Author name code: dziembowski
ADS astronomy entries on 2022-09-14
author:"Dziembowski, Wojciech A."
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Title: The complex asteroseismology of SX Phoenicis
Authors: Daszyńska-Daszkiewicz, J.; Walczak, P.; Pamyatnykh, A.;
Szewczuk, W.; Dziembowski, W.
Bibcode: 2020svos.conf...81D
Altcode: 2019arXiv191200405D
This paper presented a seismic analysis of the prototype SX Phoenicis,
and was aimed at fitting the two radial-mode frequencies and the
corresponding values of the bolometric flux amplitude (the parameter
$f$), whose empirical values were derived from multi-coulor photometric
observations. A seismic model that meets those conditions is of low
mass (M=1.05 M_⊙), has moderately effective convection in the outer
layers and described by the mixing-length parameter $\alpha_{\rm MLT}
\approx 0.7$, and a microturbulent velocity in the atmospheres of
about $\xi_{\rm t}\approx 8 \kms$. These seismic studies of a star
like SX Phe are very important for deriving constraints on outer-layer
convection because the object is borderline between very effective
and ineffective convection.
Title: VizieR Online Data Catalog: Anomalous RRd stars in Magellanic
Clouds (Soszynski+, 2016)
Authors: Soszynski, I.; Smolec, R.; Dziembowski, W. A.; Udalski,
A.; Szymanski, M. K.; Wyrzykowski, L.; Ulaczyk, K.; Poleski, R.;
Pietrukowicz, P.; Kozlowski, S.; Skowron, D.; Skowron, J.; Mroz, P.;
Pawlak, M.
Bibcode: 2018yCat..74631332S
Altcode:
The sample of anomalous RRd stars has been extracted from the OGLE
collection of 45451 RR Lyrae variables in the Magellanic Clouds
(Soszyski et al., 2016AcA....66..131S). (2 data files).
Title: VizieR Online Data Catalog: OGLE-III variable stars in Galactic
disk area (Pietrukowicz+, 2013)
Authors: Pietrukowicz, P.; Dziembowski, W. A.; Mroz, P.; Soszynski, I.;
Udalski, A.; Poleski, R.; Szymanski, M. K.; Kubiak, M.; Pietrzynski,
G.; Wyrzykowski, L.; Ulaczyk, K.; Kozlowski, S.; Skowron, J.
Bibcode: 2017yCat.120630379P
Altcode:
Observations presented in this paper were collected with the 1.3-m
Warsaw telescope at Las Campanas Observatory, Chile, during the third
phase of the Optical Gravitational Lensing Experiment (OGLE-III) in
years 2001-2009. 21 fields in the direction tangent to the Centaurus
Arm of the Galactic disk were observed. A significant majority of frames
(815-2698 per field) were collected in the standard I-band filter with
an exposure time of 120s or 180s. Additional observations, consisting
of only 3-8 measurements, were taken in the V -band filter with an
exposure time of 240s. 13 δSct objects (from OGLE-GD-DSCT-0059 to
OGLE-GD-DSCT-0071) are added after the spectroscopic follow-up described
in Pietrukowicz+ (2015AcA....65...63P ; arXiv:1503.03499)). The spectra
were obtained with the 2.5-m Irenee du Pont telescope at Las Campanas
Observatory on two nights, April 27/28 and April 28/29, 2014. (10
data files).
Title: UVSat: a concept of an ultraviolet/optical photometric
satellite
Authors: Pigulski, A.; Baran, A.; Bzowski, M.; Cugier, H.; Czerny,
B.; Daszyńska-Daszkiewicz, J.; Dziembowski, W.; Handler, G.;
Kołaczkowski, Z.; Królikowska, M.; Krzesiński, J.; Maciejewski, G.;
Michalska, G.; Molenda-Żakowicz, J.; Moskalik, P.; Niedzielski, A.;
Niemczura, E.; Ostrowski, J.; Pamyatnykh, A.; Ratajczak, M.; Rucinski,
S.; Siwak, M.; Smolec, R.; Szutowicz, S.; Tomov, T.; Wyrzykowski,
Ł.; Zoła, S.; Sarna, M.
Bibcode: 2017sbcs.conf...76P
Altcode: 2017arXiv171110366P
Time-series photometry from space in the ultraviolet can be presently
done with only a few platforms, none of which is able to provide
wide-field long-term high-cadence photometry. We present a concept of
\sat, a twin space telescope which will be capable to perform this
kind of photometry, filling an observational niche. The satellite
will host two telescopes, one for observations in the ultraviolet,
the other for observations in the optical band. We also briefly show
what science can be done with UVSat.
Title: Petersen diagram revolution
Authors: Smolec, Radoslaw; Dziembowski, Wojciech; Moskalik, Pawel;
Netzel, Henryka; Prudil, Zdenek; Skarka, Marek; Soszynski, Igor
Bibcode: 2017EPJWC.15206003S
Altcode: 2017arXiv170303029S
Over the recent years, the Petersen diagram for classical pulsators,
Cepheids and RR Lyr stars, populated with a few hundreds of new
multiperiodic variables. We review our analyses of the OGLE data, which
resulted in a significant extension of the known, and in the discovery
of a few new and distinct forms of multiperiodic pulsation. The showcase
includes not only radial mode pulsators, but also radial-non-radial
pulsators and stars with significant modulation observed on top of the
beat pulsation. First theoretical models explaining the new forms of
stellar variability are briefly discussed.
Title: Blue large-amplitude pulsators as a new class of variable stars
Authors: Pietrukowicz, Paweł; Dziembowski, Wojciech A.; Latour,
Marilyn; Angeloni, Rodolfo; Poleski, Radosław; di Mille, Francesco;
Soszyński, Igor; Udalski, Andrzej; Szymański, Michał K.;
Wyrzykowski, Łukasz; Kozłowski, Szymon; Skowron, Jan; Skowron,
Dorota; Mróz, Przemek; Pawlak, Michał; Ulaczyk, Krzysztof
Bibcode: 2017NatAs...1E.166P
Altcode: 2017NatAs...1..166P; 2017arXiv170607802P
Regular intrinsic brightness variations observed in many stars are
caused by pulsations. These pulsations provide information on the global
and structural parameters of the star. The pulsation periods range
from seconds to years, depending on the compactness of the star and
properties of the matter that forms its outer layers. Here, we report
the discovery of more than a dozen previously unknown short-period
variable stars: blue large-amplitude pulsators. These objects show very
regular brightness variations with periods in the range of 20-40 min
and amplitudes of 0.2-0.4 mag in the optical passbands. The phased
light curves have a characteristic sawtooth shape, similar to the
shape of classical Cepheids and RR Lyrae-type stars pulsating in the
fundamental mode. The objects are significantly bluer than main-sequence
stars observed in the same fields, which indicates that all of them are
hot stars. Follow-up spectroscopy confirms a high surface temperature
of about 30,000 K. Temperature and colour changes over the cycle
prove the pulsational nature of the variables. However, large-amplitude
pulsations at such short periods are not observed in any known type
of stars, including hot objects. Long-term photometric observations
show that the variable stars are very stable over time. Derived rates
of period change are of the order of 10-7 per year and,
in most cases, they are positive. According to pulsation theory, such
large-amplitude oscillations may occur in evolved low-mass stars that
have inflated helium-enriched envelopes. The evolutionary path that
could lead to such stellar configurations remains unknown.
Title: Anomalous double-mode RR Lyrae stars in the Magellanic Clouds
Authors: Soszyński, I.; Smolec, R.; Dziembowski, W. A.; Udalski,
A.; Szymański, M. K.; Wyrzykowski, Ł.; Ulaczyk, K.; Poleski, R.;
Pietrukowicz, P.; Kozłowski, S.; Skowron, D.; Skowron, J.; Mróz,
P.; Pawlak, M.
Bibcode: 2016MNRAS.463.1332S
Altcode: 2016arXiv160800576S; 2016MNRAS.tmp.1057S
We report the discovery of a new subclass of double-mode RR Lyrae
stars in the Large and Small Magellanic Clouds. The sample of 22
pulsating stars has been extracted from the latest edition of the
Optical Gravitational Lensing Experiment collection of RR Lyrae
variables in the Magellanic System. The stars pulsating simultaneously
in the fundamental (F) and first-overtone (1O) modes have distinctly
different properties than regular double-mode RR Lyrae variables
(RRd stars). The P1O/PF period ratios of our
anomalous RRd stars are within a range of 0.725-0.738, while `classical'
double-mode RR Lyrae variables have period ratios in the range of
0.742-0.748. In contrast to the typical RRd stars, in the majority
of the anomalous pulsators, the F-mode amplitudes are higher than the
1O-mode amplitudes. The light curves associated with the F-mode in the
anomalous RRd stars show different morphology than the light curves
of, both, regular RRd stars and single-mode RRab stars. Most of the
anomalous double-mode stars show long-term modulations of the amplitudes
(Blazhko-like effect). Translating the period ratios into the abundance
parameter, Z, we find for our stars Z ∈ (0.002, 0.005) - an order of
magnitude higher values than typical for RR Lyrae stars. The mass range
of the RRd stars inferred from the WI versus PF
diagram is (0.55-0.75) M⊙. These parameters cannot be
accounted for with single star evolution assuming a Reimers-like
mass-loss. Much greater mass-loss caused by interaction with other
stars is postulated. We blame the peculiar pulsation properties of
our stars to the parametric resonance instability of the 1O-mode to
excitation of the F- and 2O-modes as with the inferred parameters of
the stars 2ω1O ≈ ωF + ω2O.
Title: Nonradial oscillations in classical pulsating
stars. Predictions and discoveries
Authors: Dziembowski, W. A.
Bibcode: 2016CoKon.105...23D
Altcode: 2015arXiv151203708D
After a brief historical introduction and recalling basic concepts
of stellar oscillation theory, I focus my review on interpretation
of secondary periodicities found in RR Lyrae stars and Cepheids as a
manifestation of nonradial mode excitation.
Title: Internal Rotation of the Red-giant Star KIC 4448777 by Means
of Asteroseismic Inversion
Authors: Di Mauro, M. P.; Ventura, R.; Cardini, D.; Stello, D.;
Christensen-Dalsgaard, J.; Dziembowski, W. A.; Paternò, L.; Beck,
P. G.; Bloemen, S.; Davies, G. R.; De Smedt, K.; Elsworth, Y.; García,
R. A.; Hekker, S.; Mosser, B.; Tkachenko, A.
Bibcode: 2016ApJ...817...65D
Altcode: 2015arXiv151106160D
We study the dynamics of the stellar interior of the early red-giant
star KIC 4448777 by asteroseismic inversion of 14 splittings of
the dipole mixed modes obtained from Kepler observations. In order
to overcome the complexity of the oscillation pattern typical of
red-giant stars, we present a procedure to extract the rotational
splittings from the power spectrum. We find not only that the core
rotates from a minimum of 8 to a maximum of 17 times faster than the
surface, confirming previous inversion results generated for other
red giants (Deheuvels et al.), but we also estimate the variation of
the angular velocity within the helium core with a spatial resolution
of 0.001R and verify the hypothesis of a sharp discontinuity in the
inner stellar rotation. The results show that the entire core rotates
rigidly and provide evidence for an angular velocity gradient around
the base of the hydrogen-burning shell; however, we do not succeed in
characterizing the rotational slope, due to the intrinsic limits of
the applied techniques. The angular velocity, from the edge of the
core, appears to decrease with increasing distance from the center,
reaching an average value in the convective envelope of 68 ± 22
nHz. We conclude that a set of data that includes only dipolar modes
is sufficient to infer quite accurately the rotation of a red giant
not only in the dense core but also, with a lower level of confidence,
in part of the radiative region and in the convective envelope.
Title: Discovery of a new group of double-periodic RR Lyrae stars
in the OGLE-IV photometry.
Authors: Netzel, H.; Smolec, R.; Dziembowski, W.
Bibcode: 2015MNRAS.451L..25N
Altcode: 2015arXiv150405765N
We report the discovery of a new group of double-periodic RR Lyrae
stars from the analysis of the Optical Gravitational Lensing Experiment
- IV (OGLE-IV) Galactic bulge photometry. In 11 stars identified
in the OGLE catalogue as first overtone pulsators (RRc stars), we
detect additional longer period variability of low amplitude, in the
mmag regime. One additional star of the same type is identified in a
published analysis of the Kepler space photometry. The period ratio
between the shorter first overtone period and a new, longer period
lies in a narrow range around 0.686. Thus, the additional period is
longer than the expected period of the undetected radial fundamental
mode. The obvious conclusion that addition periodicity corresponds to
a gravity or a mixed mode faces difficulties, however.
Title: Oscillation modes in the rapidly rotating slowly pulsating
B-type star μ Eridani
Authors: Daszyńska-Daszkiewicz, J.; Dziembowski, W. A.; Jerzykiewicz,
M.; Handler, G.
Bibcode: 2015MNRAS.446.1438D
Altcode: 2014arXiv1410.6283D
We present results of a search for identification of modes
responsible for the six most significant frequency peaks detected
in the rapidly rotating slowly pulsating B-type (SPB) star μ
Eridani. All published and some unpublished photometric data are
used in our new analysis. The mode identification is carried out with
the method developed by Daszyńska-Daszkiewicz et al. employing the
phases and amplitudes from multiband photometric data and relying on
the traditional approximation for the treatment of oscillations in
rotating stars. Models consistent with the observed mean parameters
are considered. For the five frequency peaks, the candidates for the
identifications are searched amongst unstable modes. In the case of the
third frequency, which is an exact multiple of the orbital frequency,
this condition is relaxed. The systematic search is continued up to a
harmonic degree ℓ = 6. Determination of the angular numbers, (ℓ,
m), is done simultaneously with the rotation rate, Vrot,
and the inclination angle, i, constrained by the spectroscopic
data on the projected rotational velocity, Vrotsin i,
which is assumed constant. All the peaks may be accounted for with
g-modes of high radial orders and the degrees ℓ ≤ 6. There are
differences in some identifications between the models. For the two
lowest-amplitude peaks the identifications are not unique. None the
less, the equatorial velocity is constrained to a narrow range of (135,
140) km s-1. Our work presents the first application of
the photometric method of mode identification in the framework of the
traditional approximation and we believe that it opens a new promising
direction in studies of SPB stars.
Title: Bohdan Paczyński (1940-2007)
Authors: Dziembowski, Wojciech
Bibcode: 2014pas..conf...27D
Altcode:
No abstract at ADS
Title: OGLE-BLG-RRLYR-12245: An RR Lyrae Star that Switched from a
Double- to Single-Mode Pulsation
Authors: Soszyński, I.; Dziembowski, W. A.; Udalski, A.; Szymański,
M. K.; Kubiak, M.; Pietrzyński, G.; Wyrzykowski, Ł.; Ulaczyk, K.;
Poleski, R.; Kozłowski, S.; Pietrukowicz, P.; Skowron, J.; Mróz, P.
Bibcode: 2014AcA....64....1S
Altcode: 2014arXiv1403.6476S
We report the discovery of an RR Lyr star that experienced a switching
of its pulsation mode. OGLE-BLG-RRLYR-12245 was discovered as a
double-mode RRd star based on observations conducted in the years
2001-2006 during the third phase of the Optical Gravitational Lensing
Experiment (OGLE-III). The OGLE-IV observations carried out since
2010 reveal that this object is now a fundamental-mode RRab star,
with no sign of the first-overtone pulsation. The analysis of the OGLE
photometry shows that the final stage of the mode switching occurred
on a relatively short timescale of a few months in 2005. We study
the behavior of the star during this process, showing changes of the
pulsational amplitudes and periods. We also discuss possible reasons
for the mode switching in RR Lyr stars.
Title: Interpretation of the oscillation spectrum of HD 50230 -
a failure of richness
Authors: Szewczuk, Wojciech; Daszyńska-Daszkiewicz, Jadwiga;
Dziembowski, Wojciech
Bibcode: 2014IAUS..301..109S
Altcode: 2013arXiv1311.2818S
Attempts to interpret the observed oscillation spectrum of the SPB
star HD 50230 are reported. We argue that a nearly equidistant period
spacing found in the oscillation spectrum of the star is most likely
accidental. The observed period distribution requires excitation of
modes with the degree l > 4. Much more may be learned from the rich
oscillation spectrum of the star but most of the work is still ahead
of us.
Title: The evolution of the internal rotation of solar-type stars
Authors: Di Mauro, Maria Pia; Ventura, Rita; Cardini, Daniela;
Christensen-Dalsgaard, Jørgen; Dziembowski, Wojciech A.; Paternò,
Lucio
Bibcode: 2014IAUS..301..345D
Altcode:
We discuss the potential of asteroseismic inversion to study the
internal dynamics of solar-type stars and to reconstruct the evolution
of the internal rotation from the main sequence to the red-giant
phase. In particular, we consider the use of gravity and mixed modes
and the application of different inversion methods.
Title: Large Variety of New Pulsating Stars in the OGLE-III Galactic
Disk Fields
Authors: Pietrukowicz, P.; Dziembowski, W. A.; Mróz, P.; Soszyński,
I.; Udalski, A.; Poleski, R.; Szymański, M. K.; Kubiak, M.;
Pietrzyński, G.; Wyrzykowski, Ł.; Ulaczyk, K.; Kozłowski, S.;
Skowron, J.
Bibcode: 2013AcA....63..379P
Altcode: 2013arXiv1311.5894P
We present the results of a search for pulsating stars in the
7.12 deg2 OGLE-III Galactic disk area in the direction
tangent to the Centaurus Arm. We report the identification of 20
Classical Cepheids, 45 RR Lyr type stars, 31 Long-Period Variables,
such as Miras and Semi-Regular Variables, one pulsating white dwarf,
and 58 very likely δ Sct type stars. Based on asteroseismic models
constructed for one quadruple-mode and six triple-mode δ Sct type
pulsators, we estimated masses, metallicities, ages, and distance
moduli to these objects. The modeled stars have masses in the range
0.9-2.5 MSun and are located at distances between 2.5 kpc
and 6.2 kpc. Two triple-mode and one double-mode pulsators seem to be
Population II stars of the SX Phe type, probably from the Galactic
halo. Our sample also includes candidates for Type II Cepheids and
unclassified short-period (P<0.23 d) multi-mode stars which could be
either δ Sct or β Cep type stars. One of the detected variables is a
very likely δ Sct star with an exceptionally high peak-to-peak I-band
amplitude of 0.35 mag at the very short period of 0.0196 d. All reported
pulsating variable stars but one object are new discoveries. They
are included in the OGLE-III Catalog of Variable Stars. Finally,
we introduce the on-going OGLE-IV Galactic Disk Survey, which covers
more than half of the Galactic plane. For the purposes of future works
on the spiral structure and star formation history of the Milky Way,
we have already compiled a list of known Galactic Classical Cepheids.
Title: Period-luminosity relations in evolved red giants explained
by solar-like oscillations
Authors: Mosser, B.; Dziembowski, W. A.; Belkacem, K.; Goupil, M. J.;
Michel, E.; Samadi, R.; Soszyński, I.; Vrard, M.; Elsworth, Y.;
Hekker, S.; Mathur, S.
Bibcode: 2013A&A...559A.137M
Altcode: 2013arXiv1310.0839M
Context. Solar-like oscillations in red giants have been investigated
with the space-borne missions CoRoT and Kepler, while pulsations in
more evolved M giants have been studied with ground-based microlensing
surveys. After 3.1 years of observation with Kepler, it is now possible
to link these different observations of semi-regular variables.
Aims: We aim to identify period-luminosity sequences in evolved red
giants identified as semi-regular variables and to interpret them in
terms of solar-like oscillations. Then, we investigate the consequences
of the comparison of ground-based and space-borne observations.
Methods: We first measured global oscillation parameters of evolved
red giants observed with Kepler with the envelope autocorrelation
function method. We then used an extended form of the universal red
giant oscillation pattern, extrapolated to very low frequency, to fully
identify their oscillations. The comparison with ground-based results
was then used to express the period-luminosity relation as a relation
between the large frequency separation and the stellar luminosity.
Results: From the link between red giant oscillations observed
by Kepler and period-luminosity sequences, we have identified these
relations in evolved red giants as radial and non-radial solar-like
oscillations. We were able to expand scaling relations at very low
frequency (periods as long as 100 days and large frequency separation
less than 0.05 μHz). This helped us identify the different sequences
of period-luminosity relations, and allowed us to propose a calibration
of the K magnitude with the observed large frequency separation.
Conclusions: Interpreting period-luminosity relations in red giants
in terms of solar-like oscillations allows us to investigate the time
series obtained from ground-based microlensing surveys with a firm
physical basis. This can be done with an analytical expression that
describes the low-frequency oscillation spectra. The different behavior
of oscillations at low frequency, with frequency separations scaling
only approximately with the square root of the mean stellar density,
can be used to precisely address the physics of the semi-regular
variables. This will allow improved distance measurements and opens
the way to extragalactic asteroseismology with the observations of M
giants in the Magellanic Clouds.
Title: Puzzling Frequencies in First Overtone Cepheids
Authors: Dziembowski, W. A.
Bibcode: 2012AcA....62..323D
Altcode: 2012arXiv1212.0993D
The OGLE project led to the discovery of earlier unknown forms of
multiperiodic pulsation in Cepheids. Often, the observed periods may
be explained in terms of simultaneous excitation of two or rarely
three radial modes. However, a secondary variability at about 0.6
of the dominant period, detected in a number of the first overtone
(1O) pulsators inhabiting the Magellanic Clouds, seems to require a
different explanation. After reviewing a possibility of explaining
this signal in terms of radial and nonradial modes, I find that only
unstable modes that may reproduce the observed period ratio are f-modes
of high angular degrees (ℓ=42-50). I discuss in detail the driving
effect behind the instability and show that it is not the familiar
opacity mechanism. Finally, I emphasize the main difficulty of this
explanation, which requires high intrinsic amplitudes implying large
broadening of spectral line.
Title: Dipolar modes in luminous red giants
Authors: Dziembowski, W. A.
Bibcode: 2012A&A...539A..83D
Altcode: 2012arXiv1201.3762D
Context. Lots of information on solar-like oscillations in red giants
has been obtained thanks to observations with CoRoT and Kepler space
telescopes. Data on dipolar modes appear most interesting.
Aims: We study properties of dipolar oscillations in luminous
red giants to explain mechanism of mode trapping in the convective
envelope and to assess what may be learned from the new data.
Methods: Equations for adiabatic oscillations are solved by numerical
integration down to the bottom of convective envelope, where the
boundary condition is applied. The condition is based on asymptotic
decomposition of the fourth order system into components describing
a running wave and a nearly uniform shift of radiative core.
Results: If the luminosity of a red giant is sufficiently high (L
≳ 100 L⊙ at M = 2 M⊙), the dipolar modes
become effectively trapped in the acoustic cavity, which covers the
outer part of convective envelope. Energy loss caused by gravity wave
emission at the envelope base is a secondary or negligible source of
damping. Frequencies are insensitive to structure of the deep interior.
Title: Large Magellanic Cloud Cepheids in the ASAS Data
Authors: Karczmarek, P.; Dziembowski, W. A.; Lenz, P.; Pietrukowicz,
P.; Pojmański, G.
Bibcode: 2011AcA....61..303K
Altcode: 2012arXiv1201.0790K
A catalog of Cepheids in the Large Magellanic Cloud (LMC) from the
ASAS project is presented. It contains data on 65 fundamental mode
pulsators with periods longer than about 8 days. The period-luminosity
(PL) relation in the V-band does not significantly differ from the
relation determined from the OGLE-III data extended toward longer
periods but shows much larger scatter. For objects with periods longer
than 40 d there is an evidence for a shallower PL relation. The rates
of long-term period variations significant at 3σ level are found
only for seven objects. The rates for 25 objects determined with the
1σ significance are confronted with the values derived from stellar
evolution models. The models from various sources yield discrepant
predictions. Over the whole data range, a good agreement with
measurements is found for certain models but not from the same source.
Title: Nonradial Oscillations in Giants and Supergiants: An Update
Authors: Dziembowski, Wojciech
Bibcode: 2011asa..progE..16D
Altcode:
No abstract at ADS
Title: Kepler observations of δ Scuti stars
Authors: Balona, L. A.; Dziembowski, W. A.
Bibcode: 2011MNRAS.417..591B
Altcode: 2011MNRAS.tmp.1287B
We analyse observations of 1568 δ Scuti stars in the public archive
of the Kepler satellite. We compare the location of these stars
in the Hertzsprung-Russell (HR) diagram with that calculated by
linear, non-adiabatic pulsation models. There is generally a good
agreement and it appears that many of the hotter δ Scuti stars are
pulsating in overtones as high as radial order n= 8. Models predict
unstable modes of intermediate and high spherical harmonic, l, which
are trapped in the envelope. The instability strip for these modes
extends well beyond the blue edge for l≤ 4 and should be visible in
the Kepler data. However, stars with these predicted properties are
not seen. Moreover, we find that the observed frequency range does
not agree with the models. Another challenge is to understand why
more than half of the stars in the δ Sct instability strip are not
pulsating. The distribution of amplitudes argues strongly against the
notion that the pulsation amplitudes are below the Kepler detection
limit. The mode density of δ Scuti stars is surprisingly low and
suggests that modes of a high degree are not common. We do not find
any stars with mode densities as high as that found in the CoRoT
observations of HD 50844. The periodograms of stars in the same part
of the HR diagram vary widely. However, we have identified a group of
δ Scuti stars characterized by a single dominant frequency in which a
period-luminosity law is present. In many cases the dominant frequency
is accompanied by a close frequency of lower amplitude. The problem
of low frequencies in hot δ Scuti stars remains unresolved. We confirm
that these frequencies have long lifetimes and are not likely to be of
stochastic origin. By carefully examining the periodograms of δ Sct
stars in the low-frequency regime, we were able to identify a group
of δ Sct stars with low-frequency peaks closely resembling those
attributed to pure γ Dor stars. These stars lie in the region of the
HR diagram where ground-based γ Dor stars are located. None is found
hotter than the blue edge of γ Dor pulsation. We suspect that these
are genuine γ Dor/δ Sct hybrids.
Title: VizieR Online Data Catalog: VI light curves of Galactic Bulge
RR Lyrae (Soszynski+, 2011)
Authors: Soszynski, I.; Dziembowski, W. A.; Udalski, A.; Poleski,
R.; Szymanski, M. K.; Kubiak, M.; Pietrzynski, G.; Wyrzykowski, L.;
Ulaczyk, K.; Kozlowski, S.; Pietrukowicz, P.
Bibcode: 2011yCat.120610001S
Altcode:
Our observations of the Galactic bulge were obtained at Las Campanas
Observatory with the 1.3-m Warsaw telescope. The observatory is operated
by the Carnegie Institution of Washington. During the OGLE-III project
(2001-2009), theWarsaw telescope was equipped with an eight-chip mosaic
camera covering approximately 35x35arcmin in the sky with the scale
of 0.26arcsec/pixel. (5 data files).
Title: The Optical Gravitational Lensing Experiment. The OGLE-III
Catalog of Variable Stars. XI. RR Lyrae Stars in the Galactic Bulge
Authors: Soszyński, I.; Dziembowski, W. A.; Udalski, A.; Poleski,
R.; Szymański, M. K.; Kubiak, M.; Pietrzyński, G.; Wyrzykowski,
Ł.; Ulaczyk, K.; Kozłowski, S.; Pietrukowicz, P.
Bibcode: 2011AcA....61....1S
Altcode: 2011arXiv1105.6126S
The eleventh part of the OGLE-III Catalog of Variable Stars (OIII-CVS)
contains 16 836 RR Lyr stars detected in the OGLE fields toward the
Galactic bulge. The total sample is composed of 11 756 RR Lyr stars
pulsating in the fundamental mode (RRab), 4989 overtone pulsators (RRc),
and 91 double-mode (RRd) stars. About 400 RR Lyr stars are members
of the Sagittarius Dwarf Spheroidal Galaxy. The catalog includes the
time-series photometry collected in the course of the OGLE survey,
basic parameters of the stars, finding charts, and cross-identifications
with other catalogs of RR Lyr stars toward the Milky Way center. We notice that some RRd stars in the Galactic bulge show unusually
short periods and small ratio of periods, down to PF≍0.35
days and P1O/PF≍0.726. In the Petersen diagram
double-mode RRLyr stars form a parabola-like structure, which connects
shorter- and longer-period RRd stars. We show that the unique properties
of the bulge RRd stars may be explained by allowing for the wide range
of the metal abundance extending up to [Fe/H]≍-0.36. We report
the discovery of an RR Lyr star with additional eclipsing variability
with the orbital period of 15.2447 days. Some statistical features of
the RRLyr sample are presented. We discuss potential applications of
our catalog in studying the structure and history of the central region
of the Galaxy, mapping the interstellar extinction toward the bulge,
studying globular clusters and the Sagittarius Dwarf Galaxy.
Title: Acoustic oscillations in stars near the tip of the red
giant branch
Authors: Dziembowski, W. A.; Soszyński, I.
Bibcode: 2010A&A...524A..88D
Altcode: 2010arXiv1009.5989D
Context. Small amplitude oscillations are observed in red giant branch
(RGB) stars. Data on such oscillations are a source of information about
the objects, notably about properties of convection in their envelopes
and about the systems these objects inhabit. The OGLE-III catalog
contains data for about 80 thousand small amplitude variable red giants
(OSARGs) in the Large Magellanic Cloud.
Aims: We want to explain
variability oin OSARGs as the solar-like oscillation and to associate
the peaks in power spectra with frequencies of acoustic modes.
Methods: We use data on reddening-free magnitudes of the objects and
interpret them in terms of stellar physical parameters using tabulated
isochrones calculated for ages and composition parameters corresponding
to the upper RGB of the LMC. Massive data on the peak frequencies and
amplitudes are compared with expectations for stochastically excited
oscillations. The frequencies are also compared with those calculated
for radial modes in envelope models with parameters taken from the
isochrones.
Results: In stars close to the tip of the RGB,
the peaks in power spectra are found in the 0.1-1.0 μHz range,
which is consistent with extrapolation of the frequency-luminosity
relation for the solar-like oscillation. The dominant peaks occur close
to the first two radial overtones. The increase in amplitude with
luminosity is slower than linear. The exponent s ≈ 0.9 is similar
to what is found from recent analysis of CoRoT data on less luminous
red giants. Frequency separations between dominant peaks are found
to be smaller by about 20% than calculated separations between these
modes. After examining various possibilities, we left this discrepancy
unexplained.
Conclusions: The small amplitude variability of
stars at the RGB tip is likely to be caused by a stochastic excitation
of acoustic oscillations but interpreting of individual peaks in power
spectra presents a problem.
Title: Asteroseismology of rapidly rotating pulsators
Authors: Dziembowski, W. A.
Bibcode: 2010HiA....15..360D
Altcode:
Oscillation frequencies were determined for a number of rapidly
rotating main sequence stars. However, real seismic probing is still
ahead of us. I review here tools that we have for modeling pulsation
in rotating stars and their potential application to seismic sounding.
Title: Multimode Cepheids in the Large Magellanic Cloud-challenges
for theory
Authors: Dziembowski, W. A.; Smolec, R.
Bibcode: 2009AIPC.1170...83D
Altcode: 2009arXiv0907.1417D
Data on multimode Cepheids from OGLE-III catalog of the LMC Cepheids
are confronted with results from model calculations. Models whose radial
mode periods are consistent with observation are not always in agreement
with published evolutionary models. Nonradial mode interpretation
is considered for the cases of unusual period ratios. The greatest
challenge for stellar pulsation theory is explanation of double-mode
pulsators with period ratios near 0.6.
Title: Driving mechanism in massive B-type pulsators
Authors: Dziembowski, W. A.
Bibcode: 2009CoAst.158..227D
Altcode: 2008arXiv0812.2149D
After a historical introduction, I present the current status of our
understanding of the mechanism responsible for pulsation in β Cephei
and SPB stars.
Title: Double-Overtone Cepheids in the Large Magellanic Cloud
Authors: Dziembowski, W. A.; Smolec, R.
Bibcode: 2009AcA....59...19D
Altcode: 2009arXiv0903.2676D
One of the most interesting results from the OGLE-III study of the LMC
Cepheids is the large number of objects that pulsate simultaneously in
the first and second overtone (denoted 1O/2O). Double-mode Cepheids
yield important constraint on stellar evolution models. We show
that great majority of the LMC 1O/2O Cepheids have masses M=3.0±0.5
Msolar. According to current stellar evolution calculations
these masses are lower than needed for the blue loop in the helium
burning phase to reach the instability strip. On the other hand,
we found most of these stars significantly overluminous if they are
crossing the instability strip before helium ignition. A possible
solution of this discrepancy is to allow for a large overshooting from
the convective core in the main sequence phase. We also discuss origin
of double-mode pulsation. At the short period range we find two types
of resonances that are conducive to this form of pulsation. However,
at longer periods, it has a different (non-resonant) origin.
Title: Commission 35: Stellar Constitution
Authors: D'Antona, Francesca; Charbonnel, Corinne; Dziembowski,
Wojciech; Fontaine, Gilles; Larson, Richard B.; Lattanzio, John;
Liebert, Jim W.; Müller, Ewald; Weiss, Achim; Yungelson, Lev R.
Bibcode: 2009IAUTA..27..211D
Altcode:
The Commission home page <iau-c35.stsci.edu> is maintained by
Claus Leitherer and contains general information on the Commission
structure and activities, including links to stellar structure
resources that were made available by the owners. The resources
contain evolutionary tracks and isochrones from various groups,
nuclear reaction, EOS, and opacity data as well as links to main
astronomical journals. As a routine activity, the Organizing Committee
has commented on and ranked proposals for several IAU sponsored
meetings. Our Commission acted as one of the coordinating bodies of a
Symposium held at the IAU XXVI General Assembly in Prague, August 2006,
(IAU Symposium No. 239 Convection in Astrophysics, and participated in
the organization of the following Joint Discussions: JD05 Calibrating
the Top of the Stellar Mass-Luminosity Relation, JD06 Neutron Stars
and Black Holes in Star Clusters, JD08 Solar and Stellar Activity
Cycles, JD11 Pre-Solar Grains as Astrophysical Tools; JD14 Modelling
Dense Stellar Systems; and JD17 Highlights of Recent Progress in the
Seismology of the Sun and Sun-like Stars.
Title: The photometric method of mode identification for rapidly
rotating SPB stars. An application to μ Eridani
Authors: Daszyńska-Daszkiewicz, J.; Dziembowski, W. A.; Pamyatnykh,
A. A.
Bibcode: 2008JPhCS.118a2024D
Altcode:
We present an outline of a method for mode identification, which
relies on a nonadiabatic version of the traditional approximation and
involves instability and visibility considerations. Determination of
the angular degrees, (l, m), is done simultaneously with the rotation
velocity and inclination angle. The method is applied to oscillation
frequencies detected in the rapidly rotating SPB star μ Eridani.
Title: Looking into B star interiors by means of asteroseismology
Authors: Dziembowski, Wojciech A.
Bibcode: 2008AIPC.1043..351D
Altcode:
Precise modeling of upper main sequence stars is important in
various astrophysical contexts, including chemical evolution of the
Universe. However, there are still significant uncertainties resulting
mostly, but not only, from our poor understanding of transport
processes. Data on pulsating objects, in particular β Cephei stars,
provide us valuable constraints on parameters describing efficiency
of these processes and asses the overall accuracy of present stellar
models. I review methods of extracting information on stellar interior
structure and rotation from pulsation data and present some recent
results for selected objects.
Title: The two hybrid B-type pulsators: ν Eridani and 12 Lacertae
Authors: Dziembowski, W. A.; Pamyatnykh, A. A.
Bibcode: 2008MNRAS.385.2061D
Altcode: 2008arXiv0801.2451D; 2008MNRAS.tmp..406D
The rich oscillation spectra determined for the two stars, ν
Eridani and 12 Lacertae, present an interesting challenge to stellar
modelling. The stars are hybrid objects showing not only a number of
modes at frequencies typical for β Cep stars but also one mode at
frequency typical for SPB stars. We construct seismic models of these
stars considering uncertainties in opacity and element distribution. We
also present estimate of the interior rotation rate and address the
matter of mode excitation. We use both the opacity project (OP)
and opacity library Livermore (OPAL) opacity data and find significant
difference in the results. Uncertainty in these data remains a major
obstacle in precise modelling of the objects and, in particular, in
estimating the overshooting distance. We find evidence for significant
rotation rate increase between envelope and core in the two stars. Instability of low-frequency g modes was found in seismic models of
ν Eri built with the OP data, but at frequencies higher than those
measured in the star. No such instability was found in models of 12
Lac. We do not yet have a satisfactory explanation for the low-frequency
modes. Some enhancement of opacity in the driving zone is required
but we argue that it cannot be achieved by the iron accumulation,
as it has been proposed.
Title: Division Iv: Stars
Authors: Dravins, Dainis; Spite, Monique; Barbuy, Beatriz; Corbally,
Christopher; Dziembowski, Wojciech; Hartkopf, William I.; Sneden,
Christopher
Bibcode: 2007IAUTB..26..145D
Altcode:
Division IV organizes astronomers studying the characterization,
interior and atmospheric structure of stars of all masses, ages and
chemical compositions.
Title: Commission 35: Stellar Constitution
Authors: Dziembowski, Wojciech A.; D'Antona, Francesca; VandenBerg,
Don A.; Charbonnel, Corinne; Christensen-Dalsgaard, Joergen; Guzik,
Joyce A.; Langer, Norbert; Larson, Richard B.; Liebert, James W.;
Meynet, Georges; Müller, Ewald; Saio, Hideyuki
Bibcode: 2007IAUTB..26..158D
Altcode:
The session was brief and quite informal as there were only six
participants. The agenda included my report on organizational activities
of the Commission during the 2003-2006 term and Virginia Trimble's
presentation Presence of binary stars in the current astronomical
literature. I summarize below the most important part of my report.
Title: Forecasting Seismic Signatures of Stellar Magnetic Activity
Authors: Dziembowski, W. A.
Bibcode: 2007AIPC..948..287D
Altcode: 2007arXiv0709.2602D
For the Sun, a tight correlation between various activity measures
and oscillation frequencies is well documented. For other stars,
we have abundant data on magnetic activity and its changes but not
yet on its seismic signature. A prediction of the activity induced
frequency changes in stars based on scaling the solar relations is
presented. This seismic signature of the activity should be measurable
in the data expected within few years.
Title: The Optical Gravitational Lensing
Experiment. Period--Luminosity Relations of Variable Red Giant Stars
Authors: Soszyński, I.; Dziembowski, W. A.; Udalski, A.; Kubiak, M.;
Szymański, M. K.; Pietrzyński, G.; Wyrzykowski, &Lstrok.; Szewczyk,
O.; Ulaczyk, K.
Bibcode: 2007AcA....57..201S
Altcode: 2007arXiv0710.2780S
Period--luminosity (PL) relations of variable red giants in the Large
(LMC) and Small Magellanic Clouds (SMC) are presented. The PL diagrams
are plotted in three planes: log P-K_S, log P-W_JK, and log P-W_I,
where W_JK and W_I are reddening free Wesenheit indices. Fourteen
PL sequences are distinguishable, and some of them consist of three
closely spaced ridges. Each of the sequences is fitted with a linear
or quadratic function. The similarities and differences between the
PL relations in both galaxies are discussed for four types of red
giant variability: OGLE Small Amplitude Red Giants (OSARGs), Miras
and Semiregular Variables (SRVs), Long Secondary Periods (LSPs) and
ellipsoidal variables. We propose a new method of separating OSARGs
from non-variable stars and SRVs. The method employs the position in
the reddening-free PL diagrams and the characteristic period ratios of
these multiperiodic variables. The PL relations for the LMC OSARG are
compared with the calculated relations for RGB models along isochrones
of relevant ages and metallicities. We also compare measured periods
and amplitudes of the OSARGs with predictions based on the relations
valid for less luminous solar-like pulsators. Miras and SRVs
seem to follow PL relation of the same slopes in the LMC and SMC,
while for LSP and ellipsoidal variables slopes in both galaxies are
different. The PL sequences defined by LSP variables and binary systems
overlap in the whole range of analyzed wavebands. We put forward new
arguments for the binary star scenario as an explanation of the LSP
variability and elaborate on it further. The measured pulsation to
orbital period ratio implies nearly constant ratio of the star radius
to orbital distance, R/A approx 0.4, as we find. Combined effect of
tidal friction and mass loss enhanced by the low-mass companion may
explain why such a value is preferred.
Title: Bohdan Paczyński (1940-2007)
Authors: Dziembowski, W. A.
Bibcode: 2007PASP..119..836D
Altcode:
No abstract at ADS
Title: Asteroseismic signatures of stellar magnetic activity cycles
Authors: Metcalfe, T. S.; Dziembowski, W. A.; Judge, P. G.; Snow, M.
Bibcode: 2007MNRAS.379L..16M
Altcode: 2007MNRAS.tmpL..50M; 2007arXiv0704.1606M
Observations of stellar activity cycles provide an opportunity to study
magnetic dynamos under many different physical conditions. Space-based
asteroseismology missions will soon yield useful constraints on the
interior conditions that nurture such magnetic cycles, and will be
sensitive enough to detect shifts in the oscillation frequencies due to
the magnetic variations. We derive a method for predicting these shifts
from changes in the MgII activity index by scaling from solar data. We
demonstrate this technique on the solar-type subgiant β Hyi, using
archival International Ultraviolet Explorer spectra and two epochs of
ground-based asteroseismic observations. We find qualitative evidence
of the expected frequency shifts and predict the optimal timing for
future asteroseismic observations of this star.
Title: Interpretation of the Be star HD 163868 oscillation spectrum
based on the MOST observations
Authors: Dziembowski, W. A.; Daszyńska-Daszkiewicz, J.; Pamyatnykh,
A. A.
Bibcode: 2007CoAst.150..213D
Altcode:
No abstract at ADS
Title: Oscillations in main sequence B-type stars - challenges
to theory
Authors: Dziembowski, W. A.
Bibcode: 2007CoAst.150..175D
Altcode:
The current status of our understanding of the diversity of B-star
pulsation is presented with an emphasis on unsolved problems. Not all
detected modes are found unstable in standard models. The proposed way
of extending the instability by invoking an iron accumulation in the
driving zone is not free of difficulties. There are still controversies
regarding the excitation of slow modes in Be stars. Nonlinear
modelling of radial pulsations in β Cephei stars results in much
higher amplitudes than observed. There must be hidden modes involved
but we may only speculate about their nature.
Title: On the Prospects for Detection and Identification of
Low-Frequency Oscillation Modes in Rotating B Type Stars
Authors: Daszynska-Daszkiewicz, J.; Dziembowski, W. A.; Pamyatnykh,
A. A.
Bibcode: 2007AcA....57...11D
Altcode: 2007arXiv0704.2279D
We study how rotation affects observable amplitudes of high-order
g- and mixed r/g-modes and examine prospects for their detection
and identification. Our formalism, which is described in some
detail, relies on a nonadiabatic generalization of the traditional
approximation. Numerical results are presented for a number of unstable
modes in a model of SPB star, at rotation rates up to 250 km/s. It
is shown that rotation has a large effect on mode visibility in light
and in mean radial velocity variations. In most cases, fast rotation
impairs mode detectability of g-modes in light variation, as Townsend
(2003b) has already noted, but it helps detection in radial velocity
variation. The mixed modes, which exist only at sufficiently fast
rotation, are also more easily seen in radial velocity. The amplitude
ratios and phase differences are strongly dependent on the aspect,
the rotational velocity and on the mode. The latter dependence is
essential for mode identification.
Title: Commission 35: Stellar Constitution
Authors: Dziembowski, Wojciech A.; D'Antona, Francesca; Charbonnel,
C.; Christensen-Dalsgaard, J.; Guzik, J.; Langer, N.; Larson, R.;
Liebert, J.; Meynet, G.; Müller, E.; Saio, H.; Vandenberg, D.
Bibcode: 2007IAUTA..26..205D
Altcode:
Ihe triennial report from Commission 35 covers its organizational
activities and highlights accomplishments in various topics of stellar
interior physics.
Title: Division IV: Stars
Authors: Dravins, Dainis; Barbuy, Beatriz; Corbally, Christopher;
Dziembowski, Wojciech; Hartkopf, William; Sneden, Christopher;
Spite, Monique
Bibcode: 2007IAUTA..26..191D
Altcode:
The IAU Division IV (`Stars') organizes astronomers studying the
characteristics, interior and atmospheric structure, and evolution of
stars of all masses, ages, and chemical compositions.
Title: Excitation and visibility of slow modes in rotating B-type
stars
Authors: Dziembowski, W. A.; Daszyńska-Daszkiewicz, J.; Pamyatnykh,
A. A.
Bibcode: 2007MNRAS.374..248D
Altcode: 2006astro.ph.11171D; 2006MNRAS.tmp.1306D
We use the traditional approximation to describe oscillations
with frequencies comparable to the angular rotation rate. Validity
of this approximation in application to main-sequence B stars is
discussed. Numerical results regarding mode stability and visibility
are presented for a model of the Be star HD 163868. For this object,
Walker et al. detected a record number of mode frequencies using data
from the small space telescope MOST. Our interpretation of these data
differs from that of Walker et al. In particular, we interpret peaks in
the lowest frequency range as retrograde g modes. We find instability in
a large number of modes that remain undetectable because of unfavourable
aspect and/or effect of cancellation. There is no clear preference to
excitation of prograde modes.
Title: Light and Radial Velocity Variations Due to Low-Frequency
Oscillations in Rotating Stars
Authors: Daszyńska-Daszkiewicz, J.; Dziembowski, W. A.; Pamyatnykh,
A. A.
Bibcode: 2007EAS....26..129D
Altcode:
Low-frequency oscillations are unstable in a wide range of stellar
models and have been detected in a number of B-type stars. For such
modes, even a moderate rotation significantly affects amplitude
distribution over stellar surface and hence their visibility
conditions. Adopting the traditional approximation, we study effects
of rotation on relative amplitudes in photometric passbands and in
radial velocity. We present numerical results for unstable modes in a
selected stellar model of a 6 M_⊙ main sequence star. The goal is to
show how observable amplitudes, which are tools for mode identification,
depend on the rotation rate and on the aspect angle.
Title: On the nature of small amplitude peaks in delta Scuti
oscillation spectra
Authors: Daszyńska-Daszkiewicz, J.; Dziembowski, W. A.; Pamyatnykh,
A. A.
Bibcode: 2006MmSAI..77..113D
Altcode: 2005astro.ph..9449D
The standard assumption in interpretation of stellar oscillation spectra
from photometry is that the excited mode have low angular degrees,
typically ℓ< 3. Considering the case of FG Vir, the delta Scuti
star with the richest known oscillation spectrum, we show that this
assumption is not justified for low amplitude peaks. The ℓ<3
identifications have been found for 12 dominant peaks from pulsation
amplitudes and phases. However, we show that for the rest of the peaks
(55), whose amplitudes are typically below 1 mmag, much higher ℓ's
are most likely. We argue that improving amplitude resolution to the
micromagnitude level, as expected from space observations, is not likely
to be rewarded with a credible mode identifications because the spectra
will be dominated by high-ℓ modes of unknown azimuthal order, m.
Title: Progress in understanding and exploiting stellar oscillation
spectra .
Authors: Dziembowski, W. A.
Bibcode: 2006MmSAI..77..348D
Altcode: 2005astro.ph..9451D
Rich oscillation spectra of dwarf-like pulsators contain a wealth
of information about the object interiors and, in particular, about
macroscopic transport processes, which is the most difficult aspect
of stellar physics. Examples of extracting such information from data
on solar-like and opacity driven pulsators are given. Problems in
understanding new oscillation spectra are discussed. Importance of
employing various data on excited mode is emphasized.
Title: Applications of pulsation amplitudes and phases for B-type
main sequence pulsators
Authors: Daszyńska-Daszkiewicz, J.; Dziembowski, W. A.; Pamyatnykh,
A. A.
Bibcode: 2006MmSAI..77..449D
Altcode:
Combined data on pulsation amplitudes and phases from multicolour
photometry and spectroscopy yield constraints on mode identification as
well as on mean stellar parameters. The data allow for determination
of a certain complex parameter which may be compared with the model
value and thus constitute a new seismic probe of stellar interior,
specifically of its outer layers. We present here some results obtained
for the two beta Cephei stars delta Ceti and nu Eridani. We found
significant differences between models calculated with opacities from
the OPAL and OP projects.
Title: Obituary: Norman Hodgson Baker Jr (1931 - 2005)
Authors: Dziembowski, Wojciech; Gautschy, Alfred; Gough, Douglas
Bibcode: 2006MmSAI..77...33D
Altcode:
No abstract at ADS
Title: A seismic tool to study stellar structure and evolution of
differentially rotating stars
Authors: Suárez, J. C.; Goupil, M. J.; Dziembowski, W.
Bibcode: 2005ASPC..333..247S
Altcode:
Binary systems can be one of the sources producing differential rotation
inside stellar structures. Stellar evolution is thus conditioned to how
rotation modifies different regions in stellar interiors. In this work
we present the first numerical results of adiabatic frequencies in an
intermediate mass star model, using a perturbation method up to the
second order, taking into account a radial differential rotation. We
show, as we predicted, that g and mixed modes are the most affected
by differential rotation. Differences up to 3 -- 4 μHz respectively
in first and second order corrections to the frequency are found. This
can be a very important step toward the comprehension of the rotation
profile in such stars, and thus of their evolution.
Title: VizieR Online Data Catalog: IVB mag of LMC ellipsoidal
variables (Soszynski+, 2004)
Authors: Soszynski, I.; Udalski, A.; Kubiak, M.; Szymanski,
M. K.; Pietrzynski, G.; Zebrun, K.; Szewczyk, O.; Wyrzykowski, L.;
Dziembowski, W. A.
Bibcode: 2005yCatp005005403S
Altcode:
We used the OGLE-II and OGLE-III photometry of red giants in the Large
Magellanic Cloud to select and study objects revealing ellipsoidal
variability. We detected 1546 candidates for long period ellipsoidal
variables and 121 eclipsing binary systems with clear ellipsoidal
modulation. (2 data files).
Title: Constraints on parameters of B-type pulsators from combined
multicolour photometry and radial velocity data. I. β Cephei stars
Authors: Daszyńska-Daszkiewicz, J.; Dziembowski, W. A.; Pamyatnykh,
A. A.
Bibcode: 2005A&A...441..641D
Altcode: 2005astro.ph..7253D
We analyze data on pulsation amplitudes and phases for two β Cephei
stars, δ Cet and ν Eri. Strömgren photometry and radial velocity
measurements are used simultaneously to obtain constraints on mean
parameters of the stars and identification of the excited modes. The
inference about the radial mode order and mean star parameters is based
on comparison of certain complex parameter, f, determined from data,
with its counterpart derived from linear nonadiabatic modelling of
stellar oscillations. The theoretical f values are very sensitive to
the adopted opacity data. In our modelling we rely on the data from
OPAL and OP projects. Significant differences were found. New seismic
models of ν Eri were constructed with both the OPAL and OP opacities.
Title: Cluster Ages Experiment (CASE): SX Phe stars from the globular
cluster ω Centauri
Authors: Olech, A.; Dziembowski, W. A.; Pamyatnykh, A. A.; Kaluzny,
J.; Pych, W.; Schwarzenberg-Czerny, A.; Thompson, I. B.
Bibcode: 2005MNRAS.363...40O
Altcode: 2005MNRAS.tmp..797O; 2004astro.ph.12523O
We present an analysis and interpretation of oscillation spectra for all
69 SX Phoenicis stars discovered in the field of the cluster. For most
of the stars we have reliable absolute magnitudes and colours. Except
for one or perhaps two objects, the stars are cluster members. Their
pulsational behaviour is very diverse. Multiperiodic variability with
at least part of the excited modes being non-radial is most common but
there are also many cases of high-amplitude, presumably radial mode,
pulsators. In a number of such cases we have evidence for two radial
modes being excited. Parameters of radial mode pulsators are in most
cases consistent with standard evolutionary models for stars in the mass
range 0.9-1.15Msolar. However, in four cases we have evidence
that the masses are significantly lower than expected. Three objects
show frequency triplets that may be interpreted in terms of rotational
frequency splitting of l= 1 modes. Implied equatorial velocities of
rotation are from 10 to over 100 km s-1. Nearly all measured
frequencies fall in the ranges predicted for unstable modes. Two cases
of low-frequency variability are interpreted as being caused by tidal
distortion induced by close companions.
Title: Inferences from pulsational amplitudes and phases for multimode
δ Sct star FG Vir
Authors: Daszyńska-Daszkiewicz, J.; Dziembowski, W. A.; Pamyatnykh,
A. A.; Breger, M.; Zima, W.; Houdek, G.
Bibcode: 2005A&A...438..653D
Altcode: 2005astro.ph..4177D
We combine photometric and spectroscopic data on twelve modes excited
in FG Vir to determine their spherical harmonic degrees ℓ, and
to obtain constraints on the star model. The effective temperature
consistent with the mean colours and the pulsation data is about
7200 K. In six cases, the ℓ identification is unique with above 80%
probability. Two modes are identified as radial. Simultaneously with
ℓ, we determine a complex parameter f which probes subphotospheric
stellar layers. Comparing its values with those derived from models
assuming different treatment of convection, we find evidence that
convective transport in the envelope of this star is inefficient.
Title: Seismic Diagnostics of Mixing Beyond the Convective Core in
Intermediate Mass Main-Sequence Stars
Authors: Popielski, B. L.; Dziembowski, W. A.
Bibcode: 2005AcA....55..177P
Altcode: 2005astro.ph..5522P
We study prospects for seismic sounding the layer of a partial mixing
above the convective core in main-sequence stars with masses in the
1.2-1.9 M_odot range. There is an initial tendency to increase of the
convective core mass in such stars and this leads to ambiguities in
modeling. Solar-like oscillations are expected to be excited in such
objects. Frequencies of such oscillations provide diagnostics, which
are sensitive to the structure of the innermost part of the star and
they are known as the small separations. We construct evolutionary
models of stars in this mass range assuming various scenarios for
element mixing, which includes formation of element abundance jumps,
as well as semiconvective and overshooting layers. We find that the
three point small separations employing frequencies of radial and
dipole modes provide the best probe of the element distribution above
the convective core. With expected accuracy of frequency measurement
from the space experiments, a discrimination between various scenarios
should be possible.
Title: On the Nature of Regular Pulsation in Two LBV Stars of NGC 300
Authors: Dziembowski, W. A.; Slawinska, J.
Bibcode: 2005AcA....55..195D
Altcode:
Two A-type supergiants in NGC 300 exhibit periodic light variations. The
measured periods are 72.5 and 96.1 days. We show that the variability
may be explained by the excitation of strange mode driven by the opacity
mechanism acting in the layer of the iron opacity bump. The explanation
requires that the two stars are significantly undermassive. In
sequences of stellar models with decreasing luminosity-to-mass ratio,
the unstable strange modes represent continuations of strictly aperiodic
thermal modes.
Title: Seismology of triple-mode classical Cepheids of the Large
Magellanic Cloud
Authors: Moskalik, P.; Dziembowski, W. A.
Bibcode: 2005A&A...434.1077M
Altcode: 2005astro.ph..1417M
We interpret the three periods detected in OGLE LMC
Cepheids SC3-360128 and SC5-338399
as corresponding to the first three overtones of radial pulsations. This
interpretation imposes stringent constraints on parameters of the
stars and on their evolutionary status, which could only be the first
crossing of the instability strip. Evolutionary models reproducing
measured periods exist only in a restricted range of metallicities
(Z=0.004 {-} 0.007). The models impose an upper limit to the extent
of overshooting from the convective core. Absolute magnitude of each
star is confined to a narrow interval, which allows us to derive a
new estimate of the distance to the LMC. We obtain
the value of m-M ranging from 18.34 m to 18.53 m with a systematic
difference between the two stars of about 0.13 m. The rates of period
change predicted by the models are formally in conflict with the derived
observational limits, though the uncertainties of measured dot{P} may
have been underestimated. If this discrepancy is confirmed, it would
constitute a significant challenge to the stellar evolution theory.
Title: Sources of Oscillation Frequency Increase with Rising Solar
Activity
Authors: Dziembowski, W. A.; Goode, P. R.
Bibcode: 2005ApJ...625..548D
Altcode: 2005astro.ph..3266D
We analyze and interpret SOHO MDI data on oscillation frequency changes
between 1996 and 2004, focusing on differences between the activity
minimum and maximum of solar cycle 23. We study only the behavior of
the centroid frequencies, which reflect changes averaged over spherical
surfaces. Both the f-mode and p-mode frequencies are correlated with
general measures of the Sun's magnetic activity. However, the physics
behind each of the two correlations is quite different. We show
that for the f-modes the dominant cause of the frequency increase
is the dynamical effect of the rising magnetic field. The relevant
rise must occur in subphotospheric layers reaching to some 0.5-0.7
kG at a depth of about 5 Mm. However, the implied constraints also
require the field change in the atmosphere to be so small that it
has only a tiny dynamical effect on p-mode frequencies. For p-modes,
the most plausible explanation of the frequency increase is a less
than 2% decrease in the radial component of the turbulent velocity
in the outer layers. Lower velocity implies a lower efficiency of the
convective transport, hence lower temperature, which also contributes
to the p-mode frequency increase.
Title: The Optical Gravitational Lensing Experiment. Ellipsoidal
Variability of Red Giants in the Large Magellanic Cloud
Authors: Soszyński, I.; Udalski, A.; Kubiak, M.; Szymański, M. K.;
Pietrzyński, G.; Żebruń, K.; Szewczyk, O.; Wyrzykowski, Ł.;
Dziembowski, W. A.
Bibcode: 2004AcA....54..347S
Altcode: 2004astro.ph.12505S
We used the OGLE-II and OGLE-III photometry of red giants in the Large
Magellanic Cloud to select and study objects revealing ellipsoidal
variability. We detected 1546 candidates for long period ellipsoidal
variables and 121 eclipsing binary systems with clear ellipsoidal
modulation. The ellipsoidal red giants follow a period--luminosity
(PL) relationship (sequence E), and the scatter of the relation is
correlated with the amplitude of variability: the larger the amplitude,
the smaller the scatter. We note that some of the ellipsoidal
candidates exhibit simultaneously OGLE Small Amplitude Red Giants
pulsations. Thus, in some cases the Long Secondary Period (LSP)
phenomenon can be explained by the ellipsoidal modulation. We
also select about 1600 red giants with distinct LSP, which are not
ellipsoidal variables. We discover that besides the sequence D in the
PL diagram known before, the LSP giants form additional less numerous
sequence for longer periods. We notice that the PL sequence of the
ellipsoidal candidates is a direct continuation of the LSP sequence
toward fainter stars, what might suggest that the LSP phenomenon
is related to binarity but there are strong arguments against such
a possibility. About 10% of the presented light curves reveal
clear deformation by the eccentricity of the system orbits. The largest
estimated eccentricity in our sample is about 0.4. All presented
data, including individual BVI observations and finding charts are
available from the OGLE Internet archive.
Title: Nonradial Mode Excitation as the Cause of the Blazhko Effect
in RR Lyrae Stars
Authors: Dziembowski, W. A.; Mizerski, T.
Bibcode: 2004AcA....54..363D
Altcode: 2005astro.ph..1435D
A significant fraction of RR Lyr stars exhibits amplitude and/or phase
modulation known as the the Blazhko effect. The oscillation spectra
suggest that, at least in most of the cases, excitation of a nonradial
modes in addition to the dominant radial modes is responsible for the
effect. Though model calculations predict that nonradial modes may be
excited, there are problems with explaining their observed properties
in terms of finite amplitude development of the linear instability. We
propose a scenario, which like some previous, postulates energy transfer
from radial to nonradial modes, but avoids those problems. The scenario
predicts lower amplitudes in Blazhko stars. We check this prediction
with a new analysis of the Galactic bulge RR Lyr stars from OGLE-II
database. The effect is seen, but the amplitude reduction is smaller
than predicted.
Title: Various applications of multicolour photometry and radial
velocity data for multimode δ Scuti stars
Authors: Daszynska-Daszkiewicz, J.; Dziembowski, W. A.; Pamyatnykh,
A. A.; Breger, M.; Zima, W.
Bibcode: 2004IAUS..224..853D
Altcode: 2004astro.ph.11082D
In addition to revealing spherical harmonic degrees, ℓ, of excited
modes, pulsational amplitudes and phases from multicolour photometry and
radial velocity data yield valuable constraints on stellar atmospheric
parameters and on subphotospheric convection. Multiperiodic pulsators
are of particular interest because each mode yields independent
constraints. We present an analysis of data on the twelve modes observed
in FG Vir.
Title: Identification of modes in main-sequence pulsators by means
of multi-color photometry
Authors: Daszynska-Daszkiewicz, J.; Dziembowski, W. A.; Pamyatnykh,
A. A.
Bibcode: 2004CoAst.145....6D
Altcode:
We discuss two problems connected with the application of the method
of photometric amplitudes and phases for mode identification. The
first one concerns the effect of rotational mode coupling in Beta
Cep models. The second one deals with the strong dependence of the
photometric observables on convection in Delta Scuti stars.
Title: On application of multi-colour photometry of δ Scuti stars
Authors: Daszyńska-Daszkiewicz, J.; Dziembowski, W. A.; Pamyatnykh,
A. A.
Bibcode: 2004ASPC..310..255D
Altcode: 2004vslg.conf..255D; 2004IAUCo.193..255D
In δ Scuti star models the photometric amplitudes and phases exhibit
a strong dependence on convection, which enters through the complex
parameter, f, that describes the bolometric flux variation. We present
a new method of extracting l and f from multi color data and apply it
to several δ Scuti stars. The inferred values of f are sufficiently
accurate to yield a useful constraint on models of stellar convection.
Title: Asteroseismology of the β Cephei star ν Eridani:
interpretation and applications of the oscillation spectrum
Authors: Pamyatnykh, A. A.; Handler, G.; Dziembowski, W. A.
Bibcode: 2004MNRAS.350.1022P
Altcode: 2004astro.ph..2354P
The oscillation spectrum of ν Eri is the richest known for any
variable of the β Cephei type. We interpret the spectrum in terms of
normal mode excitation and construct seismic models of the star. The
frequency data combined with data on mean colours set the upper limit
on the extent of overshooting from the convective core. We use data
on rotational splitting of two dipole (l= 1) modes (g1
and p1) to infer properties of the internal rotation
rate. Adopting a plausible hypothesis of nearly uniform rotation in
the envelope and increasing rotation rate in the μ-gradient zone,
we find that the mean rotation rate in this zone is about three times
faster than in the envelope. In our standard model only the modes in
the middle part of the oscillation spectrum are unstable. To account for
excitation of a possible high-order g mode at ν= 0.43 cd-1
as well as p modes at ν > 6 cd-1 we have to invoke an
overabundance of Fe in the driving zone.
Title: Measurements of magnetic fields over the pulsation cycle in
six roAp stars with FORS 1 at the VLT
Authors: Hubrig, S.; Kurtz, D. W.; Bagnulo, S.; Szeifert, T.;
Schöller, M.; Mathys, G.; Dziembowski, W. A.
Bibcode: 2004A&A...415..661H
Altcode: 2003astro.ph..9561H
With FORS 1 at the VLT we have tried for the first time to measure the
magnetic field variation over the pulsation cycle in six roAp stars to
begin the study of how the magnetic field and pulsation interact. For
the star HD 101065, which has one of the highest photometric pulsation
amplitudes of any roAp star, we found a signal at the known photometric
pulsation frequency at the 3σ level in one data set; however this
could not be confirmed by later observations. A preliminary simple
calculation of the expected magnetic variations over the pulsation cycle
suggests that they are of the same order as our current noise levels,
leading us to expect that further observations with increased S/N have
a good chance of achieving an unequivocal detection. Based on
observations obtained at the European Southern Observatory, Paranal,
Chile (ESO programmes Nos. 69.D-0210 and 270.D-5023).
Title: Helioseismic Probing of Solar Variability: The Formalism and
Simple Assessments
Authors: Dziembowski, W. A.; Goode, P. R.
Bibcode: 2004ApJ...600..464D
Altcode: 2003astro.ph.10095D
We derive formulae connecting the frequency variations in the spectrum
of solar oscillations to the dynamical quantities that are expected to
change over the solar activity cycle. This is done for both centroids
and the asymmetric part of the fine structure (so-called even-a
coefficients). We consider the near-surface, small-scale magnetic
and turbulent velocity fields, as well as horizontal magnetic fields
buried near the base of the convective zone. For the centroids we
also discuss the effect of temperature variation. We demonstrate that
there is a full, one-to-one correspondence between the expansion
coefficients of the fine structure and those of both the averaged
small-scale velocity and magnetic fields. Measured changes in the
centroid frequencies and the even-a coefficients over the rising
phase solar cycle may be accounted for by a decrease in the turbulent
velocity of order 1%. We show that the mean temperature decrease
associated with the net decrease in the efficiency of convective
transport may also significantly contribute to the increase of the
centroid frequencies. Alternatively, the increase may be accounted
for by an increase of the small-scale magnetic field of order 100 G,
if the growing field is predominantly radial. We also show that global
seismology can be used to detect a field at the level of a few times
105 G, if such a field were present and confined to a thin
layer near the base of the convective envelope.
Title: Preliminary numerical effects of shellular rotation on
adiabatic frequencies
Authors: Suárez, J. C.; Goupil, M. J.; Michel, E.; Dziembowski,
W. A.; Lebreton, Y.; Morel, P.
Bibcode: 2004ESASP.538..431S
Altcode: 2004sshp.conf..431S
In this work we present preliminary numerical results on the adiabatic
frequencies in an intermediate mass star model, using a perturbation
method up to the second order, taking into account radial differential
rotation. We show, as we predicted, that g and mixed modes are the
most affected by differential rotation. We found differences up to 5
μHz in the first and second order corrections to the frequency. This
can be a very important step towards the understanding of the rotation
profile in intermediate mass stars.
Title: Inferring information about rotation from stellar oscillations
Authors: Goupil, M. J.; Samadi, R.; Lochard, J.; Dziembowski, W. A.;
Pamyatnykh, A.
Bibcode: 2004ESASP.538..133G
Altcode: 2003astro.ph.10360G; 2004sshp.conf..133G
The first part of this paper briefly illustrates the intense scientific
activity in the field of stellar rotation, while the second part is
devoted to discussing rotation as a perturbation effect upon oscillation
frequencies. The discussion focuses on one specific example: the
p-modes frequency small separation which provides information about
properties of the stellar inner layers. It is shown that the small
separation can be affected by rotation at the level of 0.1-0.2 μHz for
a 1.4 Msolar model rotating with an equatorial velocity of
20 km/s at the surface. This is of the same order of magnitude as the
expected precision on frequencies with a 3 months observation and must
therefore be taken into account. We show however that it is possible
to recover the small separation free of these contaminating effects
of rotation, provided enough high quality data are available as will
be with space seismic missions such as Eddington.
Title: On application of multi-colour photometry of $\delta$
Scuti stars
Authors: Daszy{ń}ska-Daszkiewicz, J.; Dziembowski, W. A.; Pamyatnykh,
A. A.
Bibcode: 2003astro.ph..9319D
Altcode:
In $\delta$ Scuti star models, the photometric amplitudes and phases
exhibit a strong dependence on convection, which enters through
the complex parameter, $f$, which describes the bolometric flux
variation. We present a new method of extracting simultaneously $\ell$
and $f$ from multi-colour data and apply it to several $\delta$ Scuti
stars. The inferred values of $f$ are sufficiently accurate to yield
an useful constraint on models of stellar convection. In addition, if
the identified mode is radial, the multi-passband data may be used to
refine global stellar parameters. Finally, the usage of radial velocity
measurements to our method improves significantly determination of
$\ell$ and $f$.
Title: Constraints on stellar convection from multi-colour photometry
of delta Scuti stars
Authors: Daszyńska-Daszkiewicz, J.; Dziembowski, W. A.; Pamyatnykh,
A. A.
Bibcode: 2003A&A...407..999D
Altcode: 2003astro.ph..5539D
In delta Scuti star models, the calculated amplitude ratios and phase
differences for multi-colour photometry exhibit a strong dependence
on convection. These observables are tools for the determination
of the spherical harmonic degree, l, of the excited modes. The
dependence on convection enters through the complex parameter f,
which describes bolometric flux perturbation. We present a method of
simultaneous determination of f and harmonic degree l from multi-colour
data and apply it to three delta Scuti stars. The method indeed
works. Determination of l appears unique and the inferred values of f
are sufficiently accurate to yield a useful constraint on models of
stellar convection. Furthermore, the method helps to refine stellar
parameters, especially if the identified mode is radial.
Title: Sunshine, Earthshine and Climate Change I. Origin of, and
Limits on Solar Variability
Authors: Goode, Philip R.; Dziembowski, W. A.
Bibcode: 2003JKAS...36S..75G
Altcode:
No abstract at ADS
Title: An interpretation of the line profile variations of RR Lyrae
Authors: Kolenberg, K.; Aerts, C.; Fokin, A.; Dziembowski, W.; Chadid,
M.; Gillet, D.
Bibcode: 2003ASPC..292..171K
Altcode: 2003ipc..conf..171K
The results of a detailed frequency analysis on high-resolution
spectroscopic observations of the Blazhko star RR Lyrae provided a
strong evidence for the presence of nonradial modes in the star. The
detection and identification of these nonradial modes make up the main
goal of this study, and would lead to a better understanding of the
origin of the Blazhko effect.
Title: Are Pulsation and Magnetic Axes Aligned in roAp Stars?
Authors: Bigot, L.; Dziembowski, W. A.
Bibcode: 2003Ap&SS.284..217B
Altcode:
It is commonly assumed that in the rapidly oscillating Ap (roAp)
stars the mode axis is aligned or nearly aligned with the magnetic
field axis. This would be possible if the field is the only important
effect causing departure from spherical symmetry. We show that even
though these stars are slow rotators, the centrifugal force cannot
be neglected. The consequence is that the modes cannot be in general
symmetric about the magnetic field. We argue that such a symmetry is
not implied by the observed coincidence between the field and pulsation
amplitude maxima.
Title: Photometric Nonadiabatic Obervables in Rotating β Cephei
Models
Authors: Daszyńska-Daszkiewicz, J.; Dziembowski, W. A.; Pamyatnykh,
A. A.
Bibcode: 2003Ap&SS.284..133D
Altcode:
We study how moderate rotation affects photometric observables in
β Cephei stars. The most important effect is the rotational mode
coupling of modes with harmonic degree, l, differing by 2 and the
same azimuthal order, m, if the frequencies are close. This is not an
uncommon situation among unstable modes in β Cep stars. Positions of
the coupled modes in the amplitude ratio vs. phase difference diagrams
are aspect, i, - and m-dependent. Inference from the diagrams becomes
more complicated.
Title: Constraints on Stellar Interior Physics from Oscillations
Authors: Dziembowski, W. A.
Bibcode: 2003ASPC..293..262D
Altcode: 2003tdse.conf..262D
Oscillating stars occupying various parts of the H-R diagram differ
in types of excited modes. In this review I distinguish four types
of oscillations encountered in stars. These are: pure g-modes; mixed
modes; high amplitude radial modes; and high-order p-modes. I discuss
separately how the frequency data of each of these types are being used
to obtain constraints on uncertain aspects of stellar physics. The
most troubling uncertainties concern convection, its efficiency in
energy transport, and the extent of element mixing beyond boundaries
of the unstable regions. There are uncertainties in the description of
angular momentum transport and various coefficient from microscopic
physics. I also give a brief survey of unsolved problems in stellar
oscillation theory awaiting progress in numerical simulations.
Title: Nonlinearity of Nonradial Modes in Evolved Stars
Authors: Nowakowski, Rafał M.; Dziembowski, Wojciech A.
Bibcode: 2003Ap&SS.284..273N
Altcode: 2003astro.ph..1504N
We show that in evolved stars, even at relatively low surface
amplitudes, nonradial modes become strongly nonlinear in the hydrogen
shell source, where the Brunt-Väisälä frequency has its absolute
maximum. The measure of nonlinearity is the product of horizontal
displacement times the radial wavenumber, |\xi_H k_r|. It becomes large
already in evolved δ-Scuti stars. This nonlinearity presents a major
problem for interpretations of amplitude modulation in RR Lyrae stars
in terms of nonradial mode excitation.
Title: On the diversity of stellar pulsations
Authors: Dziembowski, Wojciech A.
Bibcode: 2003safd.book...23D
Altcode:
Pulsation is a common phenomenon in stars. It occurs in a wide range
of their masses and in all evolutionary phases, exhibiting large
variety of forms. Stochastic driving and just two distinct instability
mechanisms are the cause of the widespread phenomenon. The diversity
of pulsation properties in stars across the H-R diagram is partially
explained in terms of differences in the ranges of unstable modes and
in terms nonlinear mechanisms of amplitude limitation. Still a great
deal remains to be explained.
Title: ν Eridani: a Very Slowly Rotating and Possibly Magnetic β
Cephei Star
Authors: Dziembowski, W. A.; Jerzykiewicz, M.
Bibcode: 2003ASPC..305..319D
Altcode: 2003mfob.conf..319D
No abstract at ADS
Title: Sources of the Solar p- and F-Mode Frequency Variations
Authors: Dziembowski, Wojciech A.; Goode, Philip R.
Bibcode: 2003IAUJD..12E..23D
Altcode:
We have derived formulae connecting the frequency variations both
centroids and the asymmetric part of the fine structure (the even-a
coefficients) in the the spectrum of solar oscillations to the dynamical
quantities that are expected to change over the solar activity cycle. We
considered the near-surface small-scale magnetic and turbulent velocity
fields as well as horizontal magnetic fields buried near the base of
the convective zone. Measured changes in the centroid frequencies
and the even even-a's over the solar cycle may be accounted for by a
decrease in the turbulent velocity of order percent going from activity
minimum to maximum. Alternatively the changes may be accounted for
by an increase of the small-scale magnetic field of order 100 G if
the growing field is predominantly radial. Both sources are localized
near photosphere. We also show discuss seismic evidence for stronger
field in deep layers. Showing that that global seismology would detect
a field at the level of a fraction of MG if such a field were present
and confined to a thin layer near the base of the convective envelope
Title: Photometric amplitudes and phases of nonradial oscillation
in rotating stars
Authors: Daszyńska-Daszkiewicz, J.; Dziembowski, W. A.; Pamyatnykh,
A. A.; Goupil, M. -J.
Bibcode: 2002A&A...392..151D
Altcode: 2002astro.ph..6109D
Effects of rotational mode coupling on photometric parameters
of stellar oscillations are studied. At moderate rotation rates,
a strong coupling between modes of spherical harmonic degree, l,
differing by 2 and of the same azimuthal order, m, takes place if the
frequencies are close. This is a common situation amongst main sequence
pulsators. Numerical results for a sequence of beta Cephei star models
are reported for the two- and three-mode couplings.\ One consequence
of mode coupling is that modes of higher degree should be considered
in photometric mode identification. Modes with nominal degree l>2
acquire substantial lle2 components and therefore are more likely to
reach detectable amplitudes. Coupled mode positions in the amplitude
ratio - phase difference diagrams, based on multicolour photometry,
become both aspect- and m-dependent. Examples of the mode path in the
diagram with varying aspect are given. The diagrams remain a useful
tool for mode identification in rotating stars but the tool must be
used with care.
Title: The oblique pulsator model revisited
Authors: Bigot, L.; Dziembowski, W. A.
Bibcode: 2002A&A...391..235B
Altcode:
The oblique pulsator model accounts for most of the pulsation properties
of the rapidly oscillating Ap (roAp) stars. The model predicts that
modes are seen as equidistant multiplets separated by the angular
frequency of rotation. The relative amplitudes of the components may
be calculated and directly compared with observations. The effects of
rotation introduce amplitude asymmetry, that is peaks corresponding
to azimuthal numbers m and -m are unequal. In this paper we propose
improvements to the model that consist of including effects of the
centrifugal force and in using a non-perturbative treatment of the
magnetic field influence. We show that in roAp stars the centrifugal
force is the primary source of the rotational frequency shift. Although
the amplitude asymmetry arises from the Coriolis force, its size is
strongly affected by the centrifugal force. For dipole modes (l=1) we
develop a simple geometrical picture of pulsation in the presence of
rotation and a magnetic field. We provide some numerical results for a
representative model of roAp stars which is applied to the case of HR
3831. We find that the mode that agrees with the observed amplitude
ratios in this star significantly departs from alignment with the
magnetic axis. We discuss problems posed by the observational data
of HR 3831, emphasizing difficulties of the standard oblique pulsator
model which assumes that the excited mode is nearly aligned with the
magnetic field.
Title: What dynamic changes in the Sun drive the evolution of
oscillation frequencies through the activity cycle?
Authors: Goode, Philip R.; Dziembowski, W. A.
Bibcode: 2002ESASP.508...15G
Altcode: 2002soho...11...15G
The frequencies of solar oscillations (f- and p-modes) evolve through
the solar cycle. The changes are driven by some combination of changes
in the magnetic field, thermal structure and velocity field. It is
unclear what is the precise combination of the three. One way or
another, this thorny issue rests on an understanding of the response
of the solar structure to increased magnetic field, but this is
complicated. As well, we do not understand the origin of the sun's
irradiance increase with increasing magnetic activity. Until recently,
it seemed that an unphysically large magnetic field change was required
to account for the frequency evolution during the cycle. However,
the problem seems to have been solved (Dziembowski, Goode &
Schou 2001). Specifically, a small-scale magnetic field was considered
assuming uncorrelated field components - allowing the vertical component
to be statistically different from the two horizontal ones. It turns
out that a purely radial random field is the most economical, as well
as being more physically sensible for other reasons. Furthermore, the
solution might have a direct bearing on the origin of the irradiance
variation. We discuss recent results and the present state of our
knowledge.
Title: On the Combined Effects of a Magnetic Field and Rotation on
Acoustic Modes
Authors: Bigot, L.; Dziembowski, W. A.
Bibcode: 2002ASPC..259..290B
Altcode: 2002IAUCo.185..290B; 2002rnpp.conf..290B
No abstract at ADS
Title: Changes in Solar Oscillation Frequencies during the Current
Activity Maximum: Analysis and Interpretation
Authors: Dziembowski, W. A.; Goode, P. R.
Bibcode: 2002ASPC..259..476D
Altcode: 2002IAUCo.185..476D; 2002rnpp.conf..476D; 2001astro.ph..9344D
We describe systematic changes in the centroid frequencies and
the splitting coefficients as found using data from MDI on board
SOHO, covering cycle 23. The data allow us to construct a seismic
map of the evolving solar activity -- covering all latitudes. At
lower latitudes, the temporal evolution closely tracks that of {\it
butterfly diagram}. The additional information from higher latitudes
in the map is of a significant activity in the polar region, peaking
at activity minimum in 1996. The most plausible source of solar
oscillation frequency changes over the solar cycle is the evolution of
the radial component of the small-scale magnetic field. The amplitude
of the required mean field changes is about 100 G at the photosphere,
and increasing going inward.
Title: Oscillations of α UMa and other red giants
Authors: Dziembowski, W. A.; Gough, D. O.; Houdek, G.; Sienkiewicz, R.
Bibcode: 2001MNRAS.328..601D
Altcode: 2001astro.ph..8337D
There is growing observational evidence that the variability of
red giants could be caused by self-excitation of global modes of
oscillation. The most recent evidence of such oscillations was reported
for αUMa by Buzasi et al. who analysed space photometric data from
the WIRE satellite. Little is understood concerning the oscillation
properties in red giants. In this paper we address the question as to
whether excited radial and non-radial modes can explain the observed
variability in red giants. In particular, we present the results of
numerical computations of oscillation properties of a model of αUMa
and of several models of a 2-Msolar star in the red giant
phase. The red giant stars that we have studied have two cavities that
can support oscillations: the inner core that supports gravity (g)
waves and the outer one that supports acoustic (p) waves. Most of the
modes in the p-mode frequency range are g modes confined in the core;
those modes with frequencies close to a corresponding characteristic
frequency of a p mode in the outer cavity are of mixed character and
have substantial amplitudes in the outer cavity. We have shown that
such modes of low degree, l=1 and 2, together with the radial (p)
modes, can be unstable. The linear growth rates of these non-radial
modes are similar to those of corresponding radial modes. In the model
of αUMa and in the 2-Msolar models in the lower regions of
the giant branch, high amplitudes in the p-mode cavity arise only for
modes with l=2. We have been unable to explain the observed oscillation
properties of αUMa, either in terms of mode instability or in terms of
stochastic excitation by turbulent convection. The modes with the lowest
frequencies, which exhibit the largest amplitudes and may correspond to
the first three radial modes, are computed to be unstable if all of the
effects of convection are neglected in the stability analyses. However,
if the Lagrangian perturbations of the turbulent fluxes (heat and
momentum) are taken into account in the pulsation calculation, only
modes with higher frequencies are found to be unstable. The observed
frequency dependence of amplitudes reported by Buzasi et al. does not
agree with what one expects from stochastic excitation. This mechanism
predicts an amplitude of the fundamental mode about two orders of
magnitude smaller than the amplitudes of modes with orders n>=5,
which is in stark disagreement with the observations.
Title: Helioseismic Role of Polar Fields
Authors: Varsik, J. R.; Goode, P. R.; Dziembowski, W. A.; Didkovsky,
L.; Woddard, M. F.
Bibcode: 2001AAS...199.8801V
Altcode: 2001BAAS...33.1433V
We examine the relationship between the change of helioseismic
frequencies at high latitudes during the solar cycle and the change
in the solar polar magnetic field. Helioseismic data from SOHO/MDI
are used to construct a helioseismic butterfly diagram, that is,
the total contributions, binned by latitude, of the p-mode splittings
decomposed into Legendre polynomials. We cover the period of the rise
of Cycle 23. We compare the frequency change contributions to polar
field strengths, in latitude bins, obtained from BBSO polar field
observations. Also we examine the residual intensity variations,
binned by latitude from the BBSO solar disk photometer.
Title: Does the Sun Shrink with Increasing Magnetic Activity?
Authors: Dziembowski, W. A.; Goode, P. R.; Schou, J.
Bibcode: 2001ApJ...553..897D
Altcode: 2001astro.ph..1473D
We have analyzed the full set of Solar and Heliospheric Observatory
(SOHO) Michelson Doppler Imager (MDI) f- and p-mode oscillation
frequencies from 1996 to date in a search for evidence of solar radius
evolution during the rising phase of the current activity cycle. Just
as Antia et al. in 2000, we find that a significant fraction of the
f-mode frequency changes scale with frequency and that if these are
interpreted in terms of a radius change, it implies a shrinking Sun. Our
inferred rate of shrinkage is about 1.5 km yr-1, which is
somewhat smaller than found by Antia et al. We argue that this rate
does not refer to the surface but, rather, to a layer extending roughly
from 4 to 8 Mm beneath the visible surface. The rate of shrinking may
be accounted for by an increasing radial component of the rms random
magnetic field at a rate that depends on its radial distribution. If
it were uniform, the required field would be ~7 kG. However, if it
were inwardly increasing, then a 1 kG field at 8 Mm would suffice. To
assess contribution to the solar radius change arising above 4 Mm, we
analyzed the p-mode data. The evolution of the p-mode frequencies may be
explained by a magnetic field growing with activity. Our finding here
is very similar to that of Goldreich et al. (1991). If the change were
isotropic, then a 0.2 kG increase, from activity minimum to maximum,
is required at the photosphere, which would grow to about 1 kG at
1 Mm. If only the radial component of the field were to increase,
then the requirement for the photospheric field increase is reduced
to a modest 60-90 G. A relative decrease in temperature of the order
of 10-3 in the subphotospheric layers, or an equivalent
decrease in the turbulent energy, would have a similar effect to the
required inward growth of magnetic field change. The implications
of the near-surface magnetic field changes depend on the anisotropy
of the random magnetic field. If the field change is predominantly
radial, then we infer an additional shrinking at a rate between 1.1
and 1.3 km yr-1 at the photosphere. If, on the other hand,
the increase is isotropic, we find a competing expansion at a rate of
2.3 km yr-1. In any case, variations in the Sun's radius
in the activity cycle are at the level of 10-5 or less and,
hence, have a negligible contribution to the irradiance variations.
Title: Resonant Excitation of Nonradial Modes in RR Lyr Stars
Authors: Nowakowski, R. M.; Dziembowski, W. A.
Bibcode: 2001AcA....51....5N
Altcode: 2001astro.ph..5141N
We study a nonlinear development of radial pulsation instability
to a resonant excitation of nonradial modes. Our theory covers the
cases of axisymmetric (m=0) modes as well as (m,-m) pairs. Adopting
a simplified treatment of the radial and nonradial mode coupling
we find that the asymptotic state is a pulsation with constant
amplitudes and we evaluate the relative amplitude of the nonradial
component. Observable consequence of the m=0 mode excitation is a small
period change and a more significant amplitude change, especially
in the case of a dipole mode (l=1). Such a mode has a fairly large
excitation probability. Significant amplitude and phase modulation is
predicted in the case of excitation of a m=+/-1 pair. We suggest that
this may explain Blazhko-type modulation in RRLyr stars. If this model
is correct, the modulation period is determined by the rotation rate
and the Brunt-Vaisala frequency in the deepest part of the radiative
interior.
Title: A model for pulsations in roAp stars
Authors: Bigot, L.; Dziembowski, W. A.
Bibcode: 2001ESASP.464..367B
Altcode: 2001soho...10..367B
The oblique pulsator model accounts for most of the pulsation properties
of the rapidly oscillating Ap (roAp) stars. It also yields predictions
for observables, from which constraints on the magnetic field may be
derived. The observables are relative amplitudes within multiplets
in frequency spectra corresponding to individual eigenmodes. Here we
propose improvements of the oblique pulsator model which consist in
including effects of centrifugal distortion of the star, and using a
non-perturbative treatment of the magnetic field.
Title: Petersen Diagram for RRd Stars in the Magellanic Clouds
Authors: Popielski, B. L.; Dziembowski, W. A.; Cassisi, S.
Bibcode: 2000AcA....50..491P
Altcode: 2000astro.ph.11442P
RRd stars from the Magellanic Clouds form a well-defined band
in the Petersen diagram. We explain this observed band with our
evolutionary and pulsation calculations with assumed metallicity
[Fe/H]=(-2,-1.3). Vast majority of RRd stars from LMC is confined to
a narrower range of (-1.7,-1.3). The width of the band, at specified
fundamental mode period, may be explained by mass spread at given
metallicity. The shape of the band reflects the path of RRd stars within
the RR Lyr instability strip. We regard the success in explaining
the Petersen diagram as a support for our evolutionary models, which
yield mean absolute magnitude in the mid of the instability strip,
<M_V>, in the range 0.4 to 0.65 mag implying distance modulus
to the LMC of 18.4 mag.
Title: Signatures of the Rise of Cycle 23
Authors: Dziembowski, W. A.; Goode, P. R.; Kosovichev, A. G.; Schou, J.
Bibcode: 2000ApJ...537.1026D
Altcode:
During the rise of Cycle 23, we have found a sizable, systematic
evolution of the Solar and Heliospheric Observatory/Michelson Doppler
Imager solar oscillation frequencies implying significant changes in the
spherically symmetric structure of the Sun's outer layers as well as in
its asphericity up to a P18 Legendre distortion. We conducted
a search for corresponding asymmetries in Ca II K data from Big Bear
Solar Observatory. We found tight temporal and angular correlations of
the respective asphericities up through P10. This result
emphasizes the role of the magnetic field in producing the frequency
changes. We carried out inversions of the frequency differences and
the splitting coefficients assuming that the source of the evolving
changes is a varying stochastic magnetic field. With respect to the
most recent activity minimum, we detected a significant perturbation in
the spherical part at a depth of 25-100 Mm, which may be interpreted
as being a result of a magnetic perturbation, <B2>,
of about (60KG)2 and/or a relative temperature perturbation
of about 1.2×10-4. Larger, although less statistically
significant, perturbations of the interior structure were found in
the aspherical distortion.
Title: Neutrinos and solar models.
Authors: Dziembowski, W. A.
Bibcode: 2000AcPPB..31.1389D
Altcode:
After summarizing principles of solar model construction and presenting
an updated prediction for the neutrino counting rates, the author
focusses this review on the question of reliability of current
models. Methods and results of seismic sounding of the solar interior
are presented in some detail. The results confirm the standard scenario
of the solar evolution. This conclusion, combined with the evidences
for neutrino oscillations, means the end of astrophysical aspect of the
solar neutrino problem. The models of the Sun interior remain important
for interpretation of the data from the neutrino detectors but the data
cannot be used to contradict the models, not even to constrain them.
Title: Non-axisymmetric oscillations of roAp stars
Authors: Bigot, L.; Provost, J.; Berthomieu, G.; Dziembowski, W. A.;
Goode, P. R.
Bibcode: 2000A&A...356..218B
Altcode:
We calculate the effect of a strong dipole magnetic field on
non-axisymmetric oscillations for roAp stars, with a typical range of
photospheric magnetic fields B_p [0.5-1.5] kG. As Dziembowski &
Goode (1996), we find that the oscillations are strongly affected by
such magnetic fields in two different ways. The first one concerns the
stability of modes, which are damped due to dissipation by Alfvénic
waves. It leads to a small imaginary part of the frequency, about
(1-15mu Hz). The real part of the frequencies is also affected and is
greater in the presence of magnetic field, with a shift of about 1-20 mu
Hz. We find that these shifts are strongly influenced by the geometry of
the mode, i.e. the value of the degree l, as it has already been shown
by Dziembowski & Goode (1996), and also by m, the azimuthal degree,
with a significant amplitude. The magnetic field, because it breaks
the spherical symmetry of the problem, raises partially the (2 l +1)
degeneracy of frequency in m. We find that the shift of both the real
and imaginary parts is always greater than in the case of axisymmetric
oscillations (m=0), except for sectoral modes (l=m), for which the
imaginary part is smaller. The second effect of large magnetic fields
is to complicate the mode identification. The perturbations cannot
be represented by pure single spherical harmonic, but by a series of
harmonics due to the angular dependence of the Lorentz force. It is
shown that this mixing of spherical harmonics also depends on the value
of m. However, our calculations do not explain the observed selection
of dipole modes in roAp stars, aligned with the magnetic axis, since
they do not minimize energy losses due to Alfvénic waves.
Title: Nonradial Modes in RR Lyrae Stars
Authors: Dziembowski, W. A.; Cassisi, S.
Bibcode: 2000ASPC..203..321D
Altcode: 2000ilss.conf..321D; 2000IAUCo.176..321D
No abstract at ADS
Title: Rotational Splitting of δ Scuti Stars
Authors: Goupil, M. -J.; Dziembowski, W. A.; Pamyatnykh, A. A.;
Talon, S.
Bibcode: 2000ASPC..210..267G
Altcode: 2000dsrs.conf..267G
No abstract at ADS
Title: Commission 35: Stellar Constitution: (Constitution Des Etoiles)
Authors: Zahn, J. -P.; VandenBerg, D.; Canal, R.; Chiosi, C.;
Dziembowski, W.; Guzik, J.; Meynet, G.; Michaud, G.; Renzini, A.;
Saio, H.; Tutukov, A.; Vauclair, G.
Bibcode: 2000IAUTA..24..201Z
Altcode:
No abstract at ADS
Title: The Petersen Diagram for RR Lyrae Stars in the Magellanic
Clouds
Authors: Popielski, B. L.; Dziembowski, W. A.
Bibcode: 2000ASPC..203..276P
Altcode: 2000IAUCo.176..276P; 2000ilss.conf..276P
We show that the spread of the Magellanic Cloud RRd stars in the
Petersen diagram may be explained in terms of spread in metallicity
by one order of magnitude.
Title: An Introduction to Helioseismology
Authors: Christensen-Dalsgaard, J.; Däppen, W.; Dziembowski, W. A.;
Guzik, J. A.
Bibcode: 2000ASIC..544...59C
Altcode: 2000vsea.conf...59C
No abstract at ADS
Title: Basic Aspects of Stellar Structure and Pulsation
Authors: Christensen-Dalsgaard, J.; Dziembowski, W. A.
Bibcode: 2000ASIC..544....1C
Altcode: 2000vsea.conf....1C
No abstract at ADS
Title: Sounding the solar interior
Authors: Dziembowski, W. A.
Bibcode: 2000ASIC..558..245D
Altcode: 2000asre.conf..245D
Contents: Before helioseismology. A brief history of solar oscillation
observations. Waves and modes in the solar model. Measuring solar
oscillations. Probing the radial structure. Measuring the rotation
rate in the Sun's interior. Time-distance helioseismology.
Title: Excitation and visibility of high-degree modes in stars
Authors: Balona, L. A.; Dziembowski, W. A.
Bibcode: 1999MNRAS.309..221B
Altcode:
Observational evidence for excitation of non-radial modes in stars
is confronted with the results of linear stability surveys for
stellar models. We consider various types of pulsators on the upper
main sequence as well as stars in the Cepheid strip. Our stability
survey covers the whole range of spherical harmonic degrees, l, where
instability is found. There is fair agreement between the theoretical
instability strip and the location of ζ Oph stars, but the observed
and calculated periods do not agree in some stars. We suggest that
either pulsation is not responsible for the ζ Oph phenomenon or else
there are serious errors in mode identification in these cases. We do
not find instability at long periods for early B-type stars, supporting
the idea that pulsation is not responsible for the periodic variations
in Be stars. The agreement between the observed and calculated periods
of high-degree modes in δ Sct stars is not very satisfactory. This
is attributed to problems in mode identification. We discuss unstable
modes of high degree in Cepheid models as a possible mechanism for
the low-amplitude radial velocities seen in some stars within the
instability strip. We find, however, that the observed periods are
at least a factor of 2 longer than the calculated periods. Finally,
we discuss the possibility of observing modes of high degree
photometrically. We suggest that a large number of high-degree modes
may become detectable by future space-borne photometric missions. The
confusion arising from these modes may greatly reduce the value of
such observations for asteroseismology. However, they will be very
important in studying the mechanism of mode selection.
Title: Nonradial Modes in RR LYR Stars
Authors: Dziembowski, W. A.; Cassisi, S.
Bibcode: 1999AcA....49..371D
Altcode: 1999astro.ph..7414D
We present a survey of nonradial mode properties in evolutionary
sequences of RR Lyr star models. Attention is focused on the modes
that may be driven by the opacity mechanism and on those that may
be excited as a consequence of the 1:1 resonance with the radial
pulsation. Qualitatively, all the models share the same properties
of nonradial modes. At the quantitative level, the properties are to
a large extent determined by the radial mode periods. There is only
weak dependence on the star metallicity and no apparent dependence
on the evolutionary status, that is on the helium exhaustion in the
convective core. In the whole range of RRab and RRc star parameters
we find unstable nonradial modes driven by the opacity mechanism. An
instability of radial pulsation to a resonant excitation of nonradial
oscillations is also a common phenomenon in both types. We discuss a
possible role of nonradial modes in amplitude modulation observed in
certain RR Lyr stars.
Title: Solar Asphericities from BBSO Synoptic Data and MDI Splittings
Authors: Goode, P. R.; