Author name code: robinson
ADS astronomy entries on 2022-09-14
=author:"Robinson, R.D." OR =author:"Robinson, Rich D." OR =author:"Robinson, Richard D." -aff:"New Zealand"
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Title: CalFUSE Version 3: A Data Reduction Pipeline for the Far
Ultraviolet Spectroscopic Explorer
Authors: Dixon, W. V.; Sahnow, D. J.; Barrett, P. E.; Civeit, T.;
Dupuis, J.; Fullerton, A. W.; Godard, B.; Hsu, J. -C.; Kaiser, M. E.;
Kruk, J. W.; Lacour, S.; Lindler, D. J.; Massa, D.; Robinson, R. D.;
Romelfanger, M. L.; Sonnentrucker, P.
Bibcode: 2007PASP..119..527D
Altcode: 2007arXiv0704.0899D
Since its launch in 1999, the Far Ultraviolet Spectroscopic Explorer
(FUSE) has made over 4900 observations of some 2500 individual
targets. The data are reduced by the principal investigator team at the
Johns Hopkins University and archived at the Multimission Archive at
STScI (MAST). The data reduction software package, called CalFUSE, has
evolved considerably over the lifetime of the mission. The entire FUSE
data set has recently been reprocessed with CalFUSE version 3.2, the
latest version of this software. This paper describes CalFUSE version
3.2, the instrument calibrations on which it is based, and the format
of the resulting calibrated data files. Based on observations made
with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer. FUSE is
operated for NASA by the Johns Hopkins University under NASA contract
NAS 5-32985.
Title: FUSE calibration pipeline version 3.0
Authors: Godard, B.; Dixon, W. V.; Dupuis, J.; Robinson, R. D.;
Sahnow, D. J.; Sonnentrucker, P.; Fullerton, A. W.; Kaiser, M. E.;
Massa, D.; Lindler, D. J.; Barrett, P.; Hsu, J. -C.; Romelfanger, M. L.
Bibcode: 2006ASPC..348..550G
Altcode:
The latest version (3.0) of the calibration pipeline for the Far
Ultraviolet Spectroscopic Explorer (FUSE) is considerably faster,
easier to maintain and more flexible for the end users than the older
versions. It represents a major design change. Data are stored as
a photon events (time of arrival, location, pulse height ...) list
throughout the pipeline. This allows time dependent corrections for such
effects as spacecraft jitter and mirror or grating motions. Moreover
users can now select and extract subsets of the data without rerunning
the pipeline.
Title: The Supergiant Wind of 22 Vul
Authors: Ake, Thomas B.; Robinson, R. D.
Bibcode: 2006AAS...208.4411A
Altcode: 2006BAAS...38R.119A
22 Vul (G3 Ib +B9 V) is the prototypical zeta Aur system with a G-type
supergiant primary and B-type main sequence companion. The secondary
star can be used as a probe of the outer atmosphere of the G star
during partial eclipse phases. FUSE observations have been obtained
during ingress and egress atmospheric eclipse phases, from a B-star
height of 2.5 RG*, until primary eclipse of the secondary
star. Compared to a normal B9 V star, 22 Vul shows added wind absorption
components in the B-star spectrum from the supergiant's wind at all
phases. The strongest lines of Fe II show wind troughs with a terminal
velocity of -200 km/s, which is substantially greater than the wind
seen in the C III profiles. The primary eclipse is not symmetrical in
the absorption lines, with line doubling occurring during ingress, but
single lines at egress. The line doubling during ingress maintains its
structure over a wide range of heights. The opacity near zero velocity
dramatically decreases in the egress spectrum as the B star goes from
1.9 to 2.3 RG*, which could be evidence of stationary coronal
loop. While the surface fluxes of C III and O VI are similar to other
late-type supergiants, both lines show optical depth effects.
Title: GALEX Observations of an Energetic Ultraviolet Flare on the
dM4e Star GJ 3685A
Authors: Robinson, Richard D.; Wheatley, Jonathan M.; Welsh, Barry Y.;
Forster, Karl; Morrissey, Patrick; Seibert, Mark; Rich, R. Michael;
Salim, Samir; Barlow, Tom A.; Bianchi, Luciana; Byun, Yong-Ik; Donas,
Jose; Friedman, Peter G.; Heckman, Timothy M.; Jelinsky, Patrick
N.; Lee, Young-Wook; Madore, Barry F.; Malina, Roger F.; Martin,
D. Christopher; Milliard, Bruno; Neff, Susan G.; Schiminovich, David;
Siegmund, Oswald H. W.; Small, Todd; Szalay, Alex S.; Wyder, Ted K.
Bibcode: 2005ApJ...633..447R
Altcode: 2005astro.ph..7396R
The Galaxy Evolution Explorer (GALEX) satellite has obtained high time
resolution ultraviolet photometry during a large flare on the M4 dwarf
star GJ 3685A. Simultaneous Near-ultraviolet (NUV, 1750-2800 Å) and
Far-ultraviolet (FUV, 1350-1750 Å) time-tagged photometry with time
resolution better than 0.1 s shows that the overall brightness in the
FUV band increased by a factor of 1000 in 200 s. Under the assumption
that the NUV emission is mostly due to a stellar continuum, and that
the FUV flux is shared equally between emission lines and continuum,
there is evidence for two distinct flare components for this event. The
first flare type is characterized by an exponential increase in flux
with little or no increase in temperature. The other involves rapid
increases in both temperature and flux. While the decay time for the
first flare component may be several hours, the second flare event
decayed over less than 1 minute, suggesting that there was little or
no confinement of the heated plasma.
Title: GALEX Ultraviolet Lightcurves of M-Dwarf Flare Stars
Authors: Wheatley, J. M.; Welsh, B. Y.; Browne, S. E.; Robinson,
R. D.; Seibert, M.; Rich, R. M.; GALEX Science Team
Bibcode: 2005AAS...206.4101W
Altcode: 2005BAAS...37Q.495W
We present several high time-resolution (0.1 s) lightcurves of bright,
short-duration (< 1000 s) flare events recorded at ultraviolet
wavelengths (1350 - 2750 A) during the first 18 months of observations
by the NASA Galaxy Evolution Explorer (GALEX). These outbursts
originate on nearby M-dwarf (dMe) flare stars, and can result in
a brightening of over 5 UV magnitudes in a period of less than 200
seconds. We compare and contrast pre and post flare behavior in
order to derive meaningful parameters that can be used as a general
diagnostic tool of the complex emission structure often exhibited by
these outbursts. Finally, we show plots of SDSS (g - r) and GALEX UV
magnitudes that can be used to identify previously unknown active dM
stars. GALEX is a NASA Small Explorer. We gratefully acknowledge
NASA's support for construction, operation, and science analysis for
the GALEX mission.
Title: FUSE observations of atmospheric eclipsing binaries
Authors: Ake, T. B.; Robinson, R. D.
Bibcode: 2005ESASP.560..381A
Altcode: 2005csss...13..381A
No abstract at ADS
Title: FUSE Observations of the Zeta Aur System 22 Vul
Authors: Ake, T. B.; Robinson, R. D.
Bibcode: 2004AAS...20510709A
Altcode: 2004BAAS...36.1525A
22 Vul (G3 Ib + B9 V; period=249d) has become a pivotal
laboratory for studying solar-type supergiants and the ζ Aur phenomenon
in general. During the atmospheric eclipse phase of these systems, the
B star can be used to directly probe different layers in the atmosphere
of the evolved supergiant primary. For 22 Vul, the primary star lies
near the dividing line in the HR diagram that separates stars with
hot, solar-like coronae from those with extended atmospheres and cool
winds. Its short period provides an opportunity to sample the entire
orbit in less than a year, compared to other ζ Aur systems which
typically have periods of several years. We have obtained FUSE
observations of 22 Vul during ingress and egress phases for the May-June
2004 eclipse. We find that strong Fe II absorption lines in the λ λ
1141-1155 region are already evident in the first observation at 2.5
RG*. These rapidly increase in strength until a height of
∼1.7 RG*, where they start to saturate. Further increases
in opacity at lower heights are compensated by strong resonance
scattering, which fills in the absorption lines and eventually leads to
emission features at totality. The continuum absorption increases slowly
with decreasing height until a height of about 1.4 RG*,
where it increases dramatically. This increase is coincident with a
factor of >10 increase in the hydrogen column mass, which can be
deduced from flux suppression due to the wing of Lyα . In totality,
the spectrum is dominated by emission lines, such as Fe II and Fe III,
arising from the ground state, caused by resonance scattering of the B
star photons in the cool star wind. We discuss current efforts underway
to model the complex features of these spectra. Support for this
research was provided by NASA research contract NNG04GL77G.
Title: The O VI and C III Lines at 1032 and 977 Å in Hyades F Stars
Authors: Böhm-Vitense, Erika; Robinson, Richard D.; Carpenter,
Kenneth G.
Bibcode: 2004ApJ...606.1174B
Altcode:
We continue our investigations into the mechanisms heating the outer
layers of cool dwarf stars. In this study we specifically seek to
determine whether in the layers with temperatures around 250,000-300,000
K, in which the O VI lines are emitted, the temperatures are determined
by heat conduction from the coronae or by the same processes that
heat the lower temperature regions. To study this we discuss here 22
spectra of Hyades F stars taken by the Far Ultraviolet Spectroscopic
Explorer (FUSE) satellite to study the O VI lines at 1032 Å and the C
III lines at 977 Å and compare them with other lower transition layer
lines, observed with HST and IUE, and with existing X-ray data. For our
targets with B-V>0.4, the X-ray fluxes of single F stars increase,
on average, slowly with increasing B-V, while the O VI line fluxes show
the same steep decrease around B-V=0.43 as previously found for the
lower temperature transition layer lines. For single stars the X-ray
fluxes decrease with increasing vsini, except for the stars with B-V
between 0.418 and 0.455, while for the O VI lines, as for the other
transition layer lines, fluxes increase with increasing vsini, if vsini
is larger than 30 km s-1. For smaller vsini, line fluxes are
independent of vsini. The B-V and vsini dependences of the O VI line
fluxes are then very different from those of the X-ray fluxes. We thus
conclude that for electron temperature Te below 300,000 K,
the transition layers for Hyades F stars are not mainly heated by
heat conduction from their coronae. Based on observations made
with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer (FUSE)
telescope, which is operated for NASA by Johns Hopkins University,
under contract NAS5-32985.
Title: High-Resolution Chandra Spectroscopy of γ Cassiopeiae (B0.5e)
Authors: Smith, Myron A.; Cohen, David H.; Gu, Ming Feng; Robinson,
Richard D.; Evans, Nancy Remage; Schran, Prudence G.
Bibcode: 2004ApJ...600..972S
Altcode: 2003astro.ph..9293S
γ Cas is the prototypical classical B0.5e star and is now known to be
the primary in a wide binary system. It has long been famous for its
unique hard X-ray characteristics, among which are variations that
correlate with changes in a number of optical light and UV line and
continuum properties. These peculiarities have led to a picture in which
processes on or near the Be star produce the observed X-ray emission. In
this paper we report on a 53 ks Chandra High Energy Transmission Grating
Spectrometer observation of this target. An inspection of our spectrum
shows that it is quite atypical for a massive star. The emission lines
appear weak because of a strong short-wavelength continuum that arises
from a hot plasma with kT=11-12 keV. The spectrum exhibits many lines,
the strongest of which are Lyα features of H-like species from Fe
through the even-Z intermediate elements (S, Si, Mg, and Ne), down to O
and N. Line ratios of the ``rif triplet'' for a variety of He-like ions
and of Fe XVII are consistent with the dominance of collisional atomic
processes. However, the presence of Fe and Si fluorescence K features
indicates that photoionization also occurs in nearby cold gas. The line
profiles indicate a mean velocity at rest with an rms line broadening
of 500 km s-1 and little or no asymmetry. An empirical
global-fitting analysis of the line and continuum spectrum suggests
that there are actually three or four plasma emission components. The
first is the dominant hot (12 keV) component, of which some fraction
(10%-30%) is heavily absorbed, while the remainder is affected by a much
lower column density of only 3×1021 cm-2. The
hot component has a Fe abundance of only 0.22+/-0.05 solar. The other
two or three major emission components are ``warm'' and are responsible
for most other emission lines. These components are dominated by plasma
having temperatures near 0.1, 0.4, and 3 keV. Altogether, the warm
components have an emission measure of about 14% of the hot component,
a low column density, and a more nearly solar composition. The 100
eV component is consistent with X-ray temperatures associated with a
wind in a typical early B star. Nonetheless, its emission measure is
a few times higher than would be expected from this explanation. The
strength of the fluorescence features and the dual-column absorption
model for the hot plasma component suggest the presence near the hot
sites of a cold gas structure with a column density of ~1023
cm-2. Because this is also the value determined by Millar
and Marlborough for the vertical column of the Be disk of γ Cas,
these attributes suggest that the X-ray-emitting sources could be
close to the disk and hence to the Be star. Finally, we discuss the
probably related issues of the origin of the warm emission components,
as well as the puzzling deficient Fe abundance in the hot component. It
is possible that the latter anomaly is related to the FIP (abundance
fractionation) effect found in certain coronal structures on the Sun
and RS CVn stars. This would be yet another indication that the X-rays
are produced in the immediate vicinity of the Be star.
Title: Surprising Differences in the Winds and Mass-Loss Rates of
Two M-Giant Stars: Gamma Cru (M3.5III) and Mu Gem (M3III)
Authors: Carpenter, K. G.; Robinson, R. D.
Bibcode: 2003AAS...203.4902C
Altcode: 2003BAAS...35Q1284C
We have assessed important parameters of the stellar winds of two very
similar M giant stars, Gamma Cru (M3.5III) and Mu Gem (M3III), using
high resolution HST/GHRS spectra and found surprising differences
in the characteristics of their winds and in their total mass-loss
rates. The wind parameters, including flow and turbulent velocities,
the optical depth of the wind above the region of photon creation,
and the mass-loss rate, have been estimated by fitting line profiles
computed using the "Sobolev with Exact Integration" (SEI) radiative
transfer code (Lamars et al. 1987), along with simple models of the
outer atmospheric structure and wind. These computed profiles are fit
to chromospheric emission lines which show self-absorptions produced
by the photon-scattering winds of these stars. The SEI code has the
advantage of being computationally fast and allows a great number
of possible wind models to be examined. The analysis is iterative in
nature: we specify estimates of the wind parameters, calculate line
profiles for the Mg II (UV1) lines and a range of unblended Fe II
lines (which have a wide range of wind opacities and therefore probe
different heights in the atmosphere), compare the computed profiles
with the observations, and modify the assumed wind properties until
the predicted profiles match the observations over as many lines as
possible.. Surprisingly, we find that the Gamma Cru wind exhibits a
significantly higher terminal velocity, wind turbulence, and mass-loss
rate than its fellow M-giant Mu Gem.
Title: FUSE Observations of the Active K Dwarf AB Doradus
Authors: Robinson, R. D.; Ake, T. B.; Dupree, A. K.; Linsky, J. L.
Bibcode: 2003csss...12..964R
Altcode:
In this paper we report on flaring activity observed in high time
resolution FUV observations of the active K dwarf star AB Doradus,
taken with the FUSE satellite. These observations include data on
the C III (λ1175) and O VI (λλ1032,1037) lines as well as the FUV
continuum. During 29 hours of observation, two large flares and >10
smaller events were observed. Here we describe the time history of
these events as well as a search for unresolved microflaring activity.
Title: Periods, cycles, and chaos in the high-energy emissions of
gamma Cas
Authors: Smith, M. A.; Robinson, R. D.
Bibcode: 2003ASPC..292..263S
Altcode: 2003ipc..conf..263S
The behavior of γ Cas (B0.5eIV) is so far unique among high-mass X-ray
sources. We review its X-ray and related optical and UV variations as
examples of periodic, cyclical, and chaotic processes. Most of this
related variability, including an apparently regular 1.1-day period in
UV continuum and line flux, 7-hour cycles in X-ray and in UV line flux,
chaotic ``shots" in the X-rays, and a variety of undulating patterns in
the X-ray and UV, can best be explained by the X-ray sources residing
near the Be star. However, a ~70-day cyclic and correlated variation of
X-ray and optical fluxes cannot be so readily explained by near-surface
generation -- nor by accretion in a binary component. Thus, we are
led to a unique picture in which these variations arise from a dynamo
excited in the Be disk.
Title: A Far Ultraviolet Spectroscopic Explorer Survey of Coronal
Forbidden Lines in Late-Type Stars
Authors: Redfield, Seth; Ayres, Thomas R.; Linsky, Jeffrey L.; Ake,
Thomas B.; Dupree, A. K.; Robinson, Richard D.; Young, Peter R.
Bibcode: 2003ApJ...585..993R
Altcode: 2002astro.ph.11363R
We present a survey of coronal forbidden lines detected in Far
Ultraviolet Spectroscopic Explorer (FUSE) spectra of nearby stars. Two
strong coronal features, Fe XVIII λ974 and Fe XIX λ1118, are
observed in 10 of the 26 stars in our sample. Various other coronal
forbidden lines, observed in solar flares, also were sought but
not detected. The Fe XVIII feature, formed at logT=6.8 K, appears
to be free of blends, whereas the Fe XIX line can be corrupted by
a C I multiplet. FUSE observations of these forbidden iron lines at
spectral resolution λ/Δλ~15,000 provides the opportunity to study
dynamics of hot coronal plasmas. We find that the velocity centroid
of the Fe XVIII feature deviates little from the stellar rest frame,
confirming that the hot coronal plasma is confined. The observed line
widths generally are consistent with thermal broadening at the high
temperatures of formation and show little indication of additional
turbulent broadening. The fastest rotating stars, 31 Com, α Aur Ab,
and AB Dor, show evidence for excess broadening beyond the thermal
component and the photospheric vsini. The anomalously large widths in
these fast-rotating targets may be evidence for enhanced rotational
broadening, consistent with emission from coronal regions extending
an additional ΔR~0.4-1.3R* above the stellar photosphere,
or represent the turbulent broadening caused by flows along magnetic
loop structures. For the stars in which Fe XVIII is detected, there is
an excellent correlation between the observed Röntgensatellit (ROSAT)
0.2-2.0 keV soft X-ray flux and the coronal forbidden line flux. As
a result, Fe XVIII is a powerful new diagnostic of coronal thermal
conditions and dynamics that can be utilized to study high-temperature
plasma processes in late-type stars. In particular, FUSE provides the
opportunity to obtain observations of important transition region
lines in the far-UV, as well as simultaneous measurements of soft
X-ray coronal emission, using the Fe XVIII coronal forbidden line.
Title: Long-duration Coherent Radio Emission from the dMe Star
Proxima Centauri
Authors: Slee, O. B.; Willes, A. J.; Robinson, R. D.
Bibcode: 2003PASA...20..257S
Altcode:
The Australia Telescope and Anglo-Australian Telescope were used in May
2000 to record the radio and optical emissions from the dMe flare star
Proxima Centauri. Eight bright optical flares over a two-day interval
resulted in no detectable excess short-term radio emission at 1.38
and 2.50GHz. However, a slowly declining 1.38GHz emission over the
two-day interval was nearly 100% right circular polarised and was
restricted to a relatively narrow bandwidth with total intensity
(I) and circular polarisation (V) varying significantly over the
104MHz receiver bandwidth. These are the first observations to show
that highly-polarised narrowband flare star emission can persist for
several days. This signature is attributed to sources of coherent radio
emission in the star's corona. Similarities with various solar radio
emissions are discussed; however, it is not possible with the existing
observations to distinguish between fundamental plasma emission and
electron-cyclotron maser emission as the responsible mechanism.
Title: A Far Ultraviolet Spectroscopic Explorer Survey of Late-Type
Dwarf Stars
Authors: Redfield, Seth; Linsky, Jeffrey L.; Ake, Thomas B.; Ayres,
Thomas R.; Dupree, A. K.; Robinson, Richard D.; Wood, Brian E.; Young,
Peter R.
Bibcode: 2002ApJ...581..626R
Altcode:
We describe the 910-1180 Å spectra of seven late-type dwarf stars
obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE)
satellite. The stars include Altair (A7 IV), Procyon (F5 IV-V), α
Cen A (G2 V), AB Dor (K1 V), α Cen B (K2 V), ɛ Eri (K2 V), and AU
Mic (M0 V). We present line identifications, fluxes, Doppler shifts,
and widths. Doppler shifts are measured with respect to heliocentric
wavelength scales determined from interstellar absorption lines, and
are compared with transition region line shifts seen in Hubble Space
Telescope (HST) ultraviolet spectra. For the warmer stars the O VI lines
extend the trend of increasing redshift with line formation temperature,
but for the cooler stars the O VI line redshifts are essentially
zero. The C III and O VI lines of most stars in the sample are best
fit with two Gaussians, and we confirm the correlation of increasing
importance of the broad component with increasing stellar activity. The
nonthermal velocities of the narrow component are subsonic and exhibit
a trend toward larger velocities with decreasing surface gravity, while
the nonthermal velocities of the broad components show no obvious trend
with stellar gravity. The C III and O VI lines of Altair show unique
broad horned profiles. Two flares were observed on AU Mic. One shows
increasing continuum flux to shorter wavelengths, which we interpret as
free-free emission from hot plasma, and relatively narrow, redshifted
C III and O VI emission. The other shows very broad line profiles.
Title: A New Design for the FUSE Science Data Calibration Pipeline
Authors: Sahnow, D. J.; Dixon, W. V.; Fullerton, A. W.; Kaiser, M. E.;
Dupuis, J.; Robinson, R. D.; Barrett, P. E.; Hsu, J. C.; Massa, D.
Bibcode: 2002AAS...201.5507S
Altcode: 2002BAAS...34.1203S
The science data calibration pipeline for the Far Ultraviolet
Spectroscopic Explorer (FUSE) was designed years before the satellite
was launched in June of 1999. After launch, a number of unexpected
instrumental features were discovered; as the FUSE team dealt with each
of them, the pipeline was modified appropriately. Eventually, these
changes made the design so cumbersome that the pipeline has become
difficult to maintain. In 2002, we began to develop a new pipeline
concept that takes into account the actual instrument characteristics,
and more closely corresponds to the path of the photons through the
instrument. This design maintains time-tagged data as photon lists
throughout the calibration process, which facilitates time-dependent
corrections and allows the user to sort, filter, and combine data
files interactively after running the pipeline. Histogram data will be
converted to a pseudo time-tagged format and processed in an identical
manner. We expect that this new pipeline design will be implemented
by the time that FUSE Cycle 4 observations begin in April 2003. We
will discuss the new design and describe the progress toward this
goal. This work is supported by NASA Contract NAS5-32985 to the Johns
Hopkins University.
Title: X-Ray and Optical Variations in the Classical Be Star γ
Cassiopeia: The Discovery of a Possible Magnetic Dynamo
Authors: Robinson, Richard D.; Smith, Myron A.; Henry, Gregory W.
Bibcode: 2002ApJ...575..435R
Altcode: 2002astro.ph..5278R
The classical B0.5e star γ Cassiopeia is known to be a unique X-ray
source by virtue of its moderate LX (1033 ergs
s-1), hard X-ray spectrum, and light curve punctuated by
ubiquitous flares and slow undulations. The peculiarities of this star
have led to a controversy concerning the origin of these emissions:
whether they are from wind infall onto a putative degenerate companion,
as in the case of normal Be/X-ray binaries, or from the Be star
itself. Recently, much progress has been made to resolve this question:
(1) the discovery that γ Cas is a moderately eccentric binary system
(P=203.59 days) with unknown secondary type, (2) the addition of RXTE
observations at six epochs in 2000, adding to three others in 1996-1998,
and (3) the collation of robotic telescope (Automated Photometric
Telescope) B- and V-band photometric observations over four seasons that
show a 3%, cyclical flux variation with cycle lengths of 55-93 days. We
find that X-ray fluxes at all nine epochs show random variations with
orbital phase, thereby contradicting the binary accretion model, which
predicts a substantial modulation. However, these fluxes correlate
well with the cyclical optical variations. In particular, the six flux
measurements in 2000, which vary by a factor of 3, closely track the
interpolated optical variations between the 2000 and 2001 observing
seasons. The energy associated with the optical variations greatly
exceeds the energy in the X-rays, so that the optical variability
cannot simply be due to reprocessing of X-ray flux. However, the strong
correlation between the two suggests that they are driven by a common
mechanism. We propose that this mechanism is a cyclical magnetic dynamo
excited by a Balbus-Hawley instability located within the inner part of
the circumstellar disk. According to our model, variations in the field
strength directly produce the changes in the magnetically related X-ray
activity. Turbulence associated with the dynamo results in changes to
the density (and therefore the emission measure) distribution within
the disk and creates the observed optical variations.
