d, respectively, which we measured by using K2 light curves. We compare
the flares of younger targets with those of TRAPPIST-1 and discuss
the possible impacts of such flares on planets in the habitable zone
of late-M dwarfs.
Title: Pathways for Ecological Change in Canadian High Arctic Wetlands
Under Rapid Twentieth Century Warming
Authors: Sim, T. G.; Swindles, G. T.; Morris, P. J.; Gałka, M.;
Mullan, D.; Galloway, J. M.
Bibcode: 2019GeoRL..46.4726S
Altcode:
We use paleoecological techniques to investigate how Canadian High
Arctic wetlands responded to a mid-twentieth century increase in growing
degree days. We observe an increase in wetness, moss diversity, and
carbon accumulation in a polygon mire trough, likely related to ice
wedge thaw. Contrastingly, the raised center of the polygon mire showed
no clear response. Wet and dry indicator testate amoebae increased
concomitantly in a valley fen, possibly relating to greater inundation
from snowmelt followed by increasing evapotranspiration. This occurred
alongside the appearance of generalist hummock mosses. A coastal
fen underwent a shift from sedge to shrub dominance. The valley and
coastal fens transitioned from minerogenic to organic-rich wetlands
prior to the growing degree days increase. A subsequent shift to moss
dominance in the coastal fen may relate to intensive grazing from
Arctic geese. Our findings highlight the complex response of Arctic
wetlands to warming and have implications for understanding their
future carbon sink potential.
Title: Origin of Radio-quiet Coronal Mass Ejections in Flare Stars
Authors: Mullan, D. J.; Paudel, R. R.
Bibcode: 2019ApJ...873....1M
Altcode: 2019arXiv190200810M
Type II radio bursts are observed in the Sun in association with
many coronal mass ejections (CMEs). In view of this association,
there has been an expectation that, by scaling from solar flares to
the flares that are observed on M dwarfs, radio emission analogous to
solar type II bursts should be detectable in association with M dwarf
flares. However, several surveys have revealed that this expectation
does not seem to be fulfilled. Here we hypothesize that the presence
of larger global field strengths in low-mass stars, suggested by recent
magnetoconvective modeling, gives rise to such large Alfvén speeds in
the corona that it becomes difficult to satisfy the conditions for the
generation of type II radio bursts. As a result, CMEs propagating in
the corona/wind of flare stars are expected to be “radio-quiet”
as regards type II bursts. In view of this, we suggest that, in the
context of type II bursts, scaling from solar to stellar flares is of
limited effectiveness.
Title: The Umbral-penumbral Boundary in Sunspots in the Context
of Magnetoconvection
Authors: Mullan, D. J.; MacDonald, J.
Bibcode: 2019ApJ...873L..10M
Altcode: 2019arXiv190209431M
Jurcak et al. have reported that, in a sample of more than 100 umbral
cores in sunspots, the umbral-penumbral boundary (UPB) is characterized
by a remarkably narrowly defined numerical value (1867 G) of the
vertical component of the magnetic field. Gough & Tayler, in
their study of magnetoconvection, showed that the onset of convection
in the presence of a magnetic field is controlled by a parameter δ,
which also depends on the vertical component of the field. Combining
the Jurcak et al. result with various empirical models of sunspots
leads us to propose the following hypothesis: the UPB occurs where the
vertical field is strong enough to increase the effective adiabatic
temperature gradient by ≥100% over its non-magnetic value.
Title: VizieR Online Data Catalog: K2 ultracool dwarfs
survey. III. M6-L0 flares (Paudel+, 2018)
Authors: Paudel, R. R.; Gizis, J. E.; Mullan, D. J.; Schmidt, S. J.;
Burgasser, A. J.; Williams, P. K. G.; Berger, E.
Bibcode: 2019yCat..18580055P
Altcode:
All the 10 targets listed in Table 1 were observed by Kepler K2
in various campaigns (see the campaign number in Table 1) in both
long-cadence mode (~30 min) and short-cadence mode (~1 min). We used
short-cadence data to study white light flares (WLFs) on all of our
targets. (3 data files).
Title: Magnetic Fields on the Flare Star Trappist-1: Consequences
for Radius Inflation and Planetary Habitability
Authors: Mullan, D. J.; MacDonald, J.; Dieterich, S.; Fausey, H.
Bibcode: 2018ApJ...869..149M
Altcode: 2018arXiv181104149M
We construct evolutionary models of Trappist-1 in which magnetic
fields impede the onset of convection according to a physics-based
criterion. In the models that best fit all observational constraints,
the photospheric fields in Tr-1 are found to be in the range 1450-1700
G. These are weaker by a factor of about 2 than the fields we obtained
in previous magnetic models of two other cool dwarfs (GJ 65A/B). Our
results suggest that Tr-1 possesses a global poloidal field that is
∼100 times stronger than the Sun's global field. In the context of
exoplanets in orbit around Tr-1, the strong poloidal fields on the star
may help to protect the planets from the potentially destructive effects
of coronal mass ejections. This, in combination with previous arguments
about the beneficial effects of flare photons in the ultraviolet and
visible portions of the spectrum, suggests that conditions on Tr-1
are not necessarily harmful to life on a planet in the habitable zone
of Tr-1.
Title: Photosynthesis on a Planet Orbiting an M Dwarf: Enhanced
Effectiveness during Flares
Authors: Mullan, D. J.; Bais, H. P.
Bibcode: 2018ApJ...865..101M
Altcode: 2018arXiv180705267M
On planets near M dwarfs, oxygenic photosynthesis (PS) will occur
with an effectiveness that depends on the supply of visible photons
with wavelengths between 400 and 700 nm. In this paper, we quantify
the effectiveness of PS in two contexts that are relevant for
M dwarfs. First, using photons from an M dwarf in its quiescent
(nonflaring) state, we find that PS on an M dwarf planet in the
habitable zone (HZ) of its parent star is less effective than on Earth
by a factor of 10 for a flare star with mid-M spectral type. For a flare
star with late-M spectral type, PS effectiveness (PSE) is smaller than
on Earth by a factor of 100 or more. Second, using photons that are
incident on the HZ planet during flares, we find that PSE can increase
by factors of 5-20 above the quiescent values. In the case of a flare
star with mid-M spectral type, we find that the PSE during a flare can
increase up to as much as 50%-60% of the values on Earth. However,
for a late-M flare star, even during flares, the PSE remains almost
one order of magnitude smaller than on Earth. We suggest that for
biological processes on M dwarf planets, the stellar activity cycle
may replace the orbital period as the “year.”
Title: K 2 Ultracool Dwarfs Survey. IV. Monster Flares Observed on
the Young Brown Dwarf CFHT-BD-Tau 4
Authors: Paudel, Rishi R.; Gizis, John E.; Mullan, D. J.; Schmidt,
Sarah J.; Burgasser, Adam J.; Williams, Peter K. G.; Berger, Edo
Bibcode: 2018ApJ...861...76P
Altcode: 2018arXiv180511185P
We present photometric measurements of two superflares observed on a
very young brown dwarf, CFHT-BD-Tau 4, observed during Campaign 13 of
the Kepler K 2 mission. The stronger of the two superflares brightened
by a factor of ∼48 relative to the quiescent photospheric level, with
an increase in Kepler magnitude {{Δ }}\tilde{{K}p}=-4.20. It
has an equivalent duration of ∼107 hr, a flare duration of 1.7 days,
and an estimated total bolometric (ultraviolet/optical/infrared) energy
up to 2.1 × 1038 erg. The weaker of the two superflares
is a complex (multipeaked) flare with an estimated total bolometric
(UV/optical/IR) energy up to 4.7 × 1036 erg. They are the
strongest flares observed on any brown dwarf so far. The flare energies
are strongly dependent on the value of the visual extinction parameter A
V used for extinction correction. If we apply a solar flare
model to interpret the two superflares, we find that the magnetic fields
are required to be stronger by as much as an order of magnitude than
previous reports of field measurements in CFHT-BD-Tau 4 by Reiners et
al. On the other hand, if we interpret our data in terms of accretion,
we find that the requisite rate of accretion for the stronger superflare
exceeds the rates that have been reported for other young brown dwarfs.
Title: White Light Flare Rates of M5-L5 dwarfs using K2 data
Authors: Paudel, Rishi R.; Gizis, John E.; Mullan, D. J.; Schmidt,
Sarah J.; Burgasser, Adam J.; Williams, Peter K. G.
Bibcode: 2018csss.confE..78P
Altcode:
The K2 mission's unique combination of wide area coverage and
continuous time coverage over months has enabled us to study white
light flare rates of cool stars of various spectral types. We have
shown that the white light flares are ubiquitous in some late-M and
early L dwarfs. Some early L dwarfs are even capable of producing
superflares with bolometric flare energies greater than 1033
erg despite having lower effective temperatures. We update our results
on the white light flare rates of very-low-mass stars with a wide
range of spectral types: M5-L5, obtained by using both short cadence
( 1 min) and long cadence ( 30 min) K2 data. We analyze the possible
relation between flare rates of very-low-mass stars in our sample and
different properties like spectral type, age, etc. Strong magnetic
fields of order of 5-10 kilogauss are required to explain the most
energetic flares. Using constraints on magnetic fields from the biggest
flares, we discuss the nature and evolution of the magnetic dynamo
on very-low-mass stars. Our results will be helpful in predicting the
number of flares on the low-mass cool stars which will be observed by
future photometric surveys like TESS.
Title: The Magnetic Binary GJ 65: A Test of Magnetic Diffusivity
Effects
Authors: MacDonald, James; Mullan, D. J.; Dieterich, Sergio
Bibcode: 2018ApJ...860...15M
Altcode: 2017arXiv171109434M
GJ 65 is an M dwarf binary system consisting of the two flare stars
BL Cet (GJ 65A) and UV Cet (GJ 65B). Two teams of investigators have
recently reported total magnetic fluxes corresponding to fields of 4.5
and 5.2 kG for GJ 65A and 5.8 and 6.7 kG for GJ 65B: for each component,
the magnetic results obtained by the two teams agree with each other to
within 1σ. For the first time, we can directly compare the predictions
of our magneto-convective models, based on fitting observed stellar
parameters, with measured field strengths. We find that our models
agree with the observed field strengths, provided the effects of
finite conductivity are accounted for. Thus, GJ 65 provides us with
an opportunity to use observations of field strengths to distinguish
between the predictions of our models that assume perfect electrical
conductivity and those that allow for finite conductivity.
Title: K2 Ultracool Dwarfs Survey. III. White Light Flares Are
Ubiquitous in M6-L0 Dwarfs
Authors: Paudel, Rishi R.; Gizis, John E.; Mullan, D. J.; Schmidt,
Sarah J.; Burgasser, Adam J.; Williams, Peter K. G.; Berger, Edo
Bibcode: 2018ApJ...858...55P
Altcode: 2018arXiv180307708P
We report the white light flare rates for 10 ultracool dwarfs using
Kepler K2 short-cadence data. Among our sample stars, two have spectral
type M6, three are M7, three are M8, and two are L0. Most of our
targets are old low-mass stars. We identify a total of 283 flares in
all of the stars in our sample, with Kepler energies in the range log
E Kp ∼ (29-33.5) erg. Using the maximum-likelihood method
of line fitting, we find that the flare frequency distribution (FFD)
for each star in our sample follows a power law with slope -α in the
range -(1.3-2.0). We find that cooler objects tend to have shallower
slopes. For some of our targets, the FFD follows either a broken power
law, or a power law with an exponential cutoff. For the L0 dwarf 2MASS
J12321827-0951502, we find a very shallow slope (-α = -1.3) in the
Kepler energy range (0.82-130) × 1030 erg: this L0 dwarf
has flare rates which are comparable to those of high-energy flares in
stars of earlier spectral types. In addition, we report photometry
of two superflares: one on the L0 dwarf 2MASS J12321827-0951502
and another on the M7 dwarf 2MASS J08352366+1029318. In the case of
2MASS J12321827-0951502, we report a flare brightening by a factor of
∼144 relative to the quiescent photospheric level. Likewise, for
2MASS J08352366+1029318, we report a flare brightening by a factor
of ∼60 relative to the quiescent photospheric level. These two
superflares have bolometric (ultraviolet/optical/infrared) energies 3.6
× 1033 erg and 8.9 × 1033 erg respectively,
while the full width half maximum timescales are very short, ∼2
min. We find that the M8 star TRAPPIST-1 is more active than the
M8.5 dwarf 2M03264453+1919309, but less active than another M8 dwarf
(2M12215066-0843197).
Title: Frequencies of Flare Occurrence: Interaction between Convection
and Coronal Loops
Authors: Mullan, D. J.; Paudel, R. R.
Bibcode: 2018ApJ...854...14M
Altcode: 2018arXiv180107708M
Observations of solar and stellar flares have revealed the presence of
power-law dependences between the flare energy and the time interval
between flares. Various models have been proposed to explain these
dependences and the numerical value of the power-law indices. Here,
we propose a model in which convective flows in granules force
the footpoints of coronal magnetic loops, which are frozen-in to
photospheric gas, to undergo a random walk. In certain conditions,
this can lead to a twist in the loop, which drives the loop unstable
if the twist exceeds a critical value. The possibility that a solar
flare is caused by such a twist-induced instability in a loop has been
in the literature for decades. Here, we quantify the process in an
approximate way with a view to replicating the power-law index. We find
that, for relatively small flares, the random walk twisting model leads
to a rather steep power-law slope that agrees very well with the index
derived from a sample of 56,000+ solar X-ray flares reported by the GOES
satellites. For relatively large flares, we find that the slope of the
power law is shallower. The empirical power-law slopes reported for
flare stars also have a range that overlaps with the slopes obtained
here. We suggest that in the coolest stars, a significant change in
slope should occur when the frozen-flux assumption breaks down due to
low electrical conductivity.
Title: The most powerful white light flare observed on a young
brown dwarf.
Authors: Paudel, Rishi R.; Gizis, John E.; Mullan, Dermott; Schmidt,
Sarah J.; Burgasser, Adam J.; Williams, Peter Kelsey George; Berger,
Edo
Bibcode: 2018AAS...23134910P
Altcode:
We have been using NASA's Kepler K2 mission short cadence (∼1 minute)
and long cadence (∼30 minutes) data to estimate the flare rates of
ultracool dwarfs of various spectral types and ages. These flare rates
will be helpful to understand the nature and evolution of magnetic
dynamo operating in cool objects. We report the detection of the most
powerful white light superflare on a very young brown dwarf. This
superflare has total bolometric energy >1036 erg
with an equivalent duration of ∼110 hours and flare duration of
∼1.84 days. This is the biggest superflare observed on any brown
dwarf. In addition to it, we also observed another superflare on same
target, which is complex (multipeaked) with total bolometric energy
>1034 erg. The occurence of two superflares in a single
K2 campaign implies a higer flare rate of this young dwarf. We compare
its flare rate with other UCDs we have been studying.
Title: Magnetic Modeling of Inflated Low-mass Stars Using Interior
Fields No Larger than ∼10 kG
Authors: MacDonald, James; Mullan, D. J.
Bibcode: 2017ApJ...850...58M
Altcode: 2017arXiv170806994M
We have previously reported on models of low-mass stars in which the
presence of inflated radii is ascribed to magnetic fields that impede
the onset of convection. Some of our magneto-convection models have
been criticized because, when they were first reported by Mullan &
MacDonald, the deep interior fields were found to be very large (50-100
MG). Such large fields are now known to be untenable. For example,
Browning et al. used stability arguments to suggest that interior fields
in low-mass stars cannot be larger than ∼1 MG. Moreover, 3D models
of turbulent stellar dynamos suggest that fields generated in low-mass
interiors may be not much stronger than 10-20 kG. In the present paper,
we present magneto-convective models of inflated low-mass stars in
which the interior fields are not permitted to be stronger than 10
kG. These models are used to fit empirical data for 15 low-mass stars
for which precise masses and radii have been measured. We show that
our 10 kG magneto-convective models can replicate the empirical radii
and effective temperatures for 14 of the stars. In the case of the
remaining star (in the Praesepe cluster), two different solutions have
been reported in the literature. We find that one of these solutions
can be fitted well with our model using the nominal age of Praesepe
(800 Myr). However, the second solution cannot be fitted unless the
star’s age is assumed to be much younger (∼150 Myr).
Title: VizieR Online Data Catalog: Rotation-Activity Correlations
in K-M dwarfs. I. (Houdebine+, 2016)
Authors: Houdebine, E. R.; Mullan, D. J.; Paletou, F.; Gebran, M.
Bibcode: 2017yCat..18220097H
Altcode:
We selected a sample of 419 late-K dwarfs on the basis of (R-I)
measurements available in the literature. Searching through databases
at the European Southern Observatory (ESO) and Observatoire de Haute
Provence (OHP), we identified spectra of 112 different stars that
are suitable for our purposes. The spectra that we use to determine
vsini came from two different echelle spectrographs: HARPS (High
Accuracy Radial velocity Planet Search, ESO; R=115000) and SOPHIE
(OHP; R=75000). (6 data files).
Title: VizieR Online Data Catalog: Rotation-Activity Correlations
in K-M dwarfs II. (Houdebine+, 2017)
Authors: Houdebine, E. R.; Mullan, D. J.; Bercu, B.; Paletou, F.;
Gebran, M.
Bibcode: 2017yCat..18370096H
Altcode:
The spectra that we use for determining the CaII and Hα equivalent
widths in the present study of dK4-dM4 stars came from three different
echelle spectrographs; HARPS (High Accuracy Radial velocity Planet
Search, ESO), SOPHIE (OHP), and FEROS (The Fiber-fed Extended Range
Optical Spectrograph). The stars in our samples include all stars
from all observing programs that have been carried out with HARPS
and SOPHIE for stars belonging to the following spectral sub-types:
dK4, dK6, dM2, and dM3. For dM4 stars, we compiled all measurements
of vsini available in the literature (see Paper I; Houdebine+ 2016,
J/ApJ/822/97). For the dK6 and dM3 samples, we also supplemented
our own measurements with measurements available in the literature,
notably for active stars (see Paper I). (7 data files).
Title: K2 Ultracool Dwarfs Survey. II. The White Light Flare Rate
of Young Brown Dwarfs
Authors: Gizis, John E.; Paudel, Rishi R.; Mullan, Dermott; Schmidt,
Sarah J.; Burgasser, Adam J.; Williams, Peter K. G.
Bibcode: 2017ApJ...845...33G
Altcode: 2017arXiv170308745G
We use Kepler K2 Campaign 4 short-cadence (one-minute) photometry to
measure white light flares in the young, moving group brown dwarfs 2MASS
J03350208+2342356 (2M0335+23) and 2MASS J03552337+1133437 (2M0355+11),
and report on long-cadence (thirty-minute) photometry of a superflare
in the Pleiades M8 brown dwarf CFHT-PL-17. The rotation period (5.24
hr) and projected rotational velocity (45 km s-1) confirm
2M0335+23 is inflated (R≥slant 0.20 {R}⊙ ) as predicted
for a 0.06 {M}⊙ , 24 Myr old brown dwarf βPic moving
group member. We detect 22 white light flares on 2M0335+23. The flare
frequency distribution follows a power-law distribution with slope
-α =-1.8+/- 0.2 over the range 1031 to 1033
erg. This slope is similar to that observed in the Sun and warmer
flare stars, and is consistent with lower-energy flares in previous
work on M6-M8 very-low-mass stars; taking the two data sets together,
the flare frequency distribution for ultracool dwarfs is a power law
over 4.3 orders of magnitude. The superflare (2.6× {10}34
erg) on CFHT-PL-17 shows higher-energy flares are possible. We detect no
flares down to a limit of 2× {10}30 erg in the nearby L5γ
AB Dor moving group brown dwarf 2M0355+11, consistent with the view
that fast magnetic reconnection is suppressed in cool atmospheres. We
discuss two multi-peaked flares observed in 2M0335+23, and argue that
these complex flares can be understood as sympathetic flares, in which
fast-mode magnetohydrodynamic waves similar to extreme-ultraviolet waves
in the Sun trigger magnetic reconnection in different active regions.
Title: LSPM J1314+1320: An Oversized Magnetic Star with Constraints
on the Radio Emission Mechanism
Authors: MacDonald, James; Mullan, D. J.
Bibcode: 2017ApJ...843..142M
Altcode: 2017arXiv170304488M
LSPM J1314+1320 (=NLTT 33370) is a binary star system consisting of
two nearly identical pre-main-sequence stars of spectral type M7. The
system is remarkable among ultracool dwarfs for being the most luminous
radio emitter over the widest frequency range. Masses and luminosities
are at first sight consistent with the system being coeval at age ∼80
Myr according to standard (nonmagnetic) evolutionary models. However,
these models predict an average effective temperature of ∼2950 K,
which is 180 K hotter than the empirical value. Thus, the empirical
radii are oversized relative to the standard models by ≈13%. We
demonstrate that magnetic stellar models can quantitatively account
for the oversizing. As a check on our models, we note that the radio
emission limits the surface magnetic field strengths: the limits
depend on identifying the radio emission mechanism. We find that the
field strengths required by our magnetic models are too strong to
be consistent with gyrosynchrotron emission but are consistent with
electron cyclotron maser emission.
Title: The Rotation-Activity Correlations in K and M Dwarfs. II. New
Constraints on the Dynamo Mechanisms in Late-K and M Dwarfs Before
and At the Transition to Complete Convection
Authors: Houdebine, E. R.; Mullan, D. J.; Bercu, B.; Paletou, F.;
Gebran, M.
Bibcode: 2017ApJ...837...96H
Altcode: 2017arXiv170107608H
We study the rotation-activity correlations (RACs) in a sample of
stars from spectral type dK4 to dM4. We study RACs using chromospheric
data and coronal data. We study the Ca II line surface fluxes-P/\sin
I RACs. We fit the RACs with linear homoscedastic and heteroscedastic
regression models. We find that these RACs differ substantially from
one spectral sub-type to another. For dM3 and dM4 stars, we find
that the RACs cannot be described by a simple model, but instead
that there may exist two distinct RAC behaviors for the low-activity
and the high-activity stellar sub-samples, respectively. Although
these results are preliminary and will need confirmation, the data
suggest that these distinct RACs may be associated with different
dynamo regimes. We also study {R}{HK}\prime
as a function of the Rossby number R 0. We find (I) for
dK4 stars, {R}{HK}\prime as a function of R
0 agrees well with previous results for F-G-K stars and
(II) in dK6, dM2, dM3, and dM4 stars, at a given R 0, the
values of {R}{HK}\prime lie at a factor of 3,
10, 20, and 90, respectively, below the F-G-K RAC. Our results suggest
a significant decrease in the efficiency of the dynamo mechanism(s)
as regards chromospheric heating before and at dM3, I.e., before and at
the transition to complete convection. We also show that the ratio of
coronal heating to chromospheric heating L X/L HK
increases by a factor of 100 between dK4 and dM4 stars. Based
on observations available at Observatoire de Haute Provence and the
European Southern Observatory databases and on Hipparcos parallax
measurements.
Title: The Age of the KIC 7177553 System
Authors: MacDonald, James; Mullan, D. J.
Bibcode: 2017ApJ...834...99M
Altcode: 2016arXiv160900942M
KIC 7177553 is a quadruple system containing two binaries of orbital
periods 16.5 and 18 days. All components have comparable masses and
are slowly rotating with spectral types of ∼G2V. The longer period
binary is eclipsing with component masses and radii M1
= 1.043 ± 0.014 M⊙, R1 = 0.940 ± 0.005
R⊙ and M2 = 0.986 ± 0.015 M⊙,
R2 = 0.941 ± 0.005 R⊙. The essentially equal
radii measurements are inconsistent with the two stars being on the man
sequence at the same age using standard nonmagnetic stellar evolution
models. Instead a consistent scenario is found if the stars are in
their pre-main-sequence phase of evolution and have an age of 32-36
Myr. We have also computed evolutionary models of magnetic stars,
but we find that our nonmagnetic models fit the empirical radii and
effective temperatures better than the magnetic models.
Title: Apparent Non-coevality among the Stars in Upper Scorpio:
Resolving the Problem Using a Model of Magnetic Inhibition of
Convection
Authors: MacDonald, James; Mullan, D. J.
Bibcode: 2017ApJ...834...67M
Altcode: 2016arXiv160802136M
Two eclipsing binaries in the USco association have recently yielded
precise values of masses and radii for four low-mass members of the
association. Standard evolution models would require these dM4.5-dM5
stars to have ages which are younger than those of more massive
stars in the association by factors which appear (in extreme cases)
to be as large as ∼3. Are the stars in the association therefore
non-coeval? We suggest that the answer is no: by incorporating the
effects of magnetic inhibition of convective onset, we show that the
stars in USco can be restored to coevality provided the four low-mass
member stars have vertical surface fields in the range 200-700 G. Fields
of such magnitude have already been measured on the surface of certain
solar-type stars in other young clusters.
Title: Rotation-Activity Correlations in K and M Dwarfs. I. Stellar
Parameters and Compilations of v sin I and P/sin I for a Large Sample
of Late-K and M Dwarfs
Authors: Houdebine, E. R.; Mullan, D. J.; Paletou, F.; Gebran, M.
Bibcode: 2016ApJ...822...97H
Altcode: 2016arXiv160407920H
The reliable determination of rotation-activity correlations (RACs)
depends on precise measurements of the following stellar parameters:
T eff, parallax, radius, metallicity, and rotational speed
v sin I. In this paper, our goal is to focus on the determination of
these parameters for a sample of K and M dwarfs. In a future paper
(Paper II), we will combine our rotational data with activity data
in order to construct RACs. Here, we report on a determination of
effective temperatures based on the (R-I) C color from
the calibrations of Mann et al. and Kenyon & Hartmann for four
samples of late-K, dM2, dM3, and dM4 stars. We also determine stellar
parameters (T eff, log(g), and [M/H]) using the principal
component analysis-based inversion technique for a sample of 105 late-K
dwarfs. We compile all effective temperatures from the literature for
this sample. We determine empirical radius-[M/H] correlations in our
stellar samples. This allows us to propose new effective temperatures,
stellar radii, and metallicities for a large sample of 612 late-K and
M dwarfs. Our mean radii agree well with those of Boyajian et al. We
analyze HARPS and SOPHIE spectra of 105 late-K dwarfs, and we have
detected v sin I in 92 stars. In combination with our previous v sin
I measurements in M and K dwarfs, we now derive P/sin I measures for
a sample of 418 K and M dwarfs. We investigate the distributions of
P/sin I, and we show that they are different from one spectral subtype
to another at a 99.9% confidence level. Based on observations
available at Observatoire de Haute Provence and the European Southern
Observatory databases and on Hipparcos parallax measurements.
Title: Mechanical Energy Fluxes Associated with Saturated Coronal
Heating in M Dwarfs: Comparison with Predictions of a Turbulent Dynamo
Authors: Mullan, D. J.; MacDonald, J.
Bibcode: 2016ApJ...818..154M
Altcode: 2016arXiv160203079M
Empirically, the X-ray luminosity LX from M dwarfs has been
found to have an upper limit of about 0.2% of the bolometric flux
Lbol. In the limit where magnetic fields in M dwarfs are
generated in equipartition with convective motions, we use stellar
models to calculate the energy flux of Alfvén waves FA
as a function of depth in the sub-surface convection zone. Since
Alfvén waves have the optimal opportunity for wave modes to reach
the corona, we suggest that FA sets an upper limit on the
mechanical flux Fmech which causes coronal heating. This
suggestion accounts quantitatively for the “saturated” values of
LX/Lbol which have been reported empirically
for M dwarfs.
Title: A Model for Interface Dynamos in Late K and Early M Dwarfs
Authors: Mullan, D. J.; Houdebine, E. R.; MacDonald, J.
Bibcode: 2015ApJ...810L..18M
Altcode:
Measurements of the equivalent width EW(CaK) of emission in the Ca ii
K line have been obtained by Houdebine et al. for stars with spectral
types from dK5 to dM4. In order to explain the observed variations
of EW(CaK) with spectral sub-type, we propose a quantitative model of
interface dynamos in low-mass stars. Our model leads to surface field
strengths Bs which turn out to be essentially linearly
proportional to EW(CaK). This result is reminiscent of the Sun, where
Skumanich et al. found that the intensity of CaK emission in solar
active regions is linearly proportional to the local field strength.
Title: Magnetoconvective models of red dwarfs: constraints imposed
by the lithium abundance
Authors: MacDonald, J.; Mullan, D. J.
Bibcode: 2015MNRAS.448.2019M
Altcode: 2015arXiv150104550M
Magnetic fields impede the onset of convection, thereby altering the
thermal structure of a convective envelope in a low-mass star: this has
an effect on the amount of lithium depletion in a magnetized star. In
order to quantify this effect, we have applied a magnetoconvective
model to two low-mass stars for which lithium abundances and precise
structural parameters are known: YY Gem and CU Cnc. For both stars,
we have obtained models which satisfy empirical constraints on the
following parameters: R, L, surface magnetic field strength, and
Li abundance. In the case of YY Gem, we have obtained a model which
satisfies the empirical constraints with an internal magnetic field
of several megagauss: such a field strength is within the range of
a dynamo where the field energy is in equipartition with rotational
energy deep inside the convection zone. However, in the case of CU
Cnc, the Li requires an internal magnetic field which is probably too
strong for a dynamo origin: we suggest possible alternatives which
might account for the reported Li abundance in CU Cnc.
Title: Dynamics of Rotation in M Dwarfs: Indications for a Change
in the Dynamo Regime in Stars at the Onset of Complete Convection
Authors: Houdebine, E. R.; Mullan, D. J.
Bibcode: 2015ApJ...801..106H
Altcode: 2015arXiv150201266H
We have measured vsin i with high precision for a sample of dM3 stars
(86 targets). We detected rotation in 82 stars (73 dM3 stars and
nine dM3e stars). We compare our measurements of vsin i for all of
the stars in our dM0, dM2, dM3, and dM4 samples to those from other
authors. We find a good agreement down to vsin i values of less than
1 km s-1. The mean of the differences between measurements
is only 0.42 km s-1. We find that the distribution of P/sini
for our dM3 stars is different from the distribution of P/sini among our
samples of dM2 and dM4 stars. The mean rotation rate for the dM3 stars
(excluding dM3e and sdM3 stars) is significantly slower (25.8 days) than
for dM2 (14.4 days) and dM4 stars (11.4 days). Analogous behavior also
emerges among the faster rotators (dMe stars): we find that a longer
rotation period also occurs at spectral subtype dM3e. Our data suggest
that, as regards the rotational properties of lower main-sequence stars,
spectral subtype dM3 stands out as exhibiting unusually slow rotation
compared to that of adjoining subtypes. Our data lead us to suggest that
the unusual rotational properties of M3 dwarfs may represent a signature
of the transition to complete convection. Based on observations
available at Observatoire de Haute Provence and the European Southern
Observatory databases and on Hipparcos parallax measurements.
Title: Surface Magnetic Field Strengths: New Tests of
Magnetoconvective Models of M Dwarfs
Authors: MacDonald, James; Mullan, D. J.
Bibcode: 2014ApJ...787...70M
Altcode:
Precision modeling of M dwarfs has become worthwhile in recent years
due to the increasingly precise values of masses and radii which
can be obtained from eclipsing binary studies. In a recent paper,
Torres has identified four prime M dwarf pairs with the most precise
empirical determinations of masses and radii. The measured radii are
consistently larger than standard stellar models predict by several
percent. These four systems potentially provide the most challenging
tests of precision evolutionary models of cool dwarfs at the present
time. We have previously modeled M dwarfs in the context of a criterion
due to Gough & Tayler in which magnetic fields inhibit the onset
of convection according to a physics-based prescription. In the
present paper, we apply our magnetoconvective approach to the four
prime systems in the Torres list. Going a step beyond what we have
already modeled in CM Dra (one of the four Torres systems), we note
that new constraints on magnetoconvective models of M dwarfs are now
available from empirical estimates of magnetic field strengths on the
surfaces of these stars. In the present paper, we consider how well our
magnetoconvective models succeed when confronted with this new test of
surface magnetic field strengths. Among the systems listed by Torres,
we find that plausible magnetic models work well for CM Dra, YY Gem,
and CU Cnc. (The fourth system in Torres's list does not yet have
enough information to warrant magnetic modeling.) Our magnetoconvection
models of CM Dra, YY Gem, and CU Cnc yield predictions of the magnetic
fluxes on the stellar surface which are consistent with the observed
correlation between magnetic flux and X-ray luminosity.
Title: The theoretical instability strip of M dwarf stars
Authors: Rodríguez-López, C.; MacDonald, J.; Amado, P. J.; Moya,
A.; Mullan, D.
Bibcode: 2014MNRAS.438.2371R
Altcode: 2013arXiv1312.2743R; 2013MNRAS.tmp.2985R
The overstability of the fundamental radial mode in M dwarf models
was theoretically predicted recently by Rodríguez-López et al. The
periods were found to be in the ranges ∼25-40 min and ∼4-8 h,
depending on stellar age and excitation mechanism. We have extended
our initial M dwarf model grid in mass, metallicity and mixing length
parameter. We have also considered models with boundary conditions from
PHOENIX NEXTGEN atmospheres to test their influence on the pulsation
spectra. We find instability of non-radial modes with radial orders up
to k = 3, degree ℓ = 0-3, including p- and g-modes, with the period
range extending from 20 min up to 11 h. Furthermore, we find theoretical
evidence of the potential of M dwarfs as solar-like oscillators.
Title: Lithium abundance and surface magnetic fields: new constraints
in magnetic models of M dwarfs
Authors: MacDonald, James; Mullan, Dermott
Bibcode: 2013arXiv1311.3246M
Altcode:
Precision modeling of M dwarfs has become worthwhile in recent years
due to the increasingly precise values of masses and radii which can be
obtained from eclipsing binary studies. Torres (2013) has identified
4 prime M dwarf pairs with the most precise empirical determinations
of masses and radii. The measured radii are consistently larger than
standard stellar models predict. We have previously modeled M dwarfs in
the context of a criterion due to Gough & Tayler in which magnetic
fields inhibit the onset of convection according to a physics-based
prescription. New constraints on the models of M dwarfs are now provided
by measurements of lithium abundances. The key aspect of Li in terms of
setting constraints on magnetic modeling is that Li burning starts at T
= 2.5 MK, and temperatures of just such magnitude are associated with
the base of the convection zone: magnetic inhibition of convective
onset can shift this base slightly closer to the surface, i.e. to
slightly lower temperatures, thereby reducing the amount of Li depletion
compared to a non-magnetic model. In the present paper, we consider how
our magneto-convection models handle the new test of stellar structure
provided by Li measurements. Among the prime systems listed by Torres,
we find that plausible magnetic models work well for CM Dra and YY Gem
but not for CU Cnc. (The fourth system in Torres's list does not yet
have enough information to warrant magnetic modeling.) For CU Cnc,
we suggest that the observed lithium may have been accreted from a
circumstellar disk. We find that our magneto-convection models of CM
Dra, YY Gem and CU Cnc yield results which are consistent with the
observed correlation between magnetic flux and X-ray luminosity.
Title: Magnetic Effects and Oversized M Dwarfs in the Young Open
Cluster NGC 2516
Authors: MacDonald, James; Mullan, Dermott J.
Bibcode: 2013ApJ...765..126M
Altcode: 2013arXiv1302.2941M
By combining rotation periods with spectroscopic determinations of
projected rotation velocity, Jackson et al. have found that the mean
radii for low-mass M-dwarfs in the young, open cluster NGC 2516 are
larger than model predictions at a given absolute I magnitude or I -
K color and also larger than measured radii of magnetically inactive
M-dwarfs. The relative radius difference is correlated with magnitude,
increasing from a few percent at MI = 7 to greater than 50%
for the lowest luminosity stars in their sample at MI ~
9.5. Jackson et al. have suggested that a two-temperature star spot
model is capable of explaining the observations, but their model
requires spot coverage fractions of at least 50% in rapidly rotating
M-dwarfs. Here we examine these results in terms of stellar models that
include the inhibiting effects of magnetic fields on convective energy
transport, with and without the effects of star spots. We find that a
pure spot model is inconsistent with the color-magnitude diagram. The
observations of radii versus color and radii versus absolute magnitude
in NGC 2516 are consistent with models which include only magnetic
inhibition or a combination of magnetic inhibition and spots. At a
given mass we find a large dispersion in the strength of the vertical
component of the magnetic field in the stellar photosphere but the
general trend is that the vertical field increases with decreasing
mass from a few hundred Gauss at 0.65 M ⊙ to 600-900 G,
depending on spot coverage, in the lowest mass stars in the sample at
0.25 M ⊙.
Title: Solar Cycle Variation of Sound Speed inside the Sun
Authors: Mullan, D. J.; MacDonald, J.; Rabello-Soares, M. C.
Bibcode: 2012ApJ...755...79M
Altcode:
Empirical radial profiles of the changes in sound speed inside the
Sun between solar minimum and solar maximum have been extracted
from Michelson Doppler Imager data by Baldner & Basu and
Rabello-Soares. Here, we compare these results with the theoretical
radial profiles predicted by a model of magnetic inhibition of
convective onset: In the model, the degree of magnetic inhibition
is characterized by a parameter δ, which is essentially the ratio of
magnetic pressure to gas pressure. We find that the theoretical profiles
overlap significantly with the empirical results in the outer half of
the convection zone. But differences in the deeper layers indicate that
the model needs to be modified there. The main result that emerges in
the present comparison is that the value of δ must be larger near the
surface than at great depth. A secondary result is that, in the course
of the solar cycle, the magnetic field magnitude at the base of the
convection zone may be out of phase with the field near the surface.
Title: Precision modelling of M dwarf stars: the magnetic components
of CM Draconis
Authors: MacDonald, J.; Mullan, D. J.
Bibcode: 2012MNRAS.421.3084M
Altcode: 2011arXiv1106.1452M; 2012MNRAS.tmp.2466M
The eclipsing binary CM Draconis (CM Dra) contains two nearly identical
red dwarfs of spectral class dM4.5. The masses and radii of the two
components have been reported with unprecedentedly small statistical
errors: for M, these errors are 1 part in 260, while for R, the errors
reported by Morales et al. are 1 part in 130. When compared with
standard stellar models with appropriate mass and age (≈4 Gyr),
the empirical results indicate that both components are discrepant
from the models in the following sense: the observed stars are larger
in R ('bloated'), by several standard deviations, than the models
predict. The observed luminosities are also lower than the models
predict. Here, we attempt at first to model the two components of CM
Dra in the context of standard (non-magnetic) stellar models using a
systematic array of different assumptions about helium abundances (Y),
heavy element abundances (Z), opacities and mixing length parameter
(α). We find no 4-Gyr-old models with plausible values of these four
parameters that fit the observed L and R within the reported statistical
error bars. However, CM Dra is known to contain magnetic fields,
as evidenced by the occurrence of star-spots and flares. Here we ask:
can inclusion of magnetic effects into stellar evolution models lead to
fits of L and R within the error bars? Morales et al. have reported that
the presence of polar spots results in a systematic overestimate of R by
a few per cent when eclipses are interpreted with a standard code. In a
star where spots cover a fraction f of the surface area, we find that
the revised R and L for CM Dra A can be fitted within the error bars
by varying the parameter α. The latter is often assumed to be reduced
by the presence of magnetic fields, although the reduction in α as a
function of B is difficult to quantify. An alternative magnetic effect,
namely inhibition of the onset of convection, can be readily quantified
in terms of a magnetic parameter δ≈B2/4πγpgas
(where B is the strength of the local vertical magnetic field). In the
context of δ models in which B is not allowed to exceed a 'ceiling'
of 106 G, we find that the revised R and L can also be
fitted, within the error bars, in a finite region of the f-δ plane. The
permitted values of δ near the surface leads us to estimate that the
vertical field strength on the surface of CM Dra A is about 500 G,
in good agreement with independent observational evidence for similar
low-mass stars. Recent results for another binary with parameters close
to those of CM Dra suggest that metallicity differences cannot be the
dominant explanation for the bloating of the two components of CM Dra.
Title: Modeling Low-mass Stars in the Presence of Magnetic Fields
Authors: Mullan, D. J.; MacDonald, J.
Bibcode: 2011ASPC..448.1209M
Altcode: 2011csss...16.1209M
In the presence of a vertical magnetic field, the onset of convection
in a star is inhibited relative to its onset in a non-magnetic
star. Following early work of Gough and Tayler for a perfectly
conducting medium, and including subsequent modifications due to finite
magnetic resistivity, we have computed models of low-mass stars using a
magnetically-altered criterion for the onset of convection. Our models
have larger radii and cooler effective temperatures than non-magnetic
models with the same mass and age. Our magnetic models provide a
unified framework for interpreting a range of observational data,
including radii which are observed to be "bloated" by as much as 10%
relative to standard models, and reduced effective temperatures,
for stars ranging in spectral types from solar to brown dwarfs.
Title: The M4 Transition: Toward a Comprehensive Understanding of
the Transition into the Fully Convective Regime
Authors: Stassun, K. G.; Hebb, L.; Covey, K.; West, A. A.; Irwin,
J.; Jackson, R.; Jardine, M.; Morin, J.; Mullan, D.; Reid, I. N.
Bibcode: 2011ASPC..448..505S
Altcode: 2010arXiv1012.2580S; 2011csss...16..505S
The difference in stellar structure above and below spectral type
∼M4 is expected to be a very important one, connected directly or
indirectly to a variety of observational phenomena in cool stars --
such as rotation, activity, magnetic field generation and topology,
timescales for evolution of these, and even the basic mass-radius
relationship. In this Cool Stars XVI Splinter Session, we aimed to
use the M4 transition as an opportunity for discussion about the
interiors of low-mass stars and the mechanisms which determine their
fundamental properties. By the conclusion of the session, several key
points were elucidated. Although M dwarfs exhibit significant changes
across the fully convective boundary, this "M4 transition" is not
observationally sharp or discrete. Instead, the properties of M dwarfs
(radius, effective temperature, rotation, activity lifetime, magnetic
field strength and topology) show smooth changes across M3-M6 spectral
types. In addition, a wide range of stellar masses share similar
spectral types around the fully convective transition. There appears to
be a second transition at M6-M8 spectral types, below which there exists
a clear dichotomy of magnetic field topologies. Finally, we used the
information and ideas presented in the session to construct a framework
for how the structure of an M dwarf star, born with specific mass and
chemical composition, responds to the presence of its magnetic field,
itself driven by a feedback process that links the star's rotation,
interior structure, and field topology.
Title: Threshold effects in assembling a cell on a young planet
Authors: Mullan, D. J.
Bibcode: 2011SPIE.8152E..14M
Altcode: 2011SPIE.8152E..29M
Molecules in the young Earth's oceans underwent a finite number C
of collisions during the time interval which elapsed between Earth's
formation and the emergence of life. The finiteness of C sets an upper
limit G on the number of constituent molecules which could have been
randomly assembled into a genetic molecule from a reservoir of the
relevant constituent molecules for use in the first cell. A viable cell
requires a certain minimum number of genes in order to be functional:
this means that the genetic material must include a minimum number M of
the constituent molecules which encode for information. The question
we address here is: is it mathematically possible for G to take on
values which are at least as large as M? The answer is Yes, provided
that we allow for the fact that there is a large amount of flexibility
("non-specificity") in the operation of biological molecules. When
this flexibility is allowed for, we find that G can be comparable to M
provided that the mass of constituent molecules which were available
in the pre-biotic Earth exceeded a certain threshold. Our estimates
indicate that a mass on the order of a few tons would satisfy the
threshold.
Title: Estimates of Densities and Filling Factors from a Cooling
Time Analysis of Solar Microflares Observed with RHESSI
Authors: Baylor, R. N.; Cassak, P. A.; Christe, S.; Hannah, I. G.;
Krucker, Säm; Mullan, D. J.; Shay, M. A.; Hudson, H. S.; Lin, R. P.
Bibcode: 2011ApJ...736...75B
Altcode: 2011arXiv1107.3997B
We use more than 4500 microflares from the RHESSI microflare data set to
estimate electron densities and volumetric filling factors of microflare
loops using a cooling time analysis. We show that if the filling factor
is assumed to be unity, the calculated conductive cooling times are
much shorter than the observed flare decay times, which in turn are
much shorter than the calculated radiative cooling times. This is likely
unphysical, but the contradiction can be resolved by assuming that the
radiative and conductive cooling times are comparable, which is valid
when the flare loop temperature is a maximum and when external heating
can be ignored. We find that resultant radiative and conductive cooling
times are comparable to observed decay times, which has been used as an
assumption in some previous studies. The inferred electron densities
have a mean value of 1011.6 cm-3 and filling
factors have a mean of 10-3.7. The filling factors are lower
and densities are higher than previous estimates for large flares,
but are similar to those found for two microflares by Moore et al.
Title: Magneto-convection and Lithium Age Estimates of the β Pictoris
Moving Group
Authors: Macdonald, J.; Mullan, D. J.
Bibcode: 2010ApJ...723.1599M
Altcode: 2010arXiv1006.1308M
Although the means of the ages of stars in young groups determined
from Li depletion often agree with mean ages determined from
Hertzsprung-Russell (H-R) diagram isochrones, there are often
statistically significant differences in the ages of individual stars
determined by the two methods. We find that inclusion of the effects of
inhibition of convection due to the presence of magnetic fields leads
to consistent ages for the individual stars. We illustrate how age
consistency arises by applying our results to the β Pictoris moving
group (BPMG). We find that, although magnetic inhibition of convection
leads to increased ages from the H-R diagram isochrones for all stars,
Li ages are decreased for fully convective M stars and increased for
stars with radiative cores. Our consistent age determination for BPMG
of 40 Myr is larger than previous determinations by a factor of about
two. We have also considered models in which the mixing length ratio is
adjusted to give consistent ages. We find that our magneto-convection
models, which give quantitative estimates of magnetic field strength,
provide a viable alternative to models in which the effects of magnetic
fields (and other processes) are accounted for by reducing the mixing
length ratio.
Title: Patterns of X-ray, Chromospheric, and Radio Emission in
Low-mass Stars: Fast and Slow Magnetic Reconnection
Authors: Mullan, D. J.
Bibcode: 2010ApJ...721.1034M
Altcode:
Magnetic reconnection events in the atmospheres of low-mass dwarf stars
can be classified as either slow or fast, depending on whether ohmic
diffusion or Hall currents dominate in the reconnection process. We
suggest that the separation of reconnection into slow and fast
categories can help to explain some systematics of low-mass dwarfs as
regards their emissions in X-rays, Hα, and radio. On the one hand, in
the warmer dwarfs (<M7), where electrical resistivities are small,
fast reconnection is permitted, and this can explain the occurrence of
flares and "quiescent" coronal heating. On the other hand, the fact
that the coolest dwarfs (>M7) are inefficient emitters in Hα and
X-rays but strong emitters in radio, may be understood in the context
that only slow reconnection is permitted to occur in those stars,
as a result of high electrical resistivity. However, even though only
slow reconnection is permitted in the latter stars, the speed of the
outflow jets from reconnection sites can serve as efficient sources of
radio emission as a result of the electron cyclotron maser instability.
Title: Magnetic Models of the Brown Dwarfs HD 130948b and HD 130948c
Authors: Mullan, D. J.; MacDonald, J.
Bibcode: 2010ApJ...713.1249M
Altcode:
Dynamical masses have been obtained by Dupuy et al. for the brown
dwarf binary HD 130948BC. The components have luminosities log (L/L
sun) of -3.82 and -3.90, and masses of 0.0555 and 0.0530
M sun. In a luminosity-age diagram, Dupuy et al. found
that L bol for both B and C are brighter than theoretical
tracks by factors of 2-3, if the age of the system is as old as their
estimate of 0.79 Gyr (based on gyrochronology). Here, we apply our
model of magnetic convection, in which the onset of convection is
impeded in the presence of a vertical magnetic field: our goal is
to replicate the observed properties not only in the luminosity-age
diagram, but simultaneously in the T eff -age diagram and
in the H-R diagram. Expressing the internal magnetic pressure as a
fraction δ of the gas pressure, we obtain evolutionary tracks which
fit both stars in an H-R diagram provided that δ lies in the range
0.007-0.038. With such values of δ, our models replicate the observed
luminosities of both B and C, provided that the age is no larger than
~0.39 Gyr. This is significantly younger than the mean age estimated by
Dupuy et al. for the primary star in the system, HD 130948A. However,
there is sufficient uncertainty in the empirical parameters that an
age as young as the one suggested by our magnetic models (~0.39 Gyr)
is marginally within the permitted range.
Title: Non-sparse Sampling of the Genomic Phase-Space of a Proto-cell
in the Young Earth
Authors: Mullan, D. J.
Bibcode: 2009ASPC..420..287M
Altcode:
We estimate the numbers of reactions which occurred between certain
molecules in liquid water during an interval of 200 My on the young
Earth. We compare this with the size of genomic phase-space for a
primitive cell whose genetic material consists of single-strand RNA. We
find that the reactions which occurred in 200 My are sufficiently
numerous to allow non-sparse sampling of a genome containing p ≍
15 genes, provided the genetic code used doublet codons. We argue on
general grounds that p ≥ 10 could give rise to a viable cell.
Title: Flares on a Bp Star
Authors: Mullan, D. J.
Bibcode: 2009ApJ...702..759M
Altcode:
Two large X-ray flares have been reported from the direction of a
magnetic B2p star (σ Ori E). Sanz-Forcada et al. have suggested that
the flares did not occur on the B2p star but on a companion of late
spectral type. A star which is a candidate for a late-type flare star
near σ Ori E has recently been identified by Bouy et al. However,
based on the properties of the flares, and based on a recent model of
rotating magnetospheres, we argue that, rather than attributing the two
flares to a late-type dwarf, it is a viable hypothesis that the flares
were magnetic phenomena associated with the rotating magnetosphere of
the B2p star itself.
Title: Big bang nucleosynthesis: The strong nuclear force meets the
weak anthropic principle
Authors: MacDonald, J.; Mullan, D. J.
Bibcode: 2009PhRvD..80d3507M
Altcode: 2009arXiv0904.1807M
Contrary to a common argument that a small increase in the strength
of the strong force would lead to destruction of all hydrogen in
the big bang due to binding of the diproton and the dineutron with
a catastrophic impact on life as we know it, we show that provided
the increase in strong force coupling constant is less than about 50%
substantial amounts of hydrogen remain. The reason is that an increase
in strong force strength leads to tighter binding of the deuteron,
permitting nucleosynthesis to occur earlier in the big bang at higher
temperature than in the standard big bang. Photodestruction of the less
tightly bound diproton and dineutron delays their production to after
the bulk of nucleosynthesis is complete. The decay of the diproton can,
however, lead to relatively large abundances of deuterium.
Title: Structural Effects of Magnetic Fields in Brown Dwarfs
Authors: MacDonald, J.; Mullan, D. J.
Bibcode: 2009ApJ...700..387M
Altcode:
In the brown dwarf (BD) binary 2M0535 - 05, Stassun et al. have reported
that the more massive primary has a lower T eff than the less
massive secondary. Here, we report results obtained by an evolutionary
code in which the criterion for the onset of convection in the primary
is modified in the presence of a magnetic field. Structural alterations
to the primary lead to a lower T eff and a larger radius
than would occur in a non-magnetic BD of the same age mass and age. The
observed value of T eff can be explained if the field in
the primary increases in strength from 120-320 G at the surface to
5-13 MG at the center. With zero field in the secondary, our models
indicate that both components can be co-eval with an age of 1.0-1.3
Myr. Because the binary is so young, the components have not yet had
time to synchronize their rotations: differences in angular velocity may
explain why one component has developed a field while the other has not.
Title: The Nonlinear Dynamics of Magnetic Reconnection and
Applications to Solar Eruptions
Authors: Cassak, P. A.; Shay, M. A.; Drake, J. F.; Eckhardt, B.;
Mullan, D. J.
Bibcode: 2008AGUFMSM23C..01C
Altcode:
It is widely accepted that magnetic reconnection plays a fundamental
role in releasing magnetic energy in solar eruptions, but what
initiates an eruption remains an unresolved problem. We present
a model for the catastrophic onset of fast magnetic reconnection
in weakly collisional plasmas, which potentially explains why
the energy release begins abruptly. In particular, we show that
magnetic reconnection is bistable: both the slow (Sweet-Parker)
and fast (Hall) modes of reconnection independently exist for a wide
range of parameters. However, the Sweet-Parker solution disappears
catastrophically at a critical condition, leading to the onset of
Hall reconnection and the accompanying increase in the rate of
magnetic energy release by many orders of magnitude. We present
theory and simulations confirming the model. We propose that the
disappearance of reconnection solutions is caused by saddle-node
bifurcations controlling the nonlinear dynamics of reconnection, and
derive a simple nonlinear model that displays this dynamical behavior
[1]. The model predicts the existence of an unstable reconnection
solution, which we demonstrate numerically. The unstable solution
reveals that the physics controlling the onset of Hall reconnection
occurs locally near the X-line as opposed to remotely at the boundaries
[2]. We discuss potential mechanisms for onset and show that the model
is consistent with results from laboratory experiments and solar and
stellar flare observations. [1] P. A. Cassak, Doctoral Thesis, 2006;
[2] P. A. Cassak et al., Phys. Rev. Lett., 98, 215001, 2007.
Title: The Role of Magnetic Reconnection in Self-Organization of
the Corona: Theory and Observations
Authors: Cassak, P. A.; Mullan, D. J.; Shay, M. A.
Bibcode: 2008AGUFMNG41A..03C
Altcode:
Based on observations that solar flares obey power law statistics, it
was suggested that the solar corona is in a state of self-organized
criticality [1]. However, the physical mechanism underlying the
dynamics is not well understood. A recent model [2] describing the
catastrophic onset of fast (Hall) magnetic reconnection in weakly
collisional plasmas may potentially contribute to this discussion. We
suggest that the condition at which the catastrophic onset of
reconnection occurs sets the critical state of the corona and the
physics of reconnection organizes the corona into this critical state
[3]. (See also [4].) The model makes a quantitative prediction for the
conditions of the corona at the onset of eruptions, which is known
to be consistent with observations of the solar corona. We present
new observational evidence from stellar flares (107 events in 37
sun-like stars) that stellar coronae are near the same critical state
at flare onset. This provides observational evidence in support of the
model and suggests that magnetic reconnection plays an active role in
constraining the conditions in solar and stellar coronae. Implications
for self-organization in coronal heating and solar eruptions will
be discussed. [1] E. T. Lu and R. J. Hamilton, Ap. J., 380, L89,
1991; [2] P. A. Cassak et al., Phys. Rev. Lett., 95, 235002, 2005;
P. A. Cassak et al., Ap. J. Lett., 644, L145, 2006; [3] P. A. Cassak
et al., Ap. J. Lett., 676, L69, 2008; [4] D. A. Uzdensky, Ap. J., 671,
2139, 2007.
Title: From Solar and Stellar Flares to Coronal Heating: Theory and
Observations of How Magnetic Reconnection Regulates Coronal Conditions
Authors: Cassak, P. A.; Mullan, D. J.; Shay, M. A.
Bibcode: 2008ApJ...676L..69C
Altcode: 2007arXiv0710.3399C
There is currently no explanation of why the corona has the temperature
and density it has. We present a model that explains how the dynamics
of magnetic reconnection regulates the conditions in the corona. A
bifurcation in magnetic reconnection at a critical state enforces
an upper bound on the coronal temperature for a given density. We
present observational evidence from 107 flares in 37 Sun-like stars
that stellar coronae are near this critical state. The model may be
important to self-organized criticality models of the solar corona.
Title: Observational Evidence of the Role of Collisionless Magnetic
Reconnection in Self- Organization of Solar and Stellar Coronae
Authors: Mullan, D.; Cassak, P. A.; Shay, M. A.
Bibcode: 2007AGUFMSH41C..03M
Altcode:
Magnetic reconnection is widely accepted to be the driver of eruptive
activity in the solar corona. However, questions about how a large
amount of magnetic energy can accumulate before an eruption without
triggering fast reconnection, why fast magnetic reconnection begins
abruptly, and under what coronal conditions an eruption occurs
remain unanswered. Observations of solar and stellar flares have
revealed similar phenomenologies and power law statistics, suggesting
that eruptions in solar and stellar coronae are driven by the same
physical process. The existence of power law statistics has prompted
the suggestion that solar and stellar coronae are self-organized, but
the physical foundation of this theory has been lacking. We present
observational evidence from 107 flare events in 37 sun-like stars
that solar and stellar coronae self-organize into a state in which the
coronal plasma is marginally collisional. This corroborates a recent
model in which coronal eruptions are initiated by a catastrophic
onset of collisionless magnetic reconnection [1]. These results
demonstrate that the dynamics of magnetic reconnection play an active
role in constraining the conditions of solar and stellar coronae. [1]
P. A. Cassak, M. A. Shay, and J. F. Drake, Phys. Rev. Lett., 95, 235002
(2005).
Title: Magnetic Cycles in the Sun: Modeling the Changes in Radius,
Luminosity, and p-Mode Frequencies
Authors: Mullan, D. J.; MacDonald, J.; Townsend, R. H. D.
Bibcode: 2007ApJ...670.1420M
Altcode:
We report on the results obtained with a stellar evolution code in
which cyclic magnetic fields are imposed in the convection zone of a 1.0
Msolar star. Magnetic effects are incorporated in two ways:
(1) the field pressure and energy density are included in the equations
of hydrostatic equilibrium and conservation of energy; and (2) the field
inhibits the onset of convection according to a prescription derived by
Gough & Tayler (1966). Inserting magnetic fields into the convection
zone with strengths comparable to the observed global fields in the
Sun, and assuming a simple depth dependence for the field strength,
we find differences in luminosity and radius between nonmagnetic and
magnetic models that are consistent in amplitude with the observed
activity-related changes in the Sun. Using the same magnetic fields,
and computing p-mode frequencies for nonmagnetic and magnetic models, we
find that the frequencies in a magnetic model are larger than those for
a nonmagnetic model. The frequency differences between nonmagnetic and
magnetic models agree in sign, and overlap in magnitude and frequency
dependence, with the shifts in frequency which have been observed
in the Sun between solar minimum and solar maximum. We find that the
luminosity variations are out of phase with the magnetic variations:
in order to help reconcile this result with empirical solar data,
we note that the global (poloidal) fields in the Sun are observed
to pass through minimum values at times that correspond roughly with
times of maximum toroidal fields.
Title: Spitzer Observations of Nearby M Dwarfs
Authors: Riaz, Basmah; Mullan, D. J.; Gizis, John E.
Bibcode: 2006ApJ...650.1133R
Altcode: 2006astro.ph..7405R
We present Spitzer IRAC and MIPS observations for a sample of eight
M dwarfs: six dMe, one dM, and one sdMe star. All of our targets
are found to have SEDs that are fitted within the error bars by a
purely photospheric spectrum out to 24 μm. We find no evidence for
IR excess. None of our targets are detected in the MIPS 70 and 160
μm bands. The estimated ages for all are >10 Myr, suggesting
that enough disk dissipation has occurred within the inner several
AU of the star. For four of these, Mullan et al. had reported IRAS
detections at 12 μm, although the reported fluxes were below the
5 σ IRAS detection limit (~0.2 Jy). Mullan et al. also pointed
out that V-K colors in dMe stars are larger than those in dM stars,
possibly because of the presence of a chromosphere. Here we suggest
that metallicity effects provide a better explanation of the V-K
data. For reasons of observational selection, our targets are not
the most active flare stars known, but being dMe stars indicates
the presence of a chromosphere. Scaling from Houdebine's model of
the AU Mic chromosphere, we have computed the free-free IR excesses
for a range of densities. Our Spitzer 24 μm data show that the
chromospheres in two of our targets are less dense than in AU Mic
by a factor of 10 or more. This is consistent with the fact that our
sample includes the less active flare stars. Our models also indicate
that the chromospheric contribution to the observed AU Mic emission
at submillimeter wavelengths is only about 2%.
Title: A Comparative Study of Flaring Loops in Active Stars
Authors: Mullan, D. J.; Mathioudakis, M.; Bloomfield, D. S.; Christian,
D. J.
Bibcode: 2006ApJS..164..173M
Altcode:
Dynamo activity in stars of different types is expected to generate
magnetic fields with different characteristics. As a result, a
differential study of the characteristics of magnetic loops in a broad
sample of stars may yield information about dynamo systematics. In the
absence of direct imaging, certain physical parameters of a stellar
magnetic loop can be extracted if a flare occurs in that loop. In
this paper we employ a simple nonhydrodynamic approach introduced by
Haisch, to analyze a homogeneous sample of all of the flares we could
identify in the EUVE DS database: a total of 134 flares that occurred
on 44 stars ranging in spectral type from F to M and in luminosity
class from V to III. All of the flare light curves that have been
used in the present study were obtained by a single instrument (EUVE
DS). For each flare, we have applied Haisch's simplified approach
(HSA) in order to determine loop length, temperature, electron density,
and magnetic field. For each of our target stars, a literature survey
has been performed to determine quantitatively the extent to which our
results are consistent with independent studies. The results obtained
by HSA are found to be well supported by results obtained by other
methods. Our survey suggests that, on the main sequence, short loops
(with lengths <=0.5R*) may be found in stars of all
classes, while the largest loops (with lengths up to 2R*)
appear to be confined to M dwarfs. Based on EUVE data, the transition
from small to large loops on the main sequence appears to occur between
spectral types K2 and M0. We discuss the implications of this result
for dynamo theories.
Title: Solar Wind Statistics at 1 AU: Alfven Speed and Plasma Beta
Authors: Mullan, D. J.; Smith, C. W.
Bibcode: 2006SoPh..234..325M
Altcode:
The phenomenon of MHD wave refraction is useful in interpreting
the properties of the magnetic fluctuations in certain parcels of
solar wind. In the physics of MHD wave refraction, variations in the
Alfvén speed VAlf play a dominant role. Here, we compile
statistics of the 1-min averages of VAlf at the location
of the ACE spacecraft during its first 5 years of operation. We
find that monthly distributions of VAlf are close to
log-normal, with standard deviations σV as small as 0.11
in the logarithm. Variations in the monthly mean VAlf are
correlated significantly with sunspot number. We also compile monthly
distributions of the plasma β parameter. The distributions of both
VAlf and β are significantly narrower than they would be if
the various solar wind parameters were statistically independent. In
the Tp-VAlf plane, we find a zone of avoidance
at low VAlf: for VAlf ≤10 - 15 km/s, there
are no samples in the 1-min data that are cooler than Tp
= 10 000 - 15 000 K. This feature can be understood in the context of
MHD wave refraction, although other explanations are also possible.
Title: Winds from OB Stars: A Two-Component Scenario?
Authors: Mullan, D. J.; Waldron, W. L.
Bibcode: 2006ApJ...637..506M
Altcode:
X-ray spectroscopy of several OB stars with massive winds has revealed
that many X-ray line profiles exhibit unexpectedly small blueshifts
and are almost symmetric. Moreover, the hottest X-ray lines appear
to originate closest to the star. These properties appear to be
inconsistent with the standard model of X-rays originating in shocked
material in line-driven spherically symmetric winds. Here we raise
the question, can the X-ray line data be understood in terms of a
two-component wind? We consider a scenario in which one component of
the wind is a standard line-driven wind that emerges from a broad range
of latitudes centered on the equator. The second component of the wind
emerges from magnetically active regions in extensive polar caps. The
existence of such polar caps is suggested by a recent model of dynamo
action in massive stars. We describe how the two-component model is
consistent with a variety of observational properties of OB star winds.
Title: Spitzer observations of active M dwarfs
Authors: Mullan, D. J.; Riaz, B.; Gizis, J. E.
Bibcode: 2005AAS...20718201M
Altcode: 2005BAAS...37.1463M
We present Spitzer Space Telescope observations of eight active M
dwarfs in the IRAC and MIPS bands. Based on their IRAC colors, we have
classified these M dwarfs as Weak T Tauri Stars (WTTS). Four of our
targets display small excesses at 160 μm. Over the wavelength range
from a few microns to tens of μm the flux densities can be fitted
well with a Rayleigh-Jeans curve. We do not detect evidence for the
presence of any appreciable dust in our target stars. We argue that any
small excess that we see may be due to a chromosphere/corona. Assuming
that the opacity is dominated by free-free processes, we interpret our
observations over the Spitzer wavelength range in terms of the rise in
temperature from the photosphere into the chromosphere/corona. In order
to model the free-free absorption as a function of height, we start
with a model chromosphere which was derived by E. R. Houdebine (1990,
Ph.D. dissertation) for the active M dwarf AU Mic. Houdebine derived a
semi-empirical model based on fitting the emission line profiles of Hα
and Hβ. In the model, the temperature is assumed to rise linearly with
log m (where m is mass loading) from a temperature minimum of 3255 K
up to T = 8000 K at log m (g cm-2) = -2.9. Above T = 8000 K,
the temperature rises abruptly to T = 3 x 105 K. We consider
a range of models in which the electron densities are allowed to be
uniformly larger or smaller than in the Houdebine model, by factors
of 0.1-100. Using these, we have obtained good fits to our Spitzer
observations, suggesting that the IR excesses in our target M dwarfs are
consistent with the existence of a moderately dense chromosphere/corona.
Title: Dynamo-generated magnetic fields at the surface of a massive
star
Authors: Mullan, D. J.; MacDonald, James
Bibcode: 2005MNRAS.356.1139M
Altcode:
Spruit has shown that an astrophysical dynamo can operate in the
non-convective material of a differentially rotating star as a
result of a particular instability in the magnetic field (the Tayler
instability). By assuming that the dynamo operates in a state of
marginal instability, Spruit has obtained formulae which predict
the equilibrium strengths of azimuthal and radial field components in
terms of local physical quantities. Here, we apply Spruit's formulae to
our previously published models of rotating massive stars in order to
estimate Tayler dynamo field strengths. There are no free parameters
in Spruit's formulae. In our models of 10- and 50-Msolar
stars on the zero-age main sequence, we find internal azimuthal fields
of up to 1 MG, and internal radial components of a few kG. Evolved
models contain weaker fields. In order to obtain estimates of the field
strength at the stellar surface, we examine the conditions under which
the Tayler dynamo fields are subject to magnetic buoyancy. We find
that conditions for Tayler instability overlap with those for buoyancy
at intermediate to high magnetic latitudes. This suggests that fields
emerge at the surface of a massive star between magnetic latitudes of
about 45° and the poles. We attempt to estimate the strength of the
field which emerges at the surface of a massive star. Although these
estimates are very rough, we find that the surface field strengths
overlap with values which have been reported recently for line-of-sight
fields in several O and B stars.
Title: Coronal and Chromospheric Heating in Active Dwarf Stars
Authors: Gizis, John; Mullan, Dermott
Bibcode: 2004sptz.prop.3562G
Altcode:
We propose to observe a sample of highly active M dwarfs with IRAC and
MIPS. Such active dwarfs are known to have large infrared excesses. We
argue that the excesses are likely to result from the hot chromospheres
and coronae above the photosphere. In this context, our proposed
measurements of the (excess) infrared spectral energy distribution
will allow us to reliably map the temperature rise in the chromosphere.
Title: Magnetic fields in massive stars: dynamics and origin
Authors: MacDonald, James; Mullan, D. J.
Bibcode: 2004MNRAS.348..702M
Altcode:
Evidence continues to accumulate in favour of the presence of magnetic
fields on the surfaces of massive stars. Some authors hypothesize that
such fields originate in a dynamo in the convective core, with buoyancy
bringing flux tubes to the surface. Here we show that, when realistic
stellar models are used, this `core dynamo hypothesis' encounters a
serious difficulty: in order for surface fields to originate in a core
dynamo, the core must create magnetic fields that are much stronger than
equipartition values. As an alternative hypothesis for the origin
of magnetic fields in OB stars, we suggest that a dynamo is operating in
shear-unstable gas in the radiative stellar envelope. Using a recently
developed code for the evolution of rotating stars, we find that in a
10-Msolar zero-age main sequence (ZAMS) star with a typical
rotation period, more than 90 per cent of the volume of the star is
subject to shear instability. We have recently proposed that dynamo
operation in shear-unstable gas helps to explain certain properties
of mass loss in cool giants. Here, in the context of hot stars, we
show that fields which originate in the shear-unstable regions of the
envelope can reach the surface without violating the constraints of
equipartition. Such fields rise to the surface on time-scales which
are short compared to main-sequence lifetimes.
Title: Further evidence of wave refraction associated with extended
rarefaction events in the solar wind
Authors: Smith, Charles W.; Mullan, Dermott J.; Ness, Norman F.
Bibcode: 2004JGRA..109.1111S
Altcode:
We examine the turbulence and low-frequency wave properties of
three extended rarefaction intervals observed by the Advanced
Composition Explorer (ACE) spacecraft in 1999 and 2002 while in orbit
at L1. The proton densities drop to values lower than 1
cm-3 and in one case reach 0.1 cm-3. During each
interval, the amplitudes of magnetic fluctuations fall to unusually low
levels. Moreover, the fluctuations that are present are more Alfvénic
in nature than in the typical low-latitude wind. The constant and near
average interplanetary magnetic field during these times results in
elevated Alfvén speeds. We suggest that the relative isolation of the
rarefaction intervals from sources of turbulence-generating transients
allows wave refraction to become a significant dynamic controlling the
evolution of the magnetized plasma. We present analyses of the intervals
that support the hypothesis that wave refraction explains the reduced
fluctuation levels and enhanced Alfvénicity during these times.
Title: Onset of Shear Instability in Rotating Red Giants
Authors: MacDonald, J.; Mullan, D. J.
Bibcode: 2003ApJ...598..560M
Altcode:
We incorporate into an evolutionary code a unified treatment of
turbulent transport due to convective and shear instabilities in a
rotating star. The effects of composition gradients are included in a
consistent way. We use the code to follow the evolution of rotating
stars from the pre-main sequence to near the tip of the red giant
branch. We find that at a certain stage in the red giant evolution
of a 1 Msolar star, when the convectively unstable region
occupies an extensive outer envelope, shear instability sets in in
a zone just below the H-burning shell. However, at a later time,
shear instability also develops in a second (outer) region that lies
between the H-burning shell and the convective envelope. The onset
of the outer region of shear instability coincides closely in time
with a well-defined feature of red giant evolution: the ``bump''
(so-called because of its effect on the luminosity function of
clusters). This evolutionary bump has recently been identified as a
significant empirical marker among red giants: cool massive winds
have been found to set in abruptly after the star evolves through
the bump. The results we report in the present paper suggest that the
onset of mass loss in red giants may be correlated with the onset of
an outer region of shear instability inside the star. We offer some
speculations concerning the possible physics of such a correlation.
Title: Structure and Evolution of Low-mass Stars: Where Do Magnetic
stars Become Completely Convective?
Authors: Mullan, D. J.; MacDonald, J.
Bibcode: 2003csss...12..893M
Altcode:
Stars on the main sequence are expected to be completely convective
if their mass lies below a certain value, Mcc. Standard
stellar structure codes suggest that Mcc is in the range
(0.3-0.4)M⊙. However, certain physical effects that are not
incorporated in standard models may alter the value of Mcc
significantly. Here we quantify the alterations that are brought
about in Mcc when we include magnetic field effects. In
particular, we modify the criterion for convective stability in the
manner prescribed by Gough and Tayler (1966). We find that magnetic
M dwarfs tend to have radii that are larger than expected for their
Teff values, or Teff values that are too low
for their radii. Available observational data provide quantitative
support for these structural findings. Moreover, we find that,
given the magnetic fields which are allowed to exist stably in
low-mass stars, Mcc may fall to values that are as small
as 0.1M⊙. We suggest that this result is pertinent to
understanding why coronae and chromospheres in active M dwarfs fail
to exhibit detectable alterations at spectral class M3-M4.
Title: Day the Solar Wind Almost Disappeared: Magnetic Field
Fluctuations and Wave Refraction
Authors: Smith, Charles W.; Mullan, Dermott J.; Ness, Norman F.;
Skoug, Ruth M.; Steinberg, John
Bibcode: 2003AIPC..679..509S
Altcode:
On May 11, 1999 the Advanced Composition Explorer (ACE) spacecraft
observed a rarefied parcel of solar wind that has come to be known as
``The Day the Solar Wind Disappeared.'' Little if any change is seen
in the large-scale interplanetary magnetic field during this time,
but the magnetic field fluctuations are depressed and significantly
more transverse to the mean field. The high Alfvén speed resulting
from the constant field intensity and low ion density enhances wave
refraction, and we examine this as a possible explanation for the
fluctuation properties.
Title: Turbulent dissipation in the solar wind and corona
Authors: Matthaeus, W. H.; Dmitruk, P.; Oughton, S.; Mullan, D.
Bibcode: 2003AIPC..679..427M
Altcode:
Models based upon anisotropic magnetohydrodynamic (MHD) cascade offer
promising explanations for observations of both interplanetary and
coronal turbulence and heating, which are reviewed here. In the standard
picture the cascade proceeds by driving at the energy-containing scales,
transfers through the inertial range, and into small scales where it
drives small-scale random turbulent reconnection events. In order to
understand more fully the heating and dissipation processes, one also
needs to understand how small-scale MHD-driven reconnection - involving
current sheets and filaments - induces kinetic plasma processes that
thermalize the fluid energy. Here we suggest that in these reconnection
sites MHD electric fields drive ion beam instabilities and nonlinear
electron dynamics involving electron solitary wave structures, in
analogy with the kinetic physics observed near parallel electric field
auroral regions by the FAST spacecraft.
Title: Onset of Mass Loss in Red Giants: Association with an
Evolutionary Event
Authors: Mullan, D. J.; MacDonald, J.
Bibcode: 2003ApJ...591.1203M
Altcode:
Stencel & Mullan used asymmetries in the Mg II k emission line
profile to determine the location of a ``velocity dividing line''
(VDL) in the H-R diagram. Stars to the right of (and above) the
VDL were observed to have asymmetries that are consistent with the
presence of cool massive winds. Stars to the left of (and below) the
VDL showed no evidence for winds. We show that the VDL lies close to a
certain event of stellar evolution on the red giant branch (RGB). The
event occurs when the hydrogen-burning shell evolves outward through
a discontinuity in molecular weight. In some low-mass stars, this
event causes a kink in the evolutionary track of an individual low-mass
star. In a cluster, the combined effects of such kinks create a ``bump''
in the luminosity function. Our result suggests that evolution through
the kink (or bump) on the RGB is associated with the onset of a cool
massive wind. Theoretical possibilities to explain this association
will be explored in a subsequent paper.
Title: Onset of mass loss in red giants: association with an
evolutionary event
Authors: Mullan, D. J.; MacDonald, J.
Bibcode: 2003AAS...202.5504M
Altcode: 2003BAAS...35R.774M
Stencel and Mullan (1980) used asymmetries in the MgII k emission
line profile to determine the location of a ``velocity dividing line"
(VDL) in the H-R diagram. Stars to the right of (and above) the VDL were
observed to have asymmetries which are consistent with the presence of
cool massive winds. Stars to the left of (and below) the VDL showed no
evidence for winds. We show that the VDL lies close to a certain event
of stellar evolution on the red giant branch (RGB). The event occurs
when the hydrogen-burning shell evolves outward through a discontinuity
in molecular weight. In some low-mass stars, this event causes a kink
in the evolutionary track of an individual low mass star. In a cluster,
the combined effects of such kinks create a ``bump" in the luminosity
function. We conclude that evolution through the kink (or bump) on the
RGB is associated with the onset of a cool massive wind. We speculate
that this evolutionary event is associated with a change in dynamo
mode. Work supported by NASA DE Space Grant.
Title: Short-Period Magnetic Fluctuations in Advanced Composition
Explorer Solar Wind Data: Evidence for Anticorrelation with Alfvén
Speed
Authors: Mullan, D. J.; Smith, C. W.; Ness, N. F.; Skoug, R. M.
Bibcode: 2003ApJ...583..496M
Altcode:
Short-period rms fluctuations δBrms in the interplanetary
magnetic field have been derived from Advanced Composition Explorer/MAG
data for 50 solar rotations between 1998 and 2001. We find that in
many cases, individual maxima in δBrms are associated
with local minima in the Alfvén speed VAlf. To determine
whether this anticorrelation persists in large data sets, we compute
the correlation coefficient between the temporal gradients (in the
spacecraft frame) δBrms and VAlf for each of
the 50 solar rotations in the data set. We find that, in 48 of the 50
rotations, δBrms is anticorrelated with VAlf. The
confidence level of the anticorrelation is greater than 90% in 35
rotations and is greater than 99.95% in 14 rotations. We find that in
some low-VAlf regions of solar wind, there is a pronounced
enhancement of non-Alfvénic (compressive) fluctuations. We suggest
that refraction effects operating on MHD waves contribute to these
aspects of the magnetic field fluctuation properties of the solar wind.
Title: Wave Refraction During the May 2002 Rarefaction Event
Authors: Smith, C. W.; Mullan, D. J.; Ness, N. F.; Skoug, R. M.
Bibcode: 2002AGUFMSH72C..08S
Altcode:
In previous work [Smith et al., 2001] we examined IMF wave refraction
during the May 1999 rarefaction interval known as ``The Day The Solar
Wind Disappeared.'' On that day, Alfvén speeds remained elevated over
an extended region. Analysis of the recorded ACE fields and plasma data
revealed depressed magnetic fluctuation levels, reduced compression in
the fluctuations, and a reduced wave-like component within the region
of elevated Alfvén speed, all consistent with wave refraction. The
May 2002 event provides a third such period (the second identified
event occured 2 weeks prior to the May 1999 period) and it again
demonstrates properties which are consistent with refraction. Smith,
C.~W., D.~J. Mullan, N.~F. Ness, R.~M. Skoug, and J.~Steinberg, Day
the solar wind almost disappeared: Magnetic field fluctuations, wave
refraction and dissipation, J. Geophys. Res., A106, 18,625--18,634,
2001. Efforts at the Bartol Research Institute were supported by CIT
subcontract PC251439 under NASA grant NAG5-6912 for support of the
ACE magnetic field experiment and by the NASA Delaware Space College
Grant. Work at Los Alamos was performed under the auspices of the
U.S. Department of Energy with financial support from the NASA ACE
program.
Title: Late-type Stars with Coronae but No Chromospheres?
Authors: Christian, D. J.; Mathioudakis, M.; Jevremovic, D.; Mullan,
D. J.; Lehner, N.
Bibcode: 2002AAS...201.1206C
Altcode: 2002BAAS...34.1113C
We present high resolution optical observations of several late-type
stars selected from ROSAT and EUVE surveys. These particular stars
show no Balmer or Ca II emission typical of late-type stars with X-ray
evidence for coronae. Since our sample shows no evidence for binarity
in their optical spectra, these stars may have no chromospheres. We
present preliminary results from simulated spectra using the MULTI
code of Carlsson for solving multi-level non-LTE radiative transfer
problems in plane parallel atmospheres. These simulations can reproduce
the observed Ca II line profiles. Stars without chromospheres are
expected to have high density coronae and transition regions, which
we will investigate with future FUV and X-ray observations.
Title: The origin of the FIP effect in solar and stellar atmospheres
Authors: Mullan, D. J.
Bibcode: 2002APS..DPPQM2001M
Altcode:
When elemental abundances are normalized to their values in the solar
photosphere, it is found that elements with low values of the First
Ionization Potential (FIP) are enhanced in abundance in the solar
corona relative to elements with high values of the FIP. In order to
account for this so-called "FIP effect", we suggest that the corona
is supplied with material from localized sites where magnetic flux
tubes of opposite polarity come into contact. When this process of
opposite-flux-tube-interaction (OFTI) occurs in a partially ionized
medium, it provides a natural method for enhancing low-FIP elements
in the corona. Numerical modelling of OFTI using a two-fluid code has
been reported by Arge and Mullan (1998): the enhancement of low-FIP
elements depends on the ratio of two time-scales, one for ions the
other for atoms. The model contains the important feature that,
because of a particular feature of the chromosphere in solar-like
stars, there is a built-in regulatory mechanism of the enhancement of
the low-FIP elements. This is especially important in the context of
recent discoveries of FIP bias in stars other than the Sun.
Title: Coronal Heating Distribution Due to Low-Frequency, Wave-driven
Turbulence
Authors: Dmitruk, P.; Matthaeus, W. H.; Milano, L. J.; Oughton, S.;
Zank, G. P.; Mullan, D. J.
Bibcode: 2002ApJ...575..571D
Altcode: 2002astro.ph..4347D
The heating of the lower solar corona is examined using numerical
simulations and theoretical models of magnetohydrodynamic turbulence
in open magnetic regions. A turbulent energy cascade to small length
scales perpendicular to the mean magnetic field can be sustained by
driving with low-frequency Alfvén waves reflected from mean density and
magnetic field gradients. This mechanism deposits energy efficiently
in the lower corona, and we show that the spatial distribution of the
heating is determined by the mean density through the Alfvén speed
profile. This provides a robust heating mechanism which can explain
observed high coronal temperatures and accounts for the significant
heating (per unit volume) distribution below 2 solar radii needed
in models of the origin of the solar wind. The obtained heating per
unit mass, on the other hand, is much more extended, indicating that
the heating on a per-particle basis persists throughout all the lower
coronal region considered here.
Title: V471 Tauri and SuWt 2: The Exotic Descendants of Triple
Systems?
Authors: Bond, Howard E.; O'Brien, M. Sean; Sion, Edward M.; Mullan,
Dermott J.; Exter, Katrina; Pollacco, Don L.; Webbink, Ronald F.
Bibcode: 2002ASPC..279..239B
Altcode: 2002esce.conf..239B; 2002IAUCo.187..239B
No abstract at ADS
Title: Detection of Coronal Mass Ejections in V471 Tauri with the
Hubble Space Telescope
Authors: Bond, Howard E.; Mullan, D. J.; O'Brien, M. Sean; Sion,
Edward M.
Bibcode: 2001ApJ...560..919B
Altcode: 2001astro.ph..6400B
V471 Tauri, an eclipsing system consisting of a hot DA white dwarf
(WD) and a dK2 companion in a 12.5 hr orbit, is the prototype of
the precataclysmic binaries. The late-type component is magnetically
active, due to its being constrained to rotate synchronously with the
short orbital period. During a program of ultraviolet spectroscopy of
V471 Tau, carried out with the Goddard High Resolution Spectrograph
(GHRS) on board the Hubble Space Telescope, we serendipitously
detected two episodes in which transient absorptions in the Si III
1206 Å resonance line appeared suddenly, on a timescale of <~2
minutes. The observations were taken in a narrow spectral region around
Lyα and were all obtained near the two quadratures of the binary
orbit, i.e., at maximum projected separation (~3.3 Rsolar)
of the WD and K star. We suggest that these transient features arise
when coronal mass ejections (CMEs) from the K2 dwarf pass across
the line of sight to the WD. Estimates of the velocities, densities,
and masses of the events in V471 Tau are generally consistent with
the properties of solar CMEs. Given our detection of two events
during 6.8 hr of GHRS observing, along with a consideration of the
restricted range of latitudes and longitudes on the K star's surface
that can give rise to trajectories passing in front of the WD as seen
from Earth, we estimate that the active V471 Tau dK star emits some
100-500 CMEs day-1, as compared to ~1-3 day-1
for the Sun. The K dwarf's mass-loss rate associated with CMEs is
at least (5-25)×10-14 Msolar yr-1,
but it may well be orders of magnitude higher if most of the silicon
is in ionization states other than Si III. Based on observations with
the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope
Science Institute, which is operated by AURA, Inc., under NASA contract
NAS 5-26555.
Title: Day the solar wind almost disappeared: Magnetic field
fluctuations, wave refraction and dissipation
Authors: Smith, Charles W.; Mullan, Dermott J.; Ness, Norman F.;
Skoug, Ruth M.; Steinberg, John
Bibcode: 2001JGR...10618625S
Altcode:
On May 11, 1999, the Advanced Composition Explorer (ACE) spacecraft
observed a rarefied parcel of solar wind. This has come to be known as
``The Day the Solar Wind Disappeared.'' Little if any change is seen
in the large-scale interplanetary magnetic field during this time,
but the magnetic field fluctuations are depressed and significantly
more transverse to the mean field. The high Alfvén speed resulting
from the constant field intensity and low ion density enhances wave
refraction, and we examine this as a possible explanation for the
fluctuation properties. The solar wind possesses a very low proton β,
thereby separating the cyclotron and ion inertial length scales and
permitting a test of possible dissipation dynamics. We find that the
test favors the ion inertial scale theories.
Title: Are Magnetically Active Low-Mass M Dwarfs Completely
Convective?
Authors: Mullan, D. J.; MacDonald, J.
Bibcode: 2001ApJ...559..353M
Altcode:
Stars on the main sequence are expected to be completely convective
if their mass lies below a certain value, Mcc. Standard
stellar structure codes suggest that Mcc is in the range
0.3-0.4 Msolar. In the present paper we examine several
nonstandard additions to stellar structure in order to quantify their
effects on the value of Mcc. We find that by including
magnetic field effects, Mcc may fall to values that are
significantly smaller than the standard range. This result is of
interest in understanding why coronae and chromospheres in active
M dwarfs fail to exhibit detectable alterations at spectral class
M3-M4. The structural properties of our magnetic models also explain
why active M dwarfs tend to have radii that are larger than expected
for their Teff values or Teff values that are
too low for their radii. Our results lead us to predict that certain
stars that are classified as late L or T stars may actually have masses
of 0.1-0.15 Msolar.
Title: Particle channels in the solar wind: Magnetic Field
Fluctuations, Wave Refraction and Dissipation
Authors: Mullan, D. J.; Ness, C. W.; Smith, N. F.; Skoug, J.;
Steinberg, R.
Bibcode: 2001ICRC....8.3349M
Altcode: 2001ICRC...27.3349M
Buttighofer has reported the existence of "channels" in the solar
wind through which solar energetic particles can propagate almost
scatter-free. We draw attention to an event observed by the ACE
spacecraft on May 11 1999 that has many of the characteristics of
Buttighofer's channels. The ACE event is known as `The Day the Solar
Wind (Almost) Disappeared'. During this unusual rarefaction event, the
solar wind density dropped by almost 2 orders of magnitude relative to
the value that is typically seen at ACE. However, the magnetic field
remained almost constant during the rarefaction. The most striking
feature of the magnetic field as measured by ACE was the fact that the
r.m.s. (root mean square) field fluctuations dropped to unusually low
values (about 0.1 nT). These low r.m.s. values are reminiscent of the
behavior reported by Buttighofer in the particle channels. Moreover,
the fluctuations that remained in the ACE rarefaction region were found
to be significantly more transverse to the mean field than usual, again
analogous to a feature reported by Buttighofer in the channels. We
suggest that the phenomenon of refraction allows us to understand the
properties of the magnetic fluctuations inside the ACE rarefaction
region, and by extension, in the Buttighofer particle channels as well.
Title: The Day the Solar Wind (Almost) Disappeared: Refraction
and Dissipation
Authors: Mullan, D. J.; Smith, C. W.; Ness, N. F.; Skoug, R. M.;
Steinberg, J.
Bibcode: 2001AGUSM..SP61A01M
Altcode:
On May 11, 1999 the ACE spacecraft spent several hours in an unusually
rarefied parcel of solar wind. During this rarefaction interval,
the Alfven speed was exceptionally high while the ion temperature
was low. These properties provide a rare opportunity to examine two
distinct physical processes that operate in the solar wind. First,
refraction of MHD waves: fast-mode waves are excluded from the rarefied
parcel, but Alfven waves are not affected. Second, dissipation of
magnetic fluctuations: when ACE is in the rarefaction interval,
the location of dissipation onset in the magnetic power spectrum
shifts towards a distinctly lower frequency. The direction of this
shift is consistent with dissipation processes that operate at the
ion inertial length-scale. But the observed shift in dissipation
frequency is not compatible with the widely discussed mechanism of
ion cyclotron resonance.
Title: Coronal Heating Distribution due to Alfvenic Driven
Magnetohydrodynamic Turbulence
Authors: Dmitruk, P.; Matthaeus, W. H.; Milano, L. J.; Oughton, S.;
Mullan, D. J.; Zank, G. P.
Bibcode: 2001AGUSM..SH22E02D
Altcode:
We establish a direct connection between the heating rate function
in the lower open corona and the background radial plasma density and
magnetic field profiles in that region. The confinement of the heating
between 1-2 solar radius is achieved through Alfven waves injected
from the base and reflected due to inhomogeneities of the medium,
which provide the necessary counter-propagating fluctuations to sustain
quasi-2D MHD turbulence. A phenomenological description is considered
as well as direct numerical simulations of the reduced MHD equations. A
simple approximation is applied to obtain an explicit relation between
the heating rate function q and the Alfven velocity profile. Some
example profiles are presented, which show a radial exponentially
decaying behavior for the heating, with a dissipation length of the
order of the medium inhomogeneity length. Other quantities, such as
cross helicity and fluctuations intensity are also analyzed.
Title: A Reduced Magnetohydrodynamic Model of Coronal Heating in
Open Magnetic Regions Driven by Reflected Low-Frequency Alfvén Waves
Authors: Oughton, S.; Matthaeus, W. H.; Dmitruk, P.; Milano, L. J.;
Zank, G. P.; Mullan, D. J.
Bibcode: 2001ApJ...551..565O
Altcode:
A reduced magnetohydrodynamic (RMHD) description is employed to
examine a suggestion made by W. H. Matthaeus and colleagues in 1999
that coronal heating might be sustained by a cascade of low-frequency
MHD turbulence. Here RMHD simulations show that the low-frequency
cascade to high transverse wavenumbers can be driven by an externally
maintained flux of low-frequency propagating Alfvén waves, in
combination with reflection caused by an inhomogeneous background
medium. The simulations show that the suggestions made previously on
the basis of a phenomenology are indeed realizable. In addition, the
effect is seen to sensitively depend on the presence of reflection,
as the background turbulence level needed to maintain the cascade
can be sustained only when reflection is imposed. The steady level
of turbulence is insensitive to the initial seed turbulence level
(provided it is nonzero). Consequences of this model for realistic
models of coronal heating in open field-line regions are discussed.
Title: The Oversized Magnetically Active dK Component of V471 Tauri
Authors: Bond, H. E.; O'Brien, M. S.; Mullan, D. J.; Sion, E. M.
Bibcode: 2001AGM....18S0709B
Altcode: 2001AGAb...18R..79B
V471 Tauri is an eclipsing binary in the Hyades cluster containing a
hot white dwarf (WD) and a dK2 companion. The K2 star is extremely
active, due to its synchronous rotation with the 12.5-hour orbital
period. During UV spectroscopy of V471 Tau, obtained with the GHRS on
the Hubble Space Telescope, we serendipitously detected two episodes
in which the Si III 1206 Å absorption line appeared suddenly in
the WD spectrum, on a timescale of <2 min. The spectra were taken
near maximum projected separation (~3.3 Rsolar) of the WD
and K star. looseness=-1 The Si III transients appear to arise when
coronal mass ejections (CME's) from the K2 star pass in front of the
WDνll. The velocities, densities, and masses of the features in V471
Tau are similar to those of solar CME's. Given our detection of 2 events
during 6.8 hr of GHRS observing, along with a consideration of the
restricted range of latitudes and longitudes on the K star's surface
that can give rise to trajectories passing in front of the WD as seen
from Earth, we estimate that the dK star emits some 100-500 CME's per
day, compared to ~1-3 per day for the Sun. The K dwarf's mass-loss
rate associated with CME's is at least (5-25)×10-14
Msolar yr-1, but could be orders of magnitude
higher. Measurements of the eclipse duration show that the radius of
the dK star is ~18% larger than that of a Hyades dwarf of the same mass
(0.93 Msolar)νll. We attribute the large radius to the dK
star's surface activity: because about 25% of the star's surface is
covered with starspots, the star must expand so that it can continue
to radiate the luminosity produced in its core.
Title: Extreme-Ultraviolet Flares in an F2 Star
Authors: Mullan, D. J.; Mathioudakis, M.
Bibcode: 2000ApJ...544..475M
Altcode:
We report on the coronal properties of two early F dwarfs. For
both stars, spectroscopic data reveal a hot corona with material at
temperatures as high as 107.1-107.2 K. The EUV
photometric observations of one of the stars (HR 120: F2) reveal two
large flares with rapid decay times. The rapid decays are in striking
contrast with previously published slowly decaying flares on a late
type F star (HR 1817: F8/9). Analysis of decay times suggests that the
loop sizes which flared in the F2 star are significantly shorter than
those which flared in the F8/9 star. We discuss these results in the
context of a proposal made by Giampapa & Rosner.
Title: Extended Atmospheres
Authors: Mullan, D.
Bibcode: 2000eaa..bookE1846M
Altcode:
During a total ECLIPSE of the Sun, when the Moon blocks out the
intensely bright disk of the Sun, a faint white `halo' can be seen
surrounding the Sun (see figure 1). This halo, officially called
the Sun's `CORONA' (i.e. `crown'), exists mainly because there are
electrons in the outer solar atmosphere: even though the disk of the
Sun is hidden from direct view, the electrons scatter a fraction o...
Title: MHD-driven Kinetic Dissipation in the Solar Wind and Corona
Authors: Leamon, R. J.; Matthaeus, W. H.; Smith, C. W.; Zank, G. P.;
Mullan, D. J.; Oughton, S.
Bibcode: 2000ApJ...537.1054L
Altcode:
Mechanisms for the deposition of heat in the lower coronal plasma
are discussed, emphasizing recent attempts to reconcile the fluid and
kinetic perspectives. Structures at magnetohydrodynamic (MHD) scales may
drive a nonlinear cascade, preferentially exciting high perpendicular
wavenumber fluctuations. Relevant dissipative kinetic processes must be
identified that can absorb the associated energy flux. The relationship
between the MHD cascade and direct cyclotron absorption, including
cyclotron sweep, is discussed. We conclude that for coronal and solar
wind parameters the perpendicular cascade cannot be neglected and may
be more rapid than cyclotron sweep. Solar wind observational evidence
suggests the relevance of the ion inertial scale, which is associated
with current sheet thickness during reconnection. We conclude that a
significant fraction of dissipation in the corona and solar wind likely
proceeds through a perpendicular cascade and small-scale reconnection,
coupled to kinetic processes that act at oblique wavevectors.
Title: The Origin of the FIP Effect in the Solar Atmosphere
Authors: Mullan, D. J.; Arge, C. N.
Bibcode: 2000ASPC..206...71M
Altcode: 2000hesp.conf...71M
No abstract at ADS
Title: Solar Physics: From the Deep Interior to the Hot Corona
Authors: Mullan, Dermott J.
Bibcode: 2000LNP...556....1M
Altcode: 2000stga.conf....1M; 2000fsga.conf....1M
We present an overview of the thermal properties of the Sun from the
hot interior to the hot corona. For pedagogical reasons, we confine
the discussion to certain relevant solutions of the energy conservation
equation. In the interior, quantitative information can be obtained by
using a polytropic equation of state: internal temperatures obtained
in this way are found to be reliable to about 10%, and we can obtain a
good estimate of the depth of the convection zone. In the chromosphere,
acoustic waves originating in the convection zone do work on the gas: as
the gas heats up, the atomic energy levels of many elements (especially
hydrogen) exert a strong thermostatic control so that the temperature
is confined to a steady value in the range 5000-104 K. In
long-lived coronal loops, a steady state balance between thermal
conduction and radiative losses causes the temperature of the electrons
to lie in the range (1-2) million K. Coronal ions are heated to greater
temperatures than electrons. In flares, processes of heating and cooling
are explicitly non-steady, and short-lived excursions to temperatures
as high as 25 million K (or more) are observed in the largest flares.
Title: Plasma Flows In Coronal Hole Regions
Authors: Landi, E.; Mullan, D.; Poletto, G.; Wilhelm, K.
Bibcode: 1999ESASP.446..417L
Altcode: 1999soho....8..417L
More than two decades after coronal holes (CH) had been recognized
to be the source regions of solar wind, we still do not know which
structures, within CH, most contribute to the solar wind mass flux. In
a recent work, Hassler et al. (1999) obtained velocity maps of a
coronal hole region, in the Ne VIII 770 Angstrom line, which suggest
that localized areas within the network might be sources of the solar
wind. On the opposite, an earlier work of Dupree et al. (1996), from
an analysis of the He I 10830 Angstrom line, suggested cell regions
as sources of the solar wind. In the present work, we reinvestigate
this issue analysing SUMER data taken in an equatorial and in a polar
coronal hole, observed respectively on 29 August 1996 and 21 September
1996. We study the behavior of Si II, C IV, O V, N V, Ne VIII, Mg X and
Fe XII ions, whose temperatures of formation range from chromospheric
(around 2x104 K) to coronal values (around 1.5x105
K). In particular, we analysed intensity and velocity distributions
of these lines, and illustrate changes in the plasma velocity pattern
as a function of the temperature regime where lines form.
Title: Gravity damping of Alfvén waves: numerical modelling
Authors: Mullan, D. J.; Khabibrakhmanov, I. K.
Bibcode: 1999ESASP.446..503M
Altcode: 1999soho....8..503M
Estimates of Alfven wave dissipation in the fluid approximation have
been in the literature for years. Here we report on a numerical study
of Alfven wave dissipation in a non-fluid context. We consider a nearly
collisionless magnetized plasma. A test particle moves in this medium
in the presence of an Alfven wave propagating along a vertical field
line in the presence of gravity. The model includes the randomizing
effects of collisions which occur infrequently enough that the particle
undergoes many gyrations between successive collisions. Our results
indicate that the test particle gains energy from the wave at a rate
which is larger than the linear predictions of Khabibrakhmanov and
Mullan (1994).
Title: Coronal Heating by Magnetohydrodynamic Turbulence Driven by
Reflected Low-Frequency Waves
Authors: Matthaeus, W. H.; Zank, G. P.; Oughton, S.; Mullan, D. J.;
Dmitruk, P.
Bibcode: 1999ApJ...523L..93M
Altcode:
A candidate mechanism for the heating of the solar corona in open
field line regions is described. The interaction of Alfvén waves,
generated in the photosphere or chromosphere, with their reflections
and the subsequent driving of quasi-two-dimensional MHD turbulence
is considered. A nonlinear cascade drives fluctuations toward short
wavelengths which are transverse to the mean field, thereby heating at
rates insensitive to restrictive Alfvén timescales. A phenomenology
is presented, providing estimates of achievable heating efficiency
that are most favorable.
Title: GHRS Observations of Cool, Low-Gravity Stars. V. The Outer
Atmosphere and Wind of the Nearby K Supergiant λ Velorum
Authors: Carpenter, Kenneth G.; Robinson, Richard D.; Harper, Graham
M.; Bennett, Philip D.; Brown, Alexander; Mullan, Dermott J.
Bibcode: 1999ApJ...521..382C
Altcode:
UV spectra of λ Velorum taken with the Goddard High Resolution
Spectrograph (GHRS) on the Hubble Space Telescope are used to probe the
structure of the outer atmospheric layers and wind and to estimate the
mass-loss rate from this K5 Ib-II supergiant. VLA radio observations at
λ=3.6 cm are used to obtain an independent check on the wind velocity
and mass-loss rate inferred from the UV observations. Parameters
of the chromospheric structure are estimated from measurements of
UV line widths, positions, and fluxes and from the UV continuum flux
distribution. The ratios of optically thin C II] emission lines indicate
a mean chromospheric electron density of logNe~8.9+/-0.2
cm-3. The profiles of these lines indicate a chromospheric
turbulence (v0~25-36 km s-1), which greatly
exceeds that seen in either the photosphere or wind. The centroids
of optically thin emission lines of Fe II and of the emission
wings of self-reversed Fe II lines indicate that they are formed in
plasma approximately at rest with respect to the photosphere of the
star. This suggests that the acceleration of the wind occurs above
the chromospheric regions in which these emission line photons are
created. The UV continuum detected by the GHRS clearly traces the
mean flux-formation temperature as it increases with height in the
chromosphere from a well-defined temperature minimum of 3200 K up to
about 4600 K. Emission seen in lines of C III] and Si III] provides
evidence of material at higher than chromospheric temperatures in the
outer atmosphere of this noncoronal star. The photon-scattering wind
produces self-reversals in the strong chromospheric emission lines,
which allow us to probe the velocity field of the wind. The velocities
to which these self-absorptions extend increase with intrinsic line
strength, and thus height in the wind, and therefore directly map
the wind acceleration. The width and shape of these self-absorptions
reflect a wind turbulence of ~9-21 km s-1. We further
characterize the wind by comparing the observations with synthetic
profiles generated with the Lamers et al. Sobolev with Exact Integration
(SEI) radiative transfer code, assuming simple models of the outer
atmospheric structure. These comparisons indicate that the wind in
1994 can be described by a model with a wind acceleration parameter
β~0.9, a terminal velocity of 29-33 km s-1, and a mass-loss
rate~3×10-9 Msolar yr-1. Modeling
of the 3.6 cm radio flux observed in 1997 suggests a more slowly
accelerating wind (higher β) and/or a higher mass-loss rate than
inferred from the UV line profiles. These differences may be due to
temporal variations in the wind or from limitations in one or both
of the models. The discrepancy is currently under investigation. Based on observations with the NASA/ESA Hubble Space Telescope,
which is operated by the Association of Universities for Research in
Astronomy, Inc., under NASA contract NAS 5-26555.
Title: Gravity damping of Alfvén waves: Theory and numerical modeling
Authors: Khabibrakhmanov, I. K.; Mullan, D. J.
Bibcode: 1999AIPC..471..349K
Altcode: 1999sowi.conf..349K
In a collisionless medium where gravity and magnetic field are parallel,
a charged particle and an electromagnetic wave can exchange energy
and momentum at a rate which is proportional to the gravitational
acceleration. As a result, plasma energization can occur. A test
particle approach to the problem has demonstrated that the total energy
of the particle increases at the rate predicted by the gravity damping
mechanism. Moreover, the test particle undergoes significant upward
motion in an exponential atmosphere. This indicates that gravity
damping can contribute to the solar wind formation in two distinct
ways. First, implicitly, through plasma heating: the pressure gradient
created in this way is the main driving force of the solar wind in
a fluid model. Second, the particle experiences direct lift due to
energy exchange with the wave and the up-down asymmetry of collision
rates in a stratified atmosphere.
Title: Heating and Acceleration of the Solar Wind via Gravity Damping
of Alfvén Waves
Authors: Cuseri, Iolanda; Mullan, Dermott; Noci, Giancarlo; Poletto,
Giannina
Bibcode: 1999ApJ...514..989C
Altcode:
In this paper we present a two-fluid model for the heating of the solar
corona and acceleration of the solar wind, based on the dissipation
of Alfvén waves by gravity damping. This mechanism was proposed by
Khabibrakhmanov & Mullan but has not previously been applied in
modeling efforts. After extending the Khabibrakhmanov & Mullan
theory to give an expression for the evolution of the Alfvén wave
amplitude as a function of the local parameters of the atmosphere,
we show how gravity damping compares with other mechanisms that have
been proposed for the dissipation of Alfvén waves. Then we introduce
the system of equations that we use for the wind model: this includes,
in the energy equation, a gravity dissipation term and, in the momentum
equation, a different wave acceleration term from that which is usually
adopted. Initial conditions for the integration of the equations
are compatible with recent Ulysses measurements, and the integration
proceeds from 1 AU toward the base of the solar corona and into the
transition region [where T=(1-2)×105 K]. Our results show
that the gravity damping of Alfvén waves heats protons in the solar
plasma to several million degrees and accelerates the solar wind
to 600-700 km s-1. Model predictions at low heliocentric
distances compare favorably with recently acquired data. One prediction
of our model is that the damping process is most effective in regions
where the Alfvén speed is low. Another prediction is that although
the energy is deposited mainly into protons, the deposition occurs
close enough to the Sun that collisional coupling also leads to
effective heating of the electrons (to Te~106
K). We compare and contrast the present model with models based on
ion-cyclotron resonant processes.
Title: HR 1817: the EUV properties of an active F dwarf
Authors: Mathioudakis, M.; Mullan, D. J.
Bibcode: 1999A&A...342..524M
Altcode:
We examine the coronal properties of the active F dwarf HR
1817. Photometric observations with EUVE show that the source is
in a near-continuous state of flare-like activity. Using IUE and
EUVE spectroscopic observations we construct the emission measure
distribution in the 10(4) -10(7.2) K temperature range. These
observations reveal a hot corona and activity levels similar to those
of RS CVn binaries. Based on Fe XXI line ratios we derive an upper
limit of 10(11.7) cm(-3) for the coronal density of HR 1817 and a
magnetic field strength of B<= 160 Gauss. A comparison of the EUVE
spectroscopic observations with synthetic spectra derived from ASCA
and ROSAT fits, shows that optimal agreement is obtained for fits with
sub-solar metal abundances. The reduced metal abundances increase the
radiative losses significantly at temperatures above 10(6.5) K where
Fe is no longer the dominant radiative cooling agent.
Title: Fluctuations, Dissipation and Heating in the Corona
Authors: Matthaeus, W. H.; Zank, G. P.; Leamon, R. J.; Smith, C. W.;
Mullan, D. J.; Oughton, S.
Bibcode: 1999SSRv...87..269M
Altcode:
Mechanisms for the deposition of heat in the lower coronal plasma
are discussed, emphasizing recent attempts to reconcile the fluid
and kinetic perspectives. Structures at the MHD scales are believed
to act as reservoirs for fluctuation energy, which in turn drive a
nonlinear cascade process. Kinetic processes act at smaller spatial
scales and more rapid time scales. Cascade-driven processes are
contrasted with direct cyclotron absorption, and this distinction is
echoed in the contrast between frequency and wavenumber spectra of the
fluctuations. Observational constraints are also discussed, along with
estimates of the relative efficiency of cascade and cyclotron processes.
Title: Nonprimordial Deuterium in the Interstellar Medium
Authors: Mullan, D. J.; Linsky, J. L.
Bibcode: 1999ApJ...511..502M
Altcode:
Contrary to a widespread assumption, deuterium is not simply destroyed
in stars: deuterium is also synthesized in the atmospheres of active
stars. This nonprimordial synthesis of D arises when protons accelerated
in flares interact with the atmosphere, create a flux of free neutrons,
and these neutrons then undergo radiative capture on atmospheric
protons. Radiative capture does not result in excess production of
Li, Be, or B. Ejection of flare-processed material contaminates the
interstellar medium (ISM), as was originally suggested by Coleman
& Worden. Estimates of the amount of flare-created D are subject
to considerable uncertainties, but we find, using stellar parameters
within permitted ranges, that flares may contribute significantly to
the current ISM D content. Observational data indicate that different
clouds of gas in the ISM exhibit variations in the value of D/H. We
suggest that contamination of the ISM by D-enriched material ejected
from stellar flares contributes to the observed D/H inhomogeneity. More
precise estimates of the efficiency of D ejection from flares into
the solar wind are required to evaluate this suggestion.
Title: Coronal Heating: a Comparison of Ion-cyclotron and Gravity
Damping Models
Authors: Cuseri, I.; Mullan, D.; Poletto, G.
Bibcode: 1999SSRv...87..153C
Altcode:
SOHO/UVCS data indicate that minor ions in the corona are heated more
than hydrogen, and that coronal heating results in T⊥
larger than T‖. Analogous behavior has been known from
in situ measurements in solar wind for many years. Here we compare
and contrast two mechanisms which have been proposed to account for
the above behavior: ion-cyclotron resonance and gravity damping.
Title: The Chromosphere, Wind, and Mass-Loss Rate of lambda Velorum
(K5 Ib) as Revealed by UV Emission Line Profiles
Authors: Carpenter, K. G.; Robinson, R. D.; Harper, G. M.; Bennett,
P. D.; Brown, A.; Mullan, D. J.
Bibcode: 1998AAS...193.4502C
Altcode: 1998BAAS...30Q1317C
The Goddard High Resolution Spectrograph (GHRS) on the HST has been
used to acquire high-quality UV spectra of the nearby K-supergiant
lambda Velorum. These spectra contain a wide variety of chromospheric
emission lines, many of which are self-reversed by wind absorption,
and thus allow us to probe the structure of both the chromosphere
and wind. The observed line widths, positions, and fluxes indicate a
chromosphere with a mean log n_e ~ 8.9 +/- -0.2 cm(-3) , a turbulence
(indicated by line cores with ~ 25 km s(-1) Doppler widths) greatly
in excess of the photospheric value, and no general systematic flows
(i.e. the wind acceleration appears to occur above the region of
photon creation). We compare synthetic Fe II line profiles from the
approximate Lamers et al. (1987) Sobelev with Exact Integration (SEI)
method, and from an exact comoving frame CRD calculation, with the
observations. The width and shape of the wind self-absorptions implies
a terminal velocity of 29--33 km s(-1) , and a wind turbulence of ~
9-21 km s(-1) . We find that the wind in the 1994 GHRS observations
can be described by a model with an acceleration parameter beta ~ 0.9
and a mass-loss rate of ~ 3 x 10(-9) Msun yr(-1) . However,
this model is not consistent with the VLA 3.6 cm radio continuum flux
observed in 1997.
Title: Modelling of magnetic interactions in partially-ionized gas:
application to the FIP effect
Authors: Arge, C. N.; Mullan, D. J.
Bibcode: 1998SoPh..182..293A
Altcode:
We have adapted the ZEUS code to model magnetic interactions in
partially ionized gas. When two regions of opposite polarity come into
contact with each other, ions drifting in response to the Lorentz force
fall into the minimum of the magnetic field, and then the drifting
ions force the neutrals to take part in the flow. Because of the finite
time required for ion-atom collisions to occur, the gas which emerges
from the interaction site has an ion/atom ratio which may be altered
relative to that in the ambient medium. In order to model this effect,
we adapt the Zeus code to a two-step iterative process involving
a cycle between the hydrodynamic (HD) and the magnetohydrodynamic
(MHD) versions of the code. The ion and atom fluids are coupled by
collisions. Our simulations show that in chromospheric conditions,
outflowing gas exhibits enhancements in ion/atom ratios which may be
as large as a factor of 10 or more. The magnitude of the enhancements
is determined by two key ratios which enter into the problem: the
degree of ionization (ni/na), and the plasma β parameter. We show
that, in the context of the mechanism we propose here, the amplitude
of the ion/atom enhancements in the solar chromosphere is subject to
a remarkable self-regulation because the ion density ni is almost
invariant over the height range of interest to us. Our results are
relevant in the context of the Sun, where the coronal abundances of
elements with low first ionization potential (FIP) are systematically
enhanced in certain magnetic structures. Although data for stars other
than the Sun are sparse, we point out that our results are also useful
for interpreting the available stellar data.
Title: Large Variations in the Winds of Single Cool Giants: λ
Velorum and γ Crucis
Authors: Mullan, D. J.; Carpenter, K. G.; Robinson, R. D.
Bibcode: 1998ApJ...495..927M
Altcode:
UV spectra of the ``noncoronal'' single K supergiant λ Vel and of
the single M giant γ Cru obtained with IUE and the Hubble Space
Telescope at various epochs indicate that the profiles of many lines
formed in the wind exhibit striking alterations in shape with time. We
parameterize the wind profiles in terms of an empirical optical depth
τemp by reflecting the red wing about line center and
comparing the reflected intensity with that of the blue wing. In the λ
Vel wind the terminal velocity v∞ was found to be close
to 40 km s-1 in 1978, 1982, and 1994, but was at least 20
km s-1 greater in 1990. The faster wind in 1990 also had
a total optical depth that was a factor of 2-6 times greater than at
the other epochs. Based on observations with the NASA/ESA Hubble
Space Telescope, which is operated by the Association of Universities
for Research in Astronomy, Inc., under NASA contract NAS 5-26555,
and on observations with the International Ultraviolet Explorer.
Title: Large-eddy simulation of astrophysical convection and acoustic
emission
Authors: Mullan, D. J.
Bibcode: 1998ASPC..138..253M
Altcode: 1998stas.conf..253M
No abstract at ADS
Title: Comparing Observations of Cyclical Variability in - and
Cool-Star Winds
Authors: Mullan, D. J.
Bibcode: 1998cvsw.conf..173M
Altcode:
No abstract at ADS
Title: A new heating and acceleration mechanism for the fast
solar wind
Authors: Cuseri, I.; Mullan, D. J.; Noci, G.; Poletto, G.
Bibcode: 1998MmSAI..69..745C
Altcode:
No abstract at ADS
Title: Periodic Modulation of X-Ray Intensity from Coronal Loops -
Heating by Resonant Absorption?
Authors: McKenzie, D. E.; Mullan, D. J.
Bibcode: 1997SoPh..176..127M
Altcode:
We have applied time-series analysis to sequences of coronal loop images
obtained with the Yohkoh soft X-ray telescope (SXT). Using images
obtained in fast-cadence mode (δ t = 4 s), we have generated X-ray
light curves and power spectra for multiple subregions of a number
of coronal loops. Our analysis indicates that out of 544 spectra
of loops which range in length L from 40 Mm to 150 Mm, there are
16 cases where the X-ray brightness is modulated periodically with
≥99.5% confidence. The periods τ range from 9.6 s to 61.6 s, and
the amplitudes range from 0.4% to 1.8%. Our result is statistically
significant at the 8σ level.
Title: Response to Comment by J. V. Hollweg
Authors: Khabibrakhmanov, I. K.; Mullan, D. J.
Bibcode: 1997ApJ...488..898K
Altcode:
The preceding comment by J. V. Hollweg presents criticisms of a paper
that had reported estimates of the damping rates of Alfvén waves in
the presence of gravitational acceleration. Here we point out that
there is actually significant overlap between the conclusions of the
former and the latter. We agree with a new point raised by the comment
concerning up/down asymmetry of gravity damping.
Title: Cyclic convection in a zone bounded by stable layers
Authors: Hossain, Murshed; Mullan, D. J.
Bibcode: 1997PhRvE..55.2769H
Altcode:
We have simulated compressible convection in two distinct but related
cases: (i) a single layer of unstable material is in contact with
impenetrable boundaries and (ii) a layer of unstable material that is
identical to that of case (i) except that it is now 'sandwiched' between
two thick stable layers. The convection is driven equally strongly in
both cases. We find that convection in the single layer is steady,
whereas in the 'sandwich' case, the convection exhibits nonsteady
behavior of a particular kind: the convection is cyclic. During one
part of the cycle, the convective flux FC falls to levels
that are too small for energy balance. During the second part of the
cycle, conduction increases in an attempt to restore energy, but this
eventually drives the fluid back to convective instability, with a
subsequent increase in FC . In the course of the cycle,
the fluctuations in FC are large (factors of 2-3). We
comment on the applicability of our results to convection in the sun.
Title: Radiative Transfer Effects on the Stability of Sound Waves
in a Polytropic Atmosphere
Authors: MacDonald, James; Mullan, Dermott
Bibcode: 1997ApJ...481..963M
Altcode:
From a perturbation analysis of the boundary value problem that
describes the behavior of optically thin disturbances to a polytropic
atmosphere initially in hydrostatic and radiative equilibrium, we derive
criteria for the overstability of acoustic modes. Our criteria are more
general than those found by Spiegel, which are strictly valid only if
the radiative cooling time is uniform over the atmosphere, i.e., if
the polytropic index m = 6. Our criteria for overstability are valid
for any value of m. Applying our results to the Sun and giant stars,
we find that sound waves in the envelope of the Sun do not appear to
be unstable but that instability may occur in the envelopes of red
giants. We note that, if acoustic waves become overstable as a star
evolves into a giant, then the acoustic power in the envelopes of cool
giants will be greater than one would have expected on the basis of the
predictions of Lighthill theory. We propose that chromospheric heating
in nonmagnetic stars (such as old giants in globular clusters) contains
a significant component that arises from acoustic overstability.
Title: Two-Dimensional Time-Dependent MHD in Partially Ionized Gas:
Application to the FIP problem.
Authors: Arge, Charles N.; Mullan, Dermott J.
Bibcode: 1997SPD....28.0240A
Altcode: 1997BAAS...29..900A
We study the two-dimensional behavior of partially ionized gas in
magnetized regions of the solar atmosphere. The highly time-dependent
nature of solar magnetic fields frequently produce interacting
oppositely directed magnetic fields. Since in partially ionized gases
the ions "feel" the field more than the neutral atoms, the gas which
emerges from such interacting fields may have a different ion/atom ratio
than the ambient medium. By extending the ZEUS-3D code to apply to a
partially ionized gas, we have quantified this effect. Our simulations
show that enhancements in ion/atom ratios may be as large as a factor of
almost 12. These results may be relevant to understanding EUV data from
SKYLAB which suggest that the abundance of elements with low FIP in the
solar corona are systematically enhanced in certain magnetic structures.
Title: Wind Properties of Evolved Stars: Effects on Particle
Acceleration
Authors: Mullan, D. J.
Bibcode: 1997cwh..conf..197M
Altcode: 2006mslp.conf..197M
No abstract at ADS
Title: Acceleration of the solar wind: effects of plasmoids
Authors: Mullan, D. J.
Bibcode: 1997AIPC..385..205M
Altcode: 1997recs.conf..205M
Magnetic plasmoids in the solar corona and wind are subject to outward
Lorentz forces in regions where the ambient magnetic field is decreasing
outwards. We compute velocity profiles V(r) for a spherically symmetric
solar wind containing plasmoids. In regions where the plasmoids occupy
a progressively larger fraction of the solar wind, the wind accelerates
rapidly, with radial gradients as much as 4-5 times steeper than the
maximum gradient in an isothermal wind. Reconnection causes plasmoids
eventually to lose their identity, and when the plasmoid fraction
becomes small, the velocity profile becomes almost flat, with radial
gradients shallower than in the isothermal wind by factors of about 2.
Title: Discrete sources of the solar wind: observability with
Solar Probe
Authors: Mullan, D. J.
Bibcode: 1997AIPC..385..235M
Altcode: 1997recs.conf..235M
There is observational evidence that X-ray bright points act as
discrete sources of solar wind: theoretical ideas suggest that the
discrete sources are in the form of magnetic plasmoids with dimensions
of order 10 Mm. If the corona is heated by nanoflare events, the
magnetic reconnection process which is at the heart of a nanoflare
may also create magnetic plasmoids which supply mass to the solar
wind in discrete events: plasmoids from nanoflares are expected to
have dimensions of a few hundred km. Estimates of the properties of
the plasmoids suggest that they will preserve their identity long
enough to be detectable by plasma instruments on board a Solar Probe
which penetrates in to 4Rsolar. Photons from individual
nanoflares should also be identifiable at closest approach.
Title: Coronal Heating in dMe and dM Stars: Clues from the X-Ray
Surface Fluxes
Authors: Mullan, D. J.; Fleming, T. A.
Bibcode: 1996ApJ...464..890M
Altcode:
The mean X-ray surface fluxes Fx from a nearly
complete sample of M dwarfs within 6-7 pc of the Sun fall into two
groups: most dMe stars have Fx above a critical value
Fc, while most dM stars have Fx below the
same Fc. The formal value of Fc, where the
distribution functions of dMe and dM stars are maximally different,
is ≍ 105.3-105.4 ergs cm-2
s-1. The value of Fc is close to an upper limit
computed by Mullan & Cheng for a model of acoustically heated
coronae in an M dwarf (Fac ≍ 105.1 cgs). These
results provide quantitative support for the hypothesis that the
coronal heating process in dMe stars cannot be predominantly acoustic in
nature. A Monte Carlo analysis of the X-ray data suggests that X-
active regions on dMe stars occupy at least 45%-50% of the surface,
while, on dM stars the X- active regions occupy no more than 10% of
the surface. This analysis leads to the conclusion that quiet coronal
regions in both dM and dMe stars may be heated acoustically.
Title: Structure of the heliospheric MHD bow shock: Effects of
ion-atom drifts
Authors: Mullan, D. J.; Arge, C. N.
Bibcode: 1996JGR...101.2535M
Altcode:
We report on the structure of a perpendicular MHD shock in the
partially ionized gas of the local interstellar medium (LISM). In
situations where the flow speed is sub-Alfvenic in the ion gas and
supersonic in the atom gas, the ions and atoms behave differently
in the shock. Ion-atom drifts provide a dissipation mechanism which
permits computation of the steady state structure of the transition
between upstream and downstream conditions. Using parameter ranges
which are appropriate for the LISM, we find that the heliospheric bow
shock may be many hundreds of AU thick.
Title: Correlating Coronal Temperature and Gravitational Potential:
A Test of the Nonthermal Boundary Hypothesis
Authors: Williams, L. L.; Mullan, D. J.
Bibcode: 1996ApJ...457L..95W
Altcode:
It has been suggested that a nonthermal boundary condition can
account for the existence of a coronal temperature inversion in
stars. The nonthermal hypothesis makes falsifiable predictions of
the maximum coronal temperature obtainable. This maximum temperature
corresponds to the depth of the potential well at the boundary,
which is effectively R*, and so varies as M*/R*. Here, we compare
this prediction with one-temperature fits to single stars in Einstein
data. We find that the coronal temperatures in dwarfs are consistent
with a nonthermal boundary condition, but the nonthermal hypothesis
cannot readily account for the coronal temperatures of class III
giants. However, interpreting the emission from the non-compliant
giants in alternate terms suggests that a dwarf companion may be
the source of the X-rays. More generally, gravitational potential is
found to be a useful quantity to ordering coronal observations: (1)
the "dividing line" near K4, beyond which giant star coronae are not
observed, corresponds to a rather precipitous factor of 10 drop in the
strength of the stellar gravitational potential; and (2) M dwarfs and
A stars of class III--V, whose coronae have been somewhat of a puzzle,
all have relatively deep gravitational potentials.
Title: Remote Sensing of the Solar Wind Using Radio Waves: Part 2
Authors: Yakovlev, O. I.; Mullan, D. J.
Bibcode: 1996IrAJ...23....7Y
Altcode:
No abstract at ADS
Title: Test of an Acoustic Mechanism for Atmospheric Heating in
Dynamo-Deficient F Stars
Authors: Mullan, D. J.
Bibcode: 1996nasa.reptU....M
Altcode:
In a qualitative sense, the heating of chromospheres and coronae has
long been ascribed to either acoustic or magnetic heating. However,
quantitative discussions of the energy balance with detailed comparison
to the fluxes of chromospheric emission lines have begun to appear only
recently. The aim of this work is to observe F stars where magnetic
effects might be expected to be rather small, thereby allowing us
hopefully to access acoustically heated atmospheres.
Title: Acoustic and Magnetic Heating of Chromospheres/Coronae:
Are There Distinct Signatures?
Authors: Mullan, D. J.
Bibcode: 1996nasa.reptT....M
Altcode:
In a qualitative sense, the heating of chromospheres and coronae has
long been ascribed to either acoustic or magnetic heating. However,
quantitative discussions of the energy balance with detailed comparison
to the fluxes of chromospheric emission lines have begun to appear
only recently. The aim of this work is to obtain more quantitative
information on the acoustic/magnetic mechanisms by comparing data
with models of acoustically heated atmospheres. Mechanical energy
in acoustic form is inevitably present in all stars with convective
envelopes. Once the acoustic waves are generated, their propagation
and dissipation in the chromosphere and corona can be computed by ab
initio models, again using the well defined equations of compressible
hydrodynamics (e.g. Mullan and Cheng: Papers I-III). In contrast to the
ubiquitous acoustic modes, magnetic modes need not be present. And even
in stars where magnetic heating is at work, the atmospheric heating
always includes an acoustic component as well. In order to evaluate
the magnetic contribution in such stars, we need to separate out the
acoustic contribution. To address the "acoustic-magnetic" mixture, and
separate the components, our strategy in this work has been to select
stars in those parts of the HR diagram where the magnetic contribution
is "turning on". By studying such stars, we hope to quantify the
acoustic component which pervades the atmospheres of all cool stars,
and characterize how the magnetic components alter the emission measure
distribution in the atmosphere. Two groups of stars are suitable for
our purposes: they are the groups which have recently been the subject
of detailed quantitative modelling as regards acoustic propagation,
i.e. the coolest dwarfs and the warm stars.
Title: Mass loss from active dwarf stars
Authors: Mullan, D. J.
Bibcode: 1996ASPC..109..461M
Altcode: 1996csss....9..461M
No abstract at ADS
Title: Remote Sensing of the Solar Wind Using Radio Waves: Part 1
Authors: Mullan, D. J.; Yakovlev, O. I.
Bibcode: 1995IrAJ...22..119M
Altcode:
No abstract at ADS
Title: Destabilization of Compressible Convection by Radiation:
Quantitative Evaluation
Authors: Cheng, Q. Q.; Hossain, Murshed; Mullan, D. J.
Bibcode: 1995ApJ...447..789C
Altcode:
We simulate time-dependent compressible hydrodynamic convection
including the effects of radiative transfer (RT). The gray RT
equation is solved along bundles of parallel rays using an integral
method. Applying a perturbation to an initial polytrope, we follow the
flows for many sound-crossing times. In the linear regime, we report
on a case where, in the presence of radiation, the critical Rayleigh
number Rac(rad) for onset of convection is at least 20%-30%
smaller than the value of Rac in the absence of radiation. In
the nonlinear regime, we report on another case where the entropy
gradients and maximum flow speeds in the presence of radiation are
larger by factors of at least 40%-60% than the corresponding quantities
in the nonradiative case.
Title: Coronal Heating in Flare Stars: Resonant MHD Absorption?
Authors: Mullan, D. J.; Johnson, M.
Bibcode: 1995ApJ...444..350M
Altcode:
We report on the occurrence of periodicity in X-rays from certain
flare stars. Statistically significant periods in the range 70-200
s are present in Einstein data for EQ Vir, Proxima Cen, and AD
Leo. Periodicities are also present (although with lower statistical
significance) in ROSAT data for AD Leo, and in EXOSAT data for EQ Vir
and AD Leo. In UV Cet, we also find periodicity in Einstein data, but
with lower statistical significance than for the above three stars:
however, the UV Cet X-ray period is close to a period discovered by
Gary et al. (1982) in microwaves in the close companion L726-8A. We
argue that the properties of magnetic loops in flare stars are such
that the variations in a single magnetic loop may be detectable in
integrated flux. The period which we find in the X-ray flux from
Proxima Cen is consistent with a global Alfven wave resonance in a
large loop whose properties were derived some years ago by Haisch
(1983) using an entirely different line of reasoning from what we use
here. The periods, amplitudes, and durations of the variations which
we have found in X-ray emission are consistent with the hypothesis
that resonant absorption of MHD waves is occurring at certain times
in coronal loops in the above four stars.
Title: Magnetic Moments and Angular Momenta of Stars and Planets
Authors: Arge, C. N.; Mullan, D. J.; Dolginov, A. Z.
Bibcode: 1995ApJ...443..795A
Altcode:
Using published data on magnetic fields, radii, masses, and rotation,
we have compiled a data set of magnetic moments mu and angular momenta
L for stars and planets. In our subsample of hotter stars (classes A,
B, and O), there are 171 objects. In the subsample of cooler stars
(classes F, G, K, and M), there are 54 objects. We include 33 white
dwarfs, of which 19 are in cataclysmic variables. The pulsar subsample
contains 32 pulsars in binaries and 429 isolated pulsars. Som subsamples
exhibit significant empirical correlations between log mu and log
L. For the hot and cool stars, the correlations are positive. However,
the hot-star correlation is significantly shallower than for the cool
stars. In the solar system subsample, the correlation has essentially
the same slope as for the cool stars, although the magnetic moments are
two to three orders of magnitude smaller for the solar system objects
at a given L value. For isolated white dwarfs, the correlations are
weak or absent. Pulsars and white dwarfs in close binaries show strong
negative correlations: the results are quantitatively consistent with
magnetically enforced synchronism with the orbital period. When we
consider the centers of gravity of the different subsamples of objects,
a significant positive correlation appears between log mu and log L.
Title: Intrinsic Time-dependence in Astrophysical Convection
Authors: Hossain, M.; Mullan, D. J.
Bibcode: 1995SPD....26..104H
Altcode: 1995BAAS...27..950H
No abstract at ADS
Title: Acoustic heating in Procyon: comparison of theory with
EUVE data
Authors: Mullan, D. J.; Cheng, Q. Q.
Bibcode: 1995IAUS..176P.208M
Altcode:
No abstract at ADS
Title: A dividing line between dM and dMe stars: X-ray surface fluxes
Authors: Mullan, D. J.; Fleming, T. A.; Schmitt, J. H. M. M.
Bibcode: 1995IAUS..176P.210M
Altcode:
No abstract at ADS
Title: Coronal loops in flare stars: heating by resonant MHD
absorption
Authors: Mullan, D. J.; Johnson, M.
Bibcode: 1995IAUS..176P.206M
Altcode:
No abstract at ADS
Title: Acoustic Heating of the Chromosphere and Cool Corona in the
F Star alpha Canis Minoris (Procyon)
Authors: Mullan, D. J.; Cheng, Q. Q.
Bibcode: 1994ApJ...435..435M
Altcode:
We report on a hydrodynamical model of acoustic wave energy deposition
in the atmosphere of the F star Procyon. The model treats radiative
losses in the photosphere by solving the continuum radiative transfer
(RT) problem; it treats radiative losses in the chromosphere by solving
the RT equation in two representative strong lines (Mg II k and Lyman
alpha); and it includes optically thin emission from the corona. We
find a temperature minimum of 4440 K and a transition region at a
height of 3500-4000 km above the photosphere. Our acoustic model
accounts for the reported fluxes of Mg II and Lyman alpha emission
lines, as well as for the X-ray flux from the cool (T less than 1 MK)
coronal component reported by Lemen et al. (1989). The differential
emission measure distribution in our model agrees quite well with
empirical results of Jordan et al. (1986).
Title: Accurate Frequencies of Polytropic Models
Authors: Christensen-Dalsgaard, J.; Mullan, D. J.
Bibcode: 1994MNRAS.270..921C
Altcode:
Observations of solar oscillations are already yielding frequencies
with a relative accuracy exceeding 10 new experiments will increase
this accuracy by about an order of magnitude during the coming
decade. Full utilization of such data requires comparable accuracy in
the computation of frequencies of stellar models. A very convenient
test of the calculation of adiabatic oscillation frequencies is
provided by the polytropic models: the models are completely well
defined and can easily be computed with the required precision;
hence their frequencies can serve as references against which to
test oscillation codes before these are applied to more realistic
models. Here we analyse the computation of such frequencies. Their
accuracy is tested in different ways, including the comparison of two
independently derived sets of results, and tables of frequencies are
given. More extensive sets of frequencies can be obtained directly
from the authors, in computer-readable form. The analysis of the
polytropic models provides interesting illustrations of a problem
in the labelling of dipolar modes, as well as of the effects on the
frequencies of the perturbation in the gravitational potential. Key
words: Sun: oscillations - stars: interiors - stars: oscillations.
Title: Gravitational Damping of Alfven Waves in Stellar Atmospheres
and Winds
Authors: Khabibrakhmanov, I. K.; Mullan, D. J.
Bibcode: 1994ApJ...430..814K
Altcode:
We consider how gravity affects the propagation of Alfven waves in a
stellar atmosphere. We show that when the ion gyrofrequency exceeds
the collision rate, the waves are absorbed at a rate proportional to
the gravitational acceleration g. Estimates show that this mechanism
can readily account for the observed energy losses in the solar
chromosphere. The mechanism predicts that the pressure at the top of
the chromosphere PTc should scale with g as PTc
proportional to gdelta, where delta approximately equals 2/3;
this is close to empirical results which suggest delta approximately
equals 0.6. Gravitational damping leads to deposition of energy at a
rate proportional to the mass of the particles. Hence, heavier ion
are heated more effectively than protons. This is consistent with
the observed proportionality between ion temperature and mass in the
solar wind. Gravitational damping causes the local g to be effectively
decreased by an amount proportional to the wave energy. This feature
affects the acceleration of the solar wind. Gravitational damping
may also lead to self-regulation of the damping of Alfven waves in
stellar winds: this is relevant in the context of slow massive winds
in cool giants.
Title: Acoustically Heated Chromospheres in M Dwarfs
Authors: Mullan, D. J.; Cheng, Q. Q.
Bibcode: 1994ASSL..187..587M
Altcode: 1994fsgb.book..587M
No abstract at ADS
Title: Test of an Acoustic Mechanism for Atmospheric Heating in
Dynamo-deficient F Stars
Authors: Mullan, Dermott
Bibcode: 1994euve.prop...29M
Altcode:
Stars in the spectral range from late A to mid F have convection zones
which are thought to be too shallow to allow for efficient dynamo
operation. As a result, such stars almost certainly do not rely
on magnetic processes to heat their chromospheres and coronae. And
yet there is evidence that chromospheres and coronae are present in
these stars. This suggests the presence of non-magnetic mechanical
heating. The nature of this heating is not yet known. Recent work
on an acoustic mechanism has led to the prediction that if acoustic
heating is at work, the differential emission measure (DEM) should have
a certain shape, with a well defined minimum at temperatures between
10^5 and 10^6 K, and steep slopes on both sides of the minimum. The
EUVE spectrometer is ideally suited to observe lines formed in the
temperature range 10^{5-6} K. We propose an observing program to
determine the DEM in 6 F stars. Comparisons with acoustic predictions
will enable us to assess critically and quantitatively how viable the
acoustic mechanism is in these stars.
Title: Acoustic Waves in M Dwarfs: Maintaining a Corona
Authors: Mullan, D. J.; Cheng, Q. Q.
Bibcode: 1994ApJ...420..392M
Altcode:
We use a time-dependent hydrodynamics code to follow the propagation
of acoustic waves into the corona of an M dwarf star. An important
qualitative difference between M dwarfs and stars such as the Sun is
that the acoustic spectrum in M dwarfs is expected to peak at periods
close to the acoustic cutoff PA: this allows more effective
penetration of waves into the corona. In our code, radiative losses in
the photosphere, chromosphere, and corona are computed using Rosseland
mean opacities, Mg II kappa and Ly alpha emission, and optically
thin emissivities respectively. We find that acoustic heating can
maintain a corona with a temperature of order 0.7-1 x 106
K and a surface X-ray flux as large as 105ergs/sq cm/s. In
a recent survey of X-rays from M dwarfs, some (20%-30%) of the stars
lie at or below this limiting X-ray flux: we suggest that such stars
may be candidates for acoustically maintained coronae.
Title: Acoustic Heating of Coronae in Inactive M Dwarfs
Authors: Cheng, Q. Q.; Mullan, D. J.
Bibcode: 1993AAS...183.1508C
Altcode: 1993BAAS...25.1314C
We have used a time-dependent hydrodynamics code to follow the
propagation of acoustic waves from the photosphere of an M dwarf star
upwards into the chromosphere and corona. An important qualitative
difference between M dwarfs and stars such as the Sun is that the
acoustic spectrum in M dwarfs is believed to peak at periods close
to the acoustic cut-off P_A: as a result of this, the waves can more
effectively penetrate into the corona, rather than being essentially
entirely dissipated in the chromosphere. In our code, radiative
losses in the photosphere, chromosphere, and corona are computed using
Rosseland mean opacities, MgII k and Ly-alpha emission, and optically
thin emissivities respectively. Our results indicate that acoustic
heating can maintain a corona in an M dwarf with a temperature of order
0.7-1 million K and a surface X-ray flux F_X up to (1-1.5) times 10(5)
ergs cm(-2) sec(-1) . We find that it is not possible for acoustic
waves to generate larger F_X values: when we attempt to raise the F_X
values by supplying more input flux at the photosphere, the effect is
to push the transition region to greater heights, thereby reducing the
emission measure of the X-ray plasma. Preliminary results from a ROSAT
study of a nearly complete sample of dK/dM stars out to several parsecs
(J. Schmitt and T. Fleming, 1993, priv. comm.) indicate that most
inactive dwarfs have log F_X no larger than 5.1-5.2, and temperature
no hotter than about 1 million K. In view of the good correspondence
between our calculations and these data, we suggest that such inactive
stars may be candidates for acoustically maintained coronae. This work
has been supported by NASA Grant NAGW-2456.
Title: Penetrative Stellar Convection
Authors: Hossain, M.; Mullan, D. J.
Bibcode: 1993AAS...18311103H
Altcode: 1993BAAS...25Q1457H
We have reported recently on a comparative study of boundary conditions
in simulations of compressible convection, and how these determine
certain aspects of the solutions (M. Hossain and D. J. Mullan, ApJ
416, 733 [1993]). In that work, the simulations were confined to a
layer which was convectively unstable throughout its extent. Since
the medium is compressible, the solutions contain not only convective
modes but also acoustic components. The results indicated that the
relative contribution of the acoustic components to the overall flows
can vary significantly depending on which boundary conditions are
applied. We are now extending the work to investigate how the addition
of convectively stable layers below and/or above the unstable layer will
affect the solutions. As far as the convective modes are concerned,
it is already known that the presence of an overlying stable layer
may increase or decrease the critical Rayleigh number depending on
the degree of stability (V. Savolainen et al. Phys. Fluids A4, 626
[1992]). Here, we use simulations to investigate how the acoustic
components are affected. To avoid numerical difficulties, both the
initial states and the transport coefficients are taken as analytic
functions of depth. Preliminary results from such attempts will be
reported. This work has been supported by NASA grant NAGW-2456.
Title: Magnetic Moments and Angular Momenta in Stars and Planets
Authors: Arge, N.; Mullan, D. J.; Dolginov, A.
Bibcode: 1993AAS...18311101A
Altcode: 1993BAAS...25.1456A
Using published data on magnetic fields, radii, masses, and rotation,
we have compiled a dataset of magnetic moment M and angular momentum
J for stars and planets. The stellar sub-sample extends from spectral
class O to class M, with members of all classes represented. Among the
hotter stars (classes A, B, and O), the sample includes 134 objects. For
the cooler stars (classes F, G, K, and M), there are 33 objects. Our
sub-sample of white dwarfs includes 33 objects, some in cataclysmic
variables. The neutron star sub-sample contains 17 pulsars in binaries
and 86 isolated pulsars. The solar system sub-sample contains seven
planets plus the Sun. We find statistically significant positive
correlations between log M and log J in the hot sub-sample and in the
cool sub-sample: however, the slopes of the correlations are quite
different for hot and cool stars. In the solar system sub-sample,
the correlation has essentially the same slope as for the cool stars,
although the intercept is almost two orders of magnitude smaller
for the solar-system objects. For the isolated pulsars, we find no
statistically significant correlation, while pulsars in binaries show
a significant anti-correlation. Anti-correlation also appears in the
white dwarf sub-sample, but this result relies heavily on five systems
where rotational information is ambiguous. This work has been supported
by NASA Grant NAGW-2456.
Title: Periodic X-ray Emission in Flare Stars: Resonant MHD
Absorption?
Authors: Mullan, D. J.; Johnson, M.
Bibcode: 1993AAS...183.1503M
Altcode: 1993BAAS...25.1313M
Magnetic fields play a role in the heating of coronae in the Sun
and cool dwarfs, but the physical processes at work are not yet
clear. Several magnetic models have been proposed for coronal
heating. In one class of models, resonant absorption of MHD waves
in closed loops is believed to be at work. In a very general sense,
resonance implies the existence of a preferred frequency. Thus,
resonant absorption models would be subject to a decisive test if
periodicities could be identified in the coronal emission. We have
searched for periodicities in the EINSTEIN X-ray fluxes of 15 cool
dwarfs. Statistically significant periodicities have been detected
in several stars, including EQ Vir, AD Leo, and Prox Cen. The period
detected in Prox Cen is consistent with MHD resonance in the loop
for which parameters were derived by Haisch in his study of X-ray
light curves in Einstein data. For AD Leo, ROSAT/HRI and EXOSAT data
also suggest the presence of periodicity at the same period as in the
EINSTEIN data. For EQ Vir, EXOSAT data suggest the presence of the same
period as in the EINSTEIN data. However, the statistical significance of
the ROSAT and EXOSAT periods is not as good as those for EINSTEIN. This
work has been supported by NASA Grant NAGW-2456, by the NASA Space
Grant College program, and by the NASA Astrophysical Data Program.
Title: Simulation of Compressible Convection: A Comparative Study
of Boundary Conditions
Authors: Hossain, Murshed; Mullan, D. J.
Bibcode: 1993ApJ...416..733H
Altcode:
Numerical simulation of compressible hydrodynamics in the context
of astrophysical convection is necessarily restricted to a limited
volume of an entire star: however, the physical parameters inside
that volume are determined by what goes on throughout the star as a
whole. The aim of boundary conditions (BCs) is to model the contact
(both mechanical and thermal) between the computational domain and the
rest of the star. Different investigators have used various combinations
of BCs. Here we explore how the choice of BC affects certain aspects
of the solutions of three-dimensional compressible convection. As
regards mechanical BCs, we examine both closed and open domains. In
the case of open domains, some of the BC sets we examine are based
on recent suggestions by Poinsot & Lele for applying the method
of characteristics to dissipative flows. Our results indicate that
the hydrodynamic solutions can be qualitatively different depending
on the choice of BC. As a specific example of the differences,
we examine how the BCs affect the relative contributions of acoustic
and incompressible components to the overall flow. In recently
reported simulations of homogeneous decaying turbulence, Ghosh &
Mattheaus have found that the relative contribution of acoustic
components depends sensitively on the choice of conditions in the
initial state. Analogously, the ratio Rai of acoustic to
incompressible power in convective simulations in statistically steady
state might be expected to depend on the choice of certain conditions
also: in convection, the controlling factors would be expected to
be the boundary conditions. To investigate that possibility, it is
necessary to treat the two modes of a stratified compressible medium
(the gravity-driven and acoustic modes) on an equal footing: both
modes are subject to instability (if radiative effects are at work)
although the criteria for onset of instability are different for the two
modes. (In the surface layers of the Sun, convective and acoustic modes
seem to have comparable amplitudes, as if both modes may be experiencing
instability.) We report results of an exploration of a small subset
of BCs which indicate that the value of Rai does exhibit
significant variation depending on BCs. In the astrophysical context,
this conclusion has implications for the modeling of chromospheres in
cool stars.
Title: MG II and LY alpha Fluxes in M Dwarfs: Evaluation of an
Acoustic Model
Authors: Mullan, D. J.; Cheng, Q. Q.
Bibcode: 1993ApJ...412..312M
Altcode:
Surface fluxes of Mg II and Ly-alpha from a sample of M dwarfs have been
reported by Panagi and Mathioudakis (PM, 1992). In order to determine if
acoustic effects can reproduce the fluxes in the least active ('basal')
stars in the PM sample, we have constructed steady state models of
plane-parallel atmospheres with T(eff) = 3000 and 4000 K including
heating by shock dissipation. The models incorporate radiative losses in
the chromosphere by solving the transfer equation for two strong lines
(Mg II k and Ly-alpha). Predictions of standard weak shock theory
are expected to overestimate the rate of energy deposition in the
chromospheres of M dwarfs compared to the rate of deposition in F,
G, or K dwarfs. In order to incorporate this effect in M dwarfs, we
compute a series of atmospheric models in which, for a given T(eff),
the standard weak shock dissipation rate is reduced by a series of
factors F(r). Although the input acoustic flux F(mech) may in principle
be determined uniquely by T(eff) and log g, the correct value to use
in any particular case is not well known. We therefore use F(mech) and
F(r) as parameters to construct a family of M dwarf models at various
T(eff) and g. Comparing with the observed fluxes in the least active M
dwarfs of the PM sample, we find that our models reproduce the observed
fluxes of both Mg II and Ly-alpha best if F(mech) is close to the value
predicted by Bohn (1984) and if F(r) is approximately equal to 10-20.
Title: Simulation of Compressible Convection: A Comparative Study
of Boundary Conditions
Authors: Hossain, M.; Mullan, D. J.
Bibcode: 1993BAAS...25.1194H
Altcode:
No abstract at ADS
Title: Mass Loss from Cool Dwarfs: Limits on Detectability
Authors: Doyle, J. G.; Mullan, D. J.
Bibcode: 1993ASSL..183..401D
Altcode: 1993pssc.symp..401D
No abstract at ADS
Title: Shock heating in inactive M dwarf atmospheres
Authors: Mullan, D. J.; Cheng, Q. Q.
Bibcode: 1992AAS...181.5105M
Altcode: 1992BAAS...24.1206M
Surface fluxes of MgII and Ly-alpha have recently been reported for
a sample of M dwarfs. We have developed a model to determine if the
minimum (basal) fluxes in stars with effective temperatures 3000-4000
K can be understood in terms of shock dissipation. Using a prescribed
input of mechanical energy at the lower boundary, the hydrodynamic
equations, including radiative and conductive losses, are evolved
to a (nearly) stationary state. Radiative losses and/or gains in the
photosphere are handled using opacities tabulated by Kurucz. In the
chromosphere, radiative losses are treated by solving the transfer
equation for the two strongest lines emitted by the chromosphere: MgII
and Ly-alpha . We find that the models are successful in reproducing the
lower envelope of the observed Ly-alpha versus MgII fluxes. In M dwarfs,
the transmission of acoustic power upwards into the corona is expected
to be more efficient than in the Sun: using plausible choices for the
coronal transmission parameters, we have explored the possibility that
the X-ray fluxes observed in the least active M dwarfs might also be
due largely to acoustic heating.
Title: Limits on Detectability of Mass Loss from Cool Dwarfs
Authors: Mullan, D. J.; Doyle, J. G.; Redman, R. O.; Mathioudakis, M.
Bibcode: 1992ApJ...397..225M
Altcode:
Recent spectroscopic evidence supports the theoretical expectation
that certain cool dwarfs may have stellar winds with M-dot values
several orders of magnitude larger than the solar rate. For large
enough values of M-dot, the emission from the wind is expected to have
a spectrum which, at low enough frequencies, becomes a power law,
S(v) about v exp alpha with alpha about 0.7. Data from IRAS and VLA
suggest that such a spectrum may in fact occur in certain M dwarfs:
a key test of the wind spectrum would be provided if the stars could
be detected at lambda about 1 mm. We show that the M-dot required to
ensure power-law emission is a few times 10 exp -10 solar mass/yr. With
M-dot of this order, fluxes at lambda about 1 mm would be tens of
mJy. Using the James Clerk Maxwell Telescope, we have tested this
prediction on several stars: the data are suggestive but are near the
limits of detection. Confirmation of our estimates will be important
for evolution and for interstellar medium (ISM) physics: if even a
few percent of all M dwarfs are losing mass at the above rates, the
mass balance of the ISM will be dominated by M dwarfs.
Title: Three-dimensional Compressible Hydrodynamic Convection in
the Sun and Stars: Erratum
Authors: Hossain, Murshed; Mullan, D. J.
Bibcode: 1992ApJ...397..353H
Altcode:
No abstract at ADS
Title: Book Review: Mechanisms of chromospheric and coronal heating /
Springer, 1991
Authors: Fontenla, J. M.; Mullan, D. J.; Fontenla, J. M.
Bibcode: 1992SoPh..139..409F
Altcode: 1992SoPh..139..409U
No abstract at ADS
Title: Book Review: The sun and cool stars: activity, magnetism,
dynamos / Springer-Verlag, 1991
Authors: Mullan, D. J.
Bibcode: 1992SoPh..139..409M
Altcode: 1992SoPh..139..409T
No abstract at ADS
Title: Limits on detectability of mass loss from cool dwarfs
Authors: Mullan, D. J.; Doyle, J. G.; Mathioudakis, M.; Redman, R. O.
Bibcode: 1992AAS...180.6006M
Altcode: 1992BAAS...24..826M
Recent spectroscopic evidence supports the theoretical expectation
that certain cool dwarfs may have stellar winds with dot {M} values
several orders of magnitude larger than the solar rate. For large
enough values of dot {M}, the emission from the wind is expected to
have a spectrum which, at low enough frequencies, becomes a power law,
S_ν ~ nu (alpha ) with alpha ~ 0.7. Data from IRAS and VLA suggest
that such a spectrum may in fact occur in certain M dwarfs: a key
test of the wind spectrum would be provided if the stars could be
detected at lambda ~ 1 mm. We show that the dot {M} required to ensure
power law emission is a few times 10(-10) Msun \ yr(-1)
. With dot {M} of this order, fluxes at lambda ~ 1 mm would be tens
of mJy. Using the James Clerk Maxwell Telescope, we have tested this
prediction on several stars: the data are suggestive but are near the
limits of detection. Confirmation of our estimates will be important
for studies of evolution of low mass stars and for interstellar medium
(ISM) physics: if even a few percent of all M dwarfs are losing mass
at the above rates, the mass balance of the ISM will be dominated by
M dwarfs. Moreover, if flares on these stars are accompanied by mass
ejecta, the ISM may become contaminated with deuterium produced in
the surface layers of the star by energetic protons.
Title: Transient Periodicities in X-Ray--active Red Dwarfs: First
Results from Mount Cuba and Interpretation with an Oscillating
Loop Model
Authors: Mullan, D. J.; Herr, R. B.; Bhattacharyya, S.
Bibcode: 1992ApJ...391..265M
Altcode:
Results from a program at the Mount Cuba Astronomical Observatory
to optically monitor transient periodicities in flare stars are
reported. The data are analyzed for periodicities by means of a modified
autocorrelation approach. A randomization technique is used to assess
the statistical significance of periods. In AD Leonis, variability
with amplitude 0.4 percent and a period of 4.1 min was detected during
one run on March 6, 1991: the probability that this periodicity is
due to chance is 10 exp -5. Further observations obtained on AD Leo
within 1-2 hr on the same night showed no significant variability
(0.01). It is shown that optical periodicities of the sort detected
may be interpreted as arising from oscillations in coronal loops.
Title: Transient Optical Periodicities in Red Dwarfs: Loop
Oscillations in the Corona
Authors: Mullan, D. J.; Herr, R. B.; Bhattacharyya, S.
Bibcode: 1992ASPC...26..306M
Altcode: 1992csss....7..306M
No abstract at ADS
Title: Millimeter; Sub-Millimeter Emission from Flare Stars
Authors: Mullan, D. J.; Doyle, J. G.; Mathioudakis, M.; Redman, R. O.
Bibcode: 1992ASPC...26..328M
Altcode: 1992csss....7..328M
No abstract at ADS
Title: Transient optical periodicities in coronal red dwarfs.
Authors: Herr, Richard B.; Mullan, Dermott J.
Bibcode: 1992MmSAI..63..747H
Altcode:
Evidence is presented for the existence of transient low-amplitude
(several tenths of a percent of total visible light) periods in the
range of several minutes, in the optical monitoring of dwarf M stars
with high X-ray luminosities using the 61-cm Cassegrain at Mt. Cuba
Observatory in Delaware (USA). The stars observed include AD Leo,
Gliese 549, and DT Vir.
Title: Surface Inhomogeneities on Late-Type Stars
Authors: Byrne, Patrick B.; Mullan, Dermott J.
Bibcode: 1992LNP...397.....B
Altcode: 1992QB843.C6S87....; 1992sils.conf.....B
No abstract at ADS
Title: Summary and Conclusions (Invited)
Authors: Mullan, D. J.
Bibcode: 1992LNP...397..233M
Altcode: 1992sils.conf..233M
No abstract at ADS
Title: Three-dimensional Compressible Hydrodynamic Convection in
the Sun and Stars
Authors: Hossain, Murshed; Mullan, D. J.
Bibcode: 1991ApJ...380..631H
Altcode:
A pseudospectral code is used to obtain numerical solutions of 3D
compressible hydrodynamic convection in a stratified medium where
the boundaries are open and where radiative losses are strongest
at the top. Results are presented for a series of three simulations
in progressively deeper boxes. The code does not make the anelastic
approximation, but follows sound waves explicitly. Spatial organization
is found in the form of vertical 'stacking' of smaller structures
on top of larger ones, with each structure being roughly Hp to a
vertical extent. Compressibility effects are apparent in the density:
the snapshots indicate that, on a horizontal plane, the density at
certain points may be of an order of 10 percent larger or smaller than
elsewhere on that plane.
Title: Transient Optical Periodicities of Red Dwarfs with High
X-Ray Luminosities
Authors: Herr, R. B.; Mullan, D. J.
Bibcode: 1991BAAS...23.1383H
Altcode:
No abstract at ADS
Title: Survival of discrete structures in the solar wind
Authors: Mullan, D. J.
Bibcode: 1991A&A...248..256M
Altcode:
Survival time scales are estimated for structures ejected from the
sun into the solar wind, which are subject to decay processes during
their transit from the sun to the earth. With the assumption that
the structures are magnetic, MHD estimates are made to determine the
time scales for reconnection into the ambient magnetic field. It is
found that for structures smaller than a few tens of Mm at the sun, the
probability of surviving the transit to the earth is low. The transition
in size between structures that survive the sun-earth transit and
those that do not (20-30 Mm at the sun) corresponds to scales of (4-6)
x 10 to the 11th cm at the earth orbit (assuming radial expansion).
Title: Corona: Coordinated research on non-thermal processes in
astrophysics
Authors: Mullan, Dermott J.
Bibcode: 1991udnw.rept.....M
Altcode:
Several problems in astrophysics are examined where departures from
purely thermal behavior are expected to produce effects which are
detectable by a variety of NASA satellites. The problems identified
as worthy of study include coronal heating in stars, deposition
of non-thermal particle energy in stars from external sources, and
turbulence in atmospheres and winds of stars. The progress in solving
these problems is summarized.
Title: Inefficient Accretion by the DA2 White Dwarf in V471 Tauri
Authors: Mullan, D. J.; Shipman, H. L.; Sion, E. M.; MacDonald, J.
Bibcode: 1991ApJ...374..707M
Altcode:
The results are reported of an analysis of eight-high-resolution
International Ultraviolet Explorer (IUE) short-wavelength primary
spectra of V471 Tauri, an eclipsing-spectroscopic, precataclysmic
binary in the Hyades. The coaddition in velocity space of regions
surrounding principle high- and low-excitation ion species on a common
velocity scale was used. Identifiable lines were found to fall into
three categories: interstellar absorption in the line of sight to the
Hyades cluster; broad features due to absorption by the stellar wind
from the K2 dwarf; and narrow high-velocity circumbinary absorption
independent of orbital phase. The white dwarf in V471 Tauri is estimated
to have accreted some 40,000 times more material than the determined
upper limits. It is proposed that the barrier presented by the white
drawf's rotating magnetosphere reduces accretion from the K2V stellar
wind to levels which are undetectable with the IUE satellite.
Title: Three-Dimensional Hydrodynamic Stellar Convection
Authors: Hossain, M.; Mullan, D. J.
Bibcode: 1991BAAS...23.1047H
Altcode:
No abstract at ADS
Title: Convection in Stars and Heating of Coronae
Authors: Mullan, D. J.
Bibcode: 1991IrAJ...20....1M
Altcode:
The properties of convection in the sun and other cool stars are
summarized. Recent studies of convection which have involved the use of
supercomputers to model the flow of compressible gas in three dimensions
are discussed. It is shown how the results of these computations may
eventualy provide an understanding of how nonthermal processes heat
coronal gas to temperatures of millions of degrees.
Title: Coronal Heating by Nanoflares: Possible Evidence of Plasmoids
in Radio Occultation Data
Authors: Mullan, D. J.
Bibcode: 1991mcch.conf..637M
Altcode:
No abstract at ADS
Title: Inefficient Accretion by the DA2 White Dwarf in V471 Tauri
Authors: Mullan, D. J.; Shipman, H. L.; Sion, E. M.; MacDonald, J.
Bibcode: 1991BAAS...23..828M
Altcode:
No abstract at ADS
Title: Globular Clusters - where have all the Light Stars Gone
Authors: Mullan, D. J.
Bibcode: 1990IrAJ...19..177M
Altcode:
No abstract at ADS
Title: On the ``Fast Electron Hypothesis'' for Stellar Flares
Authors: Mullan, D. J.
Bibcode: 1990ApJ...361..215M
Altcode:
It is pointed out that Gurzadyan's (1988) fast-electron hypothesis for
stellar flares encounters certain difficulties. The origin of the fast
electrons is obscure. Negative flares and predicted ratios of X-ray
to optical fluxes are not necessarily a proof of the fast-electron
hypothesis. When the electrons thermalize, they will yield X-ray
fluxes which are orders of magnitude too large to be consistent with
observations.
Title: Sources of the solar wind - What are the smallest-scale
structures?
Authors: Mullan, D. J.
Bibcode: 1990A&A...232..520M
Altcode:
Data obtained by a variety of observing techniques are summarized to
show that the solar wind is structured over a wide range of scale sizes
(from one to one millionth solar radius). The possible contribution
of reconnection sites as sources of momentum for the solar wind
is examined. It is pointed out that, as plasmoids ejected from the
reconnection sites traverse the stratified corona, they may expand
sufficiently to occupy all of the available volume. It is suggested
that the solar wind, at its origin, is composed essentially of these
plasmoids.
Title: New Numerical Solutions of Three-dimensional Compressible
Hydrodynamic Convection
Authors: Hossain, Murshed; Mullan, D. J.
Bibcode: 1990ApJ...354L..33H
Altcode:
Numerical solutions of three-dimensional compressible hydrodynamics
(including sound waves) in a stratified medium with open boundaries
are presented. Convergent/divergent points play a controlling role in
the flows, which are dominated by a single frequency related to the
mean sound crossing time. Superposed on these rapid compressive flows,
slower eddy-like flows eventually create convective transport. The
solutions contain small structures stacked on top of larger ones,
with vertical scales equal to the local pressure scale heights,
Hp. Although convective transport starts later in the
evolution, vertical scales of Hp are apparently selected
at much earlier times by nonlinear compressive effects.
Title: A Model of 3-Dimensional Compressible Convection
Authors: Hossain, M.; Mullan, D. J.
Bibcode: 1990BAAS...22..897H
Altcode:
No abstract at ADS
Title: Astrophysics in Antarctica : Newark, Delaware, 1989
Authors: Mullan, Dermott J.; Pomerantz, Martin A.; Stanev, Todor
Bibcode: 1990AIPC..198.....M
Altcode: 1989AIPC..198.....M; 1990asan.conf.....M
The present conference discusses the use of Antarctic bases for
very high energy gamma-ray astronomy and neutrino astronomy, design
considerations for a TeV telescope at the South Pole, the microwave
detection of ultrahigh-energy neutrinos in ice, cosmic background
radiation anisotropy and Galactic emission observations from Antarctica,
IR astronomy in Antarctica, and the atmospheric transparency over
Antarctica of the spectrum from mid-IR to cm wavelengths. Also
discussed are Antarctic detection of LF and MF high latitude terrestrial
emissions, zero-pressure balloon behavior in Antarctica, long-duration
ballooning at midlatitudes and Antarctica, solar and stellar
observations at the South Pole, South Pole photometry of selected
variable stars, and astrophysical experimentation in Antarctica.
Title: A search for evidence of wind accreation by the DA2 white
dwarf in V471 Tauri.
Authors: Sion, E. M.; Mullan, D. J.; Shipman, H. L.
Bibcode: 1990apcb.conf..155S
Altcode: 1990cvlm.proc..155S
The authors report the results of an analysis of eight high-resolution
International Ultraviolet Explorer SWP (1200 Å - 2000 Å) spectra
of the Hyades eclipsing-spectroscopic pre-cataclysmic binary V471
Tauri. The technique utilized was the coaddition in velocity space of
regions surrounding principal high-excitation and low-excitation ion
species on a common velocity scale. Certain images were compensated
for the orbital motion of the white dwarf during the exposure. The
authors' line detections fell into three categories: (1) interstellar
absorption in the line of sight to the Hyades cluster; (2) broad stellar
wind features; and (3) narrow, high-velocity circumbinary absorption
independent of orbital phase. In the absence of strong evidence for
accreted photospheric metals and/or helium at the Einstein-redshifted
velocity of the white dwarf surface layers, the authors favor, as a
possible explanation, that the white dwarf does not accrete from the
K2V stellar wind in detectable amounts due to the barrier presented
by its rotating magnetosphere.
Title: Discrete Sources of Solar Wind: What are the Smallest
Structures?
Authors: Mullan, D. J.
Bibcode: 1989BAAS...21.1187M
Altcode:
No abstract at ADS
Title: Far-Infrared Properties of Flare Stars and dM Stars
Authors: Mullan, D. J.; Stencel, R. E.; Backman, D. E.
Bibcode: 1989ApJ...343..400M
Altcode:
Results are reported from a search of the IRAS data base for flare
stars and for a control sample of dM stars. At 12 microns, 70-80
percent of both samples have been detected. The K-12 colors of flare
stars are significantly different from those of dM stars: for a given
K magnitude, a flare star is about 70 percent brighter at 12 microns
than a dM star. At 100 microns, 27 percent of the flare stars which are
sources at 12 microns have been detected, while none of the comparable
dM stars has been detected. Implications for microflaring are discussed.
Title: Discovery of a Cool Expanding Shell at -1200 Kilometers per
Second around V471 Tauri
Authors: Sion, Edward M.; Bruhweiler, Fred C.; Mullan, Dermott;
Carpenter, Ken
Bibcode: 1989ApJ...341L..17S
Altcode:
High-resolution IUE spectra of V471 Tauri reveal the presence of a
very-high-velocity cool expanding gas in the line of sight to the
binary system with an expansion velocity of -1200 km/s. The summed
strength of the coadded absorption is 125 mA + or - 25 mA, with FWHM =
30 km/s. It is suggested that the observed absorption may be related to
the narrow coadded absorption at -590 km/s noted by Bruhweiler and Sion
(1966). The large expansion velocity suggests a possible association
with an ancient nova outburst.
Title: Evidence for a Cool Wind from the K2 Dwarf in the Detached
Binary V471 Tauri
Authors: Mullan, D. J.; Sion, E. M.; Bruhweiler, F. C.; Carpenter,
K. G.
Bibcode: 1989ApJ...339L..33M
Altcode:
Evidence for mass loss from the K2 dwarf in V471 Tauri is found in the
form of discrete absorption features in lines of various elements (Mg,
Fe, Cr, Mn) and ionization stages (Mg I, Mg II, Fe I, Fe II). Resonant
Mg II absorption indicates a mass loss rate of at least 10 to the
-11th solar masses per year. The wind appears to be cool (no more than
a few times 10,000 K).
Title: Solar and Stellar Flares - Questions and Problems
Authors: Mullan, D. J.
Bibcode: 1989SoPh..121..239M
Altcode: 1989IAUCo.104..239M
Although progress has been made in understanding certain aspects of
the physics of solar and stellar flares, there are a number of topics
which, in the author's opinion, still pose a problem. We summarize
these topics here.
Title: Far-Infrared Properties of Flare Stars and dM Stars
Authors: Mullan, D. J.; Stencel, R. E.; Backman, D. E.
Bibcode: 1989BAAS...21..795M
Altcode:
No abstract at ADS
Title: G-Mode Pulsations in Polytropes: High-Precision Eigenvalues
and the Approach to Asymptotic Behavior
Authors: Mullan, D. J.
Bibcode: 1989ApJ...337.1017M
Altcode:
In an extension of previous work on oscillations of polytropes, high
precision eigenfrequencies for the g-modes are obtained. The range of
periods covered here extends from a fraction of 1 hr to 100,000 s for
a polytrope with solar mass and radius. The results are presented in
a format which indicates quantitatively how the period separations
between adjacent modes approaches the asymptotic behavior predicted
by Tassoul (1980). As before, the oscillation code is based on the
Cowling approximation (as assumed by Tassoul).
Title: V471 Tauri
Authors: Sion, E. M.; Bruhweiler, F. C.; Carpenter, K.; Mullan, D. J.
Bibcode: 1988IAUC.4667....2S
Altcode: 1988IAUC.4667....0S
E. M. Sion, Villanova University; F. C. Bruhweiler, Catholic
University of America; K. Carpenter, Goddard Space Flight Center,
NASA; and D. J. Mullan, Bartol Research Institute, report: "IUE
High-Resolution SWP and LWP spectra of the Hyades member V471 Tau
(an eclipsing-spectroscopic, pre-cataclysmic, close binary having
spectral types K2 V + DA2) reveal very-high-velocity, cool, expanding
gas (Fe II, Si II, C II, Mg II, O I), along the line-of-sight to the
binary system, with an expansion velocity of -1200 km/s. There is no
variation of the co-added absorption feature with orbital phase and
its equivalent width is 0.035 nm, with FWHM = 30 km/s. The absorption
appears in seven lines of the Fe II uv1 multiplet (258.5-261.7 nm),
co-added in velocity space for each of the 11 LWP images around the
orbit, in co-added LWP velocity plots of Mg II, Mn II, and Cr II,
as well as in SWP plots of C II (133.5 nm), Si II (126.0 nm), and O I
(130.2 nm), co-added in velocity space. This feature very likely has an
origin distinct from the persistent lower velocity (-500, -260 km/s)
absorption components of the K2 V stellar wind discovered by Mullan
et al. (1988, ESA SP-281, p. 378; Ap.J. Letters, submitted) and from
the narrow co-added feature of C II, Si II, and O I, at -590 km/s
discovered by Bruhweiler and Sion (1986, Ap.J. 202, L45). The large
expansion velocity suggests the possibility of its association with an
ancient nova outburst. Further observations in other wavelength regions,
including deep CCD imaging and direct photography, are strongly urged."
Title: Detection of a Cool Wind From the K Dwarf in V471 Tauri
Authors: Mullan, D. J.; Sion, E. M.; Bruhweiler, F.; Carpenter, K.
Bibcode: 1988BAAS...20..997M
Altcode:
No abstract at ADS
Title: V471 Tauri: The Oldest and Nearest Old Nova?
Authors: Sion, E. M.; Bruhweiler, F. C.; Carpenter, K.; Mullan, D. J.
Bibcode: 1988BAAS...20S1021S
Altcode:
No abstract at ADS
Title: Radial and Nonradial Pulsations of Polytropes: High-Precision
Eigenvalues and the Approach of p-Modes to Asymptotic Behavior
Authors: Mullan, D. J.; Ulrich, R. K.
Bibcode: 1988ApJ...331.1013M
Altcode:
In order to extract eigenfrequencies of high-order p-modes with
precisions of one part in 500,000, a careful numerical treatment
is necessary for both the input stellar model and the oscillation
code. Here, polytropes are used to avoid uncertainties associated
with the input model. Eigenfrequencies were obtained in a number of
polytropes for l-values of 0-3 and radial orders up to about 40. It
was that, in order to have the frequencies converge with the necessary
precision, the input model must contain several thousand shells,
and the oscillation code must contain at least several dozen grid
points in each loop of the eigenfunction. The outermost loop requires
many more points: the converged models contain several hundred grid
points between the last node and the surface. An important check on
the results is provided by a comparison of the asymptotic behavior
of adjacent mode separations with the limiting behavior predicted by
Tassoul (1980). The present oscillation code is based on the Cowling
(1941) approximation (as assumed by Tassoul).
Title: Mass loss from the K dwarf in V471 Tauri
Authors: Mullan, D. J.; Bruhweiler, F.; Sion, E. M.
Bibcode: 1988ESASP.281a.423M
Altcode: 1988uvai....1..423M; 1988IUE88...1..423M
Spectra of Mg II h and k were obtained for V471 Tau at phases zero (K
dwarf in front) and 0.5 (white dwarf in front). At phase zero, strong
blueshifted absorption is present, suggestive of a wind from the K dwarf
with terminal velocity 600 to 400 km/sec and mass loss rate at least 3
orders of magnitude greater than solar. Discrete blue-shifted absorption
features occur at velocities of 200 and 500 km/sec. At phase 0.5,
the blueshifted absorption is much weaker, although still detectable.
Title: Discovery of a Stellar Wind in V471 Tauri: Accretion, Magnetic
Braking and Pre-CV Evolution
Authors: Sion, E. M.; Mullan, D. J.; Bruhweiler, F. C.
Bibcode: 1988BAAS...20..706S
Altcode:
No abstract at ADS
Title: Chromospheric velocities at the location of helium 10830 Å
dark points.
Authors: Holt, Rush D.; Mullan, Dermott J.
Bibcode: 1988sscd.conf...80H
Altcode:
The authors has observed apparent outflows of several km/sec from
the solar chromosphere at the location of He dark points in coronal
holes. The flows do not appear at dark points outside coronal holes. In
addition to a summary of results from the observing program over several
years, the authors report here on observations from an individual day.
Title: Magnetohydrodynamic Modeling of Coronal Bright Points
Authors: Waldron, W. L.; Mullan, D. J.
Bibcode: 1987ApJ...319..971W
Altcode:
Coronal bright points are known to be magnetic in nature. The authors
have developed a two-dimensional MHD code to investigate the response
of a stratified atmosphere to a localized magnetic structure. If a
bright point occurs at a region where flux tubes of opposite polarity
have encountered by chance, in the presence of stratification, a net
upward mass flux results. Upflow velocities of up to one half of the
Alfvén speed are produced. The implications of these results for mass
outflows from bright points in coronal holes are discussed.
Title: Velocity Fields in a Low-Latitude Coronal Hole: Results from
the Solar Maximum Mission
Authors: Mullan, D. J.; Waldron, W. L.
Bibcode: 1987ApJ...317..487M
Altcode:
The Solar Maximum Mission (SMM) satellite has been used to observe
Doppler signatures in C IV in a low-latitude coronal hole as it
crossed the central meridian (1985 February 2-8). Scatter plots of
C IV emission intensity versus velocity do not show the pronounced
positive correlation which has been reported in other regions on the
sun. These data suggest that the coronal hole may control the gross
velocity field in the solar atmosphere at the level where C IV is
formed. Some localized regions of upflow coincide with EUV bright
points in the coronal hole.
Title: X-Rays from Globular Clusters
Authors: Mullan, D. J.
Bibcode: 1987IrAJ...18...54M
Altcode:
No abstract at ADS
Title: How to Recognize a Black-Hole
Authors: Mullan, D. J.
Bibcode: 1987IrAJ...18...52M
Altcode:
No abstract at ADS
Title: Different Types of Solar Flares
Authors: Mullan, D. J.
Bibcode: 1987IrAJ...18....1M
Altcode:
The characteristics of three forms of energy that emerge in solar
flares, hot plasma, fast particles, and bulk motion of the surrounding
gas, are described. Consideration is given to thermal X-rays from
flares, temperature variations during a flare, the origin of hard X-rays
from flares, and gamma rays from flares. The magnetic constraints and
turbulence related to bulk motion in flares are examined. Flares with
energetic ions in space and flares rich in He-3 are analyzed. The
distinction between fast electrons created by a flare at the flare
site and in interplanetary space is investigated. Electron rich flares,
solar cosmic rays, and the positive charge on solar cosmic ray ions are
studied. The relation between ion cyclotron waves and the acceleration
of certain particles and the release of magnetic energy in the sun
are discussed.
Title: Coronal Bright Points: Ca II K Line Shifts and MHD Modelling
Authors: Mullan, D. J.; Holt, R. D.; Waldron, W. L.
Bibcode: 1986BAAS...18R1042M
Altcode:
No abstract at ADS
Title: Ernst Julius Öpik, 23 October 1893 - 10 September 1985.
Authors: Wayman, P. A.; Mullan, D. J.
Bibcode: 1986QJRAS..27..508W
Altcode:
No abstract at ADS
Title: Ionic Charge States of Solar Energetic Particles: Effects of
Flare X-Rays
Authors: Mullan, D. J.; Waldron, W. L.
Bibcode: 1986ApJ...308L..21M
Altcode:
Ionic charge states of solar energetic particles (SEP) from three
flares have been reported by Luhn et al. (1984). Interpretations of the
mean charges in terms of a source temperature Ts (assuming collisional
ionization equilibrium) yield inconsistent results. For Mg, the required
Ts (up to 8 x 10 to the 6th K) are larger than for N and Si by factors
of up to 5. Here it is pointed out that flare X-rays photoionize the
ambient corona, causing apparent ionization temperatures there to exceed
the local electron temperature, Te. Using realistic flare X-ray fluxes,
it is shown that the charge data for six elements (C, N, Ne, Mg, Si,
and S) can be fitted if the source is at coronal temperatures (Te =
1-2 x 10 to the 6th K), but the ionization equilibrium is radiation
dominated. For oxygen, a slight inconsistency persists in the three
flares.
Title: Displaced narrow absorption components in the spectra of
mass-losing OB stars : indications of corotating interaction regions ?
Authors: Mullan, D. J.
Bibcode: 1986A&A...165..157M
Altcode:
The discovery of displaced narrow components (DNCs) in an increasingly
large number of stars of various spectral types suggests that
an explanation of these features may contribute significantly to
understanding of winds from stars of all types. The reported properties
of DNCs are summarized here with a view to evaluating one particular
scenario for DNC formation which involves corotating interaction regions
(CIRs) in the stellar wind. The relevant features of the CIR scenario
are summarized, and the extent to which DNC properties support the
CIR scenario is discussed.
Title: Is There a Difference Between Coronal Heating and Flaring in
the Sun
Authors: Mullan, D. J.
Bibcode: 1986IrAJ...17..518M
Altcode:
No abstract at ADS
Title: Ultraviolet observations of astronomical phenomena
Authors: Mullan, D. J.
Bibcode: 1986udnw.reptS....M
Altcode:
The purpose was to study various aspects of mass loss in stars of
different types. The observational part of the research was directed at
three Cepheid variables; the archival part of the research was directed
at hot stars (for information on corotating interaction regions) and
at cool giants (for study of variability in the mass losing part of
the atmosphere).
Title: E.J. Opik - an Astronomer's Astronomer
Authors: Mullan, D. J.
Bibcode: 1986IrAJ...17..425M
Altcode:
No abstract at ADS
Title: Winds in Hot Stars - X-Ray Driven Flip-Flops
Authors: Mullan, D. J.
Bibcode: 1986IrAJ...17..521M
Altcode:
No abstract at ADS
Title: A search for outflows from X-ray bright points in coronal holes
Authors: Mullan, D. J.; Waldron, W. L.
Bibcode: 1986udnw.reptR....M
Altcode:
Properties of X-ray bright points using two of the instruments on Solar
Maximum Mission were investigated. The mass outflows from magnetic
regions were modeled using a two dimensional MHD code. It was concluded
that mass can be detected from X-ray bright points provided that the
magnetic topology is favorable.
Title: Shifts of the Ca II K line in He I 10830 dark points.
Authors: Holt, Rush D.; Mullan, D. J.
Bibcode: 1986SoPh..107...63H
Altcode: 1987SoPh..107...63H
We investigate the velocity field of the solar chromosphere at the
location of 65 He I 10830 dark points (DP's). We have obtained spectra
of such points in the vicinity of the Ca II K line. As a measure of
differential chromospheric velocity, we use the shift of the K line
center relative to a nearby photospheric Fe I line. We find that in
He I DP's, the distribution of K line shifts is skewed towards the
blue: the blueward skewing is more pronounced in He I DP's located
in coronal holes. To the extent that He I DP's are proxies of coronal
bright points, our study is relevant to previous reports of outflows
from such bright points.
Title: Ionic charge distributions of energetic particles from
solar flares
Authors: Mullan, D. J.; Waldron, W. L.
Bibcode: 1986udnw.reptQ....M
Altcode:
The effects which solar flare X-rays have on the charge states of solar
cosmic rays is determined quantitatively. Rather than to characterize
the charge distribution by temperature alone, it is proposed that the
X-ray flux at the acceleration site also is used. The effects of flare
X-rays are modeled mathematically.
Title: Circumstellar Matter around Cool Stars
Authors: Mullan, D. J.
Bibcode: 1986IrAJ...17..364M
Altcode:
The circumstellar environment encompasses gas and dust, and in
addition magnetic fields that are difficult to adduce observational
evidence for. The existence of velocity fields is closely related
to the phenomenon of mass loss, which is perhaps the most important
data to be obtained from an evolutionary standpoint in the course of
circumstellar environment studies. Magnetic fields, however, remain
the circumstellar component about which least is known. Attention is
presently given to the location of circumstellar matter, the reason for
that position, and the mechanism of such matter's generation by a star.
Title: Models of Transition Regions in Hybrid Stars
Authors: Brosius, J. W.; Mullan, D. J.
Bibcode: 1986ApJ...301..650B
Altcode:
Models for the transition regions of six hybrid stars, four bright
giants and two supergiants, are calculated. The models include mass
loss and prescribe Alfven waves as the source of mechanical energy. The
momentum and energy deposition rates required at each level of the
atmosphere are evaluated. The final models for all six stars have
mass loss rates lying below the current VLA upper limits by factors
of two to ten, and have densities which agree with those derived by
density-sensitive line ratios. The density vs. temperature structure in
Alpha TrA agree well with that derived by Hartmann et al. (1985). Wave
amplitudes and magnetic field strengths are derived as functions of
height, and the amplitudes are found to agree well with the observed
line widths in Alpha TrA.
Title: M dwarfs: Theoretical work
Authors: Mullan, Dermott J.
Bibcode: 1986NASSP.492..455M
Altcode: 1986mts..book..455M
Theoretical work on the atmospheres of M dwarfs has progressed along
lines parallel to those followed in the study of other classes
of stars. Such models have become increasingly sophisticated as
improvements in opacities, in the equation of state, and in the
treatment of convection were incorporated during the last 15 to 20
years. As a result, spectrophotometric data on M dwarfs can now be
fitted rather well by current models. The various attempts at modeling
M dwarf photospheres in purely thermal terms are summarized. Some
extensions of these models to include the effects of microturbulence
and magnetic inhomogeneities are presented.
Title: X-Ray Stars in Our Galaxy - Further Notes from the Einstein
Satellite
Authors: Mullan, D. J.
Bibcode: 1985IrAJ...17..153M
Altcode:
No abstract at ADS
Title: Radio outbursts in RS Canum Venaticorum stars : Coronal
heating and electron runaway.
Authors: Mullan, D. J.
Bibcode: 1985ApJ...295..628M
Altcode:
Radio outbursts of RS CVn stars are sometimes thought of as analogs
of flares in red dwarf stars. The author examines the possibility that
the outbursts are not due to flares, but rather to a highly efficient
case of coronal heating: mechanical energy reaching the corona from
the convection zone of an RS CVn star gives rise to induced electric
fields which may be so large that electron runaway becomes possible. It
is proposed that gyrosynchrotron emission from fast electrons which
are produced by the process are the source of radio outbursts from RS
CVn stars.
Title: Anomalous Abundances of Solar Energetic Particles and Coronal
Gas: Coulomb Effects and First Ionization Potential (fip) Ordering
Authors: Mullan, D. J.
Bibcode: 1985ICRC....4..237M
Altcode: 1985ICRC...19d.237M
The first ionization potential (FIP) ordering of elemental abundances in
solar energetic particles and in the corona which can both be explained
Coulomb effects is discussed. Solar energetic particles (SEP) and
coronal gas have anomalous abundances relative to the photosphere. The
anomalies are similar in both cases: which led to the conclusion that
SEP acceleration is not selective, but merely preserves the source
abundances. It is argued that SEP acceleration can be selective, because
identical selectivity operates to determine the coronal abundances. The
abundance anomalies are ordered by first ionization potential (FIP).
Title: Formation of the H-alpha absorption line in the chromospheres
of cool stars
Authors: Cram, L. E.; Mullan, D. J.
Bibcode: 1985ApJ...294..626C
Altcode:
The authors discuss the theory of the formation of the Hα absorption
line in the chromospheres of cool stars and present a number of results
which demonstrate that observations of this line can provide valuable
information on certain properties of stellar chromospheres. It is
shown that the widespread existence of strong Hα absorption in
cool stars indicates the equally widespread existence of stellar
chromospheres. The calculations also reinforce the idea that the great
widths of Hα absorption lines in late-type giant stars are due to
nonthermal chromospheric velocity fields.
Title: Co-rotating Interaction Regions In Stellar Winds: Particle
Acceleration and Non-Thermal Radio Emission in Hot Stars
Authors: Mullan, D. J.
Bibcode: 1985ASSL..116...39M
Altcode: 1985rst..conf...39M
A co-rotating interaction region (CIR) forms in a stellar wind when a
fast stream from a rotating star overtakes a slow stream. CIR's have
been studied in detail in the solar wind over the past decade primarily
because they are efficient sources of particle acceleration. Here,
the author points out the usefulness of CIR's in OB star winds to
explain two properties of such winds: deposition of non-radiative
energy in the wind far from the stellar surfaces and acceleration of
non-thermal particles.
Title: Non-Thermal Radio Emission from Flare Stars and RS CVn Systems
(Invited Paper)
Authors: Mullan, D. J.
Bibcode: 1985ASSL..116..173M
Altcode: 1985rst..conf..173M
The observations of continuum radio emission from flares on red dwarf
stars and from outbursts on RS CVn Systems are reviewed. In the RS
CVn systems, the emission appears to be mainly incoherent, whereas
in the flare stars, particularly at the lower frequencies, a coherent
mechanism must be at work. In the RS CVn systems, it appears that during
a radio outburst, a large fraction of the coronal electrons become
highly energetic. This is interpreted in the context of deposition
of mechanical energy in the corona, followed by a turbulent cascade,
and it is found that the electric fields induced in the corona of an
RS CVn star may be large enough to give rise to electron runaway in
certain large magnetic loops.
Title: Observations of the Ca II K spectral line in He 10830 Å
dark points.
Authors: Holt, R.; Fortune, N.; Braun, D.; Mullan, D.
Bibcode: 1985BAAS...17..760H
Altcode:
No abstract at ADS
Title: Search for Chromospheric Velocity Signatures of HeI 10830Å
Dark Points
Authors: Holt, R.; Park, A.; Archibald, J.; Mullan, D.
Bibcode: 1985BAAS...17Q.593H
Altcode:
No abstract at ADS
Title: Search for chromospheric velocity signatures of He I 10830
Å darkpoints.
Authors: Holt, R.; Park, A.; Archibald, J.; Mullan, D.
Bibcode: 1985BAAS...17..593H
Altcode:
No abstract at ADS
Title: Observations of the Ca II K spectral line in He 10830 Å
dark points.
Authors: Holt, R.; Fortune, N.; Braun, D.; Mullan, D.
Bibcode: 1985BAAS...17..933H
Altcode:
No abstract at ADS
Title: Rotational modulation of chromospheric emission in cool giants
and "hybrid" stars.
Authors: Brosius, J. W.; Mullan, D. J.; Stencel, R. E.
Bibcode: 1985ApJ...288..310B
Altcode:
Archival data from the International Ultraviolet Explorer have been
used to study temporal variations of the Mg II h and k emission lines
in eight late-type giants. Evidence is presented that the variations
are periodic in nature. It is argued that the periodicities can be
interpreted in terms of rotation. It is found that the four fastest
rotators in the sample are 'hybrid' stars.
Title: Energy dissipation mechanisms in flare stars
Authors: Mullan, D. J.
Bibcode: 1985IAUS..107..245M
Altcode:
The author summarizes the relevant physical conditions in the atmosphere
of a flare star. He finds that even outside flares, the dissipation
of mechanical energy in the atmospheres of flare stars is in a certain
sense qualitatively different from what is observed in the sun. Further,
he discusses energy dissipation in flares and finds that, at least in
the very coolest flare stars, the distinction between the flaring and
the non-flaring states of the star becomes difficult to define.
Title: Co-rotating interaction regions in stellar winds: particle
acceleration and non-thermal radio emission in hot stars.
Authors: Mullan, D. J.
Bibcode: 1985NASCP2358..130M
Altcode: 1985onhm.rept..130M
A co-rotating interaction region (CIR) forms in a stellar wind when a
fast stream from a rotating star overtakes a slow stream. The author
points out the usefulness of CIR's in OB star winds to explain two
properties of such winds: deposition of non-radiative energy in the wind
far from the stellar surfaces and acceleration of non-thermal particles.
Title: Radio Flare Stars under a "Microscope"
Authors: Mullan, D. J.
Bibcode: 1985IrAJ...17...67M
Altcode:
No abstract at ADS
Title: A Study of Mass Loss in a Long-Period Cepheid Variable
Authors: Mullan, Dermott J.
Bibcode: 1985iue..prop.2183M
Altcode:
We propose to study mass loss in a Cepheid. The target star is unique
among Cepheids in that it lies at the center of 4-5 concentric rings
of dust. The ring structure suggests that the Cepheid (RS Puppis) has
experienced several mass loss events or shock ejection events in the
recent past. The mechanism by which Cepheids lose mass is currently
unknown, but since RS Pup shows signs of having experienced mass loss
in the past, it is a prime candidate for searching for mass loss at the
present time. With this in mind, we propose to obtain high resolution
magnesium line profiles to search for circumstellar absorption at or
beyond the escape speed. Although there is as yet no proof that it is
the stellar pulsation which provides the mechanical energy to power mass
loss (if any) from a Cepheid, it is natural to inquire if the mass loss
varies during the pulsation cycle. Therefore, we propose to obtain MgII
line profiles at specially chosen phases in the cycle (e.g. at maximum
acceleration and deceleration, and at the "bump" on the light curve)
as well as at light maximum. Because of the long exposures necessary,
this is a collaborative effort with European proposers.
Title: Co-rotating interaction regions in stellar winds.
Authors: Mullan, D. J.
Bibcode: 1984NASCP2349..458M
Altcode: 1984fiue.rept..458M
A co-rotating interaction region (CIR) forms in a stellar wind when a
fast stream from a rotating star overtakes a slow stream. CIR's have
been studied in detail in the solar wind over the past decade. Here, the
author points out their usefulness in interpreting several spectroscopic
features in stars of various types, including "hybrid" stars, OB stars,
and cool supergiants.
Title: Rotational modulation of chromospheric emission in cool giants
and "hybrid" stars.
Authors: Brosius, J. W.; Mullan, D. J.; Stencel, R. E.
Bibcode: 1984NASCP2349..476B
Altcode: 1984fiue.rept..476B; 1984IUE84......476B
The authors have used IUE archival data to study temporal variations of
the Mg II h and k emission lines in 8 late-type giants. They present
evidence that the variations are periodic in nature. They argue that
the periodicities can be interpreted in terms of rotation and find
that the four fastest rotators in their sample are "hybrid" stars.
Title: Asymmetries in stellar MG II H and K and CA II H and K line
profiles - Discrepancies between MG and CA asymmetries
Authors: Mullan, D. J.
Bibcode: 1984ApJ...284..769M
Altcode:
"Discrepant asymmetry" stars are cool giants which show the opposite
sense of asymmetry in the emission cores of Ca and Mg (i.e., red
peak dominant in Mg, blue peak dominant in Ca). The author suggests
that these discrepancies can be understood in terms of corotating
interaction regions in the winds.
Title: Corotating interaction regions in stellar winds
Authors: Mullan, D. J.
Bibcode: 1984ApJ...283..303M
Altcode:
In a stellar wind, a corotating interaction region (CIR) is formed when
a rotating star emits wind in a non-spherically symmetric manner. CIRs
have been studied in detail in the solar wind: here, their role in
stellar winds is considered. The radial distance at which CIRs form
(in terms of the stellar radius) is proportional to the ratio of wind
speed to the rotational speed at the stellar surface: in the sun, this
ratio is approximately 200, but it may be much smaller than that in
other stars. The properties of CIRs appear to be able to account for
certain features of 'hybrid stars' (i.e., cool giants with cool winds
plus 'warm' lines in their UV spectra), for X-ray emitting structures
in hot star winds, and for the appearance of extended chromospheres
in cool supergiants.
Title: On the possibility of resonant electrodynamic coupling in
the coronaeof red dwarfs.
Authors: Mullan, D. J.
Bibcode: 1984ApJ...282..603M
Altcode:
The authors suggest that, in the context of Ionson's electrodynamic
coupling theory of coronal heating, the coupling efficiency may pass
through a resonance in lower main-sequence stars. It is proposed that
this resonance contributes to the strong observed X-ray emission in
early-M dwarfs. In the coolest M dwarfs, the efficiency is expected
to decrease rapidly as conditions move away from resonance.
Title: Alfven waves in the solar wind in association with solar
energetic particles - Sunspot umbral origin?
Authors: Mullan, D. J.; Owens, A. J.
Bibcode: 1984ApJ...280..346M
Altcode:
The paper reports on a search for Alfven waves in the solar wind during
solar particle events using ISEE 3 data. During the period June 6-8,
1979, signatures of outgoing Alfven waves are found which could have
been released from the sun simultaneously with the initial release
of energetic particles. The waves have durations of 200-900 s. An
isolated Alfven wave is also seen when the first particles (with E
greater than or approximately equal to 57 MeV) are detected by ISEE 3;
the width of this wave is about 950 s. A series of five small-amplitude
'spikes' in the magnetic field are also seen, preceding the arrival
of the first particles by a few hours. The spikes are separated by
intervals of 854 + or - 20 s. It is suggested that these periodicities
may be determined by the propagation characteristics of Alfven waves
in the solar atmosphere. The features which have been detected appear
to be consistent with Alfven waves released from the umbra of a sunspot
during a flare.
Title: A note on the magnetic field strengths on the surfaces of
cool dwarfs
Authors: Mullan, D. J.
Bibcode: 1984ApJ...279..746M
Altcode:
Various techniques have been used to detect and measure magnetic fields
in cool dwarf stars. In the case of one of the stars discussed by Linsky
and Gary (1983), it is shown that if the emission is optically thick
gyrosynchrotron, then the fields at the surface of the star are in the
range from 10 to 20 kilogauss. These field strengths have been labeled
'unrealistic' by Linsky and Gary. In the present investigation, it is
shown that currently available observational data do not exclude the
possibility that fields with field strengths in the range from 10 to 20
kilogauss exist in dark spots on the surfaces of cool dwarf stars. Thus,
contrary to the statement of Linsky and Gary, photospheric fields in
the 10-20 kilogauss range need not be 'unrealistic'.
Title: Co-rotating Interaction Regions in Stellar Winds
Authors: Mullan, D. J.
Bibcode: 1984BAAS...16Q.522M
Altcode:
No abstract at ADS
Title: Rotational Modulation of Chromospheric Emission in Cool Giants
and `Hybrid' Stars
Authors: Brosius, J. W.; Mullan, D. J.; Stencel, R. E.
Bibcode: 1984BAAS...16..491B
Altcode:
No abstract at ADS
Title: Acceleration of particles in shocked magnetic neutral sheets
Authors: Mullan, D. J.; Perez-Peraza, J.; Galvez, M.; Alvarez, M.
Bibcode: 1984AdSpR...4b.157M
Altcode: 1984AdSpR...4..157M
We report on a study of particle acceleration in a magnetic neutral
sheet which has been struck by a passing MHD shock, such as occurs in
the sun when a flare occurs near a helmet streamer. Using trajectory
calculations, we derive energy spectra. We propose that long-lived
particle events with energies up to 10-100 MeV following solar flares
may be explained by the mechanism described here.
Title: Do Magnetic Fields Drive Mass Loss in Hot and Cool Stars?
Authors: Mullan, D. J.
Bibcode: 1983IrAJ...16..107M
Altcode:
It is pointed out that since the original discoveries of 'quiescent'
mass loss in cool stars and in hot stars, the study of mechanisms
for driving such mass loss has tended along different directions
for hot and cool stars. The present investigation is concerned with
a possible common feature in the winds of hot and cool stars, taking
into account magnetic field effects. Difficulties with current models
of mass loss are examined, giving attention to the sun, cool giants,
and hot stars. The role of magnetic fields is studied, and it is shown
that certain aspects of magnetic fields in the atmospheres of both
hot and cool stars might aid the mass loss process. Unstable magnetic
loops are considered along with the motion of plasmoid in ambient
field, interactions between fast and slow streams in stellar winds,
X-ray photon driven winds, and origins of magnetic loops in hot stars.
Title: Momentum flux invariance in the solar wind
Authors: Mullan, D. J.
Bibcode: 1983ApJ...272..325M
Altcode:
On the basis of data from the ISEE C satellite, the results of Steinitz
and Eyni (1980), Eyni and Steinitz (1981), and Steinitz (1981) are
corroborated. In addition, the scope is extended to another phase of
the solar cycle when solar wind conditions are quite different. The
temperature of the wind is seen to be related to the velocity in a
way that alters with time. This indicates that the rate of energy
deposition, or its distribution between thermal and kinetic forms,
fluctuates during the solar cycle. On the other hand, the coefficients
in the density-velocity relation have apparently remained unchanged
between solar minimum and solar maximum conditions, thereby keeping the
momentum flux remarkably constant. Barring the possibility of chance
coincidence, the momentum flux in the solar wind may be even more
invariant than Steinitz and Eyni suspected. This strengthens their
conclusion that the momentum-flux invariance derives from initial
constraints determining the evolution of the solar wind.
Title: Solar particle corotating events from shocked neutral sheets.
Authors: Mullan, D. J.; Perez-Peraza, J.; Galvez, M.; Alvarez, M.
Bibcode: 1983ICRC...10..307M
Altcode: 1983ICRC...18j.307M
The authors study the effect of a travelling shock wave on a magnetic
neutral sheet. Specifically, they consider the interaction between a
flare shock and a large coronal neutral sheet, such as that which is
associated with a helmet-streamer.
Title: Solar Particle Corrotating Events from Shocked Neutral Sheets
Authors: Mullan, D. J.; Perez-Peraza, J.; Galvez, M.; Alvarez-Madrigal,
M.
Bibcode: 1983ICRC....4...25M
Altcode: 1983ICRC...18d..25M
No abstract at ADS
Title: Release of solar cosmic rays from the corona - Rayleigh-Taylor
instability and reconnection
Authors: Mullan, D. J.
Bibcode: 1983ApJ...269..765M
Altcode:
An analytical model for coronal propagation and the release of solar
cosmic rays (SCR) is formulated and compared with data correlating type
II bursts with SCR enhancements in interplanetary space. Consideration
is given to the various time scales of acceleration of the particles
and electrons at flare sites. The bottle model is discussed, which
regards the SCR and flare ejecta as being formed on initially closed
magnetic field lines. The possibilities that either the onset of a
Rayleigh-Taylor instability (RT) or reconnection of open and closed
magnetic field lines open the bottle are examined. It is found that
solar P events involve flare conditions which can feature an RT that
has sufficient stability for opening to occur. Energetic electrons
may be emitted by the same process that opens the bottle.
Title: Isotopic anomalies among solar energetic particles Contribution
of preacceleration in collapsing magnetic neutral sheets
Authors: Mullan, D. J.
Bibcode: 1983ApJ...268..385M
Altcode:
Anomalous abundances of elements among solar flare particles
have previously been interpreted in terms of a contribution from
pre-acceleration in collapsing magnetic neutral sheets. Here, we examine
the isotopic anomalies which are predicted by this pre-acceleration
mechanism. We find enhancements of neutron-rich isotopes, but the
enhancements are not a monotonic function of the mass ratio. Thus,
enhancement of 22Ne/20Ne need not be accompanied
by any significant enhancements in, say, 18O/16O
or 26Mg/24Mg. High temperatures (such as those
in flares) favor enhancement of 22Ne/20Ne as the
dominant isotopic anomaly. These results may help to account for some
of the observed isotopic anomalies among solar energetic particles.
Title: Closed and open magnetic fields in stellar winds
Authors: Mullan, D. J.; Steinolfson, R. S.
Bibcode: 1983ApJ...266..823M
Altcode:
A numerical study of the interaction between a thermal wind and a
global dipole field in the sun and in a giant star is reported. In
order for closed field lines to persist near the equator (where a
helmet-streamer-like configuration appears), the coronal temperature
must be less than a critical value Tc, which scales as M/R. This
condition is found to be equivalent to the following: for a static
helmet streamer to persist, the sonic point above the helmet must not
approach closer to the star than 2.2-2.6 stellar radii. Implications
for rapid mass loss and X-ray emission from cool giants are pointed
out. The results strengthen the case for identifying empirical dividing
lines in the H-R diagram with a magnetic topology transition locus
(MTTL). Support for the MTTL concept is also provided by considerations
of the breakdown of magnetostatic equilibrium.
Title: Alfven waves in the solar wind in association with solar
energetic particles: Sunspot umbral origin
Authors: Mullan, D. J.; Owens, A. J.
Bibcode: 1983udnw.rept.....M
Altcode:
We report on a search for Alfven waves in the solar wind during
solar particle events. During the period June 6-8, 1979, we find
clear signatures of outgoing Alfven waves which could have been
released from the sun simultaneously with the initial release of
energetic particles. The waves are soliton-like, with durations of
200-900 seconds. An isolated square Alfven solution is also seen when
the first particles (with E approximately 57 MeV) are detected: the
width of this solution is approximately 950 seconds. A series of five
small amplitude spikes in the magnetic field are also seen, preceding
the arrival of the first particles by a few hours. The spikes are
separated by intervals of 854 + or - 20 seconds. We suggest that these
periodicities are determined by conditions at the sun. The features
which we have detected are consistent with Alfven waves released from
the umbra of a sunspot during a flare.
Title: Closed and open magnetic fields in stellar atmospheres Effects
on mass loss from cool giant stars
Authors: Mullan, D. J.
Bibcode: 1983IAUS..102..487M
Altcode:
The author proposes that the onset of rapid mass loss among cool giants,
and the absence of hot coronal material from their atmospheres, is
associated with a transition in the large-scale magnetic topology
of the atmosphere from closed to open. According to this view, field
loops in the atmospheres of giants of spectral class K and later cannot
find equilibrium, but are in a state of dynamical evolution throughout
their lifetime in the atmosphere.
Title: Models of spots and flares
Authors: Mullan, D. J.
Bibcode: 1983ASSL..102..527M
Altcode: 1983IAUCo..71..527M; 1983ards.proc..527M
Reference is made to laboratory experiments carried out in recent
years showing that there are many more ways to drive a plasma out of
equilibrium than to preserve equilibrium. With this in mind, it may be
easier to understand why flares should occur in a stellar atmosphere
(where convective jostling of field lines creates potential for driving
a large number of instabilities) than why a long-lived feature such as a
dark spot should persist. Work done on the equilibrium structure of cool
spots in the sun and stars is summarized. Since spots involve complex
interaction between convective flows and magnetic fields, observations
are used as an aid in identifying the dominant processes that should
enter into the modeling. The summary begins with a discussion of certain
relevant properties of spots in the solar atmosphere. Attention is
then given to the concentration of magnetic fields, the stability of
spots, spot cooling and missing flux, and the relationship between
spots and flares.
Title: Activity in Red Dwarf Stars
Authors: Mullan, D. J.
Bibcode: 1982IrAJ...15..288M
Altcode:
Recent observational and theoretical investigations of activity in red
dwarfs are reviewed in a summary of papers given at the IAU Colloquium
in Catania, Sicily, in August, 1982. Consideration is given to the
physical properties of active dwarfs, such as effective temperature,
opacity, stellar structure, convection and chromospheric heating,
coronal heating, rotation, starspots, and magnetic fields. Observations
discussed include optical, UV, and X-ray data on flares (F), spots,
and precursors; activity cycles in stars; activity on stars other
than red dwarfs; and applicable (mainly VLA) observations of the
sun. Theoretical studies of such problems as the high-temperature
extension of the 'nebular' model, coronal heating versus F, energy
release and dissipation in F, electrodynamic coupling between
photosphere and corona, the role of the photosphere in F, the
fast-electron hypothesis of F light, the gas dynamics of a stellar
atmosphere during F and starspots are summarized.
Title: 'Hybrid' Stars and the Onset of Magnetically Driven Winds in
Cool Giants
Authors: Mullan, D. J.
Bibcode: 1982BAAS...14..894M
Altcode:
No abstract at ADS
Title: Onset of rapid mass loss in cool giant stars - Magnetic
field effects
Authors: Mullan, D. J.
Bibcode: 1982A&A...108..279M
Altcode:
The possibility that closed magnetic field loops exist in steady state
in stellar atmospheres in the HR diagram is examined. A model derived by
Pneuman (1968) for helmet streamers in the solar corona is applied using
a semi-empirical technique, to find that long-lived closed loops exist
only below a certain boundary in the HR diagram. The region below this
boundary is occupied by stars which are known to have hot coronae and
slow mass loss. It is suggested that rapid mass loss sets in when closed
field loops can no longer exist in steady state in the atmosphere.
Title: Model chromospheres of RS CVn stars : Balmer line profiles
in lam And.
Authors: Mullan, D. J.; Cram, L. E.
Bibcode: 1982A&A...108..251M
Altcode:
Two models have been constructed for the chromosphere of the RS CVn
star Gamma And, one with a low-pressure transition zone, and one with
a high pressure transition zone. The high pressure model predicts an
H alpha line profile which agrees fairly well with high resolution
observations of Gamma And, without the need to include nonthermal
line broadening. The low pressure model predicts an H alpha profile
which agrees very well with the observations, after application of
macroturbulent broadening with a gaussian velocity parameter of about 30
km/sec. Methods which could distinguish between the two alternatives
are discussed, and it is suggested that if the low-pressure model
is valid, the large macroturbulence may be associated with unstable
magnetic flux loops in the atmosphere.
Title: Magnesium emission variability among late-type giant stars.
Authors: Mullan, D. J.; Stencel, R. E.
Bibcode: 1982ApJ...253..716M
Altcode:
Profiles of the Mg II h and k emission features in the spectra of 21
late-type giant stars were obtained. Emission strengths were separately
measured in the shortward (S) and longward (L) components. Variations
in total emission intensity (S + L) can be interpreted as evidence
for variations in the rate of mechanical energy deposition in the
chromosphere. Mass loss processes in the corona/outer atmosphere may
be strong enough to affect the ratio of S/L: thus, rapid mass loss
causes S/L to be less than unity. Rapid mass loss is likely caused by
deposition of mechanical energy by stellar wind. Variations in S/L are
a measure of variations in the rate of mechanical energy deposition
in the corona/outer atmosphere. The stellar sample variations were
divided into four classes: (1) variations in S/L; (2) variations in the
circumstellar absorption components; (3) variations in the total flux;
and (4) no evidence for variations found on the time scales used.
Title: Theoretical studies of the RS Canum Venaticorum stars
Authors: Mullan, D. J.
Bibcode: 1982udnw.rept.....M
Altcode:
The activity in RS Canum Venaticorum (CVn) is investigated. Models
for chromospheric structure are developed and the role of magnetic
fields both in the photosphere as well as in the chromosphere and
upper atmosphere are examined. T Tau stars are also studied from the
same points of view. The properties of magnetic field loops are used
to help understand the atmospheric structure in RS CVn stars. The
concepts developed in the case of these stars appear to be applicable
over a much broader region of the HR diagram. The absence of stable
magnetic loops in the atmospheres of late type giant stars suggests
that the atmospheres of RS CVn active components are qualitatively
distinct from the solar atmosphere.
Title: Isotopic anomalies in cosmic rays: effects of pre-acceleration
in collapsing magnetic neutral sheets.
Authors: Mullan, D. J.
Bibcode: 1982BAAS...14..577M
Altcode:
No abstract at ADS
Title: Coronal Holes - Mass Loss Driven by Magnetic Reconnection
Authors: Mullan, D. J.; Ahmad, I. A.
Bibcode: 1982SoPh...75..347M
Altcode:
We propose that bubbles of matter ejected from magnetic reconnection
sites in polar plumes drive the solar wind in coronal holes.
Title: Caution! High winds beyond this point.
Authors: Mullan, D. J.
Bibcode: 1982Ast....10a..74M
Altcode:
No abstract at ADS
Title: "Discrepant asymmetry" stars: the role of unsteady magnetic
flux loops in the atmospheres of late-type giant stars.
Authors: Mullan, D. J.; Stencel, R. E.
Bibcode: 1982NASCP2238..235M
Altcode: 1982auva.nasa..235M; 1982IUE82......235M; 1982NASCP2338..235M
A number of spectroscopic peculiarities of K giants and other
stars which lie in a wedge in the HR diagram are discussed. These
peculiarities can be understood in terms of unsteady magnetic flux
loops emerging into the stellar atmosphere from beneath the surface.
Title: Preacceleration in collapsing magnetic neutral sheets and
anomalous abundances of solar flare particles
Authors: Mullan, D. J.; Levine, R. H.
Bibcode: 1981ApJS...47...87M
Altcode:
Levine's (1974) concept of a collapsing magnetic neutral sheet
which can accelerate ambient protons to several times the mean
thermal speed, provided that the collapse time scale is shorter
than the proton Coulomb loss time, is applied to heavier elements
in an investigation of the composition of the particles which can
be accelerated by such a sheet. Tables of ionization equilibrium are
combined with the thermal structure of a constant pressure loop in order
to calculate phix (the fraction of an element x which is
accelerated) of 18 heavy elements, from carbon to nickel which satisfy
a maximum-ionization criterion. Normalizing phix to oxygen,
it is found that relative to the composition of the ambient materials,
C and N can be depleted by factors of up to 2-10, while other heavy
elements, along with hydrogen, are enhanced. The numerical results
obtained are qualitatively similar to anomalous abundances reported
among solar flare particles.
Title: X-Rays from Stars
Authors: Mullan, D. J.
Bibcode: 1981IrAJ...15..147M
Altcode:
No abstract at ADS
Title: The solar "constant".
Authors: Mullan, D. J.
Bibcode: 1981IrAJ...15..143M
Altcode:
No abstract at ADS
Title: Enhanced emission of Alfvén waves from sunspots during
proton flares
Authors: Mullan, D. J.
Bibcode: 1981SoPh...70..381M
Altcode:
Thomas (1978) has shown that, if Alfvén waves exist in a sunspot umbra,
they are normally reflected so strongly by the temperature minimum as
to be essentially undetectable in the upper solar atmosphere. However,
it is known that in many proton flares, chromospheric emission overlies
the umbra of a sunspot, indicating that the transition region (TR)
between chromosphere and corona in the umbral flux tube has moved
down to lower altitudes. As a result of this lowering, umbral Alfvén
waves have readier access to the corona: the coronal leakage depends
exponentially on the altitude of the TR. We find that the Alfvén
wave flux which leaks out of the umbra into the corona can exceed
107 ergs cm-2 s-1. A flux of this
magnitude is expected to dissipate rapidly in the corona, thereby
contributing to a positive feedback loop which ensures prolonged (∼1
hr) leakage of the umbral Alfvén waves into the corona. We propose
that these Alfvén waves may contribute significantly to prolonged
energization of proton flares in which umbral coverage occurs.
Title: Are Discrepant Asymmetry Red Giants Necessarily Hybrid Stars?
Authors: Mullan, D. J.; Stencel, R. E.
Bibcode: 1981BAAS...13..886M
Altcode:
No abstract at ADS
Title: Large Macroturbulence in the Chromosphere of an RS CVn Star
Authors: Cram, L. E.; Mullan, D. J.
Bibcode: 1981BAAS...13..514C
Altcode:
No abstract at ADS
Title: Variable mass loss and magnetic topology in cool giant stars.
Authors: Mullan, D. J.; Stencel, R. E.
Bibcode: 1981BAAS...13..547M
Altcode:
No abstract at ADS
Title: Theoretical studies of the RS cannum venaticorum stars
Authors: Mullan, D. J.
Bibcode: 1981udnw.rept.....M
Altcode:
Four areas of research were investigated: chromospheric modelling;
starspot modelling; supersonic transition locus (STL) crossing; and
STL crossing and T Tauri phenomena. Relationships among these areas
of research are presented. Stellar structure and mass ejection for
these stars were examined along with chromospheric analysis.
Title: The Origin of Cosmic Rays
Authors: Mullan, D. J.
Bibcode: 1981IrAJ...15....9M
Altcode:
No abstract at ADS
Title: Mass loss from warm giants - Magnetic effects
Authors: Mullan, D. J.
Bibcode: 1981ASSL...88..355M
Altcode: 1981pprg.work..355M
(Previously announced in STAR as N80-34332)
Title: Obs. of Discr Chromo Em Line Prof Asymm & Var Asymm in
UV Spectra of Late-Type Stars
Authors: Mullan, Dermott J.
Bibcode: 1981iue..prop..795M
Altcode:
Based on our observing programs dealing with the Mg II chromospheric
emission features among cool post-main-sequence stars, we are in a
position to continue the search for asymmetry and profile variations
among the three kinds of asymmetry variables identified to date: (i)
the radical asymmetry variables which exhibit discrepant asymmetries
in Mg II K (V/R < 1) relative to Ca II K (V/R > 1); (11) the
circumstellar variables which exhibit changes in their circumstellar
thickness and velocities along the line of sight; and (iii) the
"non-variables" which may in fact be variables over time scales greater
than the one year so far spent in any such investigation. Stars in group
(i) also show discrepant Wilson-Bappu line-widths. The radical asymmetry
variables occur among objects slightly cooler and more luminous than
a Magnetic Topology Transition Locus (M.T.T.L.) lying close to, but
distinct from, a previously defined transition locus (formerly known
as the Supersonic Transition Locus). The MTTL separates solar-type
objects which possess static high temperature coronal material and
circulation-type asymmetries, from the non-solartype objects which
possess lower temperature outer atmospheres, substantial mass loss,
and outflow asymmetries. The MTTL segregates stars according to whether
the magnetic field topology dominating their outer atmospheres is open
or closed. The radical asymmetry variables, lying close to the MTTL,
afford our best opportunity to see the interplay between the archetypal
magnetic configurations and to understand the physics involved.
Title: Pre-Acceleration in Collapsing Magnetic Neutral Sheets and
Composition Anomalies among Solar Flare Particles
Authors: Mullan, D. J.; Levine, R. H.
Bibcode: 1981ICRC....3..410M
Altcode: 1981ICRC...17c.410M
In flares occurring in magnetic neutral sheets, the collapsing of the
magnetic fields toward the neutral sheet results in their acting as
mirrors in a first-order Fermi acceleration process and constitutes
an intrinsic source of particle pre-acceleration. Although modest, the
acceleration injects particles into the main flare acceleration with a
composition which is different from that of the ambient corona. Upon
calculating the fractions of 18 elements which are accelerated in
the collapsing mirrors, covering the series from C to N, and then
normalizing to O, peak abundances are found at Si and Fe, with local
minima at N and in the S-Ca region. There is a qualitative similarity
between these relative abundances and the relative enhancement factors
of the elements among solar flare particles.
Title: High resolution flux profiles of the Mg II h & k lines
in evolved F8 to M5 stars.
Authors: Stencel, R. E.; Mullan, D. J.; Basri, G. S.; Linsky, J. L.
Bibcode: 1981NASCP2171..317S
Altcode: 1980IUE80......317S; 1981uviu.nasa..317S; 1981NASCP3171..317S
The central results of a survey of the Mg II resonance line
emission in a sample of over 50 evolved late type stars, including
spectral-luminosity type F8 to M5 and La to IV are presented. Observed
and surface fluxes are derived and correlations noted. The major
findings include: (1) Mg II k emission core asymmetry transition near
K1 III, analogous to that known for Ca II K; (2) a small gravity and
temperature dependence of the Mg II chromospheric radiative loss rate.
Title: Possible Evidence for Attenuation of AN MHD Shock by a Magnetic
Neutral Sheet in the Solar Corona
Authors: Mullan, D. J.
Bibcode: 1981ICRC....3...51M
Altcode: 1981ICRC...17c..51M
The possibility that H-alpha filaments in the solar chromosphere
are markers beneath regions where coronal shocks experience severe
attenuation, and are thus the areas where particles can be ejected
from the corona, is examined from data from flares of Aug. 4, 1972 and
Nov. 9, 1979. Both flares produced gamma rays with 2.2 MeV energy, and
data exists for the energetic protons received at earth stations. The
efficiency of transport was high for the August event and two magnitudes
lower for the November event. H-alpha filtergrams of the two flares show
that in the second flare the shock propagation crossed two or three
filaments lying in its path. The low efficiency of particle transport
is taken to indicate a degraded shock in the region above the filaments,
indicating that the particles needed a different route to escape.
Title: The outher atmospheres of cool stars. VII. High resolution,
absolute flux profiles of the MG II H and K lines in stars of spectral
types F8 to M5.
Authors: Stencel, R. E.; Mullan, D. J.; Linsky, J. L.; Basri, G. S.;
Worden, S. P.
Bibcode: 1980ApJS...44..383S
Altcode:
We present high-resolution lUE spectra of the emission cores of the
Mg II resonance doublet at 280 nm in a selection of 54 stars covering
a range of spectral type from F8 to MS and of luminosity class from
supergiant (Ia) to subgiant (IV). These spectra were obtained with the
LWR echelle system onboard the IUE satellite, and have been calibrated
in absolute flux units using OAO 2 photometry of Eta UMa as a standard,
plus the Barnes and Evans relations for stellar angular diameters. The
uncertainty in flux is probably of order 20%. We discuss the qualitative
line profile groupings, as determined by Basri and Linsky, and derive
chromospheric radiative losses in the h and k lines; we discuss
these loss rates as functions of effective temperature and luminosity
class. We make further comparisons of these rates with rates derived
for the Ca II H and K lines by Linsky and his colleagues. Chromospheric
velocity fields and indicators of circumstellar envelopes are discussed
in terms of profile asymmetries and other diagnostics. Line width
measures and velocity shifts of the central reversals are tabulated,
among other quantities, and several correlations noted. Finally, we
discuss the relation of the Wilson K index and stellar coronae to Mg
II emission, and note the occurrence of Fe II emission lines in the
middle range of the UV of late-type stars.
Title: Magnetohydrodynamic shock propagation in the vicinity of a
magnetic neutral sheet
Authors: Steinolfson, R. S.; Mullan, D. J.
Bibcode: 1980ApJ...241.1186S
Altcode:
This paper reports a numerical investigation of the propagation of
magnetohydrodynamic (MHD) shocks in the vicinity of magnetic neutral
sheets. The attenuation of a shock after passing through a neutral sheet
has been evaluated (assuming infinite electrical conductivity). In a
parameter study, values of shock speed, polytropic index, plasma beta,
and neutral-sheet thickness which are representative of solar coronal
conditions have been examined. If solar cosmic rays are accelerated
in association with a flare-induced shock (as seems most likely),
then our results suggest that the spatial structure of solar particle
sources will be influenced by helmet streamers. Such streamers are most
readily detectable by H alpha filaments in the underlying chromosphere.
Title: Addendum - Detection of Mass Loss in Stellar Chromospheres
Authors: Stencel, R. E.; Mullan, D. J.
Bibcode: 1980ApJ...240..718S
Altcode:
No abstract at ADS
Title: Mass Loss from Coronal Holes Driven by Magnetic Reconnection
Authors: Ahmad, I. A.; Mullan, D. J.
Bibcode: 1980BAAS...12..918A
Altcode:
No abstract at ADS
Title: A Search for Mg II Asymmetry Variations Among Cool Evolved
Stars
Authors: Mullan, D.; Stencel, R.
Bibcode: 1980BAAS...12..801M
Altcode:
No abstract at ADS
Title: Detection of mass loss in stellar chromospheres.
Authors: Stencel, R. E.; Mullan, D. J.
Bibcode: 1980ApJ...238..221S
Altcode:
IUE observations of 47 cool giants have been made in a search for
the onset of expansion in the Mg II h and k emission cores. It
is found that, in a statistical sense, the longward emission peak
becomes dominant above a velocity dividing line in the H-R diagram,
which lies close to a temperature dividing line reported by Linsky
and Haisch (1979). Also discussed are asymmetries in emission cores,
collected asymmetry data including the evaluation of absolute visual
magnitude by MK classification, Ca K emission width, and Mg k emission
width, and evolutionary implications for the hypothesized supersonic
transition locus (Mullan, 1978).
Title: Stochastic acceleration of solar cosmic rays in an expanding
coronal magnetic bottle
Authors: Mullan, D. J.
Bibcode: 1980ApJ...237..244M
Altcode:
It is noted that the major apparent difficulty with the 'magnetic
bottle' proposed by Schatten and Mullan is that expansion of the
closed model might have a severe cooling effect on the cosmic rays
trapped inside. The present paper examines this difficulty by applying
the equation for stochastic acceleration to an expanding bottle. The
scattering centers are taken to be small-scale magnetic inhomogeneities
which are present in the corona prior to the flare, and which are
set into turbulent motion when a flare induced shock passes by. It
is found that acceleration efficiencies can be high enough to offset
expansive cooling. It is concluded that the results indicate that the
flux of particles which are accelerated to 100 MeV is very sensitive
to shock speed if this speed is less than about 1000 km per s.
Title: Coronal Propagation of a Magnetohydrodyamic Shock in the
Vicinity of a Magnetic Neutral Sheet
Authors: Mullan, D. J.; Steinolfson, R. S.
Bibcode: 1980IAUS...91..323M
Altcode:
No abstract at ADS
Title: Non-Thermal Stellar Winds in Cool Stars
Authors: Mullan, D. J.
Bibcode: 1980SAOSR.389..189M
Altcode: 1980csss....1..189M
No abstract at ADS
Title: Variable Mass Loss Among Stars at or Near the Supersonic
Transition Locus
Authors: Mullan, Dermott J.
Bibcode: 1980iue..prop..483M
Altcode:
We propose to utilize the IUE to continue LWR observations of G, K and
M stars as was done during our proposal during the second year of IUE
operation. The Mg II emission cores were observed in a sample of 47 cool
giants, and we found that in a statistical sense, the longward emission
peak becomes dominant above a certain locus in the HR diagram. This
locus lies close to a "temperature dividing line" reported by Linsky
and Haisch. Combining the Mg results with earlier Ca II K data and with
circumstellar shell data, a fundamental change in the structure of the
upper atmosphere of cool giants, consistent with the predictions of
a supersonic transition locus-hypothesis advanced by Mullan, has been
determined. Two major questions remain which can only be answered by
further IUE observations: (1) how sharp is this STL division in the
HR diagram?; and (2) do the stars nearest the STL exhibit variable
asymmetry due to instabilities in their upper atmospheres which alter
the coupling between corona and chromosphere? The first of these
questions can be addressed with observations of new target stars
which will Improve the statistics. The second can be addressed with
repeat observations of selected objects close to the STL to search
for variations on a 1 year timescale.
Title: Mass loss from warm giants: Magnetic effects
Authors: Mullan, D. J.
Bibcode: 1980STIN...8034322M
Altcode:
Among warm giant stars, rapid mass loss sets in along a well defined
velocity dividing line (VDL). Hot corona also disappear close to the
VDL and thermal pressure cannot drive the observed rapid mass loss in
these stars. The VDL may be associated with magnetic fields changing
from closed to open. Such a change is consistent with the lack of
X-rays from late-type giants. A magnetic transition locus based on
Pneuman's work on helmet streamer stability agrees well with the
empirical VDL. The change from closed to open fields not only makes
rapid mass loss possible, but also contributes to energizing the mass
loss in the form of discrete bubbles.
Title: Model chromospheres of flare stars.I. Balmer-line profiles.
Authors: Cram, L. E.; Mullan, D. J.
Bibcode: 1979ApJ...234..579C
Altcode:
In preparation for using the Balmer lines in flare stars as
chromospheric diagnostics, we have computed a grid of model
chromospheres by superposing prescribed temperature rises on published
models for M dwarf photospheres. The chromospheric equation of state
is dominated by the non-LTE ionization of hydrogen, which is treated
by using a model hydrogen atom with five bound levels. The radiative
transfer equation is solved explicitly for the Lyman continuum and
the Balmer lines Ha, Hfl, and H)'. In the absence of a chromosphere,
Ha, Hfl, and H)' appear as weak absorption lines. As the amount of
chromospheric material (between temperatures Te = 5500 K and 50,000 K)
increases, these absorption lines first become deeper, then develop
emission peaks on the outer edges of their wings, and finally, when the
chromosphere is sufficiently massive, the Balmer lines become strong
emission lines. The results obtained here will be used to interpret
the profiles and decrements of the Balmer lines in flare stars, in both
quiescent and flaring states. Subject headings: line profiles - stars:
atmospheres - stars: chromospheres - stars: flare - stars: late-type
Title: Can flare stars be the source of galactic cosmic rays?
Authors: Mullan, D. J.
Bibcode: 1979ApJ...234..588M
Altcode:
Lovell has proposed that flare stars may provide a substantial
contribution to the energy in galactic cosmic rays (GCR). Here his
arguments are reexamined, taking into account recent improvements
in stellar statistical data and in our understanding of the processes
involved in acceleration of protons in association with solar flares. It
is found that flare stars contribute no more than 0.0002 of the GCR
energy. A result of this order had previously been derived by Edwards
and McQueen on the basis of scaling X-ray fluxes and energetic particle
fluxes in solar flares; however, the present work is not subject to
the uncertainties associated with X-ray and particle flux scaling.
Title: Long-lived sources of solar cosmic rays
Authors: Mullan, D. J.
Bibcode: 1979AIPC...56..163M
Altcode: 1979pama.work..163M
The high correlation between prompt solar cosmic rays and
a flare-induced MHD shock is well known. We point out that the
propagation properties of such a shock cause shock heating of the solar
atmosphere to be confined to a unipolar magnetic region. As a result,
if particles can be accelerated within the shock-processed part of
the corona, the fluxes of suc particles will exhibit sharp spatial
gradients near quiescent filaments. The passage of an MHD shock leads
to the rapid collapse of magnetic neutral regions which prior to shock
passage were collapsing too slowly to accelerate particles. We suggest
that these newly triggered magnetic acceleration regions provide a
third phase of solar flare acceleration regions provide a third phase
of solar flare acceleration which may persist for many days after a
flare. Collapsing magnetic regions with lengths scales of order 100
km can explain a variety of coronal phenomena.
Title: Heating of Chromospheres and Coronae in Cool Stars
Authors: Mullan, D. J.
Bibcode: 1979IrAJ...14...73M
Altcode:
Recent evidence is summarized that suggests that neither of two
traditional views of chromospheric heating (that the flux of energy
required to heat the solar chromosphere) is only a small fraction of
the total radiative energy emerging from the solar interior and that
chromospheres are heated by acoustic waves generated in the convection
zone (which lies close to the surface of the star) is valid. Particular
attention is given to cool stars, both dwarfs and giants. The directions
in which research is currently heading in attempting to understand
heating of chromospheres and coronae in these stars are indicated. Hot
stars are excluded from consideration because radiation pressure plays
an important role in their atmospheric heating. It is concluded that the
role of magnetic fields in giants is different from the role in dwarfs.
Title: On the detectability of starspot magnetic fields.
Authors: Mullan, D. J.
Bibcode: 1979ApJ...231..152M
Altcode:
A starspot is considered as the surface manifestation of a submerged
magnetic dipole. It seems likely that in red dwarfs the dipole is
generated by dynamo action within the outer convection zone. As
a result, a starspot-associated dipole is highly decentered. By
integrating the field of a decentered dipole over the disk of a spotted
star, the effective longitudinal field, the effective transverse
field, and the mean surface field are derived. If the axial field in
a starspot is 10 kilogauss, it is found that in most favorable cases,
the effective longitudinal field is about 150 gauss, the effective
transverse field is approximately 1.1 kilogauss, and the mean surface
field is roughly 1.3 kilogauss.
Title: Motion of solar cosmic rays in the coronal magnetic field.
Authors: Mullan, D. J.; Schatten, K. H.
Bibcode: 1979SoPh...62..153M
Altcode:
Trajectories of solar cosmic rays have been calculated in a static
ninth-order coronal magnetic field. It is found that as a result of
field curvature and gradients, protons drift across the field lines at
a rate of up to 200 γβ2 deg hr−1. These drift
rates are of the same order as, but somewhat smaller than, empirically
derived rates. Localized enhancements of magnetic field have been
inserted into the ninth-order field in order to model (in a highly
idealized manner) the effects of the small-scale magnetic features
which give rise to X-ray bright points. The motions of the particles
in the presence of these scattering centers can be parameterized
approximately by a cross-field diffusion coefficient. Our estimates
of this coefficient, although crude, overlap with empirical values
which have been deduced over a wide range of energies.
Title: Coronal Propagation of AN MHD Shock in the Vicinity of a
Magnetic Neutral Sheet
Authors: Steinolfson, R. S.; Mullan, D. J.
Bibcode: 1979ICRC....5...15S
Altcode: 1979ICRC...16e..15S
No abstract at ADS
Title: Proton Flares and the Missing Energy in Sunspots
Authors: Mullan, D. J.
Bibcode: 1979ICRC....5...22M
Altcode: 1979ICRC...16e..22M
No abstract at ADS
Title: Pre-Main Sequence Stars and Galactic Cosmic Rays
Authors: Mullan, D. J.; Owens, A. J.
Bibcode: 1979ICRC....2...98M
Altcode: 1979ICRC...16b..98M
No abstract at ADS
Title: Can Solar Flares BE the Cause of Ancient Catastrophes?
Authors: Mullan, D. J.; Kent, D. W.
Bibcode: 1979ICRC....5..323M
Altcode: 1979ICRC...16e.323M
No abstract at ADS
Title: Observable Influence of Stellar Winds on Late-Type
Chromospheres
Authors: Mullan, Dermott J.
Bibcode: 1979iue..prop..224M
Altcode:
No abstract at ADS
Title: Can Flare Stars BE the Source of Galactic Cosmic Rays?
Authors: Mullan, D. J.
Bibcode: 1979ICRC....2...92M
Altcode: 1979ICRC...16b..92M
No abstract at ADS
Title: Supersonic stellar winds and rapid mass loss in cool stars.
Authors: Mullan, D. J.
Bibcode: 1978ApJ...226..151M
Altcode:
Rapid mass loss from cool giants and supergiants via supersonic stellar
winds is considered. It is proposed that when a star evolves to a
state in which its wind, driven by hydrodynamic expansion, becomes
supersonic at the base of the corona, the rate of mass loss from the
star should increase essentially discontinuously by an amount equal
to the ratio of gas densities at the top of the chromosphere and the
base of the corona. Empirically determined chromospheric gas pressures
are combined with the minimum-flux coronal model of Hearn (1975) in
order to derive the locus in the H-R diagram along which stellar winds
undergo a transition from transonic (fed by purely coronal material) to
supersonic (fed by chromospheric material). This locus is found to agree
well with the boundary of the domain occupied by circumstellar-shell
stars (in which rapid mass loss is known to occur). An estimate of
the mass-loss rate under supersonic conditions is made and shown to
be consistent with observed rates.
Title: Fast azimuthal transport of solar cosmic rays via a coronal
magnetic bottle
Authors: Schatten, K. H.; Mullan, D. J.
Bibcode: 1977JGR....82.5609S
Altcode:
Key observations pertaining to the fast azimuthal propagation of solar
cosmic ray particles are reviewed. Briefly, protons and electrons with
a wide range of energies from 40° to 60° heliolongitude on either
side of a flare site have access to the earth-sun interplanetary
field line within an hour of flare onset. We propose that coronal
magnetic bottles, produced by flares, serve as temporary traps for
solar cosmic rays in some instances. It is the expansion of these
bottles at velocities of 300-500 km/s which allows fast azimuthal
propagation of solar cosmic rays independent of energy. As a result
of the Rayleigh-Taylor instability, cosmic rays originally trapped
inside the bottle are released into interplanetary space at a time of
the order of 0.5-1 hour after the flare.
Title: Dust from the sun?
Authors: Mullan, D. J.
Bibcode: 1977A&A....61..369M
Altcode:
It was previously proposed that the sun might be the source of
rare-earth-rich submicron particles detected in rocket flights from
Kiruna, Sweden, but not in flights originating from sites at lower
latitudes. The problem of sputtering in the solar atmosphere is
examined and found to be serious enough that a nonsolar origin must
be considered for the particles. The motion of grains in a variety of
solar model atmospheres is calculated, and it is shown that sputtering
rates in active regions are probably so large that submicron grains
of rare-earth-rich material are reduced to such small sizes that they
cannot be repelled from the sun by radiation pressure. It is suggested
that the rare-earth-rich submicron particles detected at Kiruna but
nowhere else are the product of local mining of rare-earth-rich ores
in the vicinity of Kiruna.
Title: Rapid Mass Loss and Supersonic Stellar Winds.
Authors: Mullan, D. J.
Bibcode: 1977BAAS....9..649M
Altcode:
No abstract at ADS
Title: Solar and stellar flares.
Authors: Mullan, D. J.
Bibcode: 1977SoPh...54..183M
Altcode:
Short-lived increases in the brightness of many red dwarfs have been
observed for the last 30 yr, and a variety of more or less exotic
models have been proposed to account for such flares. Information about
flares in the Sun has progressed greatly in recent years as a result
of spacecraft experiments, and properties of coronal flare plasma
are becoming increasingly better known. In this paper, after briefly
reviewing optical, radio and X-ray observations of stellar flares, we
show how a simplified model which describes conductive plus radiative
cooling of the coronal flare plasma in solar flares has been modified
to apply to optical and X-ray stellar flare phenomena. This model
reproduces many characteristic features of stellar flares, including
the mean UBV colors of flare light, the direction of flare decay in
the two-color diagram, precursors, Stillstands, secondary maxima, lack
of sensitivity of flare color to flare amplitude, low flux of flare
X-rays, distinction between so-called spike flares and slow flares,
Balmer jumps of as much as 6-8, and emission line redshifts up to 3000
km s−1. In all probability, therefore, stellar flares
involve physical processes which are no more exotic (and no less!) than
those in solar flares. Advantages of observing stellar flares include
the possibilities of (i) applying optical diagnostics to coronal flare
plasma, whereas this is almost impossible in the Sun, and (ii) testing
solar flare models in environments which are not generally accessible
in the solar atmosphere.
Title: A model for nonmonotonic optical light curves of stellar
flares.
Authors: Mullan, D. J.
Bibcode: 1977ApJ...212..171M
Altcode:
An idealized model of flare decay is considered, including both
radiative and conductive cooling of the hot expanding plasma. The
optical light curve has contributions from both conductive power and
X-ray heating. In contrast to the monotonic decay predicted when only
radiative cooling is included, the present results include features
which may be analogous to observed stillstands and secondary maxima and
to a certain class of precursor events. Stillstands, secondary maxima,
and precursors are here considered to be different manifestations of
the same physical phenomenon.
Title: Influence of stellar flare X-rays on the optical light curve.
Authors: Mullan, D. J.; Tarter, C. B.
Bibcode: 1977ApJ...212..179M
Altcode:
The steady-state interaction of an incident flux of bremsstrahlung
X-rays with the optically thick atmosphere of a flare star is
computed. A significant fraction of the incident flux is converted
into nonionizing photons. The emerging optical radiation has colors
which, when combined with previously estimated colors of radiation
emitted directly by the flare region, are in the correct sense to
account for the evolution of decaying flare light in the two-color
diagram. Emission lines contribute up to 10% of the intensity in the
B filter. Red asymmetries of the emission lines may be associated with
radiation pressure.
Title: Heterogeneity of the solar atmosphere.
Authors: Mullan, D. J.
Bibcode: 1977oehs.conf..377M
Altcode:
No abstract at ADS
Title: Mean colors of stellar flare continuum.
Authors: Mullan, D. J.
Bibcode: 1976ApJ...210..702M
Altcode:
Shmeleva and Syrovatskii have shown that under certain circumstances
the temperature structure in the hotter regions (T> 2 x l0 K)
of a solar flare is characterized by two universal functions: one
for constant density conditions, the second for constant pressure
conditions. Here we show that the U - B, B - V colors of optically
thin thermal bremsstrahlung emitted by both of these temperature
structures are consistent with the mean colors of stellar flares near
maximum light and also with the observed evolution of flare light
in the two-color diagram during flare cooling. We suggest that the
transition which occurs in the character of stellar flare light from
mostly continuum emission near flare maximum to mostly line emission
later in the flare is related to the transition which must occur from
the constant density regime to the constant pressure regime on a time
scale of order 1-2 minutes. The two types of flares (spike flares and
slow flares) identified by Moffett are ascribed to these two different
regimes. The flare light-curve model described here resembles in
some respects a model previously proposed by Andrews, but there are
differences in detail. Subject headings: stars: flare - Sun: flares
Title: Second-stage acceleration in solar flares.
Authors: Mullan, D. J.
Bibcode: 1976A&A....52..305M
Altcode:
A model proposed by Chevalier and Scott (1975) to account for cosmic-ray
acceleration in an expanding supernova remnant is applied to the case
of a shock wave injected into the solar corona by a flare. Certain
features of solar cosmic rays can be explained by this model
Title: Properties of minimum-flux coronae in dwarfs and giants.
Authors: Mullan, D. J.
Bibcode: 1976ApJ...209..171M
Altcode:
Hearn's (1975) minimum-flux corona model is applied to main-sequence and
giant stars. In this model, the corona is assumed to have a strictly
radial magnetic field and to adjust itself in such a way that for
a given pressure at its base, the sum of radiative, conductive, and
stellar-wind energy fluxes from it is a minimum. Numerical results for
main-sequence stars of spectral type M6 V through O5 V are presented
in terms of coronal temperatures and efficiency factors for converting
thermal energy to mechanical energy. Similar results are given for
the giant stars Alpha Aur, Beta Gem, Alpha Boo, and Alpha Tau. It is
shown that: (1) coronae of red dwarfs must be up to three times cooler
than the solar corona unless the efficiency of conversion increases
by many orders of magnitude in later spectral types relative to the
efficiency of the sun; (2) coronae of upper-main-sequence stars may
be twice as hot as the solar corona; (3) the coronae of Beta Gem and
Alpha Aur are hot enough to explain the presence of O V, O VI, Si III,
and N V lines; (4) the coronae of Alpha Boo and Alpha Tau are so cool
that insignificant O V emission is expected; and (5) line asymmetries
due to stellar winds are most easily detectable in Alpha Boo and Alpha
Aur because of the larger mass-loss rates per unit area in these stars.
Title: Flare Stars: II. Physical Characteristics of the Flares
Authors: Mullan, D. J.
Bibcode: 1976IrAJ...12..277M
Altcode:
No abstract at ADS
Title: Flares on white dwarfs and gamma-ray bursts.
Authors: Mullan, D. J.
Bibcode: 1976ApJ...208..199M
Altcode:
Using the known properties of stellar flares on red dwarfs, the
characteristics of flares on magnetic white dwarfs are predicted. On
the basis of these predictions, the hypothesis that gamma-ray bursts
are thermal radiation from a high-temperature analog of normal stellar
flares occurring on magnetic white dwarfs, with surface fields of 10
to the 7th to 10 to the 8th gauss is examined. It appears unlikely
that main-sequence stars are the sites of the bursts. Various observed
properties of the gamma-ray bursts are shown to be analogous to known
properties of normal stellar flares. In Particular, multiple peaks
within a burst are ascribed to sympathetic flares. Independent arguments
suggest that the gamma-ray bursts occur on white dwarfs at distances
of order 100-300 pc. It is proposed that fluctuations in the luminosity
of certain white dwarfs may be related to starspots and flare activity.
Title: Thermal X-rays from stellar flares: reevaluation of scaling
from solar flares.
Authors: Mullan, D. J.
Bibcode: 1976ApJ...207..289M
Altcode:
A simple method of estimating the ratio of thermal X-ray luminosity
to optical luminosity in stellar flares is presented which is valid at
times close to and following peak flare luminosity. It is proposed that
at such times, the X-ray and optical luminosities are, respectively,
the energy radiated by the plasma and the energy conducted out of
the plasma. If the conductive energy directed downward toward the
photosphere and lower chromosphere is regarded as the energy source
for optical flares, then the luminosity ratio is equal to the ratio
of conductive to radiative cooling times in the hot flare plasma. A
self-consistent model for red-dwarf flares is discussed in which the
decay time of the optical light curve is controlled by the time scale
for thermal conduction from the hot flare plasma. The condition that the
two times must be approximately equal leads to estimates of electron
densities in flares, provided the flare temperatures are known; it is
argued that stellar flare temperatures are 1 to 4 times higher than
the mean temperature of solar flares. Numerical results are given for
the luminosity ratio in flares of seven UV Ceti stars. It is concluded
that if the solar luminosity ratio is used in attempting to predict
thermal X-ray fluxes from stellar flares, the predicted X-ray flux
accompanying an optical flare of given amplitude will provide only an
upper limit on the actual thermal X-ray flux.
Title: Erratum: Sympathetic Stellar Flares and Electron Precipitation
as Probes of Coronal Structure in Flare Stars
Authors: Mullan, D. J.
Bibcode: 1976ApJ...206..672M
Altcode:
No abstract at ADS
Title: Flare Stars: Note added in Proof
Authors: Mullan, D. J.
Bibcode: 1976IrAJ...12..231M
Altcode:
No abstract at ADS
Title: Flare Stars: I. Physical Characteristics
Authors: Mullan, D. J.
Bibcode: 1976IrAJ...12..161M
Altcode:
The properties of flare stars - a group of dwarf stars of late spectral
type characterized by intermittent short-lived increases in brightness -
are examined. Particular attention is given to properties which pertain
to the generation of strong magnetic fields, since hypotheses according
to which stellar flares derive their energy ultimately from magnetic
field energy appear to be the most attractive.
Title: Sympathetic stellar flares and electron precipitation as
probes of coronal structure in flare stars.
Authors: Mullan, D. J.
Bibcode: 1976ApJ...204..530M
Altcode:
We have previously suggested that sympathetic stellar flares might
be triggered by a hydromagnetic (Moreton) wave propagating from one
starspot to another. Electron cyclotron waves (whistlers) are expected
to propagate with speeds exceeding Moreton wave speeds by factors of
up to 30. We suggest that short-period (13 s) oscillations observed
by Rodono during a large flare in the Hyades flare star H II 2411 are
the whistler analog of sympathetic flares, and are due to electron
precipitation triggered by whistlers propagating between starspots
at opposite rotation poles of the star. According to this model,
if starspot fields are as strong ( 10 gauss) as a recent starspot
model suggests, then the coronal density at about 7-8 stellar radii
in H II 2411 is in the range (0.5-10) x 106 . These densities are
larger by factors of than the densities in the solar corona at 7-8
solar radii. Adopting ne = 2 x 106 at 7.5 stellar radii in the corona
of a flare star thought to be very similar to H II 2411 (YZ CMi), we
find that the coronal temperature in the flare star is lower than in
the solar corona by a factor of 3-10. Using the Kennel-Petschek model
originally developed for precipitation of electrons from the terrestrial
radiation belts, we interpret Rodono's flare observations to mean that
spectral indices of the differential energy spectrum of electrons in
three flares on H II 2411 lie in the range 2.5 .0. Subject headings:
stars: coronae - stars: flare
Title: Possible evidence for the occurrence of magnetic fields of
order 10 kilogauss in the red dwarf star BY Draconis.
Authors: Mullan, D. J.; Bell, R. A.
Bibcode: 1976ApJ...204..818M
Altcode:
Koch and Pfeiffer have reported the presence of linear polarization in
broad-band observations of the light from the spotted dwarf BY Dra. Here
we examine the hypothesis that the polarization is produced by the
cumulative effects of the Zeeman patterns of individual absorption
lines within the bandpass. We compute the polarization expected in a
cool (0e = 1.5) dwarf observed in UBVR filters. Comparing our results
with observations of BY Dra, we find that the surface field strength on
this star is at times of order 10 kilogauss. Peak linear polarization
is observed to occur when the site previously occupied by a large
starspot is optimally visible. We suggest that the presence of fields
of order 10 kilogauss in an old bipolar region is consistent with a
recent model of starspots on a cool dwarf. Subject headings: stars:
late-type - stars: magnetic
Title: Properties of Minimum Flux Coronae in Dwarfs and Giants
Authors: Mullan, D. J.
Bibcode: 1976BAAS....8..307M
Altcode:
No abstract at ADS
Title: A Comment on 'A Comment on the Damping of Magnetohydrodynamic
Waves' by Bibhas R. De
Authors: Mullan, D. J.
Bibcode: 1976Ap&SS..44L...9M
Altcode:
We point out that in the case of sunspots, when Coulomb collisions
are included, the reduction in Lundqvist number due to anisotropic
conductivity is several orders of magnitude less than the reduction
estimated by Dc. This result suggests that, contrary to De's conclusion,
sunspot conditions do permit efficient MHD wave propagation
Title: Polarized light from lower main-sequence stars: is it due to
synchrotron emission?
Authors: Mullan, D. J.
Bibcode: 1975ApJ...201..630M
Altcode:
Broadband plane polarization (0.1-10%) has been observed in the
light emitted by flare stars outside flares. The possibility that the
polarization might be due to synchrotron radiation is examined. On
the basis of the electron energy spectrum which would be required,
it is suggested that synchrotron radiation is probably not responsible
for the observed polarization.
Title: E. M. Lindsay, An All-Ireland Astronomer
Authors: Mullan, D. J.
Bibcode: 1975IrAJ...12..110M
Altcode:
No abstract at ADS
Title: Are stellar flares energized by the missing energy in
starspots?
Authors: Mullan, D. J.
Bibcode: 1975ApJ...200..641M
Altcode:
It is suggested that the energy released in stellar flares is provided
by a steady accumulation of a fraction of the missing energy in
starspots. The analysis is based on the assumption that starspots are
necessary for flare activity since they are highly efficient generators
of nonthermal energy. The rate of occurrence of flares of given absolute
magnitude on a star of given absolute visual magnitude is determined,
and an expression is obtained for the dependence of the cooling time
of optically thick flare plasma on the absolute magnitude and gravity
of a star. The results are found to be in excellent agreement with
the observed statistical properties of stellar flares over a range of
almost 10 magnitudes in intrinsic visual luminosity. It is noted that
the formulas can be applied in an approximate way to solar flares and
sunspots, and that the lack of flare stars in spectral types earlier
than KO may mean that starspots become very small in late G-type stars.
Title: Magnetic fields in dMe stars: how effective is the battery
mechanism?
Authors: Mullan, D. J.
Bibcode: 1975PASP...87..455M
Altcode:
Although we agree with Worden (1974) that the surfaces of dMe stars
may be the sites of magnetic fields as large as 1-100 kilogauss,
we disagree with his conclusion that Biermann's battery mechanism is
responsible for generating such large fields. We believe that it is
more probable that field generation occurs in convective conditions,
where the battery cannot operate efficiently. We also disagree with
the argument used by Worden to relate magnetic energies with observed
properties of flare stars. Key words: dMe stars - magnetic fields -
flare stars
Title: On the possibility of magnetic starspots on the primary
components of W Ursae Majoris type binaries.
Authors: Mullan, D. J.
Bibcode: 1975ApJ...198..563M
Altcode:
We examine the hypothesis that magnetic starspots occur in W
UMa stars. Crude estimates of toroidal field strength in these
rapidly-rotating stars suggest that the fields may be large enough
(2-10 kilogauss) to permit spots to form. The probability of spot
formation is larger on the primary component than on the secondary,
especially in Rucinski's W type systems. It is shown that the existence
of starspots on the primary component in W UMa systems can account
for the apparent temperature excess of the secondary relative to the
primary in W type systems, and for the much smaller variations in the
light curve exhibited by Rucinski's A type systems. We predict upper
limits on the amplitude of the distortions which can occur in the
light curve of these systems due to starspot activity. The presence
of starspots leads to flare activity, but flares with amplitudes as
large as Amv = 1 mag are expected to occur very rarely, only once every
few years in W UMa. Flares of amplitude 0.01 mag, may, however, occur
sufficiently frequently to explain short-term deviations from mean
light curves occurring within a single night's observations. Cycles
of starspot activity appear to have periods of 5-10 years in two of
these systems. Tidal effects may be important in determining preferred
starspot longitudes. Subject heading: W Ursae Majoris stars
Title: Magnetic fields and dense chromospheres in dMe stars.
Authors: Mullan, D. J.
Bibcode: 1975A&A....40...41M
Altcode:
The hypothesis is investigated that dense chromospheres of dMe stars are
heated by dissipation of hydromagnetic waves which may be generated in
active regions where the nonspot magnetic field strength can be as large
as 5 to 10 kG. It is proposed that dMe stars are a set of magnetic stars
on the lower main sequence which have strong fields generated by dynamo
action in deep convective envelopes, while dM stars are nonmagnetic
or weakly magnetic stars having no starspots on their surfaces. The
combination of magnetic fields and dense chromospheres in dMe stars is
shown to provide consistent evidence for several conclusions, including:
(1) the dMe stars which are most likely to be flare stars are those
with hydrogen emission lines and (2) propagation of flare-initiated
coronal waves can trigger sympathetic stellar flares. It is suggested
that grain formation occurs in starspots of dMe stars and that such
grains in a circumstellar shell are responsible for the systematic IR
excesses of dMe stars relative to dM stars.
Title: The Relation Between Flares and the Missing Energy in Spots
Authors: Mullan, D. J.
Bibcode: 1975BAAS....7S.362M
Altcode:
No abstract at ADS
Title: Broad-band Polarization Expected in Magnetic M Dwarfs.
Authors: Mullan, D. J.; Bell, R. A.
Bibcode: 1975BAAS....7..271M
Altcode:
No abstract at ADS
Title: Magnetic fields in the sun.
Authors: Mullan, D. J.
Bibcode: 1974FrInJ.298..341M
Altcode:
The observed properties of solar magnetic fields are reviewed, with
particular reference to the complexities imposed on the field by
motions of the highly conducting gas. Turbulent interactions between
gas and field lead to heating or cooling of the gas according as
the field energy density is less or greater than the maximum kinetic
energy density in the convection zone. The field strength above which
cooling sets in is 700 G. A weak solar dipole field may be primeval,
but dynamo action is also important in generating new flux. The dynamo
is probably not confined to the convection zone, but extends throughout
most of the volume of the sun. Planetary tides appear to play a role
in driving the dynamo.
Title: Is Magnetic Convection Important in the Sun?
Authors: Mullan, D. J.
Bibcode: 1974SoPh...38....9M
Altcode:
Although magnetic convection in all probability does operate inside the
Sun, the energy flux which it can carry is 5-6 orders of magnitude
smaller than the total solar flux. Thus the effects of magnetic
convection on the internal temperature structure of the Sun are
probably small.
Title: Starspots on flare stars.
Authors: Mullan, D. J.
Bibcode: 1974ApJ...192..149M
Altcode:
Sizes of starspots on flare stars can be derived from the author's
convection-cell hypothesis. The sizes are in fair agreement with
those observed on YY Gem, CC Eri, and BY Dra by Bopp and Evans. The
hypothesis predicts that periodic brightness variations due to
starspots are restricted to stars brighter than a critical absolute
visual magnitude. A convective model of a starspot on YY Gem has been
computed, assuming that the missing flux is in the form of Alfve'n
waves. It is found that the surface field must exceed 10 gauss, and is
probably less than about 3 x 10 gauss. With a surface field of 2 x 10
gauss, the effective temperature of the spot is in the range Te = K,
depending on the field gradient. These figures are to be compared with
Te = 2000 K estimated from observations by Bopp and Evans. Efficient
dynamo action is shown to be a possible mechanism for generating
such large surface fields. There is a possibility that tidal effects
may influence starspot formation. Subject headings: flare stars -
hydromagnetics - late-type stars - magnetic stars
Title: Magnetic Fields on Spotted Red Dwarfs.
Authors: Mullan, D. J.
Bibcode: 1974BAAS....6..333M
Altcode:
No abstract at ADS
Title: Sunspot Models with Alfvin Wave Emission
Authors: Mullan, D. J.
Bibcode: 1974ApJ...187..621M
Altcode:
Sunspot models have been computed on the assumption that the missing
flux is transported by undissipated Alfve'n waves. In order to
estimate the flux of these waves, we propose an extension of C)pik's
cellular model of convection to include the effects of a vertical
magnetic field on horizontal gas flow. Horizontal motions, not vertical
motions, are impeded more or less severely depending on the electrical
conductivity, and this reduces the convective flux. These motions,
however, shake the field lines; and this is assumed to be a source
of Alfve'n waves, compensating for the reduction in the convective
flux. The free parameter D/H (ratio of cell diameter to cell depth) is
adjusted such that the total sum of radiative, convective, and Alfve'n
wave fluxes remains constant at all depths and equals the undisturbed
solar flux entering the spot from below. Our treatment is speculative
to some extent, but it leads to a unique spot model for a given field
strength. We derive the depth-dependence of the effective temperature T,
in a spot, and find that it increases nonmonotonically from a low value
at the surface (2750 K in a spot with magnetic field B = 3000 gauss) to
the solar value (5780 K) at depth z. The latter turns out to lie within
1 percent of the depth of the solar convection zone in all cases of
interest. Convection cells in sunspots are found to be narrow cylinders
(DIH < 0.2 and decreasing, the stronger the field) aligned along
the field lines. The diameter of a cell at the top of a spot with B =
3000 gauss is about 80 km. The depth ZA where the Alfve'n wave flux
has a maximum is found to be 841 km in a spot with B = 3000 gauss, and
ZA decreases with decreasing B. The values of ZA seem to correspond
with the "Wilson depression. Radiative influx of heat from the walls
of a spot is found to be minimal for B = gauss, and increases rapidly
outside this range. Thus spots with B in this range are expected to live
longest, and the most common observed field strengths are expected to
lie in this range. If radiative heating is responsible for the lower
limit on the permissible field strength in spots (1200 gauss), then
there should also be an upper limit on spot field strength at 5300
gauss. Subject headings: hydromagnetics - sunspots
Title: Sunspots, Supergranules, and the Depth of the Solar Convection
Zone
Authors: Mullan, D. J.
Bibcode: 1973ApJ...186.1059M
Altcode:
It is suggested that a large sunspot or a sunspot group can be regarded
as a special kind of convection cell penetrating the entire depth of
the solar convection zone (analogous to a supergranule, according to
the author's model). Theoretical results of Vickers are used to show
that the diameters of supergranules and large sunspots should lie in the
range 18,000-65,000 km. This agrees well with results of Bumba et al.,
who found preferred spot sizes in large groups ranging from 18,000
to 54,000 km. Spots with diameters less than 18,000 km are probably
shallower than the convection zone, i.e., they have depths less than
10,000 km. Implications for flare stars are noted. Subject headings:
convection - granules, solar - sunspots
Title: Comments on Papers by P. R. Wilson Concerning Sunspots
Authors: Mullan, D. J.
Bibcode: 1973SoPh...32..441M
Altcode:
No abstract at ADS
Title: Flare Triggering by Coherent Oscillations
Authors: Mullan, D. J.
Bibcode: 1973ApJ...185..353M
Altcode:
Energy fluxes in coherent oscillations observed by Tanaka in Ha plages
are estimated to be very large, (0.954.50) x 1010 ergs cm-2 5 1 During
their lifetime ( 10 s) these oscillations transmit up to 1 0 ergs to
the chromosphere, sufficient to provide for a large flare even if the
efficiency of conversion into electromagnetic and corpuscular energies
is only 1 percent. If these oscillations do not in fact provide the
energy released in a flare, they are expected to be at least important
in triggering a flare, as Tanaka has observed. The coherent oscillations
are thought to be free modes of the Sun (as described by C. L. Wolff),
preferentially excited in plages near sunspots where subphotospheric
magnetic fields modify the structure of the convection zone. Subject
headings: flares, solar - plages, solar - solar atmospheric motions
Title: Fast Rotation of Metal-poor Stars
Authors: Mullan, D. J.
Bibcode: 1973A&A....27..379M
Altcode:
Summary. Mass loss in stellar winds is expected to be a sensitive
function of maximum convective velocity in the outer convection zone of
stars. Metal-poor stars are expected to have low mass loss rates, and
therefore their rotational velocities are not braked as efficiently
as are those of metal-rich stars. This explains an observational
correlation described by Danziger and Faber (1972). Key words: stellar
rotation - metal abundance
Title: Correction of Sunspot Intensities for Scattering Light
Authors: Mullan, D. J.
Bibcode: 1973SoPh...32...65M
Altcode:
Correction of sunspot intensities for scattered light usually
involves fitting theoretical curves to observed aureoles (Zwaan, 1965;
Staveland, 1970, 1972). In this paper we examine the inaccuracies in the
determination of scattered light by this method. Earlier analyses are
extended to examine uncertainties due to the choice of the expression
for limb darkening, including terms up to sin12θ. For the
spread function we consider Lorentzians and Gaussians for which analytic
expressions for the aureole can be written down. Lorentzians lead to
divergence and normalization difficulties, and should not be used in
scattered light determinations. Gaussian functions are more suitable.
Title: Earthquake Waves and the Geomagnetic Dynamo
Authors: Mullan, Dermott J.
Bibcode: 1973Sci...181..553M
Altcode:
It is proposed that earthquake waves energize the geomagnetic
dynamo. Fluid motions generated by earthquakes may have enough energy
to be in equipartition with fields as large as 100 gauss. Seismic waves
from meteoritic impacts with energies sufficient to reverse the field
occur every 170,000 years.
Title: On the Possibility of Constructing a Radiative Sunspot Model
in Magnetohydrostatic Equilibrium
Authors: Mullan, D. J.
Bibcode: 1973SoPh...30...75M
Altcode:
It is currently believed that it is impossible to construct a
radiative sunspot model in magnetohydrostatic equilibrium unless
magnetic fields below the surface are excessively large (> 100
kG). This belief is based on results obtained using the mixing length
theory of convection. We wish to point out that by using a different
theory of convection, due to Öpik (1950), it is possible to compute
a radiative sunspot model in which the field becomes no greater than
9000 G. By applying two boundary conditions, (i) depth of spot equals
depth of convection zone, (ii) magnetic field has zero gradient at
the base of the spot, we show that a radiative spot has a unique
effective temperature for a given Wilson depression, Δ. For Δ = 650
km, we find Te = 3800K ; for Δ = 150 km, Te =
3950K. According to our model, spots having Te cooler than
these values should not exist.
Title: Can Oscillations Grow in a Sunspot Umbra?
Authors: Mullan, D. J.; Yun, H. S.
Bibcode: 1973SoPh...30...83M
Altcode:
Umbral flashes and running penumbral waves have been attributed by
Moore (1972) to overstable oscillations in the umbra. His numerical
results were derived by inserting physical conditions at two particular
depths beneath the umbral surface. Seven variables must be specified
at each point. We have extended Moore's analysis to examine the
depth-dependence of overstable oscillations in a recently computed
umbral model. Electrical conductivity is evaluated taking full account
of partial ionization and magnetic fields. In the surface layers,
within 250 km of the top of the umbral convection zone, the conductivity
is so low that Joule dissipation is more rapid than the growth rate
of oscillations. In these layers, Moore's results are therefore not
applicable. At greater depths, oscillations can grow and we agree
with Moore that both umbral flashes and penumbral waves may be due
to overstable oscillations. However, we suggest that both phenomena
can arise at the same depth in the spot, and not in two layers, as
Moore suggests.
Title: Are Penumbral Filaments Convection Rolls?
Authors: Mullan, D. J.
Bibcode: 1973A&A....24..103M
Altcode:
Summary. The occurrence of strong magnetic fields in dark filaments
in sunspot penumbrae is shown to be only marginally consistent with
the hypothesis of penumbral convection rolls. Convection rolls are
not permitted if the field in dark filaments exceeds that in bright
filaments by more than 2 %. The upper limit to this excess may be as
small as 0.8 %, if not in fact negative. Key words: sunspot penumbra -
magnetic fields in filaments - convection rolls
Title: Convection in A Stars: Suppressed in Magnetic Stars?
Authors: Mullan, D. J.
Bibcode: 1973IrAJ...11...32M
Altcode:
No abstract at ADS
Title: Seven Color Photometry of Umbral Cores with the Bartol Coudé
Telescope.
Authors: Mullan, D. J.; Wyller, A. A.
Bibcode: 1973BAAS....5...20M
Altcode:
No abstract at ADS
Title: Transition from Shallow to Deep Convection Zones in Stars
Authors: Mullan, D. J.
Bibcode: 1972ApL....12...13M
Altcode:
No abstract at ADS
Title: Thin Solar Convection Zone and Sunspots
Authors: Mullan, D. J.
Bibcode: 1972NPhS..235...58M
Altcode: 1972Natur.235...58M
THERE is a remarkable coincidence between the depth of a sunspot
(1.05 × 104 km according to Hong Sik Yun1)
and my recent determination2 of the depth of the solar
convection zone, namely 1.04 × 104 km.
Title: New Variable Stars in the Large Magellanic Clouds
Authors: Andrews, A. D.; Mullan, D. J.
Bibcode: 1971IrAJ...10..149A
Altcode:
No abstract at ADS
Title: Sequences for Binocular Variables
Authors: Mullan, D. J.
Bibcode: 1971JBAA...81..454M
Altcode:
No abstract at ADS
Title: Books Reviewed or Received: "Cosmic Gamma Rays" by
F. W. Stecker
Authors: Mullan, D. J.
Bibcode: 1971IrAJ...10..113M
Altcode: 1971IrAJ...1O..113M
No abstract at ADS
Title: Books Reviewed or Received: "Radio Astronomy and the Galactic
System", edited by H. van Woerden
Authors: Mullan, D. J.
Bibcode: 1971IrAJ...10..114M
Altcode: 1971IrAJ...1O..114M
No abstract at ADS
Title: The Space Between the Stars
Authors: Mullan, D. J.
Bibcode: 1971IrAJ...10....1M
Altcode:
No abstract at ADS
Title: News and Comments: Possible Identifications of Molecular
Helium in White Dwarfs
Authors: Mullan, D. J.
Bibcode: 1971IrAJ...10...25M
Altcode:
No abstract at ADS
Title: Effects of Rotation on Solar Convection: Cyclones in the
Solar Atmosphere
Authors: Mullan, D. J.
Bibcode: 1971IrAJ...10...12M
Altcode:
No abstract at ADS
Title: Book Review: "Stellar Physics" by Hong-Yee Chiu
Authors: Mullan, D. J.
Bibcode: 1971IrAJ...10..151M
Altcode: 1971IrAJ...1O..151M
No abstract at ADS
Title: Cellular convection in model stellar envelopes
Authors: Mullan, D. J.
Bibcode: 1971MNRAS.154..467M
Altcode:
Model envelopes have been constructed using a theory of cellular
convection due to Opik, in which turbulent heat exchange between
rising and falling gas is allowed for by introducing an experimentally
determined coefficient. Radiative heat losses are also included. A model
of the solar convection zone is found to be only x io km deep, with a
bottom temperature of x , and a gas pressure log Pg = . Supergranules
are considered to be convection cells penetrating the entire convection
zone. Temperature inhomogeneities in the upper convection zone exhibit
a plateau in the region 0.5<7 < I 3. In models of main sequence
stars, the maximum convective velocity is found to have a peak value
( km s-') at spectral type A6, with a rapid drop at earlier spectral
types. Supergranule cells increase in size abruptly by a factor of
almost a between spectral types Go and Ga. Metal-poor stars have lower
convective velocities than metalrich stars in the deeper layers.
Title: The structure of transverse hydromagnetic shocks in regions
oflow ionization
Authors: Mullan, D. J.
Bibcode: 1971MNRAS.153..145M
Altcode:
The compression of a magnetic field by a shock in a predominantly
neutral gas is discussed. By solving the fluid equations, a steady-state
structure is determined for a one-dimensional shock front propagating
through a partially ionized gas in a direction perpendicular to
the field lines. Temperatures and velocities of ions, electrons,
and atoms are calculated as a function of the spatial coordinate
moving with the shock frame. When the degree of ionization is small,
field compression occurs at the expense of the momentum carried by
the neutral atoms. The neutral momentum is transferred indirectly to
the field by way of the ions. To effect the transfer, many ion-atom
collisions are required. Shock widths from several hundred to several
times I0 ion- atom mean free paths are required. Within the broad
profile, a narrow atom shock is imbedded near the hot end if the
shock speed exceeds a critical value. Within the broad region of
changing field, the transient momentum in the ions causes them to drift
relative to the atoms, and to become hotter than the atoms. Shocks in
interstellar clouds of predominantly neutral hydrogen are discussed
in detail. Within such shocks, the ion temperature rises to several
hundred degrees hotter than the atom temperature, and ions drift
relative to atoms with speeds of the order of a few kilometres per
second. Detection of these ion-atom differences may be possible in
the case of weak shocks, for it is only in weak, broad shocks that
radiation carries a significant fraction of the energy flux. In the
weakest shock for which numerical results are available, the radiative
flux amounts to about ao per cent of the convected flux. Suggested
lines for observation of ion-atom differences are the ionized carbon
line at I 6 microns and the neutral oxygen line at 147 microns. It
is suggested that ion-atom velocity differences within shocks lead to
efficient removal of the magnetic field from a contracting protostar,
thereby permitting fragmentation into masses as small as one solar
mass. Application of the results to a model which has been proposed
for solar flares, and to conditions in laboratory experiments are noted.
Title: Cellular convection in stellar envelopes.
Authors: Mullan, D. J.
Bibcode: 1971IrAJ....9..310M
Altcode:
No abstract at ADS
Title: Cellular Convection in Stellar Envelopes
Authors: Mullan, D. J.
Bibcode: 1970IrAJ....9..310M
Altcode:
No abstract at ADS
Title: Identification of Parkes Radio Sources on ADH plates
Authors: Mullan, D. J.
Bibcode: 1970IrAJ....9..246M
Altcode:
No abstract at ADS
Title: The Interstellar Medium as a Detector of Cosmic Rays
Authors: Mullan, D. J.
Bibcode: 1970IrAJ....9..250M
Altcode:
No abstract at ADS
Title: Flares of red dwarf stars.
Authors: Gershberg, R. E.; Mullan, D. J.
Bibcode: 1970frds.book.....G
Altcode:
No abstract at ADS
Title: The structure of hydromagnetic shocks in regions of very
low ionization.
Authors: Mullan, D. J.
Bibcode: 1969PhDT.......156M
Altcode:
No abstract at ADS
Title: The Structure of Hydromagnetic Shocks in Regions of Very
Low Ionization.
Authors: Mullan, Dermott Joseph
Bibcode: 1969PhDT........10M
Altcode:
No abstract at ADS
Title: First List of Emission Objects in the LMC
Authors: Lindsay, E. M.; Mullan, D. J.
Bibcode: 1963IrAJ....6...51L
Altcode:
No abstract at ADS