Title: Estimation of Mass-Loss Rates for M Giants from UV Emission
Line Profiles
Authors: Carpenter, K. G.; Robinson, R. D.
Bibcode: 2001AAS...199.9205C
Altcode: 2001BAAS...33.1441C
The photon-scattering winds of M giants produce absorption features
in the strong chromospheric emission lines. These provide us with an
opportunity to assess important parameters of the wind, including
flow and turbulent velocities, the optical depth of the wind above
the region of photon creation, and the star's mass-loss rate. We
have used the Lamers et al. (1987) Sobolev with Exact Integration
(SEI) radiative transfer code, along with simple models of the outer
atmospheric structure and wind, to determine the wind characteristics of
two M-giant stars, γ Cru (M3.4) and μ Gem (M3IIIab). The SEI code has
the advantage of being computationally fast and allows a great number
of possible wind models to be examined. The analysis procedure involves
specifying wind parameters and then using the program to calculate
line profiles for the Mg II (UV1) lines and a range of unblended Fe II
lines. These lines have a wide range of wind opacities and therefore
probe different heights in the atmosphere. The assumed wind properties
are iterated until the predicted profiles match the observations
over as many lines as possible. We present estimates of the wind
parameters for these stars and offer a comparison to wind properties
previously-derived for low-gravity K stars using the same technique.
Title: Correlated X-ray and optical variations in γ Cas (B0.5e)
Authors: Smith, M. A.; Robinson, R. D.; Henry, G. W.
Bibcode: 2001AAS...19914308S
Altcode: 2001BAAS...33Q1524S
γ Cas is a classical B0.5e star known to be a unique X-ray source
by virtue of its moderate Lx (1033 ergs
s-1), hard X-ray spectrum, and light curve punctuated
by ubiquitous flares and slow undulations. There is not a complete
consensus on the origin of these emissions, whether from the Be star
or from wind infall onto a putative degenerate binary companion. In
the last year much progress has been made to resolve this question:
(1) the discovery by Harmanec et al. (2000) that γ Cas is a moderately
eccentric binary system (P = 203.6 d) with unknown secondary type,
(2) the addition of RXTE observations at 6 epochs in 2000, adding
to 3 others in 1996-8, (3) the collation of robotic telescope (APT)
B, V-band photometric observations over 4 seasons which show a 3%,
cyclical flux variation with cycle lengths of 55-85 days. We find that
X-ray fluxes at all 9 epochs show random variations with orbital phase,
thereby contradicting the binary accretion model. However, these fluxes
correlate well with the optical variations. In particular, the six
flux levels in 2000, which vary by a factor of three, closely track
the interpolated optical variations between the 2000 and 20001 optical
seasons. This result suggests that the optical variations originate
from processing of X-rays, e.g. from modulated irradiation of the Be
star's photosphere. Since the optical and X-ray variations are not
strictly periodic, we suggest that their origin lies in a dynamo,
either within the star or in the dense circumstellar Be disk.
Title: Far-Ultraviolet Observations of Flares on the dM0e Star
AU Microscopii
Authors: Robinson, Richard D.; Linsky, Jeffrey L.; Woodgate, Bruce E.;
Timothy, John G.
Bibcode: 2001ApJ...554..368R
Altcode:
We report on far-ultraviolet (FUV) observations of flares on the
dM0e star AU Microscopii obtained on 1998 September 6 with the
Space Telescope Imaging Spectrograph onboard the Hubble Space
Telescope. The data consist of medium-resolution echelle spectra
covering the wavelength region from 1170 to 1730 Å with a spectral
resolution of λ/Δλ=45,800. The observations were obtained using
the TIME-TAG observing mode in which the time and position of each
detected photon is recorded. This allows a study of variability that
is constrained only by counting statistics. During a total on-source
time of 10,105 s, we observed numerous microflare bursts as well as
four well-defined flare events. These flares lasted between 10 s and
3 minutes and were most easily detected in the FUV continuum and the
Si IV and C IV resonance lines. Variations in both the cooler (e.g.,
Lyα, C II, O I) and hotter (O V, N V, Fe XXI) emission lines were
much less pronounced. We examined the physical characteristics of the
flare events, including the time history of the wavelength-integrated
fluxes in the continuum and the various emission lines. In particular,
we searched for, but did not find, evidence for increased emission in
the red wing of the Lyα line, which would indicate the presence of
moderately energetic proton beams. We integrated the emission over
the entire time of major flare activity to investigate the average
line and continuum properties of the time-averaged spectrum, including
flows and turbulence as a function of temperature. We also considered
the shape of the FUV continuum, which may actually be caused by the
enhancement of numerous weak emission lines. Unfortunately, none of
the events were strong enough to allow a detailed examination of line
profiles as a function of time.
Title: Mg II Emission Lines of Hyades F Stars
Authors: Böhm-Vitense, Erika; Mena-Werth, Jose; Carpenter, Kenneth
G.; Robinson, Richard D.
Bibcode: 2001ApJ...550..457B
Altcode:
With the Hubble Space Telescope (HST) we have observed Hyades
F stars, using the Space Telescope Imaging Spectrograph (STIS),
in order to get more information about the heating mechanism(s)
for the chromospheres and transition layers and their dependence
on rotation and age. In this paper we study the Mg II lines at 2800
Å. We include earlier observations with the International Ultraviolet
Explorer (IUE) satellite. The Mg II emission lines become observable
for B-V>0.3. The emission line fluxes increase steeply until
B-V~0.40. For single stars there is a steep decrease in flux between
B-V=0.41 and B-V=0.44, similar to the behavior of the Ca II emission
line cores. For larger B-V the Mg II emission line fluxes again
increase, but much more slowly than for the Ca II lines. Generally,
the low point of the emission is reached between B-V=0.43 and B-V=0.45,
i.e., similar to the Ca II emission cores. For the Hyades F stars there
appears to be a difference between the emissions for single stars and
those for binaries. We find that for Hyades stars with surface line
fluxes larger than 106 ergs cm-2 s-1
the emission line fluxes decrease with increasing vsini. For smaller
fluxes they may increase with increasing vsini. We have only three
stars that perhaps show this. We study the flux ratios of the Mg II
k and h lines at 2795.7 and 2802.5 Å in order to determine where
the lines fall on the curve of growth. For the earliest F stars
studied here the ratio is close to 2, as expected for optically thin
lines. Generally, it seems that the optical depths in the line centers
are less than 10. There remain problems in understanding the size of
the line widths. We discuss the interpretation of the Wilson-Bappu
effect. For the Hyades F stars there is a strong dependence of the
line width on the effective temperature. Based on observations with
the NASA/ESA Hubble Space Telescope obtained at the Space Telescope
Science Institute, which is operated by the Association of Universities
for Research in Astronomy. Incorporated, under NASA contract NAS5-26555.
Title: HST/STIS Quiescent Spectrum of AU Microscopii (CD-ROM
Directory: contribs/pagano)
Authors: Pagano, I.; Linsky, J. L.; Carkner, L.; Robinson, R. D.;
Woodgate, B.; Timothy, G.
Bibcode: 2001ASPC..223.1039P
Altcode: 2001csss...11.1039P
No abstract at ADS
Title: Far-UV Echelle Spectroscopy of Arcturus with HST STIS (CD-ROM
Directory: contribs/ayres)
Authors: Ayres, T. R.; Brown, A.; Harper, G. M.; Bennett, P. D.;
Linsky, J. L.; Carpenter, K. G.; Robinson, R. D.
Bibcode: 2001ASPC..223.1079A
Altcode: 2001csss...11.1079A
No abstract at ADS
Title: Estimation of Mass-Loss Rates from Emission Line Profiles in
the UV Spectra of Cool Stars (CD-ROM Directory: contribs/carpente)
Authors: Carpenter, K. G.; Robinson, R. D.; Harper, G. M.
Bibcode: 2001ASPC..223.1591C
Altcode: 2001csss...11.1591C
No abstract at ADS
Title: Coordinated Optical, Radio and EUV Observations of a Flare
on YZ CMi (CD-ROM Directory: contribs/robinson)
Authors: Robinson, R. D.; Airapetian, V.; Slee, O. B.; Mathioudakis,
M.; Carpenter, K. G.
Bibcode: 2001ASPC..223.1151R
Altcode: 2001csss...11.1151R
No abstract at ADS
Title: Ultraviolet Emission Lines in BA and Non-BA Giants
Authors: Böhm-Vitense, Erika; Carpenter, Kenneth G.; Robinson,
Richard D.
Bibcode: 2000ApJ...545..992B
Altcode:
With the Hubble Space Telescope (HST) and the Goddard High Resolution
Spectrograph we have observed four barium and three weak barium stars
in the ultraviolet spectral region, together with two nonpeculiar giant
standard stars. An additional suspected Ba star was observed with HST
and the Space Telescope Imaging Spectrograph. In the H-R diagram,
three of the observed Ba stars lie on the same evolutionary tracks
as the Hyades giants. Using International Ultraviolet Explorer (IUE)
spectra of previously studied giants together with our HST spectra,
we investigate whether the chromospheric and transition layer
emission-line spectra of the Ba stars are different from those of
nonpeculiar giants and from those of giants with peculiar carbon and/or
nitrogen abundances. Except for the Ba star HD 46407 and the suspected
Ba star HD 65699, the Ba star and mild Ba star emission-line fluxes are,
for a given effective temperature and for a given luminosity, lower
than those for the nonpeculiar giants observed with IUE. In comparison
with the HST-observed standard stars, the C IV λ1550-to-C II λ1335
line flux ratios are smaller, but not necessarily so in comparison
with all IUE-observed nonpeculiar giants. However, the C IV-to-C II
line flux ratios for the Ba stars decrease with increasing carbon
abundances. This shows that the energy balance in the lower transition
layer is influenced by the carbon abundance. The temperature gradient
appears to be smaller in the C II line-emitting region. There does not
seem to be a difference in chromospheric electron densities for the
Ba and non-Ba stars, though this result is rather uncertain. Based on
observations with the NASA/ESA Hubble Space Telescope obtained at the
Space Telescope Science Institute, which is operated by the Association
of Universities for Research in Astronomy, Inc., under NASA contract
NAS 5-26555.
Title: A Search for Rotational Modulation of X-Ray Centers on the
Classical BE Star γ Cassiopeiae
Authors: Robinson, Richard D.; Smith, Myron A.
Bibcode: 2000ApJ...540..474R
Altcode:
In this paper we discuss X-ray observations of γ Cas obtained in
1998 November with the Rossi X-Ray Timing Explorer (RXTE). The data
were obtained nearly continuously over 54 hr, which is about twice
the expected rotational period. An earlier RXTE light curve obtained
in 1996 March over a 27 hr period showed X-ray flux arising from
short-duration shots (flares) superimposed on an undulating ``basal''
component that was anticorrelated with fluctuations of the UV continuum
over a timescale of ~10 hr. The object of the present study was to
(1) examine the long-term variations of the X-ray characteristics
through comparisons with this earlier data and (2) to determine
whether variations of the basal flux repeat during a second rotation
period. A comparison of the results with the 1996 data set shows a
number of similarities and differences in the X-ray behavior: (a)
the mean X-ray level in 1998 was only 60% of the 1996 level, (b)
the basal fluxes in 1998 vary over shorter timescales (less than 2
hr) than in 1996, (c) the shots were found to show a slightly softer
(cooler) mean color than the basal component in 1998, although they
were slightly hotter in 1996, (d) fluctuations in the colors of the
shot and basal fluxes generally track one another in both data sets,
(e) cyclical patterns of X-ray flux decrease with a period of about
7.5 hr occurred in both data sets, and (f) the frequency of shots
with a given integrated energy was found to decrease exponentially
with energy, although the rate of decrease in 1996 was slower than in
1998. There was only marginal evidence for a repetition during the
second half of the time sequence of long-term basal flux variations
seen during the first half of the observations. We suspect, however,
that the large intrinsic variability of the X-ray source would have
masked a true replication. We also present archival IUE data that
shows the presence of UV continuum variations in 1982 with similar
characteristics to those seen in 1996. This suggests that the regions
responsible for the UV variability are very long lived. The data also
provide the basis for a refined but still tentative rotational period
of 1.12277 days. Assuming a flare paradigm and a very simple electron
beam model, we examine the atmospheric heating expected for the shot
events. We conclude that it is possible to explain how the measured shot
temperature can be smaller than the temperature deduced for the basal
X-ray emission. We also discover that if the beam model is correct
then the electrons within the beam have relatively high energies
(>200 keV) and are nearly monoenergetic. In three appendices we
discuss arguments, first, against the idea that the X-ray emission
from γ Cas arises from mass accretion onto a hypothetical white dwarf
companion or from an active late-type star and, second, in favor of
its origin from near the surface of γ Cas.
Title: A Multiwavelength Campaign on γ Cassiopeiae. IV. The Case
for Illuminated Disk-enhanced Wind Streams
Authors: Cranmer, Steven R.; Smith, Myron A.; Robinson, Richard D.
Bibcode: 2000ApJ...537..433C
Altcode:
On 1996 March 14-15 we conducted a campaign with the Hubble Space
Telescope GHRS to observe the Si IV λλ1394, 1403 lines of the B0.5e
star γ Cas at high temporal and spectral resolution. As a part of this
~22 hr campaign, the Rossi X-Ray Timing Explorer (RXTE) was also used
to monitor this star's copious and variable X-ray emission. In this
fourth paper of a series we present an analysis of the rapid variations
of the discrete absorption components (DACs) of the Si IV doublet. The
DACs attain a maximum absorption at -1280 km s-1, taper
at higher velocities, and extend to -1800 km s-1. The DACs
in this star's resonance lines have been shown to be correlated with
a >~6 yr cycle in the Balmer line emission V/R ratio, and in 1996
this DAC strength was near its maximum. We derive hydrogen densities of
109-1010 cm-3 in the DAC material using
a curve-of-growth method and find that the plasma becomes marginally
optically thick near -1280 km s-1. The ``mean DAC'' probably
represents a broad ``plateau'' with a volume density intermediate
between the star's midlatitude wind and equatorial disk. We also follow
the blueward evolution of subfeatures in the DACs. These features
appear to emanate primarily from one or two discrete azimuths on the
star and accelerate much more slowly than expected for the background
wind, thereby exhibiting an enhanced opacity spiral stream pattern
embedded within the structure forming the DAC. In the first two papers
in this series, we suggested that active X-ray centers are associated
with at least two major cool clouds forced into corotation. Several
correlations of flickering in the Si IV DACs are found in our data,
which support the idea that changes in X-ray ionizing flux cause
changes in the ionization of material at various sectors along the
spiral pattern. We demonstrate that similar flickering is visible in
archival IUE data from 1982 and may also be responsible for earlier
reports from Copernicus of rapid changes in this star's UV and optical
lines. Finally, we discovered that flickering of the DAC fluxes in the
1982 data is correlated with rotation phase and shows a modulation
with a 7.5 hr cyclical cessation of X-ray flares that was observed
recently by RXTE. This confirms our basic picture that lulls in X-ray
activity close to the star's surface cause both a lower Si V ionization
fraction and an increase in Si IV variability within the DAC structures.
Title: HST/STIS Echelle Spectra of the dM1e Star AU Microscopii
Outside of Flares
Authors: Pagano, Isabella; Linsky, Jeffrey L.; Carkner, Lee; Robinson,
Richard D.; Woodgate, Bruce; Timothy, Gethyn
Bibcode: 2000ApJ...532..497P
Altcode:
We report on observations of the dM1e flare star AU Microscopii
obtained with the Space Telescope Imaging Spectrograph (STIS) on 1998
September 6. A total of 10,105 s of observations were obtained with
the medium-resolution E140M grating. We report here on the 9200 s of
``quiescent'' data when the star did not obviously flare. In this
data set, we identify 142 emission lines from 28 species, including
low-temperature chromospheric lines (e.g., C I, O I), transition-region
lines (e.g., C II-IV, N IV, O III-V, Si II-IV), and the coronal line Fe
XXI 1354 Å. There are a number of intersystem lines that are useful for
measuring electron densities. We discuss line redshifts and the broad
wings of transition-region lines that provide evidence of microflare
heating. We derive the emission-measure distribution and compare it
with the solar one. We also discuss the shape and formation mechanisms
of the He II 1640 Å lines. The analysis of the flare data and of the
interstellar absorption features seen in the H I, D I, C II, and O
I resonance lines will be published elsewhere. Based on observations
made with the NASA/ESA Hubble Space Telescope, obtained at the Space
Telescope Science Institute, which is operated by the Association of
Universities for Research in Astronomy, Inc., under NASA contract NAS
5-26555. These observations are associated with proposal ID 7556.
Title: The Changing X-Ray Properties of γ Cas
Authors: Smith, M. A.; Robinson, R. D.
Bibcode: 2000ASPC..214..352S
Altcode: 2000IAUCo.175..352S; 2000bpet.conf..352S
No abstract at ADS
Title: Winds from Luminous Late-Type Stars. I. The Effects of
Nonlinear Alfvén Waves
Authors: Airapetian, V. S.; Ofman, L.; Robinson, R. D.; Carpenter,
K.; Davila, J.
Bibcode: 2000ApJ...528..965A
Altcode:
We present the results of magnetohydrodynamic (MHD) modeling of winds
from luminous late-type stars using a 2.5-dimensional, nonlinear MHD
computer code. We assume that the wind is generated within an initially
hydrostatic atmosphere and is driven by torsional Alfvén waves
generated at the stellar surface. Two cases of atmospheric topology
are considered: case I has longitudinally uniform density distribution
and isotropic radial magnetic field over the stellar surface, and case
II has an isotropic, radial magnetic field with a transverse density
gradient, which we refer to as an ``atmospheric hole.'' We use the
same set of boundary conditions for both models.The calculations
are designed to model a cool luminous star, for which we assume an
initial hydrostatic pressure scale height of 0.072 R*,
an Alfvén wave speed of 92 km s-1 at the surface, and a
wave period of 76 days, which roughly corresponds with the convective
turnover time. For case I the calculations produce a wind with terminal
velocity of ~22 km s-1 and a mass loss rate comparable to the
expected value of 10-6 Msolar yr-1. For
case II we predict a two-component wind: a fast (25 km s-1)
and relatively dense wind outside of the atmospheric hole and a slow
(15 km s-1), rarefied wind inside of the hole.
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: A Multiwavelength Campaign on γ Cassiopeiae. III. The Case
for Magnetically Controlled Circumstellar Kinematics
Authors: Smith, Myron A.; Robinson, Richard D.
Bibcode: 1999ApJ...517..866S
Altcode:
In two previous papers, we have discussed simultaneous Rossi X-Ray
Timing Explorer (RXTE) and Hubble Space Telescope GHRS observations
made of γ Cas (B0.5e) over a full day on 1996 March 14-15. The light
curves generated from these data show features that anticorrelate
and led us to conclude that the star has multiple surface activity
centers that rotate into view every rotational cycle of 1.123 days. In
a second paper we found that dips in the UV continuum (UVC) light
curve are probably caused by the passage of cool, co-orbiting clouds
that are suspended above surface X-ray-active centers. In this paper
we use difference spectra from our >21 hr GHRS time series to
investigate <=2% spectral variations within the photospheric Si
IV 1394-1403 Å lines as well as smaller variations from features
in neighboring wavelengths at 1382-1386 and 1404-1417 Å. Several
difficulties arise in interpreting these variations as signatures of
surface inhomogeneities, so we have studied both types of variations
in the context of the kinematics of occulting circumstellar (CS)
structures. By means of model-atmospheres codes and up-to-date line
lists, we computed a grid of cloud opacity for various temperatures
in our spectral range. Using these synthetic spectra, we are able to
identify features as optically thick absorptions due to Fe II, Cr II,
and C I lines from ``cool'' (T<10,000 K) plasma, of Si IV, Si III,
S IV, and Ni II lines from ``warm'' plasma (~10,000-18,000 K), and of
Si IV and Fe V lines from hot plasma (>=30,000 K). The variations
of the cool- and hot-plasma lines are in phase with the UVC light curve
while the warm-plasma line curves lead these curves by 3-4 hr. The cool-
and warm-plasma lines participate in the blue-to-red deceleration and
appear to be analogues of the ``migrating subfeature'' pattern found
in optical lines by previous observers. The velocity range for these
lines is consistent with limits of +/-Vsini, suggesting that they are
formed in corotating cloudlets that are distinct from the cool clouds we
studied in a previous paper. In contrast, warm and hot-plasma lines are
``ultrasharp features'' (``USFs'') that maintain a constant velocity
for several hours. The USFs are visible over a wide velocity range, in
some cases having a velocity of at least +1500 km s-1. Both
cooling and heating of circumstellar plasma is consistent with the
existence of strong nonradiative processes operating in some regions
above γ Cas. Additionally, the migrating subfeatures find a ready
explanation in circumstellar cloudlets forced by magnetic forces into
corotation, as was also inferred from the UVC light curve in Smith,
Robinson, & Hatzes. The existence of the stationary ultrasharp
absorption features, particularly at large positive velocities, is
difficult to explain unless one invokes interactions between magnetic
loops from the star and a putative field in the circumstellar disc. This
picture holds the potential of explaining the hot, flaring character of
this star's X-rays and predicts the existence of other X-ray-emitting
γ Cas analogues as magnetic Be stars having dense CS discs.
Title: STIS UV Atlas of the Red Giant Arcturus(1)
Authors: Ayres, T. R.; Brown, A.; Harper, G. M.; Bennett, P. D.;
Linsky, J. L.; Carpenter, K. G.; Robinson, R. D.
Bibcode: 1999AAS...194.6701A
Altcode: 1999BAAS...31..930A
The Space Telescope Imaging Spectrograph (STIS) conducted a series
of observations of the archetype ``noncoronal'' red giant Arcturus
(HD 124897; alpha Boo; K1 III) on 24 August 1998, 17--23 UT. The STIS
program was the result of a failed cycle 5 pointing, which could not
be rescheduled during the abbreviated cycle 6, and was carried over
to cycle 7. Three grating settings---E230H (t_exp= 1340 s), E230M
(2493 s), and E140M (5208 s)---covered the ultraviolet spectrum
from 1150--2850 Angstroms, at resolutions between R ~ 4*E(4) -
1*E(5) , with essentially no gaps. The resulting spectrograms are
stunning. The resolution is very high, as is the S/N; the spectral
coverage is broad and comprehensive. The far-UV interval (1150--1700
Angstroms) is a rich emission line spectrum, dominated by the broad
resonance transitions of atomic hydrogen (lambda 1215) and oxygen
(lambda 1305 triplet). There are numerous narrow emissions, mostly
from low-excitation species such as Si I, Fe II, and fluoresced bands
of carbon monoxide. Surprisingly, high-excitation species---Si IV
(lambda 1393: 6*E(4) K) and C IV (lambda 1548: 1*E(5) K)---are present
as well (as seen in the earlier ``failed'' GHRS spectra). The mid-UV
(1700--2600 Angstroms) shows additional emission lines, particularly [C
II] and [Si II] in the 2325 Angstroms region; the photospheric continuum
rises strongly toward the longer wavelengths. The 2600--2850 Angstroms
interval is mostly a photospheric absorption spectrum, although the
bright chromospheric emission doublet of Mg II lords over the 2800
Angstroms region. We present a comprehensive spectral atlas based on
our reductions of the STIS echellograms. We discuss the processing
strategies, line identifications, and some of the preliminary results
from our analysis of this windy, noncoronal giant. (1) This work was
supported by grants GO-06066.01-94A from STScI, and NAG5-3226 from
NASA. Observations were from the NASA/ESA HST, collected at the STScI,
operated by AURA, under contract NAS5-26555.
Title: A Search for Microflaring Activity on DME Flare
Stars. II. Observations of YZ Canis Minoris
Authors: Robinson, R. D.; Carpenter, K. G.; Percival, J. W.
Bibcode: 1999ApJ...516..916R
Altcode:
We report on time-series photometric observations of the dM4.5e flare
star YZ Canis Minoris obtained in 1993 November with the High Speed
Photometer on board the Hubble Space Telescope. The data consist of five
30 minute time sequences with a sampling rate of 0.01 s that were taken
through the F240W filter (centered at 240 nm with an 80 nm width). At
these wavelengths the stellar photospheric background is small,
so relatively small flares can be detected. The observations show a
stellar background of 120 counts s-1 on which are superposed
54 flare events ranging in integrated flux from 2.0×1028
to 3.0×1030 ergs, as well as longer term variations with an
amplitude of up to 50% of the average continuum intensity and timescales
ranging from several minutes to hours. A statistical analysis of this
background suggests that it may be composed of unresolved microflaring
activity that has an energy distribution considerably steeper than
that deduced for the larger flare events. This is consistent with
previous observations as well as the self-organized criticality and
reconnecting current sheet flare theories. These results are compared
with data from the dM8e flare star CN Leonis, which was obtained
earlier with the same experimental setup. CN Leo has both a smaller
stellar background and a lower flare occurrence rate than YZ CMi. The
fact that CN Leo also has a quiescent X-ray flux that is less than
10% of the YZ CMi emission suggests a link between chromospheric and
coronal heating. 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 NAS5-26555.
Title: Lines of OIV and SIV in the Goddard High-Resolution
Spectrograph spectrum of RR Tel: constraints on atomic data
Authors: Harper, G. M.; Jordan, C.; Judge, P. G.; Robinson, R. D.;
Carpenter, K. G.; Brage, T.
Bibcode: 1999MNRAS.303L..41H
Altcode:
High signal-to-noise ratio spectra of RR Tel obtained at medium
resolution with the Goddard High-Resolution Spectrograph (GHRS) on the
Hubble Space Telescope (HST) are used to test available atomic data for
the OIV 2s^22p ^2P-2s2p^2 ^4P multiplet (UV 0.01). The fine-structure
intervals of the 2s2p^2 ^4P term given by Moore (1983) appear to need
revision. The flux ratios of lines within multiplet UV (0.01), which
have a common upper level, depend only on transition probabilities. The
observed flux ratio of lines from the ^4P_3/2 level differs from that
predicted by theory, but this difference cannot be attributed to a
blend with a line of SIV]. At the electron densities in the RR Tel
nebula, other flux ratios give information on the relative electron
excitation rates between the ^2P and ^4P fine-structure levels. Using
the collision strengths calculated by Zhang, Graziani & Pradhan,
the rate to the ^4P_5/2 level, relative to the rates to the other J
states, appears to be underestimated by ~ 10 per cent, which is within
the expected uncertainty of 20 per cent. We also discuss the SIV 3s^23p
^2P-3s3p^2 ^4P multiplet.
Title: Winds and mass-loss from evolved, low-gravity cool stars.
Authors: Carpenter, Kenneth G.; Robinson, Richard D.
Bibcode: 1999NewAR..43..471C
Altcode:
We summarize results from several programs utilizing the Goddard High
Resolution Spectrograph (GHRS) on the Hubble Space Telescope (HST) to
study winds and mass-loss from evolved, low-gravity cool stars. We have
found that: (i) the photons for thermally and fluorescently excited
UV emission lines are created below the region of wind acceleration,
(ii) the self-reversals in optically thick emission lines indicate an
outflowing wind with mean velocities of 9-25 km/s, (iii) the profiles
of optically thin emission lines indicate a mean chromospheric
macroturbulence of 24-35 km/s, anisotropically distributed along
the radial-tangential directions, (iv) significant emission from
hot material (≈10 5 K) is seen in both non-coronal and
hybrid stars to the right of the Linsky-Haisch dividing line, (v)
the weakness of Fe II emission lines in the carbon stars, combined
with the presence of the Fe I 2807 Å feature only in carbon stars,
suggests that the ionization fraction of iron is significantly lower in
the outer atmospheres of carbon stars than in O-rich stars, and (vi)
Fe II line profile variations indicate changes in mass-loss rate and
wind opacity on a timescale of several years in two typical late-type,
low-gravity stars.
Title: Estimation of Mass-Loss Rates from Emission Line Profiles in
the UV Spectra of Cool Stars
Authors: Carpenter, K. G.; Robinson, R. D.; Harper, G. M.
Bibcode: 1999STIN...0027505C
Altcode:
The photon-scattering winds of cool, low-gravity stars (K-M giants and
supergiants) produce absorption features in the strong chromospheric
emission lines. This provides us with an opportunity to assess important
parameters of the wind, including flow and turbulent velocities,
the optical depth of the wind above the region of photon creation,
and the star's mass-loss rate. We have used the Lamers et al. Sobolev
with Exact Integration (SEI) radiative transfer code along with simple
models of the outer atmospheric structure to compute synthetic line
profiles for comparison with the observed line profiles. The SEI
code has the advantage of being computationally fast and allows a
great number of possible wind models to be examined. We therefore
use it here to obtain initial first-order estimates of the wind
parameters. More sophisticated, but more time-consuming and resource
intensive calculations will be performed at a later date, using the
SEI-deduced wind parameters as a starting point. A comparison of
the profiles over a range of wind velocity laws, turbulence values,
and line opacities allows us to constrain the wind parameters,
and to estimate the mass-loss rates. We have applied this analysis
technique (using lines of Mg II, 0 I, and Fe II) so far to four stars:
the normal K5-giant alpha Tau, the hybrid K-giant gamma Dra, the K5
supergiant lambda Vel, and the M-giant gamma Cru. We present in this
paper a description of the technique, including the assumptions which
go into its use, an assessment of its robustness, and the results of
our analysis.
Title: FUV Observations of Flares on the dM0e star AU MIC
Authors: Robinson, R. D.; Linsky, J. L.; Woodgate, B.; Carkner, L.;
Timothy, G.
Bibcode: 1998AAS...193.9906R
Altcode: 1998BAAS...30R1396R
We report on FUV observations of stellar flares on the dM0e star AU
Microscopii obtained on 1998 Sept 06 with the Space Telescope Imaging
Spectrograph (STIS) aboard the Hubble Space Telescope. The data consist
of medium resolution echelle observations covering the wavelength region
from 1170-1730 Angstroms with a resolution of 45,800. The observations
were obtained using the TIME-TAG observing mode in which the time and
position of each detected photon is recorded. This allows an analysis
of variability which is limited only by the counting statistics of the
data. During a total on-source time of 10,000 s we observed numerous
microflare bursts as well as 4 well defined flare events. These flares
lasted between 1 and 3 minutes and and were most easily observed in
the FUV continuum and the Si IV and C IV resonance lines. Variations
in both the cooler (e.g. Lyalpha , C II, O I) and hotter (O V, N V, Fe
XXI) emission lines were much less pronounced. In this paper we will
examine the physical characteristics of the main flare events. This
discussion will include the time history of the wavelength integrated
fluxes in the continuum and various emission lines. In particular, we
search for increased emission in the red wing of the Lyalpha line which
would indicate the presence of moderately energetic proton beams. We
also integrate the time sequences over the entire flare duration and
investigate the average line and continuum properties during each of
the events, including flows and turbulence as a function of temperature
as well as the shape of the FUV continuum. Unfortunately, none of
the events was strong enough to allow a detailed examination of line
profiles as a function of time. This work is supported by NASA grants
to the Catholic University of America and the University of Colorado.
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: HST/STIS Echelle Spectra of the dM0e Star AU MIC Outside
of Flares
Authors: Linsky, J. L.; Robinson, R. D.; Carkner, L.; Woodgate, B.;
Timothy, G.
Bibcode: 1998AAS...193.4510L
Altcode: 1998BAAS...30.1318L
We report on observations of the dM0e flare star AU Microscopii
(HD197481) obtained with the Space Telescope Imaging Spectrograph
(STIS) on 1998 Sept 6. We analyze 10105 seconds of observations
obtained with the medium resolution E140M grating, which covers the
complete 1170--1730 Angstroms spectral range with a resolution of
45,800 (corresponding to 6.6 km/s). The data were obtained in the
TIME-TAG mode to isolate time intervals of flaring. We report here
on the 9200 seconds of data for which the star did not have a large
flare. In this beautiful data set we identify more than 70 emission
lines including low temperature chromospheric lines (e.g., C I, O I),
transition region lines (e.g., C II--IV, N IV, O III-V, Si II-IV),
and the coronal line Fe XXI 1354 Angstroms. There are a number of
intersystem lines that are useful for measuring electron densities. We
discuss line redshifts and the broad wings of transition region lines
that provide evidence of microflare heating. We also discuss the shapes
of the Lyman-alpha and He II 1640 Angstroms lines, and the interstellar
absorption features seen in the H I, D I, C II, and Mg II resonance
lines. This work is supported by NASA grants to the University of
Colorado and the Catholic University of America.
Title: Evidence for Magnetically-Controlled Flows above Gamma CAS
(B0.5e)
Authors: Smith, M. A.; Robinson, R. D.
Bibcode: 1998AAS...193.9902S
Altcode: 1998BAAS...30Q1395S
To understand the cause of reported rapid UV variations from the B0.5e
star gamma Cas, we obtained continuous HST/GHRS large-aperture spectra
centered on the SiIV 1394A, 1403A lines over a timespan of 22.5 hours
and at a cadence of 1 per minute. In previous work we found that the
star has multiple UV/X-ray activity centers which rotate into view
every rotation cycle, estimated as 1.12 days. An ultraviolet light
curve constructed from our spectra shows two 1 clouds forced into
corotation. We examine herein spectral absorption line variations apart
from the photospheric/DAC SiIV lines. These features vary on timescales
from minutes to several hours. From Hubeny model atmosphere codes, we
identify these features as optically thick absorptions from several
ions formed in ``cool" (T < 10,000K), ``warm" (10,000--18,000K),
and ``hot" (> 30,000K) sources. The cool/hot plasma lines vary in
phase with the light curve whereas the light curves from warm ions
lead this curve by 3 hours. All these lines exhibit either of two
types of kinematical behavior: (1) blue-to-red motion consistent
with plasma trapped incorotating clouds or (2) uniform motion of
hot plasma over several hours at either low or high ( +1500 km/s)
velocities. The existence of cool/hot plasmoids above gamma Cas
implies the importance of surface magnetic processes. We suggest
that the high constant-velocity features arise from mass ejections
triggered by interactions between magnetic loops from the star and
the circumstellar decretion disk. This picture might explain the hot,
flaring character of this star's X-rays and predicts the existence
of other gamma Cas-analogs: magnetic, X-ray emitting Be stars with
dense disks.
Title: A Multiwavelength Campaign on γ Cassiopeiae. II. The Case
for Corotating, Circumstellar Clouds
Authors: Smith, Myron A.; Robinson, Richard D.; Hatzes, Artie P.
Bibcode: 1998ApJ...507..945S
Altcode:
Simultaneous X-ray/UV observations over a full day on 1996 March
14-15 have been made of the prototypical B0.5e star γ Cas using the
Rossi X-Ray Timing Explorer satellite and the Goddard High Resolution
Spectrograph (GHRS) on board the Hubble Space Telescope. The GHRS
spectra, taken in the region of the Si IV λλ1394-1403 doublet,
also permitted the construction of an extremely precise light curve
from a nearby ``pseudocontinuum'' region. The continuum UV and X-ray
light curves reveal a pair of X-ray maxima ~10 hr apart that coincide
in time with UV continuum flux ``dips'' of ~1%. In the first paper in
this series we attributed the long-term X-ray variations to magnetic
activity sites on the surface of the star that undergo rotational
modulation on a ~1.125 days period. In the current study we find that
flux and color curves generated from a 33 hr sequence of International
Ultraviolet Explorer (IUE) echellegrams obtained in 1996 January display
dip features similar to those in the GHRS data. Comparing the timings of
the continuum flux dips and the Si IV line strength variations in both
the GHRS and IUE data sets gives a slightly revised period of 1.123 days
for both the UV and X-ray activities. This strengthens the argument that
high-energy activity on γ Cas is modulated by rotation of long-lived
structures close to its surface. Analysis of the pseudocontinuum
light curves constructed from the GHRS and IUE light curves shows at
least two surprising characteristics for the flux dips: (1) the dips
last only ~0.3 cycles, which is too brief for rotation modulation of
surface features, and (2) their amplitudes increase from long to short
wavelengths, which attain a maximum near 1206 Å. The character of the
variations of the photospheric Si IV line profiles is unexpected in
that the equivalent width fluctuations do not correlate with the slow
undulations of the continuum flux. Moreover, the profile variations do
not show an expected blue-to-red migration of microfeatures. We show
that the continuum characteristics and absence of migration of features
in the Si IV lines can be explained by the presence of very cool,
optically thin clouds that corotate with the star. Assuming a tilt
of the rotational axis of +45° to the observer's line of sight, our
model simulations of the two major dips in the UV light curves indicate
that the clouds have radii of a few tenths of a stellar radius and are
attached to points on the surface at low to mid-latitudes on the near
hemisphere. These findings support the conclusion of the first paper
in this series that γ Cas is a member of a small group of OB stars
that have magnetospheres associated with X-ray activity.
Title: Toward a Mass for the High Luminosity Cepheid T MON
Authors: Evans, Nancy Remage; Robinson, Richard D.; Carpenter,
Kenneth G.; Vinko, Jozsef; Massa, Derck; Wahlgren, Glenn M.
Bibcode: 1998AAS...192.8204E
Altcode: 1998BAAS...30.1155E
The long period (27(d) ) Cepheid T Mon was shown by Coulson (1983,
MNRAS, 203, 925) to have orbital motion. A full orbit has not yet been
observed since the first velocities nearly a century ago. However, the
ratio of the orbital velocity changes during the last decade provide the
mass ratio between the Cepheid and its hot companion. We have obtained
new ground-based velocities of the Cepheid which constrain the orbital
velocity amplitude. To measure the companion velocities, we have IUE
high resolution spectra and also a Hubble Space Telescope Goddard High
Resolution Spectrograph spectrum (1840 to 1880 Angstroms). Use of the
new data to constrain the mass of the Cepheid will be discussed.
Title: Goddard High-Resolution Spectrograph Observations of Cool
Low-Gravity Stars. IV. A Comparison of the K5 III stars α Tauri
and γ Draconis
Authors: Robinson, Richard D.; Carpenter, Kenneth G.; Brown, Alexander
Bibcode: 1998ApJ...503..396R
Altcode:
The Goddard High-Resolution Spectrograph (GHRS) has observed the K5 III
star α Tau in the 2330 Å region on three separate occasions. These
spectra show marked changes with time, with the UV continuum varying
by a factor of 2, and with the emission lines changing in flux by 30%
or more, with the amount of change dependent upon the opacity of the
line. The variations suggests a restructuring of the atmosphere rather
than simply a change in the surface area covered by chromospheric
material. Surprisingly, there was no detectable change in the
chromospheric turbulence on timescales of hours or years. On average,
the lower part of the atmosphere was found to be fairly static, with
a slight infall of 1-2 km s-1. At higher altitudes, probed
by observation of the stronger Fe II lines as well as of the O I (UV 2)
and Mg II (UV 1) resonance lines, there is evidence for the acceleration
of a slow wind, similar to that seen in the M giants γ Cru and μ
Gem. This wind is much less massive than for the later type giants,
however, since its effects are seen in only the most optically thick
of the Fe II lines. Comparison of the α Tau observations with similar
data for the K5 III hybrid star γ Dra shows remarkable similarity in
the photosphere and lower chromosphere. Both stars have pronounced
UV continua, identical turbulences and chromospheric densities, and
very similar line fluxes and profiles for all lines formed in the
lower chromosphere, including C II], Co I, Si II], and Fe II. A deep
exposure near 1500 Å also shows the first evidence for hot plasma
in the atmosphere of α Tau through the detection of the C IV (UV 1)
doublet with a surface flux about 30% of that observed in γ Dra. Most
of the evidence for the stellar wind is in the Mg II (UV 1) and O I
(UV 2) resonance lines. Modeling these lines using the Sobolev with
Exact Integration (SEI) radiative transfer code shows that the wind in
γ Dra accelerates faster and reaches a higher terminal velocity than
does the wind in α Tau. However, the wind turbulent velocity in γ Dra
is only about one-third of the value seen in α Tau. We conclude that
the observations support the suggestion by Judge & Stencel that the
processes that heat the chromosphere are distinct from those that drive
the stellar winds. 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: A Multiwavelength Campaign on γ Cassiopeiae. I. The Case
for Surface X-Ray Flaring
Authors: Smith, Myron A.; Robinson, Richard D.; Corbet, Robin H. D.
Bibcode: 1998ApJ...503..877S
Altcode:
In 1996 March we obtained simultaneous Rossi X-Ray Timing Explorer
RXTE Proportional Counter Array (PCA) and Hubble Space Telescope (HST)
Goddard High-Resolution Spectrograph (GHRS) light curves for the B0.5e
star γ Cas in order to compare its X-ray and ultraviolet continuum
flux behaviors. The GHRS data set consisted of a nearly continuous
sequence of UV spectra covering a 21+ hr interval. Each 40 Å spectrum
was centered on the Si IV λλ1394-1403 lines and registered 8100
counts in each 1 s exposure. Combining spectra and integrating over
>100 continuum pixels allowed us to define a UV continuum light
curve binned to 1 minute with a signal-to-noise ratio of a few thousand
pixel-1. We found that the light curve exhibited variations
over a time comparable to the rotation period of the star, showing
two broad minima 10 hr apart, which had depths of 0.8% and 1.8%. The
long-term trends in the UV are anticorrelated with the X-ray fluxes,
with the X-rays exhibiting increases of ~10% and ~40% during times of
UV flux minima. The stability of the long-term X-ray variations on γ
Cas is supported by phasing our March data with contemporaneous ASCA
data, suggesting a possible period of 1.125 days (or a close alias). We
also get agreement of dip patterns for an assumed 1.123 day period by
phasing the GHRS continuum flux curve with IUE light curves in various
wavelengths from 2 months earlier. We take this as an estimate of the
star's rotational period. We conclude that the X-ray emission from
γ Cas probably consists of two components. The first is a slowly
varying ``basal'' flux representing the minimum level seen during any
given phase. Superimposed on this are rapid fluctuations (``shots'')
that have lifetimes ranging from <10 s to >=10 minutes. The
character of these components varies from one spacecraft orbit to
the next, indicating that the emissions are not produced in a truly
``stationary'' chaotic environment. Moreover, both the number and
amplitude of the shots increase during UV minima. The shot profiles
are typically symmetric and can have decay times of a few seconds
or less. The shots also have a slightly harder flux distribution
than the basal component, suggesting that the two emission regions
are not cospatial. The time-averaged X-ray spectrum indicates a quasi
temperature of ~108 K, in agreement with earlier studies. We
present a picture in which magnetically generated structures on and
over the star's surface are responsible for the basal and shot X-ray
components. The energies and luminosities of the shots are so high
that even the weakest events we measure are comparable in strength to
the most luminous flares on cool active stars. Using general cooling
relations for a thermal plasma, one finds that the source region
for the shots probably have a size scale of <=104 km
and densities of ~1013-14 cm-3. From a simple
flare model, we find that generally only a small fraction of the shot
energy is radiated during the event itself. The remainder of the hot
plasma expands to fill a confined volume, possibly a magnetic loop,
connected to the original flare site. A collection of these loops may
then account for the basal emission. With this model, we estimate that
the individual loops have a characteristic density of ~1011
cm-3 and dimensions of >=0.1R*. We note that
the magnetic interpretation for the shot and basal emission poses
several theoretical questions, such as how complex, dynamic fields
can exist on a star that does not have a convective envelope. These
results suggest that γ Cas is a member of an arguably new group of
hot stars that flare continuously in X-rays. This group may represent
an extension of the hotter Bp stars to high values of rotation.
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: The Goddard High Resolution Spectrograph: Post-COSTAR
Characteristics
Authors: Robinson, R. D.; Ake, T. B.; Lindler, D. J.; Heap, S. R.;
Carpenter, K. G.; Leckrone, D. S.; Maran, S. P.; Smith, A. M.; Brandt,
J. C.; Beaver, E. A.; Boggess, A.; Ebbets, D. C.; Hutchings, J. B.;
Jura, M.; Linsky, J. L.; Savage, B. D.; Trafton, L. M.; Walter, F. M.;
Weymann, R. J.
Bibcode: 1998PASP..110...68R
Altcode:
We review changes to the characteristics of the Goddard High Resolution
Spectrograph (GHRS) which resulted from the installation of the
Corrective Optics Space Telescope Axial Replacement (COSTAR) on the
Hubble Space Telescope. The introduction of two new optical elements
into the light path altered the spectral distribution of the light,
decreasing the amount of light striking the instrument by about 30%
at wavelengths greater than 1200 Å and effectively eliminated all
radiation at wavelengths less than 1130 Å. However, at the longer
wavelengths the improved focus offset this loss when the Large Science
Aperture (LSA) was used and increased the overall throughput of the
Small Science Aperture (SSA) by a factor of 2. The improved focus also
enhanced the spectral resolution of LSA observations and improved the
ability of the instrument to observe in crowded fields.
Title: Ultraviolet Absorption Lines from High-Velocity Gas in the
VELA Supernova Remnant: New Insights from Space Telescope Imaging
Spectrograph Echelle Observations of HD 72089
Authors: Jenkins, Edward B.; Tripp, Todd M.; Fitzpatrick, Edward L.;
Lindler, Don; Danks, Anthony C.; Beck, Terrence L.; Bowers, Charles W.;
Joseph, Charles L.; Kaiser, Mary Elizabeth; Kimble, Randy A.; Kraemer,
Stephen B.; Robinson, Richard D.; Timothy, J. Gethyn; Valenti, Jeff
A.; Woodgate, Bruce E.
Bibcode: 1998ApJ...492L.147J
Altcode: 1997astro.ph.10279J
The star HD 72089 is located behind the Vela supernova remnant and
shows a complex array of high- and low-velocity interstellar absorption
features arising from shocked clouds. A spectrum of this star was
recorded over the wavelength range 1196.4-1397.2 Å at a resolving
power of λ/Δλ = 110,000 and a signal-to-noise ratio of 32 by the
Space Telescope Imaging Spectrograph on the Hubble Space Telescope. We
have identified seven narrow components of C I and have measured their
relative populations in excited fine-structure levels. Broader features
at heliocentric velocities ranging from -70 to 130 km s-1 are
seen in C II, N I, O I, Si II, S II, and Ni II. In the high-velocity
components, the unusually low abundances of N I and O I, relative
to S II and Si II, suggest that these elements may be preferentially
ionized to higher stages by radiation from hot gas immediately behind
the shock fronts. Based on observations with the NASA/ESA Hubble
Space Telescope obtained at the Space Telescope Science Institute,
which is operated by the Association of Universities for Research in
Astronomy (AURA), Inc., under NASA contract NAS5-26555. The analysis
was supported by NASA grant NAG5-30110 to Princeton University.
Title: Critical IUE Coverage of a Rotational Cycle of Gamma CAS
Authors: Smith, M. A.; Robinson, R. D.
Bibcode: 1998ESASP.413..281S
Altcode: 1998uabi.conf..281S
No abstract at ADS
Title: Fluorescence in the Chromosphere of alpha Tau (K5 III)
Authors: McMurry, A. D.; Jordan, C.; Carpenter, K. G.; Robinson, R. D.
Bibcode: 1998ASPC..154.1293M
Altcode: 1998csss...10.1293M
The Goddard High Resolution Spectrograph (GHRS) on the Hubble Space
Telescope has been used to obtain spectra of alpha Tau covering many
parts of the uv spectrum between 1200AA and 2000AA. Here we concentrate
on the region between 1320AA and 1390AA, which is covered by 2 medium
resolution exposures. Most of the lines appearing in these spectra have
been identified, including 2 bands of fluorescent CO lines. Preliminary
modelling of these CO lines has been carried out, and the results are
compared with the observations.
Title: The On-Orbit Performance of the Space Telescope Imaging
Spectrograph
Authors: Kimble, R. A.; Woodgate, B. E.; Bowers, C. W.; Kraemer,
S. B.; Kaiser, M. E.; Gull, T. R.; Heap, S. R.; Danks, A. C.; Boggess,
A.; Green, R. F.; Hutchings, J. B.; Jenkins, E. B.; Joseph, C. L.;
Linsky, J. L.; Maran, S. P.; Moos, H. W.; Roesler, F.; Timothy,
J. G.; Weistrop, D. E.; Grady, J. F.; Loiacono, J. J.; Brown, L. W.;
Brumfield, M. D.; Content, D. A.; Feinberg, L. D.; Isaacs, M. N.;
Krebs, C. A.; Krueger, V. L.; Melcher, R. W.; Rebar, F. J.; Vitagliano,
H. D.; Yagelowich, J. J.; Meyer, W. W.; Hood, D. F.; Argabright,
V. S.; Becker, S. I.; Bottema, M.; Breyer, R. R.; Bybee, R. L.;
Christon, P. R.; Delamere, A. W.; Dorn, D. A.; Downey, S.; Driggers,
P. A.; Ebbets, D. C.; Gallegos, J. S.; Garner, H.; Hetlinger, J. C.;
Lettieri, R. L.; Ludtke, C. W.; Michika, D.; Nyquist, R.; Rose, D. M.;
Stocker, R. B.; Sullivan, J. F.; Van Houten, C. N.; Woodruff, R. A.;
Baum, S. A.; Hartig, G. F.; Balzano, V.; Biagetti, C.; Blades, J. C.;
Bohlin, R. C.; Clampin, M.; Doxsey, R.; Ferguson, H. C.; Goudfrooij,
P.; Hulbert, S. J.; Kutina, R.; McGrath, M.; Lindler, D. J.; Beck,
T. L.; Feggans, J. K.; Plait, P. C.; Sandoval, J. L.; Hill, R. S.;
Collins, N. R.; Cornett, R. H.; Fowler, W. B.; Hill, R. J.; Landsman,
W. B.; Malumuth, E. M.; Standley, C.; Blouke, M.; Grusczak, A.; Reed,
R.; Robinson, R. D.; Valenti, J. A.; Wolfe, T.
Bibcode: 1998ApJ...492L..83K
Altcode:
The Space Telescope Imaging Spectrograph (STIS) was successfully
installed into the Hubble Space Telescope (HST) in 1997 February,
during the second HST servicing mission, STS-82. STIS is a versatile
spectrograph, covering the 115-1000 nm wavelength range in a variety
of spectroscopic and imaging modes that take advantage of the angular
resolution, unobstructed wavelength coverage, and dark sky offered by
the HST. In the months since launch, a number of performance tests and
calibrations have been carried out and are continuing. These tests
demonstrate that the instrument is performing very well. We present
here a synopsis of the results to date.
Title: Winds and Mass-Loss from Evolved, Low-Gravity Cool Stars
Authors: Carpenter, K. G.; Robinson, R. D.
Bibcode: 1998HiA....11..378C
Altcode:
No abstract at ADS
Title: Searching for the Cause of Hybrid Star Activity
Authors: Robinson, Richard D.; Carpenter, Kenneth G.; Brown, Alexander
Bibcode: 1998ASPC..154.1307R
Altcode: 1998csss...10.1307R
UV spectra for a wide range of chromospheric diagnostics have been
obtained for the non-coronal giant alpha Tau and the `hybrid' star
gamma Dra using the Goddard High Resolution Spectrograph. These stars
have very similar spectral types and are very close to one another in
mass, luminosity, size, effective temperature and gravity. A detailed
comparison shows that the photosphere and chromosphere of the stars
are very similar. The two stars show the same level of UV continuum
emission and chromospheric turbulence and have Fe 2 emission profiles
which are nearly identical. The amount of transition region plasma,
as measured by the C 4 surface flux, is also nearly the same. The winds
of these two stars, however, are significantly different. Preliminary
models based on the O 1 (UV 2) and Mg 2 (UV 1) profiles show that
the wind for alpha Tau has a terminal velocity of ~30 km s^{-1} and
a much slower acceleration than the wind of gamma Dra, which has a
terminal velocity of ~65 km s^{-1}. However, despite the different wind
properties, the mass loss rate from these two stars is very similar.
Title: Atmospheric Dynamics of Luminous Late-Type Stars
Authors: Airapetian, V. S.; Ofman, L.; Robinson, R. D.; Carpenter,
K.; Davila, J.
Bibcode: 1998ASPC..154.1569A
Altcode: 1998csss...10.1569A
We present first results of magnetohydrodynamic (MHD) calculations of
winds from luminous late-type stars using an existing, 2.5D, non-linear
MHD code recently developed by Ofman & Davila (e.g., Ofman &
Davila 1997). We assume that the wind is initiated in a hydrostatic
atmosphere with an isothermal pressure scale height of 0.072 R* and a
``chromospheric hole'' modeled by a transverse density structure and
a radial magnetic field. To ensure that we are accurately assessing
the terminal velocity of the wind, we carried out the calculations
to a height of 20 stellar radii. We find that in the higher density
(low Alfven velocity) regions outside of the ``chromospheric hole'' the
Alfven waves are freely propagating. Ponderomotive forces associated
with these waves drive radial, compressive motions and contribute to
stellar wind acceleration. The compressive motions then excite slow
magnetosonic waves which non-linearly steepen into solitary waves that
propagate on top of a background flow. This situation is similar to
solar coronal hole models. In the lower density ``chromospheric hole''
region the Alfven wave are strongly reflected, and produce a substantial
outflow, with both radial and azimuthal velocities approaching the
local Alfven speed. Our results are in qualitative agreement with
observational signatures of winds in cool, luminous late-type stars.
Title: HST Observations of Carbon Stars
Authors: Carpenter, Kenneth G.; Robinson, R. D.; Johnson, H. R.
Bibcode: 1998ASPC..154.1578C
Altcode: 1998csss...10.1578C
Ultraviolet spectra obtained with the Goddard High Resolution
Spectrograph (GHRS) on the HST of the carbon stars TX Psc, TW Hor,
and UU Aur are presented and compared with each other and with spectra
of the oxygen-rich cool giants mu Gem and 30 Her. For TX Psc, we
discuss the inferred outer atmospheric flow and turbulent velocities,
the relative and absolute strength of emission from C 2 and Fe 2,
the ionization fraction of iron, and variations of its spectrum with
time. We also discuss two fluorescence processes operating in the
atmospheres of carbon stars and compare in detail the Mg 2 profiles
seen in both carbon and O-rich stars.
Title: Digging Deeper in the Coronal Graveyard
Authors: Ayres, Thomas R.; Brown, Alexander; Harper, G. M.; Bennett,
P. D.; Linsky, J. L.; Carpenter, K. G.; Robinson, R. D.
Bibcode: 1997ApJ...491..876A
Altcode:
Soft X-ray detections of stellar coronae (T ~ 106 K) are
rare in the giant branch redward of ~K1 III. We have conducted a less
direct--but more sensitive--search using the Hubble Space Telescope
Goddard High-Resolution Spectrograph to probe for far-ultraviolet
proxies of the hot coronal gas in representative ``noncoronal''
red giants. In every target so far examined, we find weak but
statistically significant Si IV emission, as well as, commonly,
C IV and, in some cases, N V. Si IV is not affected by the CNO
anomalies produced by the first dredge-up, which can deplete the
carbon abundance and weaken C IV. In the low-activity giants,
the λ1393 component of the Si IV doublet must be corrected
for sharp absorptions, which we believe are caused by carbon
monoxide in overlying cool material. The normalized flux ratios
(\Rscr≡f/fbol) of Si IV and X-rays among the ``coronal''
yellow giants (lying just blueward of the ``noncoronal'' zone) fall on a
uniform track, \RscrX~\Rscr2SiIV. In
the noncoronal zone, however, the Si IV index is nearly constant
(\RscrSiIV~10-8), independent of \RscrX
(which ranges from ~10-8 to <~10-10). The
mechanism that diminishes X-ray activity in the red giants is highly
sensitive to an as yet unidentified stellar property. Photoelectric
absorption by cool gas might play a more important role than previously
suspected, particularly if hot magnetic loops are partly or completely
buried in the chromosphere.
Title: The Case for X-Ray Flaring in gamma CAS
Authors: Smith, M. A.; Robinson, R. D.; Corbet, R. H. D.
Bibcode: 1997AAS...191.9304S
Altcode: 1997BAAS...29Q1362S
To address whether the well-known, 10(8) K X-rays from gamma Cas are
emitted from the Be star or a degenerate companion, we have conducted
simultaneous observations with the RXTE/PCA and HST/GHRS satellites
over nearly a full day on 1996 March 14-15. A quasi-continuum light
curve formed at lambda 1400 shows two ~ 1% dips separated by about
10 hours. The X-ray flux curve is dominated by two variabilities,
slow undulations on the rotational timescale and a rapid one with a ~
1/f power spectrum. The slow X-ray undulations shows two peaks that
coincide in time with two UV light dips, which suggests the X-rays
arise from rotational modulation. The stability of this component
has been verified by phasing our data with contemporaneous ASCA
data, resulting in a period of 1.125 days. Also, IUE observations
two months earlier show flux and color variations near the phases
of the GHRS observations. These match if the period is adjusted to
1.123 days and we take this as the star's rotational period. The
X-ray emission consists of a ``basal" component, representing the
minimum flux observed at any phase, and of rapidly varying ``shots"
with lifetimes ranging from <=10 s to several minutes. The shots
have a slightly harder energy distribution, so these two components are
probably not emitted cospatially. The average symmetric profile and the
short lifetimes of the shots indicates they are formed in a high density
region. From a simple flare model, we show that the site of the shots
has a density of >=10(14) cm(-3) and a scale of <=10(4) km. Most
of the shot energy is not directly radiated but rather expands into a
basal-emitting "canopy" with a density of <=10(11) cm(-3) and a scale
<=0.1R_{*}. These results argue that gamma Cas is a member of a new
group of upper main sequence stars which continuously flare in X-rays.
Title: Pondering Puzzling Profiles: MG II Emission from Carbon Stars
Authors: Carpenter, K. G.; Robinson, R. D.; Johnson, H. R.
Bibcode: 1997AAS...191.1306C
Altcode: 1997BAAS...29.1230C
We have obtained spectroscopic observations of the Mg II h & k
emission lines from three carbon stars, using the moderate resolution
(R>=20,000) G270M grating of the Goddard High Resolution Spectrograph
(GHRS) on the Hubble Space Telescope (HST). The emission profiles
from all three stars are very broad and heavily mutilated by overlying
absorption. The profiles from two of the stars, TX Psc (N0; C6,2) and
TW Hor (N0; C7,2), are very similar. However, the profiles from the
third star, UU Aur (N3; C5,3) are dramatically different from the other
two stars on the blue side, with strong emission persisting out to ~
-200 km/s (cf. to only weak emission out to ~ -100 km/s in TX Psc and
TW Hor). If the intrinsic (i.e. chromospheric) width of the emission in
the carbon stars is comparable to that in oxygen-rich giants, then the
spectra of the first two carbon stars are easily understood in terms of
additional overlying absorptions, but the amount of excess emission from
the blue side of the UU Aur profiles is difficult to explain. If, on
the other hand, the intrinsic width of the Mg II emission in the carbon
stars is more comparable to that of the supergiant oxygen-rich stars
(e.g. alpha Ori), then all the carbon stars have tremendous overlying
absorption on both the blue and red wings of the profiles and UU Aur
is notable only because it has significantly less absorption on the
blue-side of the profiles, relative to the other carbon stars. In this
paper, we present an analysis of the spectra and our interpretation,
based on all the available data, of these fascinating and complex line
profiles and their formation.
Title: Hubble Space Telescope Spectroscopy of the Carbon Star
TX Piscium
Authors: Carpenter, Kenneth G.; Robinson, Richard D.; Johnson, Hollis
R.; Eriksson, Kjell; Gustafsson, Bengt; Pijpers, Frank P.; Querci,
Francois; Querci, Monique
Bibcode: 1997ApJ...486..457C
Altcode:
Ultraviolet spectra obtained with the Hubble Space Telescope of the
carbon star TX Piscium (HR 9004) are presented, along with analysis
providing information on its outer atmosphere, including flow and
turbulent velocities, line formation mechanisms, and variations with
time. Both thermal (collisionally excited) and fluorescent emission
from the chromosphere of the star appear to be formed near the stellar
rest velocity, i.e., in a region below that in which the stellar wind is
accelerated. Absorption self-reversals in the Mg II emission confirm the
presence of an outflowing stellar wind at a mean velocity of about 9-10
km s-1. Circumstellar absorption features (Mn I and Fe I)
overlying the Mg II emission indicate a cool shell expanding at about
5-6 km s-1 relative to the photosphere. The widths (FWHM)
of various emission lines indicate that the chromospheric turbulence is
at least 16 km s-1, but that it may increase with altitude
to as much as 34 km s-1. Three hours of integration on
the C II] lines are examined for any signs of variability that might
indicate the presence of shocks, but no statistically significant
variations are seen. A previous identification (in spectra of UU Aur)
of an emission line at 2807 Å, seen only in spectra of carbon stars,
as belonging to Fe I multiplet UV45 pumped by the C II] line at 2325 Å
is confirmed by the discovery of an absorption feature corresponding
exactly to the wavelength of the pumped transition (Fe I UV13) near
2325 Å. Lines from Fe II UV165, previously seen in solar off-limb
spectra and in Goddard High-Resolution Spectrograph spectra of α Tau,
are clearly present. The normally much stronger Fe II UV32, 62, and
63 multiplets are seen but are weaker relative to both the UV165 lines
and the intercombination lines of C II] and Si II] than in α Tau. The
weakness of these Fe II lines is indicated both by their absolute flux
levels and by their narrow, single-peaked profiles, which are in sharp
contrast to the broad, double-peaked profiles seen in oxygen-rich cool
giant and supergiant stars. The weakness of the Fe II lines and the
presence of the Fe I 2807 Å line suggest that the ionization fraction
of iron (Fe II/Fe I) is significantly lower in the outer atmospheres of
carbon stars. Fluxes in emission lines of Fe II and Mg II are >=2-3
times lower than in a 1984 IUE spectrum of TX Psc, confirming that
the latter was obtained at an epoch of unusual UV brightness for the
star. The Mg II profiles are heavily mutilated by overlying absorption,
even more so than in 1984. The TX Psc profiles are very similar to
those seen in the carbon star TW Hor but are dramatically different
than those in another carbon star, UU Aur, whose lines show violet
wing emission out to much shorter wavelengths than in the other two
stars. 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 at the Haute-Provence Observatory.
Title: GHRS Observations of Cool Low-Gravity Stars. III. Plasma
Flows and Turbulence in the Outer Atmosphere of α Orionis (M2 Iab)
Authors: Carpenter, Kenneth G.; Robinson, Richard D.
Bibcode: 1997ApJ...479..970C
Altcode:
We present the results of a Goddard High Resolution Spectrograph (GHRS)
study of flow and turbulent velocities in the outer atmosphere of the
M2 Iab supergiant α Ori. Ions with observed chromospheric emission
features include C I, C II, Si I, Si II, O I, Co II, Al II, Mg II,
Cr II, Fe I, and Fe II, while the photospheric absorption spectrum
is dominated by Fe I. The widths of optically thin lines of C II],
Si II], Co II, and Fe II indicate an average chromospheric turbulent
velocity (Doppler FWHM) in the range of 31-35 km s-1, which
is substantially above the expected chromospheric sound velocity. The
shape of the C II] profiles indicates that this turbulence is probably
anisotropically distributed, with velocities preferentially directed
along and/or perpendicular to the radial direction. The radial
velocity of near-UV Fe I photospheric absorption lines averaged 18
km s-1, somewhat smaller than the systemic RV of α Ori
itself (21 km s-1) and significantly smaller than the RV
of the optical Fe I lines (23 km s-1) at the time of the
HST observations. The various components of the chromospheric emission
lines had radial velocities in the range 7-36 km s-1. The
emission wings for most of the ionic species averaged ~20 km
s-1, i.e., they were nominally at rest with regard to
the star. The central absorption features in these lines, however,
showed lower velocities, averaging ~16 km s-1. The radial
velocity of the central reversals in the stronger lines was also found
to be correlated with the opacity of the lines, changing from ~20 km
s-1 for the lower opacity lines to ~14 km s-1
for the higher opacity lines. This implies that we are directly viewing
the acceleration of the stellar wind in the chromosphere from rest to
about 7 km s-1. Contrary to the results reported earlier
by Carpenter (1984) on the basis of IUE data, there is no indication
of a deceleration of the wind at large distances from the star.
Title: Fishing in the Coronal Graveyard
Authors: Ayres, T. R.; Brown, A.; Harper, G. M.; Bennett, P. D.;
Linsky, J. L.; Carpenter, K. G.; Robinson, R. D.
Bibcode: 1996AAS...189.7815A
Altcode: 1996BAAS...28.1379A
Hot coronae (T ~ 10(6) K) are thought to be rare among single giant
stars to the right of the ``Linsky--Haisch dividing line'' near K0 in
the H--R diagram. K and M giants are such slow rotators that absence of
dynamo generated magnetic activity would be natural. Nevertheless,
gamma Dra (K5 III) unexpectedly was detected in FUV coronal
proxies---hot lines Si IV lambda 1393 and C IV lambda 1548---by HST
/GHRS during Science Verification, and subsequently was discovered
as a faint X-ray source in a deep ROSAT /PSPC pointing. Is gamma Dra
anomalous, or is the lack of coronal detections among the K giants
simply a matter of insufficient sensitivity? We have used the GHRS
low resolution mode to search for additional examples of hot lines
among inactive single red giants. Si IV provides a clean diagnostic
of subcoronal material because it falls near the peak sensitivity
of the G140L mode and does not suffer from abundance depletions that
can affect C IV in red giants. X-ray/Si IV ratios are such that HST
can reach to much fainter limiting ``coronal'' magnitudes than even
very deep ROSAT pointings. In every target so far examined, we find
weak---but statistically significant---Si IV emission. These include:
the ancient red giant Arcturus (alpha Boo: K1 III), recorded at the
end of Cycle 5; and epsilon Crv (K2.5 III) and epsilon Sco (K2 III)
observed in Cycle 6. X-ray/Si IV ratios of red giants (for which
measurements, or upper limits, of both diagnostics are available)
fall on a uniform track, extending downward from active K0 ``Clump''
giants like beta Ceti all the way to Arcturus itself, in the depths of
the ``coronal graveyard.'' The systematic behavior argues that magnetic
dynamo action continues even when long term angular momentum loss has
slowed the stellar spin to a crawl. This work was supported by grant
GO-06066.01-94A from STScI.
Title: Observing Stellar Coronae with the Goddard High Resolution
Spectrograph. II. The RS CVn Binary System HR 1099
Authors: Robinson, R. D.; Airapetian, V. S.; Maran, S. P.; Carpenter,
K. G.
Bibcode: 1996ApJ...469..872R
Altcode:
We report time series observations of the RS CVn star HR 1099 taken
with the Goddard High Resolution Spectrograph onboard the Hubble Space
Telescope. The data cover a wavelength range from 1342 to 1375 Å and
show a measurable continuum, as well as emission lines of O I, C I,
CII, Fe II, O V, and Fe XXI. The chromospheric and transition region
features are seen only in the active K1 IV component of the binary
system, while the Fe XXI (1O7 K) flux may come from both
components, with the active component having the stronger flux. There is
no indication of Fe MI emission, formed at 1.3 x 106 K. The
width of the Fe XXI profile indicates that the corona of the primary
is unlikely to extend to heights greater than 2.3R⋆,
while other indicators suggest that the average loops are really
much smaller, having a length of ∼3 x 1010 cm with an
electron density on the order of 1010 cm-3. Some
evidence for atmospheric turbulence is detected in all of the observed
emission lines. This turbulence initially increases with height, going
from less than 30 km s-1 in the chromosphere to as much
as 150 km s-1 in the transition region. The turbulence
then decreases in the corona, where velocities of less than 65 km
s-1 are indicated. Theoretical fits to the O V profile
also suggest that this turbulence is anisotropically distributed,
with motions directed primarily along or perpendicular to the radial
direction. While admitting the possibility that the atmosphere is
heated by microflare events, we examine an alternative heating process
that involves the damping of MHD turbulence, which might be generated
by nonlinear Alfvén waves or by shocks. Simple calculations indicate
that the observed turbulence is sufficient to account for the transition
region and coronal heating.
Title: Coordinated HST-EUVE observations of YZ CMi
Authors: Robinson, Richard D.
Bibcode: 1996csc..reptQ....R
Altcode:
The observations were taken as part of a coordinated observing campaign
which was designed to study flares on the dMe star YZ CMi. The campaign
also included rapid spectral time sequences in the optical (using the
3.9 m Anglo-Australian Telescope) and UV (using the GHRS aboard the
Hubble Space Telescope and the IUE satellite) and radio observations
taken with the Australia Telescope and VLA. The goal was to investigate
the physical characteristics of the flares in different temperature
regimes with a time resolution which is high enough to investigate
the dynamics of the events.
Title: HST Studies of Carbon and K-M Giant/Supergiant Stars
Authors: Carpenter, K. G.; Robinson, R. D.
Bibcode: 1996swhs.conf..418C
Altcode:
No abstract at ADS
Title: Rapid UV spectroscopy of flares on YZ CMi
Authors: Robinson, R. D.; Woodgate, B. E.; Carpenter, K. G.
Bibcode: 1996ASPC..109..285R
Altcode: 1996csss....9..285R
No abstract at ADS
Title: Identification and analysis of UV emission lines observed near
1550 Angstroms in the spectrum of alpha Tau obtained with the GHRS
Authors: McMurry, A. D.; Jordan, C.; Rowe, A. K.; Carpenter, K. G.;
Robinson, R. D.
Bibcode: 1996ASPC..109..271M
Altcode: 1996csss....9..271M
No abstract at ADS
Title: Examining the coronal heating on the RS CVn binary HR 1099
Authors: Airapetian, V.; Robinson, R. D.; Maran, S. P.; Carpenter,
K. G.
Bibcode: 1996ASPC..109..249A
Altcode: 1996csss....9..249A
No abstract at ADS
Title: The Transition Region, Corona, Chromosphere, and Wind of the
K5 Giant gamma Draconis
Authors: Brown, A.; Carpenter, K. G.; Robinson, R. D.; Harper, G. M.;
Deeney, B. D.; Ayres, T. R.
Bibcode: 1995AAS...18710303B
Altcode: 1995BAAS...27.1430B
Gamma Draconis (K5 III) is the first single, normal late K giant
located on the red side of the coronal ``dividing line'' known to
show conclusive evidence for both hot ( ~ 10(5) K) transition \
region (TR) and coronal (> 10(6) K) plasma. We present HST GHRS
ultraviolet spectra of gamma Dra obtained on 1995 July 20 and 1991
April 6/18. These observations include spectra obtained at low, medium,
and echelle resolution that provide a full set of chromospheric and
TR emission line fluxes and profiles. These are combined with ROSAT
PSPC observations to measure the TR and coronal properties, such
as emission measure distribution, electron density, and nonthermal
velocity fields, of this star. The high temperature emissions of gamma
Dra are compared to those of a sample of hybrid-chromosphere bright
giants and supergiants. This work is supported by Space Telescope
Science Institute grant GO-06068.01-94A and NASA grants NAG5-1792 and
NAGW-4529 to the University of Colorado.
Title: The Astrophysical Laboratory RR Tel: Observations
Authors: Harper, G. M.; Brown, A.; Robinson, R. D.; Jordan, C.;
Carpenter, K. G.; Shore, S. N.
Bibcode: 1995AAS...187.1910H
Altcode: 1995BAAS...27.1313H
The rich emission line spectrum of the bright symbiotic nova RR
Tel provides one of the best laboratories for determining empirical
values of atomic parameters, including branching ratios, Einstein
A-values and electron collision strengths. In addition to providing
atomic parameters not measurable in the laboratory, the observations
also test recent theoretical calculations of these parameters. In July
1995 we obtained high quality low and medium resolution, large aperture
spectra of RR Tel using the Goddard High Resolution Spectrograph (GHRS)
on board the Hubble Space Telescope. These spectra sample wavelengths
from 1100 Angstroms to 2680 Angstroms and contain numerous lines of
astrophysical interest which will be used to derive empirical atomic
data (e.g. branching ratios in C II], N III], O III] and O IV])
as well as lines from which the local temperature and density can
be determined. The GHRS spectra have greater dynamic range and lower
background noise than available high resolution IUE data, permitting
detailed evaluation of spectral diagnostics even for the weak emission
features from some intercombination and forbidden transitions. In this
paper we show examples of the spectra, discuss the atomic transitions
for which atomic data will be derived and describe the methods used in
determining these parameters. RR Tel and AG Peg, both of which have been
observed with the GHRS, are fundamental for studying time variability
of spectra of a symbiotic nova and the GHRS observations will be
compared with previous IUE results to investigate this variability. We
will also discuss the observed flows and turbulence found from the
line profiles as well as the shape of the UV continuum. Support for
this work was provided by NASA through grant number GO-05863.01-94A
from the Space Telescope Science Institute, which is operated by the
Association of Universities for research in Astronomy, Inc., under
NASA contract NAS5-26555.
Title: A Search for Microflaring Activity on dMe Flare
Stars. I. Observations of the dM8e Star CN Leonis
Authors: Robinson, R. D.; Carpenter, K. G.; Percival, J. W.;
Bookbinder, J. A.
Bibcode: 1995ApJ...451..795R
Altcode:
Microflares are frequent, short-duration, energetically weak
disturbances occurring in the nonradiatively heated regions of the
Sun and other magnetically active stars. They are thought to be the
low-energy extension of flares commonly seen on active dMe stars and
may be a major source of heating the chromosphere and corona of cool
stars in general. In this paper we describe rapid time sequence UV
photometry of the dMe star CN Leo taken with the High Speed Photometer
(HSP) aboard the Hubble Space Telescope (HST). The filter was centered
at 240 nm, near wavelengths at which flares are expected to have
maximum intensity and the stellar background is small. During 2 hr of
on-source observing, a total of 32 flarelike events were detected,
with integrated counts ranging from 12 to more than 14,000. In most
cases the events had integrated energy ranging between 1027
and 1028 ergs and can be classified as microflares. A
considerable fine structure was seen in these events, with substantial
variations sometimes occurring on timescales of less than 1 s. The
occurrence rates for the smaller events showed a power-law distribution,
with a slope comparable to that seen for larger events observed from
the ground. Extrapolating the occurrence rate relation to nanoflare
energies indicates a predicted count rate that is significantly smaller
than that observed, suggesting that the nanoflares have a different
energy distribution than the larger events.
Title: The Goddard High Resolution Spectrograph: In-Orbit Performance
Authors: Heap, S. R.; Brandt, J. C.; Randall, C. E.; Carpenter, K. G.;
Leckrone, D. S.; Maran, S. P.; Smith, A. M.; Beaver, E. A.; Boggess,
A.; Ebbets, D. C.; Garner, H. W.; Hutchings, J. B.; Jura, M.; Linsky,
J. L.; Savage, B. D.; Cardelli, J. A.; Trafton, L. M.; Walter, F. M.;
Weymann, R. J.; Ake, T. B.; Crenshaw, D. M.; Malumuth, E. M.; Robinson,
R. D.; Sandoval, J. L.; Shore, S. N.; Wahlgren, G. M.; Bruhweiler,
F.; Lindler, D. J.; Gilliland, R. L.; Hulbert, S. J.; Soderblom, D. R.
Bibcode: 1995PASP..107..871H
Altcode:
The in-orbit performance of the Goddard High Resolution Spectrograph
onboard the Hubble Space Telescope (HST) is presented. This report
covers the pre-COSTAR period, when instrument performance was
limited by the effects of spherical aberration of the telescope's
primary mirror. The digicon detectors provide a linear response
to count rates spanning over six orders of magnitude, ranging from
the normal background flux of 0.01 counts diode ^-1 s^-1 to values
larger than 10^4 counts diode^-1 s^-1. Scattered light from the
first-order gratings is small and can be removed by standard background
subtraction techniques. Scattered light in the echelle mode is more
complex in origin, but it also can be accurately removed. Data
have been obtained over a wavelength range from below 1100 A to
3300 A, at spectral resolutions as high as R = lambda/delta-lambda =
90,000. The wavelength scale is influenced by spectrograph temperature,
outgassing of the optical bench, and interaction of the magnetic field
within the detector with the earth's magnetic field. Models of these
effects lead to a default wavelength scale with an accuracy better
than 1 diode, corresponding to 3 km s^-1 in the echelle mode. With
care, the wavelength scale can be determined to an accuracy of 0.2
diodes. Calibration of the instrument sensitivity functions is tied into
the HST flux calibration through observations of spectrophotometric
standard stars. The measurements of vignetting and the echelle
blaze function provide relative photometric precision to about 5% or
better. The effects of fixed-pattern noise have been investigated,
and techniques have been devised for recognizing and removing it
from the data. The ultimate signal-to-noise ratio achievable with the
spectrograph is essentially limited only by counting statistics, and
values approaching 1000:1 have been obtained. (SECTION: Astronomical
Instrumentation)
Title: An Atlas of Alpha Orionis Obtained with the Goddard High
Resolution Spectrograph on the Hubble Space Telescope
Authors: Brandt, J. C.; Heap, S. R.; Beaver, E. A.; Boggess,
A.; Carpenter, K. G.; Ebbets, D. C.; Hutchings, J. B.; Jura, M.;
Leckrone, D. S.; Linsky, J. L.; Maran, S. P.; Savage, B. D.; Smith,
A. M.; Trafton, L. M.; Walter, F. M.; Weymann, R.; Snow, M.; Randall,
C. E.; Ake, T. B.; Robinson, R. D.; Wahlgren, G.
Bibcode: 1995AJ....109.2706B
Altcode:
An atlas of observations of the late-type supergiant a Orionis taken
with the Goddard High Resolution Spectrograph in 1992 September is
presented. We have included identifications of the major features along
with the fully reduced spectrum. The 33 exposures consist of 3 high
resolution (R∼80,000) and 30 medium resolution (R ∼20,000∼35,000)
observations. The latter provide complete wavelength coverage from
1980 to 3300 Å.
Title: GHRS Observations of Cool, Low-Gravity Stars. II. Flow
and Turbulent Velocities in the Outer Atmosphere of gamma Crucis
(M3.4 III)
Authors: Carpenter, Kenneth G.; Robinson, Richard D.; Judge, Philip G.
Bibcode: 1995ApJ...444..424C
Altcode:
The Goddard High Resoulution Spectrograph (GHRS) on the Hubble Space
Telescope (HST) has been used to obtain medium (R = 20,000) and high
(R = 85,000) resoultion UV spectra of chromosphere emission features
for the M3.4 III star gamma Cru. Small Science Aperture (SSA) G270M and
Echelle-B spectra of selected regions in the 2300-2850 A range were
obtained to determine the kinematics of the chromosphere using lines
of C2), Fe2, Co2, Si1/2), Ni2, Mn2, and Mg2. Profiles of C2) (UV 0.01)
lines and fluorescently excited lines of low optical depth indicate
average turbulent velocities (Doppler FWHM) of 30.2 +/- 1.3 and 28.8 +/-
1.3 km/s, respectively. The fluorescent emission lines (mean RV = 21.3
+/- 0.9 km/s) and the wings of the emission components of Fe2 lines
(mean RV = 22.8 +/- 0.4 km/s) are approximately at rest relative to
the radial velocity of the star (21 km/s), while the C2) lines show
a modest inflow (mean RV = 23.1 +/- 0.9 km/s). The more opaque lines
of Fe2 and Mg2 exhibit complex profiles resulting from line formation
in an optically thick, extended expanding atmosphere. The emission
wings of these lines are broadened by multiple scattering, and they are
centered near the photospheric radial velocity. Closer to line center,
these strong lines show a strong blueshifted self-absorption feature
(already seen in IUE data), indicative of formation in an expanding
chromosphere, and a previously unseen dip in the profiles on the red
side of line center. The absorption components, when extracted using
simple Gaussian fits, show strong correlations with the relative
optical depths of the lines. The derived absorption flow velocities
converge to the photospheric velocity as one examines spectra features
formed deeper in the atmosphere. The blueward abosrption velocity
increases in magnitude from about 7 to 14 km/s with increasing line
optical depth - the strong absorptions directly map the acceleration
of the outflowing stellar wind, while the interpretation of the
weaker redshifted absorptions is more ambiguous, indicating either an
inflow of material or formation in an extended, spherically expanding
outflow. The Mg2 and Fe2 profiles, taken together, imply that the wind
speed decreases between the atmospheric layers where the Mg2 and Fe2
self-absorption components are formed. Interstellar absorptions are seen
in the resonance lines of Mg2 (UV 1) and Fe2 (UV1) with zero-volt lower
levels, at about -3 km/s, consistent with models of the interstellar
medium in the direction of gamma Cru. Finally, we have detected the Mg2
'satellite lines' seen in solar spectra obtained above the limb. In
gamma Cru these lines are probably fluorescently excited by H Ly beta.
Title: Outer Layers of a Carbon Star: The View from the Hubble
Space Telescope
Authors: Johnson, Hollis R.; Ensman, Lisa M.; Alexander, David R.;
Avrett, Eugene H.; Brown, Alexander; Carpenter, Kenneth G.; Eriksson,
Kjell; Gustafsson, Bengt; Jorgensen, Uffe G.; Judge, Philip D.; Linsky,
Jeffrey L.; Luttermoser, Donald G.; Querci, Francois; Querci, Monique;
Robinson, Richard D.; Wing, Robert F.
Bibcode: 1995ApJ...443..281J
Altcode:
To advance our understanding of the relationship between stellar
chromospheres and mass loss, which is a common property of carbon stars
and other asymptotic giant branch stars, we have obtained ultraviolet
spectra of the nearby N-type carbon star UU Aur using the Hubble
Space Telescope (HST). In this paper we describe the HST observations,
identify spectral features in both absorption and emission, and attempt
to infer the velocity field in the chromosphere, upper troposphere,
and circumstellar envelope from spectral line shifts. A mechanism
for producing fluoresced emission to explain a previously unobserved
emission line is proposed. Some related ground-based observations are
also described.
Title: MG II H and K Profiles in High-Luminosity, Late-Type Stars
Authors: Robinson, R. D.; Carpenter, K. G.
Bibcode: 1995ApJ...442..328R
Altcode:
Using high-resolution spectroscopic data taken with the Goddard High
Resolution Spectrographic (GHRS) aboard the Hubble Space Telescope
(HST) and with the International Ultraviolet Explorer (IUE) satellite,
we compare the profiles of the Mg II h and k lines seen in stars with
spectral types ranging from early K through mid-M and luminosities
from giants to supergiants. For all of these stars the lines are
broad emission features with a central absorption. When plotted on a
velocity scale the absorption features of the h and k lines agree very
well in both shape and position, as do the blue wings of the emission
component. The red wings of the emission, however, show a pronounced
difference, with the k line wing consistently shifted to the red of the
h line wing. At present the reason for this discrepancy is unknown,
but we suggest several possibilities, including radiative transfer
effects and high-speed stellar winds.
Title: Fe II Line Profiles as Stellar Wind Diagnostics in M Giant
and Supergiant Stars
Authors: Carpenter, K. G.; Robinson, R. D.
Bibcode: 1995AAS...186.2220C
Altcode: 1995BAAS...27..844C
Detailed study of high resolution, high signal-to-noise chromospheric
Fe II emission line profiles, obtained with the Goddard High Resolution
Spectrograph on the Hubble Space Telescope, provides an opportunity
to probe the flow velocities in the outer atmospheres/winds of cool,
low-gravity stars. We have previously reported seeing the acceleration
of the outflowing wind in such data from several M-giant stars,
including gamma Cru (M3.4 III), mu Gem (M3 IIIab), and alpha Tau (K5
III). Early attempts to do the same with the M-supergiant alpha Ori
(M2 Iab) met with limited success, due to the high-opacity of its outer
atmosphere and the significant differences in the character and behavior
of the observed line profiles in the supergiant spectrum. Further
analysis of the GHRS observations of Fe II lines in alpha Ori has
now enabled us to detect the accelerating outflow in it's wind as
well. A detailed comparison of the observed Fe II profiles and the
flows inferred from those profiles, with those for the less-luminous,
but similar Teff M-giant gamma Cru are presented.
Title: Heating and Ionization of Stellar Chromospheres by Nonthermal
Proton Beams: Implications for Impulsive Phase, Redshifted Lyman-
alpha Radiation in Stellar Flares
Authors: Brosius, Jeffrey W.; Robinson, Richard D.; Maran, Stephen P.
Bibcode: 1995ApJ...441..385B
Altcode:
We investigate the physical basis for the timescale of impulsive-phase,
redshifted Lyman-alpha emission in stellar flares on the assumption
that it is determined by energy losses in a nonthermal proton beam that
is penetrating the chromosphere from above. The temporal evolution
of ionization and heating in representative model chromospheres
subjected to such beams is calculated. The treatment of 'stopping'
of beam protons takes into account their interactions with (1)
electrons bound in neutral hydrogen, (2) nuclei of neutral hydrogen,
(3) free electrons, and (4) ambient thermal protons. We find that, for
constant incident beam flux, the system attains an equilibrium with the
beam energy input to the chromosphere balanced by radiative losses. In
equilibrium, the beam penetration depth is constant, and erosion of the
chromosphere ceases. If the redshifted, impulsive-phase stellar flare
Lyman-alpha emission is produced by downstreaming hydrogen formed
through charge exchange between beam protons and ambient hydrogen,
then the emission should end when the beam no longer reaches neutral
hydrogen. The durations of representative emission events calculated
on this assumption range from 0.1 to 14 s. The stronger the beam, the
shorter the timescale over which the redshifted Lyman-alpha emission
can be observed.
Title: Investigating the Post-COSTAR Point Spread Function for
the GHRS
Authors: Robinson, R. D.
Bibcode: 1995chst.conf..193R
Altcode:
No abstract at ADS
Title: Line Identifications in Goddard High Resolution Spectrograph
Observations of Cool, High-Luminosity Stars
Authors: Carpenter, K. G.; Robinson, R. D.
Bibcode: 1995ASPC...81..559C
Altcode: 1995lahr.conf..559C
No abstract at ADS
Title: Observations of Fe XXI on the RS CVn star HR 1099: deducing
the coronal properties
Authors: Robinson, R. D.; Airapetian, V. S.; Maran, S. P.; Carpenter,
K. G.
Bibcode: 1995IAUS..176P.191R
Altcode:
No abstract at ADS
Title: GHRS and IUE Observations of the Symbiotic Binary CI Cygni
Authors: Jalakas, M.; Stencel, R. E.; Carpenter, K. G.; Robinson, R. D.
Bibcode: 1994AAS...185.2111J
Altcode: 1994BAAS...26.1345J
The interacting symbiotic binary star, CI Cygni, is a system
which consists of a hot star orbiting a red giant. Matter from
the larger star is being transfered to the smaller one. Both IUE
and more recently, the Goddard High Resolution Spectrograph on HST
have detected variable ultraviolet emission lines produced by this
transfer. Phase-dependent changes to the doppler velocity of the
matter allow us to map the direction of the flow. Using the HST
snd IUE spectrometers, we re-examined two different types of lines
(resonance and intercombination) which had been reported to show
velocity differences. We will present an analysis of these data showing
the relative locations of high and low density features in the matter
transfer in this system. The lead authors are pleased to acknowledge
partial support for this effort through NASA grant NAG5-2368 to the
University of Denver.
Title: Coronal Heating Mechanisms: A Search for Microflare Activity
on YZ CMi
Authors: Robinson, R. D.; Carpenter, K. G.; Percival, J. W.
Bibcode: 1994AAS...185.4506R
Altcode: 1994BAAS...26.1380R
We report on time series photometric observations of the dM4.5e flare
star YZ CMi taken in 1993 Nov with the High Speed Photometer aboard
the Hubble Space Telescope. The data consist of five 30 minute time
sequences, with a sampling rate of 0.01s, which were taken through
the F240W filter (centered at 240nm with a 800 Angstroms width). This
setup was selected since the flare energy is expected to be large and
the stellar background is small at these wavelengths. The observations
show a stellar background of 120 counts s(-1) on which are superimposed
well defined flare events ranging in integrated energy from 10(28)
to 10(30) ergs, as well as longer term variations with an amplitude
of up to 50% of the continuum intensity and time scales ranging from
several minutes to hours. These results are compared with data from
the dM8e flare star CN Leo, obtained earlier with the same experimental
setup. CN Leo has a quiescent X-ray flux which is 1/30 that of YZ CMi,
but had a higher occurrence rate of microflares. On the other hand,
the stellar UV background, which must be chromospheric in origin,
is less than 1/25th that seen on YZ CMi. This suggests a link between
chromospheric and coronal heating.
Title: Spectroscopy of the young cluster Blanco 1.
Authors: Panagi, P. M.; O'dell, M. A.; Collier Cameron, A.; Robinson,
R. D.
Bibcode: 1994A&A...292..439P
Altcode:
We present spectroscopic observations of some 115 stars of the cluster
Blanco 1, extending from the Ca II(H,K) region to the Ca II(I-R)
triplet, supporting an age similar to that of the young cluster α
Persei. The Hα absorption equivalent width vs (B-V) diagram forms a
well-defined locus, with decreasing absorption equivalent width for
decreasing effective temperature, akin to solar neighbourhood dwarfs. A
large spread in the Ca II surface flux, as a function of (B-V), also
indicates the presence of a high degree of surface inhomogeneity,
synonymous with high magnetic activity in young stars. A drop-off in
the Ca II flux at (B-V)=1.0 is also similar to the solar neighbourhood
stars, and shows that the primary chromospheric cooling changes from
the Ca II and Mg II lines to the Balmer lines. The mean chromospheric
temperature for stars at 4800K lies between 8000K and 10000K, based
on theoretical models, which is somewhat higher than the older solar
neighbourhood dwarfs. The high mean Ca II surface flux of the sample
is also consistent with that of other young clusters. We were able
to measure the equivalent width of the Li(6708) line, whose strength
as a function of (B-V) indicates an age similar to the young cluster
α Persei. The lithium abundance decreases with decreasing effective
temperature, consistent with the premise of lithium depletion in stars
with larger convection zones. Using published photometry and a recent
ZAMS fitting method, we also re-define the distance to the cluster to
be 246pc.
Title: An Optical Region Elemental Abundance Analysis of the
Chemically Peculiar HgMn Star chi LUPI
Authors: Wahlgren, Glenn M.; Adelman, Saul J.; Robinson, Richard D.
Bibcode: 1994ApJ...434..349W
Altcode:
The optical spectrum of the chemically peculiar HgMn type binary star
chi Lupi has been analyzed to determine atmospheric parameters and
elemental abundances. Echelle spectra were obtained with the 3.9
m Anglo-Australian telescope to exploit the extreme shape-lined
nature of the spectrum. This study was undertaken in support of
ultraviolet analyses currently underway that utilize echell spectra
obtained with the Hubble Space Telescope. For the B9.5 V primary
star we obtain Teff = 10650 K, log g = 3.9, and xi =
0 km/s, while for the A2 V secondary, Teff = 9200 K,
log g = 4.0, and xi = 2 km/s. Most of the elemental abundances are
typical of HgMn stars with similar Teff showing an overall
iron-peak elemental abundance distribution that is basically solar in
nature with enhancement of the light elements Si, P, and S, as well
as all detected elements heavier than the iron group. Abundances for
several elements have been determined for the first time in this star,
including several of the rare-earths. The secondary star spectrum shows
Am star characteristics. We also discuss the relative merits of the
equivalent width and synthetic spectrum techniques in determining the
elemental abundances, concluding that the synthetic spectrum technique
is necessary for obtaining abundances with the utmost accuracy.
Title: GHRS Observations of Cool, Low-Gravity Stars. I. The
Far-Ultraviolet Spectrum of alpha Orionis (M2 Iab)
Authors: Carpenter, Kenneth G.; Robinson, Richard D.; Wahlgren,
Glenn M.; Linsky, Jeffrey L.; Brown, Alexander
Bibcode: 1994ApJ...428..329C
Altcode:
We present far-UV (1200-1930 A) observations of the prototypical red
supergiant star alpha Ori, obtained with the Goddard High Resolution
Spectrograph (GHRS) on the Hubble Space Telescope (HST). The
observations, obtained in both low- (G140L) and medium- (G160/200M)
resolution modes, unambiguously confirm that the UV 'continuum'
tentatively seen with (IUE) is in fact a true continuum and is not due
to a blend of numerous faint emission features or scattering inside
the IUE spectrograph. This continuum appears to originate in the
chromospheric of the star at temperatures ranging from 3000-5000 K,
and we argue that it is not related to previously reported putative
companions or to bright spots on the stellar disk. Its stellar origin
is further confirmed by overlying atomic and molecular absorptions from
the chromosphere and circumstellar shell. The dominant structure in
this spectral region is due to nine strong, broad absorption bands of
the fourth-positive A-X system of CO, superposed on this continuum in
the 1300-1600 A region. Modeling of this CO absorption indicates that it
originates in the circumstellar shell in material characterized by T =
500 K, N(CO) = 1.0 x 1018 per sq cm, and Vturb
= 5.0 km per sec. The numerous chromospheric emission features are
attributed mostly to fluorescent lines of Fe II and Cr II (both
pumped by Lyman Alpha) and S I lines, plus a few lines of O I, C I,
and Si II. The O I and C I UV 2 multiplets are very deficient in flux,
compared to both the flux observed in lines originating from common
upper levels but with markedly weaker intrinsic strength (i.e., O I
UV 146 and C I UV 32) and to the UV 2 line fluxes seen in other cool,
less luminous stars. This deficiency appears to be caused by strong
self-absorption of these resonance lines in the circumstellar shell
and/or upper chromosphere of alpha Ori. Atomic absorption features,
primarily due to C I and Fe II are clearly seen in the G160M spectrum
centered near 1655 A. These Fe II features are formed at temperatures
that can occur only in the chromosphere of the star and are clearly
not photospheric or circumstellar in origin.
Title: Examining the Corona of HR 1099 with the Hubble Space Telescope
Authors: Robinson, R. D.; Maran, S. P.; Carpenter, K. G.; Brandt,
J. C.; Linsky, J. L.
Bibcode: 1994AAS...184.0515R
Altcode: 1994BAAS...26Q.865R
A pilot program for the study of stellar coronae through ultraviolet
diagnostic lines is in progress with the Goddard High Resolution
Spectrograph on the Hubble Space Telescope. An initial result of this
program was the detection of the coronal emission line of Fe XXI at 1354
Angstroms in the quiescent spectrum of the dMe star AU Mic (ApJ 421,
800, 1994). The line appeared to be symmetrical, had no measurable
radial velocity with respect to the stellar rest frame, and had a
width which was consistent with a thermal gas of temperature 10(7) K,
suggesting very small turbulence within the formation region. In March
1994, we observed the RS CVn binary HR 1099 = HD 22468 (K1 IV + G5 IV )
using instrumental settings identical to those employed with AU Mic,
but with the added benefit of the recently-installed HST corrective
optics (COSTAR). Each exposure was bracketed by observations of an
on-board wavelength calibration lamp. The results of the series of
eight 27-minute integrations, obtained at one-orbit intervals, will
be presented and compared with the AU Mic spectrum.
Title: GHRS Observations of the M-Giant MU GEM
Authors: Carpenter, K. G.; Robinson, R. D.; Johnson, H. R.; Ensman,
L. M.
Bibcode: 1994AAS...184.0505C
Altcode: 1994BAAS...26..863C
The ultraviolet spectrum of the M-giant mu Gem (M3 IIIab) is sampled
at selected important wavelengths with the Goddard High Resolution
Spectrograph (GHRS) on the Hubble Space Telescope (HST). The wavelength
regions are chosen to include lines sensitive to turbulence and flow
velocities in the stellar chromosphere and wind acceleration region, as
well as some fiducial photospheric absorption features. The dynamics of
the outer atmosphere of mu Gem are examined using the observed profiles
and positions of chromospheric emission lines of C II, Fe II, Mg II, Fe
I, referenced to photospheric absorption lines of Fe I. The content of
the spectra and the inferred flow and turbulent velocities are compared
to those found for the other cool, low-gravity stars observed with GHRS,
including alpha Ori (M2 Iab) and gamma Cru (M3.4 III).
Title: Examining the corona of HR 1099 with the Hubble Space
Telescope.
Authors: Robinson, R. D.; Maran, S. P.; Carpenter, K. G.; Brandt,
J. C.; Linsky, J. L.
Bibcode: 1994BAAS...26..865R
Altcode:
No abstract at ADS
Title: HST Observations of the Chromosphere of a Carbon Star
Authors: Ensman, L. M.; Johnson, H. R.; Carpenter, K. G.; Robinson,
R. D.; Luttermoser, D.
Bibcode: 1994AAS...184.0506E
Altcode: 1994BAAS...26..863E
Ultraviolet spectra from the first Hubble Space Telescope observations
of a carbon star are presented, as well as line identifications and an
initial analysis of the velocity structure of the emitting region. Two
spectra of UU Aur (HD 46687, type N3; C5,3) have been obtained. In the
lower-resolution FOS spectrum (2310 -- 3275 Angstroms), UV emission
lines of Mg I, Mg II, and Fe II, fluoresced lines of Fe I and II,
semi-forbidden emission from C II, Si II, and Al II, absorption lines of
Fe I and Ti II, and absorption by CH and CaCl have been identified. In
the high resolution GHRS spectrum, which covers a 50 Angstroms region
around the Mg II h and k emission lines, overlying Mg II and Mn I
absorption and two fluoresced Fe I lines have been resolved. The Mg
II, and perhaps the Ti II, absorption appears to be interstellar. UV
emission lines of predominantly singly ionized metals indicate the
existence of a chromosphere at ~ 5000 K. In UU Aur, the Mg II emission
clearly arises in material which is flowing away from the star at 35--60
km/s. Shifts of the Fe II emission lines support the conclusion that
the chromosphere is expanding at highly supersonic velocities. However,
overlying Mn I absorption must be due to stationary material further
out. The forbidden and fluoresced lines also come from a region(s)
with negligible velocity. These data imply that shocks may form in the
atmospheres of carbon stars and thus be a mechanism for chromospheric
heating and mass loss. In fact, previous semi-empirical chromospheric
models of the carbon star TX Psc (Luttermoser, Johnson, Avrett, and
Loeser 1989) indicate a similar shock-like velocity field. Luttermoser,
D., Johnson, H. R., Avrett, G., and Loeser, R. 1989, ApJ 345, 543.
Title: Ultraviolet and Radio Observations of the Young Rapidly
Rotating k0 Dwarf Star HD197890
Authors: Robinson, R. D.; Carpenter, K. G.; Slee, O. B.; Nelson,
G. J.; Stewart, R. T.
Bibcode: 1994MNRAS.267..918R
Altcode:
We present the results of UV observations taken with the International
Ultraviolet Explorer (IUE) satellite and microwave observations
obtained with the Australia Telescope during an observing campaign
of the rapidly rotating K0 dwarf star III) 197890, nicknamed `Speedy
Mic'. This star was recently recognized as a powerful, transient EUV
source by the ROSAT WFC, and subsequent investigation showed it to
be a ZAMS or possibly a PMS dwarf which may be a member of the Local
Association. Our observations show it to have strong, variable UV
emission lines near the `saturation' levels. The radio observations
show a level of `quiescent' emission consistent with other rapidly
rotating stars, but there is no evidence for the large flux variations
that normally characterize the time history of such objects.
Title: Observing Stellar Coronae with the Goddard High Resolution
Spectrograph. I. The dMe Star AU Microscopii
Authors: Maran, S. P.; Robinson, R. D.; Shore, S. N.; Brosius, J. W.;
Carpenter, K. G.; Woodgate, B. E.; Linsky, J. L.; Brown, A.; Byrne,
P. B.; Kundu, M. R.; White, S.; Brandt, J. C.; Shine, R. A.; Walter,
F. M.
Bibcode: 1994ApJ...421..800M
Altcode:
We report on an observation of AU Mic taken with the Goddard High
Resolution Spectrograph (GHRS) aboard the Hubble Space Telescope. The
data consist of a rapid sequence of spectra covering the wavelength
range 1345-1375 A with a spectral resolution of 10,000. The observations
were originally intended to search for spectral variations during
flares. No flares were detected during the 3.5 hr of monitoring. A
method of reducing the noise while combining the individual spectra
in the time series is described which resulted in the elimination of
half of the noise while rejecting only a small fraction of the stellar
signal. The resultant spectrum was of sufficient quality to allow the
detection of emission lines with an integrated flux of 10-15
ergs/sq cm(sec) or greater. Lines of C I, O I, O V, Cl I, and Fe XXI
were detected. This is the first indisputable detection of the 1354
A Fe XXI line, formed at T approximately = 107 K, on a
star other than the Sun. The line was well resolved and displayed
no significant bulk motions or profile asymmetry. From the upper
limit on the observed line width, we derive an upper limit of 38 km/s
for the turbulent velocity in the 107 K plasma. An upper
limit is derived for the flux of the 1349 A Fe XII line, formed at T
approximately = 1.3 x 106 K. These data are combined with
contemporaneous GHRS and International Ultraviolet Explorer (IUE) data
to derive the volume emission measure distribution of AU Mic over the
temperature range 104-107 K. Models of coronal
loops in hydrostatic equilibrium are consistent with the observed
volume emission measures of the coronal lines. The fraction of the
stellar surface covered by the footprints of the loops depends upon the
loop length and is less than 14% for lengths smaller than the stellar
radius. From the upper limit to the estimated width of the Fe XXI line
profile we find that the we cannot rule out Alfven wave dissipation
as a possible contributor to the required quiescent loop heating rate.
Title: Search for CO Absorption Bands in IUE Far-Ultraviolet Spectra
of Cool Stars
Authors: Gessner, S. E.; Carpenter, K. G.; Robinson, R. D.
Bibcode: 1994AJ....107..747G
Altcode:
Observations of the red supergiant (M2 Iab) alpha Ori with the Goddard
High Resolution Spectrograph (GHRS) on board the Hubble Space Telescope
(HST) have provided an unambiguous detection of a far-ultraviolet
(far-UV) chromospheric continuum on which are superposed strong
molecular absorption bands. The absorption bands have been identified
by Carpenter et al. (1994) with the fourth-positive A-X system of
CO and are likely formed in the circumstellar shell. Comparison of
these GHRS data with archival International Ultraviolet Explorer
(IUE) spectra of alpha Ori indicates that both the continuum and
the CO absorption features can be seen with IUE, especially if
multiple IUE spectra, reduced with the post-1981 IUESIPS extraction
procedure (i.e., with an oversampling slit), are carefully coadded
to increase the signal to noise over that obtainable with a single
spectrum. We therefore initiated a program, utilizing both new and
archival IUE Short Wavelength Prime (SWP) spectra, to survey 15 cool,
low-gravity stars, including alpha Ori, for the presence of these two
new chromospheric and circumstellar shell diagnostics. We establish
positive detections of far-UV stellar continua, well above estimated
IUE in-order scattered light levels, in spectra of all of the program
stars. However, well-defined CO absorption features are seen only in
the alpha Ori spectra, even though spectra of most of the program stars
have sufficient signal to noise to allow the dectection of features of
comparable magnitude to the absorptions seen in alpha Ori. Clearly if
CO is present in the circumstellar environments of any of these stars,
it is at much lower column densities.
Title: A Search For Microflaring Activity on dMe Flare Stars
Authors: Robinson, R. D.; Carpenter, K. G.; Percival, J. W.
Bibcode: 1994ASPC...64..456R
Altcode: 1994csss....8..456R
No abstract at ADS
Title: Lyman-Alpha Emission as a Diagnostic of Superthermal Proton
Properties in Stellar Flares
Authors: Brosius, J. W.; Robinson, R. D.; Maran, S. P.
Bibcode: 1994ASPC...64..360B
Altcode: 1994csss....8..360B
No abstract at ADS
Title: GHRS Observations and Analysis of the 0 I and C I Resonance
Lines in the UV Spectrum of alpha ORI (M2 Iab)
Authors: Carpenter, Kenneth G.; Robinson, Richard D.; Judge, Philip
G.; Ebbets, Dennis C.; Brandt, John C.
Bibcode: 1994ASPC...64...56C
Altcode: 1994csss....8...56C
No abstract at ADS
Title: Calibration of GHRS Burst Noise Rejection Techniques
Authors: Beaver, E. A.; Cohen, R. D.; Diplas, A.; Garner, H.; Heap,
S. R.; Loveland, M.; Robinson, R. D.
Bibcode: 1994chst.conf..304B
Altcode:
No abstract at ADS
Title: Determining the Vignetting and Echelle Blaze Function for
the GHRS
Authors: Robinson, R. D.
Bibcode: 1994chst.conf..291R
Altcode:
No abstract at ADS
Title: A Search for Proton Beams during Flares on AU Microscopii
Authors: Robinson, R. D.; Carpenter, K. G.; Woodgate, B. E.; Maran,
S. P.
Bibcode: 1993ApJ...414..872R
Altcode:
We report the results of a coordinated observing campaign on the active
M dwarf star AU Mic. The purpose of the campaign was to search for
evidence of proton beams during the impulsive phase of stellar flares
and to determine whether the energy contained in these beams represented
a significant fraction of the energy budget of the flare. During a total
of 3.5 hr of monitoring a small flare was observed simultaneously by
the HST, IUE, and the AAT. This event, which had a total optical + UV
emission of 1.3 x 10 exp 32 ergs, occurred during the decay phase of
a much larger event and showed no evidence for a proton beam with an
energy greater than a few times 10 exp 29 ergs/s. This is comparable
to the maximum energy flux released by the flare, though this energy
release rate must occur over a time interval much shorter than that
of the impulsive phase itself. We conclude that the proton beams may
be capable of transporting some energy during the impulsive phase
of a flare, but that they are unlikely to be the major contributor,
at least for this particular event.
Title: The chromospheres of late-type stars. II. an
atlasofchromospheric lines for selected early-K stars.
Authors: Thatcher, John D.; Robinson, Richard D.
Bibcode: 1993MNRAS.262....1T
Altcode:
High-resolution spectra of the chromospheric Na I D lines, Ca I 4227,
Mg I 4571, 5167, and 5172, the Ca II H and K resonance and IR 8542.144
and 8662.170 lines, and H-alpha and H-beta, all observed simultaneously
at the AAT, are presented. These data are presented as the observational
basis for the self-consistent, semiempirical modeling of the outer
photospheres and chromospheres of the target stars. Stellar activity is
found in the low-chromosphere lines as core-filling and, in some stars,
as line-broadening. Integrated fluxes are derived from the difference
spectra, formed by subtracting quiet from active spectra, in the
Ca II H and K resonance and IR 8542 and 8662 lines and in H-alpha,
which are presented as chromospheric activity indicators. All the
activity indices exhibit strong correlations, with the exception of
the equivalent widths of H-alpha and H-beta, which are found to be
relatively poor activity discriminators.
Title: MG II H and K Profiles in Luminous, Cool Stars
Authors: Robinson, R. D.; Carpenter, K. G.
Bibcode: 1993AAS...182.4619R
Altcode: 1993BAAS...25..877R
As part of an investigation of the velocity fields in the atmospheres of
cool, luminous stars we used the Goddard High Resolution Spectrograph
(GHRS) aboard the Hubble Space Telescope to obtain a high resolution
spectrum of the Mg II h and k lines in the M supergiant alpha Ori. These
strong, centrally reversed emission lines have been well studied by the
IUE. While the h line in alpha Ori has nearly equal intensity in the red
and blue peaks, the blue peak of the k line is always much weaker than
the red peak. This asymmetry is normally explained as resulting from
absorption by Fe I and Mn I in the circumstellar shell. An examination
of the GHRS spectrum, however, reveals that the asymmetry results from
a velocity shift between the emission wings and the central absorption
core of the line. Further, while the central absorption profiles for
the h and k lines agree very well when plotted on a velocity scale, the
centroid of the k line emission wings was observed to be redshifted by
approximately 23 km/s with respect to that of the h line. Examination
of other high resolution GHRS observations of Mg II showed a similar,
though less pronounced, relative redshift of the k line centroid on
the normal M giant gamma Cru and the hybrid K giant gamma Dra. To date,
no concrete model has been proposed to explain these shifts. Examining
well exposed, high resolution IUE observations of alpha Ori shows a
close agreement with the GHRS results. This gives us confidence that
we can use IUE spectra to study the relative shapes and velocities
of the Mg II h and k lines and we are currently involved in a program
to use the IUE archive to quantify the behavior of the Mg II h and k
profiles as a function of luminosity and spectral type for luminous,
cool stars. The results of this program will be presented.
Title: Elemental Abundances of the B6 IV Star XI Octantis
Authors: Adelman, Saul J.; Robinson, Richard D.; Wahlgren, Glenn M.
Bibcode: 1993PASP..105..327A
Altcode:
An elemental abundance study utlized AAT echelle spectrograms of the
ultrasharp-lined superifically normal B6 IV star xi Octantis. The
derived abundances fall within the trends of values derived for normal
B main sequence band stars. On average they are 0.28 dex less than
solar. (SECTION: Stars)
Title: Observations of 3C 273 With the Goddard High Resolution
Spectrograph on the Hubble Space Telescope
Authors: Brandt, J. C.; Heap, S. R.; Beaver, E. A.; Boggess, E. A.;
Carpenter, K. G.; Ebbets, D. C.; Hutchings, J. B.; Jura, M.; Leckrone,
D. S.; Linsky, J. L.; Maran, S. P.; Savage, B. D.; Smith, A. M.;
Trafton, L. M.; Walter, F. M.; Weymann, R. J.; Snow, M.; Randall,
C. E.; Lindler, D. J.; Shore, S. N.; Morris, S. L.; Gilliland, R. L.;
Lu, L.; Robinson, R. D.
Bibcode: 1993AJ....105..831B
Altcode:
The observations of the quasar 3C 273 taken with the Goddard High
Resolution Spectrograph in 1991 February are presented here. We have
included both the reduced raw data, and smoothed and deconvolved
spectra. Also, a list of observed absorption lines is presented. The
data comprise 11 spectra, including 1 low resolution observation and 10
medium resolution observations. The wavelength region covered ranged
from about 1150 to 2820 A, but was not all inclusive. The procedures
used to obtain and reduce the data, including corrections for fixed
pattern noise, compensation for the effects of spherical aberration
in the HST primary mirror, and objective detection of weak absorption
lines, are described. We also have included a short discussion on the
detection of galactic Ni II and Virgo cluster metal lines.
Title: Coordinated HST - EUVE Observations of YZ CMi
Authors: Robinson, Richard D.
Bibcode: 1993euve.prop...36R
Altcode:
We propose to use the EUVE satellite in a coordinated campaign with
the Hubble Space Telescope and other satellite and ground based
observatories to observe flares on the active dMe star YZ CMi. In
addition to EUV and UV wavelengths, the campaign will monitor the star
at optical, radio and X-ray wavelengths. The objectives are to better
understand the energy transport mechanisms and total energy balance
during a stellar flare.
Title: Observations of a Flare on AU MIC
Authors: Robinson, R. D.; Carpenter, K. G.; Woodgate, B. E.
Bibcode: 1992AAS...181.5104R
Altcode: 1992BAAS...24.1206R
We report the results of a coordinated observing campaign of the active
dMe flare star AU Mic, carried out on 1992 Sept 9. Participating
instruments include the HST and IUE satellites and the 3.9m
Anglo-Australian Telescope. During this campaign a long duration,
1.5 mag (U band) flare was detected, with the maximum phase observed
simultaneously with the IUE and AAT and the decay phase monitored by
all three instruments. The AAT observations consisted of a time series
of CCD exposures covering the wavelength range 3650-4450 Angstroms
and having a temporal resolution of 6 seconds. The impulsive phase
lasted for about 1 minute and was therefore well resolved by these
observations. The flare showed pronounced broadening of the Balmer
series, with the Balmer line fluxes increasing with the continuum
during the impulsive phase but having a much longer decay time. A
pronounced HeII (4026 Angstroms ) emission was seen during the flare,
with some indication that this line increased in absorption strength
about 30 seconds before the continuum increase. The CaII H and K lines
decreased in strength during the impulsive and early decay phase of the
event, but recovered during the later decay phase. A low resolution LWP
exposure was taken by the IUE during the peak of the flare. Combining
this with the optical data gives an almost complete coverage of the
flare spectrum from 2200 to 4500 Angstroms . We discuss the implications
of these observations in relation to current flare models.
Title: Detection of a Proton Beam during the Impulsive Phase of a
Stellar Flare
Authors: Woodgate, Bruce E.; Robinson, Richard D.; Carpenter, Kenneth
G.; Maran, Stephen P.; Shore, Steven N.
Bibcode: 1992ApJ...397L..95W
Altcode:
A transient event consistent with the predicted temporal and spectral
signatures of an energetic proton beam was detected in the impulsive
phase of a small flare on the red dwarf star AU Microscopii. It
consisted of a prominent increase in the flux in the red wing of
Lyman-alpha near 1223 A, simultaneously with the peak of a flare
observed in the 1206 A transition region line of Si III. The probability
that the red wing event was a chance fluctuation is one chance in
2.5 x 10 exp 4. This observation represents a confirmation of the
prediction by Orrall and Zirker (1976) in which downstreaming protons
accelerated during the impulsive phase of a flare charge exchange with
ambient neutral hydrogen and emit Lyman-alpha radiation from 1 to 15
A redward of line center.
Title: The Ultraviolet Photospheric Spectrum of Alpha Orionis
Authors: Wahlgren, G. M.; Robinson, R. D.; Carpenter, K. G.
Bibcode: 1992AAS...18110010W
Altcode: 1992BAAS...24.1280W
No abstract at ADS
Title: Spectroscopic investigation of cool giants and the authenticity
of their reported microwave emission.
Authors: Jones, K. L.; Robinson, R. D.; Slee, O. B.; Stewart, R. T.
Bibcode: 1992MNRAS.256..535J
Altcode:
Surface velocities and metal abundances for 19 red giant stars in
the spectral range G5 to M3 are derived on the basis of AAT echelle
spectroscopy data. Attention is given to the question of whether the
stars reported to emit radio bursts had different physical properties
(rotation rate, macroturbulence, microturbulence, and metal abundance)
from those without the radio bursts, which might explain why they
were radio emitters. The various velocities had values consistent
with those previously found for other similar stars. There was
an observed increase in both macroturbulent and, less definitely,
microturbulent velocities with lateness of spectral type at K3 and
later. A weak correlation between surface velocities and 8.4-GHz radio
surface fluxes was found. No connection between iron abundances and
radio surface fluxes was detected. It is concluded that few, if any,
of the cool giants are radio emitters.
Title: Detection of a Proton Beam during the Impulsive phase of a
Stellar Flare
Authors: Woodgate, B. E.; Robinson, R. D.; Carpenter, K. G.; Maran,
S. P.; Shore, S.
Bibcode: 1992AAS...180.3010W
Altcode: 1992BAAS...24..777W
We present evidence for the detection of a proton beam during the
impulsive phase of a flare on the Dme star AU Microscopii. At 1991
Sept 3 04h 56m 10s UT, the Goddard High Resolution Spectrograph on the
Hubble Space Telescope observed an increase in the flux in the red
wing of the hydrogen Lyman alpha line at 1223A, unaccompanied by an
increase in the blue wing, simultaneously with an increase in the flux
of the SiIII 1206A transition region line. The 1223A flux increase was
2.2 x 1027 ergs s-1 A-1, with 3s duration. The chance probability of
the observation is 1 in 105. This effect was predicted by Orrall and
Zirker (1976), as a downstreaming beam of protons charge exchanging
with ambient neutral hydrogen and emitting Lyman alpha radiation at
the downstreaming velocity. The energy in the beam is estimated as 7
x 1029 ergs s-1.
Title: The Chromosphere and Circumstellar Shell of α Orionis as
Observed with the Goddard High Resolution Spectrograph
Authors: Carpenter, K. G.; Robinson, R. D.; Wahlgren, G. M.; Linsky,
J. L.; Brown, A.
Bibcode: 1992ESOC...44..387C
Altcode: 1992swhs.conf..387C
No abstract at ADS
Title: Detection of a Proton Beam during the Impulsive Phase of a
Stellar Flare
Authors: Robinson, R. D.; Woodgate, B. E.; Carpenter, K. G.; Maran,
S. P.; Shore, S. N.
Bibcode: 1992ESOC...44..447R
Altcode: 1992swhs.conf..447R
No abstract at ADS
Title: HST Observations of the Flare Star AU MIC
Authors: Robinson, R. D.; Shore, S. N.; Carpenter, K. G.; Woodgate,
B. E.; Maran, S. P.; Brandt, J. C.; Kundu, M. R.; White, S. M.;
Linsky, J. L.; Walter, F. M.
Bibcode: 1992ASPC...26...31R
Altcode: 1992csss....7...31R
No abstract at ADS
Title: A Search for Energetic Transient Activity in Cool, Giant Stars
Authors: Robinson, Richard D.
Bibcode: 1992iue..prop.4291R
Altcode:
A recent 8.4 GHz radio survey of 82 nearby cool giant stars detected
radio emission from 24 of the objects. In all cases the radio
emissions detected were variable and non-thermal in origin. suggesting
a flare-like process. An optical follow-up to this survey showed that
the radio emitters were apparently normal field giants, with standard
metal abundance, macroturbulence and rotational velocities. The only
significant anomaly was a larger than average value of Lithium abundance
for the radio sources, suggesting youth. The existence of radio bursts
from such stars is unexpected and the physical processes causing the
bursts are probably different from those operating on the active RS
CVn and FE Comae giants. Here we propose a detailed examination Of
One of the most active of the field giants to search for UV signatures
related to radio flares.
Title: Molecular Absorption in the UV Spectrum of A ORI
Authors: Wahlgren, G. M.; Robinson, R. D.; Carpenter, K. G.
Bibcode: 1992ASPC...26...37W
Altcode: 1992csss....7...37W
No abstract at ADS
Title: Inhomogeneous structure in the chromospheres of dwarf M stars.
Authors: Turner, N. J.; Cram, L. E.; Robinson, R. D.
Bibcode: 1991MNRAS.253..575T
Altcode:
Linear combinations of observed spectra of the H-alpha and Ca-II
resonance and IR lines from the chromospheres of a quiet (Gl 1) and
an active (Gl 735) dwarf-M star are compared with the corresponding
spectra from a star of intermediate activity (Gl 887). It is shown that
the intermediate spectra cannot be explained as a simple juxtaposition
of the extreme chromospheric states. It is concluded that the range
of observed strengths of chromospheric activity indicators in dwarf-M
stars is due, at least in part, to changes in the radial structure
of the chromospheric heating function and not to changes in the area
filling factor.
Title: An Investigation of the Flare Star AU Mic with the Goddard
High Resolution Spectrograph on the Hubble Space Telescope
Authors: Maran, S. P.; Woodgate, B. E.; Carpenter, K. G.; Robinson,
R. D.; Shore, S. N.; Linsky, J. L.; Brown, A.; Byrne, P. B.; Kundu,
M. R.; White, S.; Brandt, J. C.; Shine, R. A.; Walter, F. M.
Bibcode: 1991BAAS...23.1382M
Altcode:
No abstract at ADS
Title: Coordinated IUE/HST Observations of the Flare Star AU mic:
Results from IUE
Authors: Carpenter, K. G.; Maran, S. P.; Brown, A.; Linsky, J. L.;
Robinson, R. D.; Byrne, P. B.; Judge, P. G.
Bibcode: 1991BAAS...23.1383C
Altcode:
No abstract at ADS
Title: Molecular Absorption in the Ultraviolet Spectrum of Alpha Ori
Authors: Wahlgren, G. M.; Robinson, R. D.; Carpenter, K. G.
Bibcode: 1991BAAS...23.1386W
Altcode:
No abstract at ADS
Title: Time Resolved Observations of the Lyman-Alpha Region in AU
Mic with the Goddard High Resolution Spectrograph
Authors: Woodgate, B. E.; Maran, S. P.; Carpenter, K. G.; Robinson,
R. D.; Shore, S. N.; Linsky, J. L.; Byrne, P. B.; Kundu, M. R.
Bibcode: 1991BAAS...23.1383W
Altcode:
No abstract at ADS
Title: Erratum - Detection of a Magnetic Region on HR:1099
Authors: Donati, J. F.; Semel, M.; Rees, D. E.; Taylor, K.; Robinson,
R. D.
Bibcode: 1991A&A...248..337D
Altcode:
No abstract at ADS
Title: First Results from the Goddard High-Resolution Spectrograph:
The Chromosphere of alpha Tauri
Authors: Carpenter, Kenneth G.; Robinson, Richard D.; Wahlgren,
Glenn M.; Ake, Thomas B.; Ebbets, Dennis C.; Linsky, Jeffrey L.;
Brown, Alexander; Walter, Frederick M.
Bibcode: 1991ApJ...377L..45C
Altcode:
The K5 III star Alpha Tau was observed with the Goddard High Resolution
Spectrograph on November 27, 1990 as part of the Science Assessment
Program for the HST. The spectra show intersystem and permitted
chromospheric emission lines of semiforbidden C II and Si II, Fe II,
Fe I, Ni II, and Co II. Resolved profiles of the semiforbidden C II
lines indicate a complex chromospheric turbulent velocity distribution
with mean value of roughly 24 km/s, while their observed wavelengths
indicate a 4 km/s downflow of the semiforbidden C II plasma. Twenty-five
new emission lines have been found in the 2320-2370 A region, 17 of
which have been identified with the aid of Skylab data obtained above
the solar limb, including four lines from Co II (UV 8) and an Fe I
(UV 12) line.
Title: The chromospheres of late-type stars. I. EPS Eridani as a
test case of multiline modelling.
Authors: Thatcher, John D.; Robinson, Richard D.; Rees, David E.
Bibcode: 1991MNRAS.250...14T
Altcode:
A new model of the lower chromosphere of the dwarf K2 star Epsilon
Eridani is derived by matching flux profiles of the Ca IR triplet
lines 8498 and 8542 A H-alpha and H-beta lines and the Na D lines
(all observed simultaneously at the AAT), and the Ca II K line. The
coupled non-LTE equations of statistical equilibrium and radiative
transfer are solved under the constraint of hydrostatic equilibrium
using the Carlsson (1986) code. Within the framework of the model,
the Na D lines are an important photospheric diagnostic, and the Ca
IR triplet lines can be used to locate the temperature minimum. The
computed H-alpha and H-beta depths are highly sensitive constraints
on the transition zone gradients and base pressures allowing us to
derive a pressure at the base of the transition zone of 0.9 dyn/cm.
Title: Ghrs/ Goddard High Resolution Spectroscopy Chromospheric
Emission Line Spectra of the Red Giant Alpha-Tauri
Authors: Carpenter, K. G.; Robinson, R. D.; Ebbets, D. C.; Brown,
A.; Linsky, J. L.
Bibcode: 1991fyho.conf..212C
Altcode:
No abstract at ADS
Title: Structure and Dynamics of HD32918
Authors: Robinson, Richard D.
Bibcode: 1991iue..prop.4020R
Altcode:
The star HD32918 is a member of the class of rapidly rotating,
apparently single giants collectively called the FK Comae stars. HD32918
is one of the most active members of this group and has shown remarkable
changes in its atmospheric structure, the most significant being
indicated by the change in Ho from an approximately symmetric emission
profile in Dec 1980 to an inverse P Cyg profile in Aug 1987. The IUE
observations proposed here are planned as part of a major observational
campaign designed to understand the atmospheric structure of this
star as well. as changes in that structure over both short and long
timescales. Particular attention will be paid to the characteristics
of the long lasting starspot region known to be present on the star.
Title: Fast spectroscopic variations on rapidly rotating, cool dwarfs
III. Masses of circumstellar absorbing clouds on AB Doradus.
Authors: Collier Cameron, A.; Duncan, D. K.; Ehrenfreund, P.; Foing,
B. H.; Kuntz, K. D.; Penston, M. V.; Robinson, R. D.; Soderblom, D. R.
Bibcode: 1990MNRAS.247..415C
Altcode:
We present new time-resolved Hα, Ca II H and K and Mg II h
and k spectra of the rapidly rotating KO dwarf star AB Doradus
(= HD 36705). The transient absorption features seen in the Hα
line are also present in the Ca II and Mg II resonance lines. New
techniques are developed for measuring the average strength of the
line absorption along lines-of-sight intersecting the cloud. These
techniques also give a measure of the projected cloud area. The
strength of the resonance-line absorption provides new constraints
on the column densities, projected surface areas, temperatures and
internal turbulent velocity dispersions of the circumstellar clouds
producing the absorption features. At any given time the star appears
to be surrounded by between S and 20 clouds with masses in the range
2-6 × 1017 g. The clouds appear to have turbulent internal
velocity dispersions of order 3-20 km s-1, comparable with
the random velocities of discrete filamentary structures in solar
quiescent prominences. Night-to-night changes in the amount of Ca II
resonance line absorption can be explained by changes in the amplitude
of turbulent motions in the clouds. The corresponding changes in the
total energy of the internal motions are of order 1029
erg per cloud. Changes of this magnitude could easily be activated by
the frequent energetic (∼1034 erg) X-ray flares seen on
this star.
Title: Chromospheric H alpha and CA II Lines in Late-Type Stars
Authors: Robinson, Richard D.; Cram, Lawrence E.; Giampapa, Mark S.
Bibcode: 1990ApJS...74..891R
Altcode:
Observations of H-alpha and Ca II H and K in the chromospheres of 50
main-sequence K and M stars are reported. The photospheric contribution
to the integrated core flux in these lines is found to be more important
than previously thought, and earlier estimates of the basal flux of cool
dwarf stars are revised. The H-alpha data confirm the presence of both
an upper and a lower limit to the H-alpha equivalent width attained
at any given spectral type. The maximum H-alpha absorption strength
decreases gradually toward cooler stars while the maximum 'saturated'
quiescent emission strength increases. The Ca II emission strength
is related to the strength of the emission or absorption feature at
H-alpha, but there is not a one-to-one correlation. The main results may
be explained in terms of photoionization and collisional control of the
non-LTE H-alpha source function. Several aspects of the structure and
heating of the outer atmospheres of dwarf K and M stars are discussed.
Title: Detection of a magnetic region of HR 1099.
Authors: Donati, J. -F.; Semel, M.; Rees, D. E.; Taylor, K.; Robinson,
R. D.
Bibcode: 1990A&A...232L...1D
Altcode:
The paper reports results of a magnetic field measuring campaign on the
active K component of the RS CVn system HR 1099, using Zeeman-Doppler
imaging. A localized magnetic field is detected near quadrature
(phase 0.85). Assuming this magnetic region has a circular shape, its
longitude and latitude are, respectively, estimated to be 86 + or -
4 deg and 5 + or - 5 deg. This equatorial region is largely monopolar,
has a mean field strength of 985 + or - 270 G, covers 18 + or - 3
percent of the total stellar surface and may be colocalized with a
bright photospheric spot.
Title: Fast Hα variations on a rapidly rotating, cool main-sequence
star-II. Cloud formation and ejection.
Authors: Collier Cameron, A.; Robinson, R. D.
Bibcode: 1989MNRAS.238..657C
Altcode:
Time resolved H-alpha spectra of AB Doradus are presented that confirm a
model in which large prominence-like condensations of neutral material
are trapped in corotation with the stellar magnetic field. The results
suggest that the entire confining field is deformed outward by the
increasing centrifugal force acting on the corotating cloud materials
as the cloud density increases. The angular momentum loss rate from
the cloud ejections is shown to be sufficient to brake the rotation
of the star's convective envelope on a time-scale of no more than 10
to the 8th yr.
Title: Radio and optical observation of an intense flare on HD 32918.
Authors: Bunton, J. D.; Large, M. I.; Slee, O. B.; Stewart, R. T.;
Robinson, R. D.; Thatcher, J. D.
Bibcode: 1989PASA....8..127B
Altcode: 1989PASAu...8..127B
During routine monitoring of active-chromosphere stars in August 1987
an intense radio flare on the FK Comae star HD 32918 was detected
by the Parkes radiotelescope. Other observations were notified,
enabling data to be collected at 843 MHz, 1.4 GHz, 8.4 GHz and optical
frequencies. The flare at radio frequencies was largely similar to a
previous event on this star, reaching a peak radio power of 5 x 10 to
the 12th W/Hz at 8.4 GHz. During the flare the spectrum varied as about
v to the 1.4 and later flattened as the flare progressed. Compared to
the previous flare, circular polarization was higher. Ca II and H alpha
optical spectra taken at the AAO 3.9-meter telescope show the star to
have a highly active chromosphere and a strong wind, with a terminal
velocity of about 230 km/s. While much of this activity appears to be
long lived, there is good evidence for an increase in the strength of
the chromospheric lines during the radio flare event.
Title: Fast H-alpha variations on a rapidly rotating cool main
sequence star- I. Circumstellar clouds.
Authors: Collier Cameron, A.; Robinson, R. D.
Bibcode: 1989MNRAS.236...57C
Altcode:
High-resolution CCD spectra of the H-alpha profile of AB Doradus reveal
transient absorption features whose radial velocities relative to the
underlying stellar spectrum increase monotonically with time. It is
proposed that these features originate in cool dense clouds embedded
in and corotating with the hot extended corona. Such clouds could form
through thermal instabilities at the summits of large magnetic loops.
Title: Chromospheres of M dwarf stars with incipient H-alpha emission.
Authors: Robinson, R. D.; Cram, L. E.
Bibcode: 1989PASA....8..132R
Altcode: 1989PASAu...8..132R
Observations are presented of H-alpha and Ca II resonance lines
in 4 stars having H-alpha features which place them intermediately
between the non-(e) and (e) classifications of dK/dM stars. There is
considerable variety in the shape of the H-alpha line, presumably due in
part to differences in rotation rates. As expected, the energy fluxes
in the Ca II emission lines lie between those typical of non-(e) and
(e) stars. There is some evidence (especially from the binary Gl 876A)
that the energy fluxes in the Ca II and H-alpha emission lines do
not vary from star to star according to a simple proportionality. An
intriguing result is the apparent detection of spatially displaced
chromospheric emission in Gl 907.1 and Gl 890.
Title: Optical chromospheric spectral lines in K and M dwarf stars.
Authors: Robinson, R. D.; Cram, L. E.
Bibcode: 1989sasf.confP..75R
Altcode: 1989IAUCo.104P..75R; 1988sasf.conf...75R
Observations are reported of the Ca II resonance lines and Hα in dK and
dM stars, made with high S/N ratio and high spectral resolution. Ca II
emission is found in all stars observed, and those having weak Ca II
exhibit marked Hα absorption. It is found that the strengths of the
two kinds of chromospheric lines are not tightly correlated, an effect
which can be shown to be independent of the effective temperature
of the stars. The result implies that a one-parameter description
(e.g. heating rate) of the chromospheres is not viable. While lateral
inhomogeneities are likely to be an important second parameter,
the authors also suggest that the Hα line may be formed in a region
considerable higher than that in which the Ca II lines are formed.
Title: Spectral observations of a large stellar flare.
Authors: Robinson, R. D.
Bibcode: 1989sasf.confP..83R
Altcode: 1988sasf.conf...83R; 1989IAUCo.104P..83R
High time resolution, moderate spectral resolution spectra were taken
during the decay phase of a ΔU > 3 magnitude flare on the dM5.5e
star Wolf 424. This flare shows intense, broad Balmer line emission with
extended wings, narrow Ca II lines and numerous weak emission lines from
neutral and singly ionized metals. The time history shows substantial,
short-lived enhancements in the line emission. These variations are
not always seen in association with continuum enhancements.
Title: Coronal Mass Ejections from a Rapidly-Rotating KO Dwarf Star
Authors: Collier Cameron, A.; Robinson, R. D.
Bibcode: 1988ASIC..241..399C
Altcode: 1988felm.conf..399C
No abstract at ADS
Title: Herringbone bursts associated with type II solar radio
emission.
Authors: Cairns, I. H.; Robinson, R. D.
Bibcode: 1987SoPh..111..365C
Altcode:
We report detailed observations of the herringbone (HB) fine structure
on type II solar radio bursts. Data from the Culgoora radiospectrograph,
radiometer and radioheliograph are analyzed. We determine the
characteristic spectral profiles, frequency drift rates and exciter
velocities, fluxes, source sizes, brightness temperatures, and
polarizations of individual HB bursts. Correlations between individual
bursts within the characteristic groups of bursts and the properties
of the associated type II bursts are examined. Our data are compatible
with HB bursts being radiation at multiples of the plasma frequency
generated by electron streams accelerated by the type II shock. We
conclude that HB bursts are physically distinct phenomena from type
II and type III bursts, differing significantly in emission processes
and/or source conditions; this conclusion indicates that many of the
presently available theoretical ideas for HB bursts are incorrect.
Title: The Australian radio star survey.
Authors: Wright, A. E.; Slee, O. B.; Nelson, G. J.; Stewart, R. T.;
Jauncey, D. L.; White, G. L.; Vaughan, A. E.; Lim, J.; Large, M. I.;
Bunton, J. D.; Thompson, K.; Coates, D. W.; Innis, J. L.; Peters,
W. L.; Ryan, S. G.; Robinson, R. D.; Cropper, M.; Allen, D. A.; Page,
A. A.
Bibcode: 1987PASA....7..159W
Altcode: 1987PASAu...7..159W
The authors present an overview of the survey for radio emission from
active stars that has been in progress for the last six years using
the observatories at Fleurs, Molonglo, Parkes and Tidbinbilla. The
role of complementary optical observations are also outlined. The
authors describe the different types of star that have been included
in the survey and discuss some of the problems in making the radio
observations.
Title: Properties of Metre-Wavelength Solar Bursts Associated with
Coronal Mass Ejections
Authors: Robinson, R. D.; Stewart, R. T.; Sheeley, N. R., Jr.; Howard,
R. A.; Koomen, J.; Michels, D. J.
Bibcode: 1986SoPh..105..149R
Altcode:
An investigation is made to determine the relationship between a
coronal mass ejection (CME) and the characteristics of associated
metre-wave activity. It is found that (1) the CME width and leading
edge velocity can be highly influential in determining the intensity,
spectral complexity and frequency coverage of both type II and continuum
bursts; (2) the presence of a CME is possibly a necessary condition for
the production of a metric continuum event and (3) metric continuum
bursts as well as intense, complex type II events are preferentially
associated with strong, long lasting soft X-ray events.
Title: Simultaneous optical, infrared and microwave observations of
the flare star AT Mic.
Authors: Nelson, G. J.; Robinson, R. D.; Slee, O. B.; Ashley, M. C. B.;
Hyland, A. R.; Tuohy, I. R.; Nikoloff, I.; Vaughan, A. E.
Bibcode: 1986MNRAS.220...91N
Altcode:
The first extensive observations of the binary dMe flare star AT Mic
made simultaneously at optical, infrared and microwave wavelengths are
reported. The observations have failed to show decreases in infrared
flux at the time of optical flares. Such decreases are expected if
the inverse Compton scattering mechanism proposed by Gurzadyan (1980)
is dominant during the flares. On the contrary, one large flare showed
a significant increase in the K-band flux consistent with an energy
spectrum which is flat, at least in the range from U to K bands. The
J-band flux, however, showed no significant average change during a
large number of smaller flares. These results are discussed in terms of
a number of thermal and nonthermal flare models. It is concluded that
the data can be best accounted for in terms of a hot (about 100,000 K)
plasma flare model which produces a relatively flat energy-spectrum
over a range of wavelengths determined by the temperature, density
and depth of the source region. This range may therefore vary from
flare to flare and from star to star. Microwave observations during
a large number of optical flares indicate a variation of at least
several orders of magnitude in the ratio of optical-to-microwave
flare luminosities. This is taken to imply that different plasmas are
responsible for the two emissions.
Title: The Relation Between Flare-Related Metric Continuum Bursts
and Coronal Mass Ejections
Authors: Robinson, R. D.
Bibcode: 1986SoPh..104...33R
Altcode:
We present the results of a study to determine the relationship
between flare-related metre wavelength continuum outburts and coronal
mass ejection events. Using radio data obtained with the Culgoora
radioheliograph and optical data from the SOLWIND experiment aboard
the P-78 satellite we find a good temporal and positional overlap
between the two types of events. In most cases the radio bursts could
be classed as either a type-II-related Flare Continuum (FC II) or a
Slow Drift Continuum (SDC). Comparing properties for the events it was
found that the continuum radio bursts were preferentially associated
with the broader, more rapidly moving CME transients.
Title: The variation of lithium equivalent width in active cool stars.
Authors: Robinson, R. D.; Thompson, K.; Innis, J. L.
Bibcode: 1986PASA....6..500R
Altcode: 1986PASAu...6..500R
In an attempt to determine whether the lithium line at 6707 Å comes
mainly from the quiet photosphere or from the cooler spots on spotted
active stars, the authors have measured line equivalent widths due to
Li and also due to Ca I at 6717 Å at various rotation phases on three
fast rotating stars, AB Doradus, PZ Telescopium and V1005 Orionis. The
results for AB Dor and PZ Tel show there might be some small rotational
modulation which can be linked to the best estimate of the spot
position from the available photometry. This modulation is less than
that expected on the basis of Giampapa's work on sunspot spectra. The
Li and Ca I line equivalent widths were found to anticorrelate.
Title: Fast Hα variations on a rapidly rotating spotted star.
Authors: Robinson, R. D.; Collier Cameron, A.
Bibcode: 1986PASA....6..308R
Altcode: 1986PASAu...6..308R
The observations presented here are part of a program designed to
investigate magnetically related activity on lower main sequence
stars. The data consist of a time series of spectra taken in the region
of Hα with high temporal and spectral resolution. The Hα line was
observed in order to study plage and flare like phenomena which might
be associated with the spots. In analysing these data it was found
that the Hα profile showed rapid time variations, with properties
substantially different from any previously reported on any active
chromosphere star. In this paper the authors describe these variations
and discuss their possible implications.
Title: Activity on cool stars.
Authors: Robinson, R. D.; Slee, O. B.
Bibcode: 1986AuJA....1..105R
Altcode:
The authors examine the characteristics of solar-like activity, which
is a term applied to a wide range of magnetically related phenomena
occurring in the atmospheres of cool stars. The authors describe in
general terms four basic types of activity: (1) active chromospheres,
(2) activity in transition regions and coronae, (3) starspots, and
(4) flares. Each of these phenomena are observable over nearly the
entire electromagnetic spectrum, and, when possible, the authors
attempt to show how data in various wavelength regions relate to
one another. They then examine the stellar parameters which seem to
influence the creation of activity and how the form of this activity
changes as the star evolves.
Title: The measurement of stellar photospheric magnetic fields
Authors: Robinson, R. D.
Bibcode: 1986HiA.....7..417R
Altcode:
A procedure for calculating the degree of magnetic splitting
is described. The procedure involves comparing the profile of a
magnetically sensitive spectral line with a magnetically insensitive
line; the development of the reference profile is discussed. The
limitations of the field measurement technique used to determine
the degree of magnetic splitting are considered. The technique is
applied to the measurement of the magnetic fields on cool stars and
the presence of kilogauss fields on a wide variety of cool stars is
confirmed. A summary of stellar magnetic field strengths and filling
factors observed to date is presented. It is noted that the strength
and/or area coverage of the fields vary, and the stellar activity is
related to the presence of magnetic fields.
Title: A Positional Comparison Between Coronal Mass Ejection Events
and Solar Type-II Bursts
Authors: Robinson, R. D.; Stewart, R. T.
Bibcode: 1985SoPh...97..145R
Altcode:
An investigation is made to determine the positional relation
between the leading edge of the coronal mass ejection (CME) and
the source region of associated solar type II radio bursts. A
preliminary relation between the optical and radio activity was
first established for each event using projected starting times and
positional data. Height - time plots were then deduced for the radio
activity using radiospectrograph observations in conjunction with a
variety of coronal density models. These plots were then compared with
height - time plots for the leading edge of the associated CME events,
which has been observed with the SOLWIND experiment aboard the P78-1
satellite. In 31 well-observed events a total of 13 (42%) had type II
bursts which could confidently be placed near the leading edge of the
CME. In these events the density model which gave the best agreement
between CME and type II positions was five times the Saito (1970) quiet
Sun model. The existence of these closely related events was further
confirmed by direct positional comparisons for the event of 1979,
May 4. In a further nine events the type II burst was seen within the
CME but was located well behind the leading edge, suggesting that they
were created by a blast wave. The remaining nine events had height -
time plots which could not be accurately compared. The observations are
discussed in relation to models for the CME and type II activity. We
suggest that the type II is generated when the shock wave is formed
within the closed field structure near the leading edge of the CME or,
in the case of a blast wave, interacts with closed fields in the body
of the transient.
Title: Velocities of Type-II Solar Radio Events
Authors: Robinson, R. D.
Bibcode: 1985SoPh...95..343R
Altcode:
Radial velocities for 144 simple but representative type II bursts
were determined from measured frequency time histories. The velocity
distribution is peaked in the region between 500 and 700 km s
−1 (with the exact value dependent upon the coronal
density model assumed) and skewed towards the larger velocities. In
85 % of the cases it was found that the velocities were constant with
height. In the remaining 15 % the drift rate decreased drastically
at low frequencies. This tended to occur for events having high
initial velocities. The measured velocity is dependent upon the
properties of the flare event but does not appear to be related to
other characteristics of the radio burst. Comparisons show that the
group of type II events studied had a velocity distribution which
was comparable with that for coronal mass ejection events seen in
association with type II bursts. The measured velocities were however
statistically smaller than those of interplanetary type II bursts.
Title: Flare continuum.
Authors: Robinson, R. D.
Bibcode: 1985srph.book..385R
Altcode: 1985srph.conf..385R
Flare-related metrewave continua can be divided into four distinct
classes. These are the two types of flare continuum (FCE and FCII),
the moving type IV and the slow-drift continuum (IVmB). The author
deals with the stationary continuum sources, including the FCE, the
FCII and the IVmB.
Title: A study of the rapidly rotating variable star HD 36705
(AB Doradus).
Authors: Innis, J. L.; Robinson, R. D.; Coates, D. W.; Thompson, K.
Bibcode: 1985PASA....6..156I
Altcode: 1985PASAu...6..156I
Photoelectric B and V light curves and high disperssion spectroscopic
observations were obtained in 1985 February for the rapidly rotating
spotted star HD 36705. The visual light range was about 0.09 magnitude,
with a well correlated B-V change of approximately 0.04 magnitude,
the star being redded when faintest. There is evidence for a broadband
flare of about 0.05 magnitude in V and 0.07 magnitude in B. This
occurred near maximum light. Spectroscopic observations show a
variation in the equivalent width of the Ca K emission by a factor
of about two in antiphase with the photometric variations, maximum
emission corresponding to minimum brightness. This is probably due
to bright plages and enhanced chromospheric heating associated with
the photospheric starspots. High dispersion, high signal-to-noise
spectroscopic observations confirm the presence of lithium 6708 A as
reported by Rucinski (1982, 1985). Several interpretations of the nature
of HD 36705 are discussed; however, at present, none is completely
satisfactory and further observations are required. As the star is
probably at less than 100 pc, a parallax determination may be possible,
and could help answer many of the questions concerning HD 36705.
Title: The RGO spectrograph
Authors: Robinson, R. D.
Bibcode: 1985rgsp.book.....R
Altcode:
No abstract at ADS
Title: Associations between coronal mass ejections and metric type
II bursts
Authors: Sheeley, N. R., Jr.; Howard, R. A.; Michels, D. J.; Robinson,
R. D.; Koomen, M. J.; Stewart, R. T.
Bibcode: 1984ApJ...279..839S
Altcode:
A statistical comparison of metric type II bursts and coronal
mass ejections (CMEs) during 1979-1982 was carried out. Type II
bursts without CMEs were associated with short-lived (0.5 hr) soft
X-ray events, but not with interplanetary shocks at the Helios 1
spacecraft. Type II bursts with CMEs were associated with longer-lived
X-ray events (3 hr on the average) and interplanetary shocks, and
the CMEs had speeds greater than 400 km/s. CMEs without metric type
II bursts were divided equally into groups faster and slower than 455
km/s. The faster CMEs were associated with interplanetary shocks, some
of which originated on the visible disk where metric type II bursts
should have been observed if they had occurred. These results suggest
that (1) shocks without CMEs have a relatively impulsive origin and may
die out sooner than many shocks with CMEs which are piston driven, and
(2) either some fast CMEs do not reach shock-producing super-Alfvenic
speeds until they leave the lower corona where the metric emission
originates, or these CMEs form shocks that are unable to excite type
II emission in the lower corona.
Title: Properties of Metre-Wavelength Solar Bursts Associated with
Interplanetary Type-II Emission
Authors: Robinson, R. D.; Stewart, R. T.; Cane, H. V.
Bibcode: 1984SoPh...91..159R
Altcode:
A statistical analysis is used to determine the properties of
metre-wavelength events which are associated with interplanetary type
II bursts. It is found that the likelihood of an interplanetary type
II burst is greatly increased if: (a) an associated metre-wavelength
type II has a starting frequency less than 45 MHz; (b) a strong
metre-wavelength continuum is present; (c) the type II contains
herringbone fine structure; and (d) the metre-wavelength activity is
accompanied by strong, long-lasting Hα and soft X-ray events.
Title: Scattering of radio waves in the solar corona
Authors: Robinson, R. D.
Bibcode: 1983PASA....5..208R
Altcode: 1983PASAu...5..208R
The effect of small-scale overdense structures, referred to as fibers,
on the scattering of radio waves in the solar corona is examined in
a quantitative manner. First, the ray tracing calculations and the
method used to incorporate the fibers are discussed. The parameters
of the source and coronal models used are then described, and examples
of ray paths traced through various coronal models are given. Finally,
the results of calculations are presented.
Title: A catalogue of major metre-wavelength solar events recorded
by the Dapto and Culgoora solar radio observatories (1961-1981)
Authors: Robinson, R. D.; Tuxford, J. M.; Sheridan, K. V.; Stewart,
R. T.
Bibcode: 1983PASA....5...84R
Altcode: 1983PASAu...5...84R
A catalog of 668 major, meter-wavelength radio events derived from
dynamic spectra obtained with a radio spectrograph, which recorded
radio emission intensity as a function of both frequency and time
is presented. The first of the catalog's two tables updates the
earlier catalog of Boorman et al (1961), listing the times of the
three (Type III, Type II, and continuum) components of a major radio
event. In addition, time, location and importance of the associated
optical flare, and the time and location of the short wave fadeout,
are tabulated. In the second table, the earlier catalog is expanded
by describing the morphological properties of the various phases of
the radio event. Comments, especially concerning associated microwave
activity and the tabulated optical flares, are also given.
Title: Sunspot size variations.
Authors: Boice, D. C.; Robinson, R. D.
Bibcode: 1983srca.conf...27B
Altcode: 1983srca....8...27B
No abstract at ADS
Title: Size Variations in Regular Sunspots
Authors: Robinson, R. D.; Boice, D. C.
Bibcode: 1982SoPh...81...25R
Altcode:
A study was made of the time development for 31 simple sunspots. The
growth and decay rates varied between spots, but were approximately
constant during the lifetime of individual, relatively shortlived
spots. Long-lived spots showed oscillations in umbral size, with periods
of three to five days. Occasionally, rapid changes in the spot size
were observed.
Title: On the generation of magnetic fields in late-type stars -
A local time-dependent dynamo model
Authors: Robinson, R. D.; Durney, B. R.
Bibcode: 1982A&A...108..322R
Altcode:
We assume that the magnetic field of late-type stars is generated in
the lower part of the star's convection zone and study this generation
mechanism with the help of local (in latitude) dynamo equations. For
the spectral types GO, GS, KO, KS, MO, M2, and M5 we evaluate the
magnetic field and period of the cycles as a function of rotation and
compare them with the available observational data.
Title: On an estimate of the dynamo-generated magnetic fields in
late-type stars.
Authors: Durney, B. R.; Robinson, R. D.
Bibcode: 1982ApJ...253..290D
Altcode:
The principal objective of the present investigation is related to a
prediction of the variation of magnetic fields with stellar type and the
role of pertinent variables (such as rotation and differential rotation)
with respect to the field properties. This is accomplished by estimating
a typical amplification time for the magnetic field, and a typical 'time
of rise' for the magnetic field due to magnetic buoyancy. It is assumed
that the magnetic field is generated principally in the lower part of
the stellar convection zone. Local (in latitude) dynamo equations are
considered. The selected approach consists basically in an estimate of
the typical magnitude of the magnetic field as predicted by the local
dynamo equations. The employed approach constitutes only a first step
towards the evaluation of magnetic fields in stars other than the sun.
Title: A study of multiple type II solar radio events
Authors: Robinson, R. D.; Sheridan, K. V.
Bibcode: 1982PASA....4..392R
Altcode: 1982PASAu...4..392R
The authors present a study of solar Type II events in which the bands
are not related to one another in the normal manner. The events have the
appearance of several separate Type II bursts, sometimes overlapping in
time; they have been termed multiple Type II events. Primary emphasis
in this study has been placed on the frequency evolution of each band
and their relation to each other and to the optical flaring phenomena.
Title: Multiple-frequency measurements of a flare continuum event
Authors: Robinson, R. D.
Bibcode: 1982PASA....4..389R
Altcode: 1982PASAu...4..389R
An analysis of a meter-wavelength continuum observed during a Type II
solar burst is presented. Spectral recordings were made in the 400-2000
MHz range, followed by spectral spreading and an exponential intensity
increase, after a Type I event had subsided. The continuum source was
characterized as an arch-like structure oriented in an N-S direction,
with the southern footprint situated in the same place as that of the
eastern footprint of the previous storm. The continuum is suggested
to have been caused by electrons confined and accelerated in a region
behind the shock front. A coronal mass transient is identified as the
most likely continuum source. The sequence of the FC II event observed
is qualitatively modeled as a shock wave delineating the outer edge
of the transient activity and preheating the plasma. The high levels
of magnetic turbulence present in the shock are caused by the shock
interacting with the ambient magnetic fields.
Title: A preflare diminution in the quiescent flux of EQ Pegasi.
Authors: Giampapa, M. S.; Africano, J. L.; Klimke, A.; Parks, J.;
Quigley, R. J.; Robinson, R. D.; Worden, S. P.
Bibcode: 1982ApJ...252L..39G
Altcode:
The occurrence of a remarkable flare event on EQ Peg is reported as
recorded by high speed photometry in the Johnson U band: a stellar
flare event is immediately preceded by a well-defined decline in
the quiescent flux of the star. The U band flux decays to a minimum
level that is 75% of the stellar quiescent flux, and the duration of
the so-called negative flare event is 2.7 minutes. A description of
the observation is presented and hypotheses are discussed that may
eventually account for this phenomenon.
Title: A preliminary interpretation of stellar chromospheric CA II
emission variations within the framework of stellar dynamo theory.
Authors: Durney, B. R.; Mihalas, D.; Robinson, R. D.
Bibcode: 1981PASP...93..537D
Altcode:
Recent stellar chromospheric Ca II emission data are analyzed
and interpreted within the framework of simple concepts of dynamo
theory. From an examination of the rotation rates and B-V indexes of
26 stars as presented by Vaughn at el. (1981) and the background flux
values derived by Wilson (1978) for 18 reference stars, an empirical
relation is derived between dynamo number, calculated from the B-V
index and rotation rate, and stellar chromospheric emission flux. The
Ca-emission cycle morphology of the sample stars is then examined,
and differences between the four morphological classes identified
are explained in terms of the correlation of large dynamo numbers
with the presence of several interfering magnetic modes of different
spatial scales, which do not exhibit a marked cyclic behavior, and
small numbers with the excitation of only a single mode. The gap
noted by Vaughn and Preston (1980) in the relation between the log
of the emission flux with (B-V) is then interpreted as representing
a transition from a multiple-mode dynamo to a single-mode dynamo as
the dynamo number decreases.
Title: High Resolution Spectroscopy of Solar Activity - Part One -
Observing Procedures
Authors: Cram, L. E.; Robinson, R. D.; Mauter, H. A.; Mann, G. R.;
Phillis, G. L.
Bibcode: 1981SoPh...71..237C
Altcode:
We describe an observing program designed to obtain high spatial
resolution photographic spectra of solar active region phenomena,
with time resolution as short as 6 s. The Vacuum Tower Telescope
and Echelle Spectrograph at Sacramento Peak Observatory are used to
make observations simultaneously in Hα, He D3, Ca II K, Mg
b1, the CN bandhead at λ3883, and the magnetically-sensitive
line Fe I λ6302. Images reflected from the slit jaw are exposed
simultaneously in white-light and Hα. Observations of chromospheric
heating, following a high-velocity infall along an Hα superpenumbral
filament, are presented to illustrate the capabilities of the program.
Title: Possible radio flaring activity on a late-type giant star,
alf Cet.
Authors: Boice, D. C.; Kuhn, J. R.; Robinson, R. D.; Worden, S. P.
Bibcode: 1981ApJ...245L..71B
Altcode:
A search is made for radio flare events on 15 late-type stars using the
Arecibo radio telescope at 430 MHz. About 50 hr of interference-free
data are obtained and one impulsive event is detected for the M2
III star Alpha Cet. The observed radio event is consistent with the
hypothesis that it originated in a solar-flare-type disturbance in
the stellar atmosphere.
Title: On an Estimate of the Dynamo-Generated Magnetic Fields in
Late-Type Stars
Authors: Durney, B. R.; Robinson, R. D.
Bibcode: 1981BAAS...13..791D
Altcode:
No abstract at ADS
Title: On an Estimate of the Dynamo-Generated Magnetic Fields in
Late-Type Stars
Authors: Durney, B. R.; Robinson, R. D.
Bibcode: 1981BAAS...13..906D
Altcode:
No abstract at ADS
Title: Spatial and temporal correlation of high and low temperature
solar flare emissions.
Authors: Rust, D. M.; Buhmann, R. W.; Dennis, B. R.; Robinson, R. D.;
Willson, R. F.; Simon, M.
Bibcode: 1980BAAS...12..752R
Altcode:
No abstract at ADS
Title: Observations of magnetic fields on two late-type dwarf stars.
Authors: Robinson, R. D.; Worden, S. P.; Harvey, J. W.
Bibcode: 1980ApJ...236L.155R
Altcode:
The detection of magnetic fields on the G8 V star Xi Boo A and on
the K0 B star 70 Oph A is reported. A method has been developed
and applied which provides estimates of the total field strength
and fraction of the visible stellar surface covered by fields. The
essence of this method is to perform the Fourier deconvolution of a
nonmagnetically sensitive line profile from a magnetically sensitive
one. This method is insensitive to magnetic field geometry and does not
depend on polarization measurements. Results indicate fields of 2550 +
or 390 gauss covering 20-45% Xi Boo A, and that fields of 1880 + or -
350 gauss may cover 10% of 70 Oph A. Comparison observations of solar
active regions show fields of 1800 + or - 550 gauss covering 10% of
the solar active regions, in line with previous work. Tests of solar
quiet regions and sunspots also support the validity of the stellar
measurements.
Title: Stellar Magnetic Field Measurements: A New Method
Authors: Robinson, R. D.
Bibcode: 1979BAAS...11..633R
Altcode:
No abstract at ADS
Title: Detection of Magnetic Fields on Late-type Stars
Authors: Robinson, R. D.; Boice, D.; Worden, S. P.
Bibcode: 1979BAAS...11..648R
Altcode:
No abstract at ADS
Title: A study of solar flare continuum events observed at metre
wavelengths.
Authors: Robinson, R. D.
Bibcode: 1978AuJPh..31..533R
Altcode:
This paper presents observations and an interpretation of solar
flare continuum radiation. Two types are distinguished: FCM which
often precedes a moving type IV burst, and FCII which often follows a
type II burst. Theoretical models are proposed for the two types of
flare continuum event. For FCM events, the electrons are assumed to
be injected into coronal arches, where they are confined by Coulomb
scattering and wave-particle interactions. The radiation is due to
either gyrosynchrotron or Langmuir wave processes. For FCII events,
the electrons are assumed to be accelerated by large amplitude Alfvenic
turbulence, the resultant radiation being caused by the conversion of
Langmuir waves to electromagnetic radiation at the fundamental.
Title: A discussion of radio wavelength radiation generated by
gyrosynchrotron and Langmuir-wave processes
Authors: Robinson, R. D.
Bibcode: 1978AuJPh..31..547R
Altcode:
Synchrotron and Langmuir-wave emission processes are compared, with
special emphasis being placed on meter-wavelength radiation from the
solar corona. Numerical calculations are presented which relate to
brightness temperature, circular polarization, and source structure. It
is found that both gyrosynchrotron and Langmuir processes can produce
brightness temperatures up to about 2-3 GK; larger temperatures
require induced Langmuir-wave emission. It is further shown that
gyrosynchrotron source positions observed at different frequencies
may have a dispersion in height within an inhomogeneous background
medium. Although many observations may be explained exclusively by
one of the two processes, there are numerous circumstances in which
both processes are equally likely.
Title: Observations and Interpretation of Moving Type IV Solar
Radio Bursts
Authors: Robinson, R. D.
Bibcode: 1978SoPh...60..383R
Altcode:
Properties of 23 moving type IV bursts observed with the Culgoora
Radioheliograph are summarized. Both shock and plasmoid models
are examined. It is found that the theories invoking shocks have
limited application and that plasmoid models have several problems
with regard to plasmoid formation as well as with explanations for
multiple sources and large values of circular polarization. While the
synchrotron radiation mechanism is the most widely accepted for both
shock and plasmoid models, it is possible that Langmuir wave emission
processes may be important, at least in some events. To overcome some
of the difficulties of the plasmoid theory, a new source model is
proposed. This model involves synchrotron radiation from electror ;
confined by rapid scattering through hydromagnetic wave particle
interactions.
Title: The emission of Langmuir waves in the solar corona.
Authors: Robinson, R. D.
Bibcode: 1978ApJ...222..696R
Altcode:
The emission of Langmuir waves is extremely important in relation to
the generation of meter wavelength radio bursts. The Langmuir wavenumber
spectrum emitted from several representative electron distributions is
discussed. For electrons having isotropic pitch angle distributions,
the spectrum produced is well defined and determined primarily by the
electron energy distribution. For anisotropic pitch angle distributions,
induced emission may result, with consequent interaction between
particles and waves. Both loss-cone and streaming distributions may
cause Langmuir waves to be amplified. For the streaming distribution,
this generation is primarily in the direction of streaming; for the
loss-cone distribution, wave generation perpendicular to the field
lines is favored.
Title: A study of type V solar radio bursts. II. A theoretical model.
Authors: Robinson, R. D.
Bibcode: 1978SoPh...56..405R
Altcode:
A model for the solar Type V event is developed. This model assumes
that the basic difference between Type III and Type V bursts is
the evolution of the electron beam. For a Type V this beam rapidly
elongates, so that it takes progressively longer times to pass higher
plasma levels. Physical process influencing the beam development,
including Coulomb collisions, non-linear interactions with Langmuir
waves and wave-particle scattering from various hydromagnetic wave
modes is discussed. The model is compared with previously derived
models and with observations.
Title: A study of type V solar radio bursts. I: Observations.
Authors: Robinson, R. D.
Bibcode: 1977SoPh...55..459R
Altcode:
Results of an observational study of Type V bursts are
presented. Observations were made using the C.S.I.R.O. radioheliograph
at Culgoora. Source parameters studied included flux evolution,
polarization, size, shape, position, motions and brightness temperature
at 160, 80 and 43 MHz. Comparisons of source characteristics observed
at different frequencies are made.
Title: White light and radio studies of the coronal transient of 14
15 September 1973
Authors: Smerd, S. F.; Dulk, G. A.; MacQueen, R. M.; Gosling,
J. T.; Magun, A.; Stewart, R. T.; Sheridan, K. V.; Robinson, R. D.;
Jacques, S.
Bibcode: 1976SoPh...49..369S
Altcode:
Observations of a coronal transient event were obtained in
white light by the Skylab coronagraph and at metric wavelengths
by the radioheliograph and spectrograph at Culgoora and the
spectrograph-interferometer at Boulder. The continuum radio burst was
found to originate above the outward-moving white light loop - a region
of compressed material headed by a bow wave. The computed density
in the region of radio emission, based on either gyro-synchrotron or
harmonic plasma radiation mechanisms, was approximately 10 times the
ambient coronal density; this is compatible with the density deduced
from the white light observations. The magnetic energy density derived
from the radio observations was greater than 10 times the thermal
energy density, marginally larger than the kinetic energy density in
the fastest moving portion of the transient, and considerably larger
in most other regions. The ambient medium, the white light front, the
compression region, the loop, and the slower, massive flow of material
behind are each examined. It is found that the plasma was magnetically
controlled throughout, and that magnetic forces provided the principal
mechanism for acceleration of the transient material from the Sun.
Title: White Light and Radio Studies of the Coronal Transient of
14-15 September 1973. I: Material Motions and Magnetic Field
Authors: Dulk, G. A.; Jacques, S.; Smerd, S. F.; MacQueen, R. M.;
Gosling, J. T.; Steward, R. T.; Sheridan, K. V.; Robinson, R. D.;
Magun, A.
Bibcode: 1976SoPh...49..369D
Altcode:
Observations of a coronal transient event were obtained in white
light by the Skylab coronagraph and at metric wavelengths by the
radioheliograph and spectrograph at Culgoora, Australia, and the
spectrograph-interferometer at Boulder, Colo. The continuum radio burst
was found to originate above the outward-moving white-light loop, a
region of compressed material headed by a bow wave. The computed density
in the region of radio emission, based on either gyrosynchrotron or
harmonic plasma radiation mechanisms, was approximately 10 times the
ambient coronal density; this is compatible with the density deduced
from the white-light observations. The magnetic-energy density derived
from the radio observations was greater than 10 times the thermal
energy density, marginally larger than the kinetic energy density in
the fastest-moving portion of the transient, and considerably larger
in most other regions. The ambient medium, the white-light front, the
compression region, the loop, and the slower massive flow of material
behind are each examined. It is found that the plasma was magnetically
controlled throughout and that magnetic forces provided the principal
mechanism for acceleration of the transient material from the sun.
Title: Radio emission from a source near the flare star AD Leonis.
Authors: Robinson, R. D.; Slee, O. B.; Little, A. G.
Bibcode: 1976ApJ...203L..91R
Altcode:
A weak, probably variable, source has been located 1.85 min southeast of
the flare star, AD Leo. Characteristics of this source are discussed,
and an interpretation is given based on plasma emission from a stellar
companion to the flare star.
Title: Three Frequency Radioheliograph Observations of Solar
Continuum Events
Authors: Magun, A.; Stewart, R. T.; Robinson, R. D.
Bibcode: 1975PASA....2..367M
Altcode: 1975PASAu...2..367M
The paper reports observations of seven solar continuum bursts which
were made with a radioheliograph at 160, 80, and 43 MHz. The following
properties of the bursts are especially noted: (1) their duration
varies between 10 and 100 min; (2) the radiation is emitted over
a large range of heights, but each frequency peaks near the second
harmonic plasma level; (3) the peak brightness temperature varies
between 3 million and 5 billion K in all but one case; (4) three
events are unpolarized; and (5) the polarization varies from 20% to
70% in the remaining four events. In every case, the radio emission is
found to originate from stationary sources whose heights increase with
decreasing frequency. Based on this observation, it is concluded that
both plasma and synchotron radiation are possible emission mechanisms.
Title: Multi-Frequency Heliograph Observations of Type II Bursts
Authors: Nelson, G. J.; Robinson, R. D.
Bibcode: 1975PASA....2..370N
Altcode: 1975PASAu...2..370N
Results are presented for analysis of multifrequency observations
of five solar type II bursts which were made with a radioheliograph
at frequencies of 160, 80, and 43.25 MHz. The spectra of all five
bursts are found to exhibit split-banded harmonic structure. It is
shown that (1) fundamental sources are, on the average, twice as large
and twice as bright as harmonic sources; (2) fundamental sources are
much higher than harmonic ones; (3) both types of sources increase
in size with decreasing frequency; (4) the sizes of related upper
and lower fundamental and harmonic sources are essentially equal,
but the lower sources are twice as bright; (5) the observed sources
are nearly circular and only slightly elongated; and (6) the maximum
observed brightness temperature increases with decreasing frequency in
the present frequency range. These results indicate that the portion
of a wide-angle MHD disturbance which becomes a type II-emitting shock
wave does not generally propagate radially or along the local density
gradient; instead, it tends to move away from large sunspots. It
is concluded that the large sizes and source separations observed
in this study are incompatible with the predictions made by current
scattering theories.
Title: Solar Flare Continua at the Metre Wavelengths
Authors: Robinson, R. D.; Smerd, S. F.
Bibcode: 1975PASA....2..374R
Altcode: 1975PASAu...2..374R
The term 'flare continuum' (FC) has been used by Wild (1970) to
describe a strong, stationary continuum source at the meter wavelengths
which occurs early in some flare events, particularly those of high
energy. Two classes of flare continuum are distinguished from Culgoora
radio spectrograph and heliograph observations of transient solar radio
events. The first class (FCII) is the continuum which occurs during
and following a type II burst. The second class (FCM) is that which
starts with or slightly before the flash phase and often precedes a
related IVM burst. The two are considered related if the FCM source
seems to become the IVM source or if the IV source seems to emerge
from the FC source. All FC bursts are considered to be radiated
by suprathermal electrons magnetically confined in a loop or arch
configuration. Possible differences in the plasma-field configurations
and electron energies involved in the two types of FC are examined.
Title: On the Polarization Characteristics of Moving Type IV Solar
Radio Bursts
Authors: Robinson, R. D.
Bibcode: 1974PASA....2..258R
Altcode: 1974PASAu...2..258R
Any theory dealing with type IV solar radio bursts must explain the
observed evolution of their brightness temperature and polarization. The
behaviour of isolated, moving type IV sources is characterized by a
long period of constant and low polarization during the major part of
their lifetime, followed by a substantial increase in the degree of
circular polarization during the declining phase. This behaviour puts
severe constraints on and gives valuable clues as to the evolution of
the physical conditions within the source.
Title: An Analysis of Transient Events Observed in the Fe XIV Coronal
Emission Line
Authors: Demastus, H. L.; Wagner, W. J.; Robinson, R. D.
Bibcode: 1974fpsw.conf...17D
Altcode:
No abstract at ADS
Title: Coronal Disturbances. I: Fast Transient Events Observed in
the Green Coronal Emission Line During the Last Solar Cycle
Authors: Demastus, H. L.; Wagner, W. J.; Robinson, R. D.
Bibcode: 1973SoPh...31..449D
Altcode:
Time lapse movies acquired with the Sacramento Peak Observatory
6-in filter coronagraph very infrequently show dramatic transient
events. Many of these events (at 1.04 < r/r⊙ < 1.20)
are correlated with Hα activity at the limb, especially with large
surges and eruptive prominences. Metric or centrimetric radio bursts
are found associated with certain types of λ5303 coronal events. The
heights to which such disturbances will propagate can often be estimated
from the green line movie data. We report the results of a study of
all the examples of such transients observed during the last 16 years.
Title: Fast Transient Events Observed in the Green Coronal Emission
Line
Authors: Demastus, H. L.; Wagner, W. J.; Robinson, R. D.
Bibcode: 1973BAAS....5S.270D
Altcode:
No abstract at ADS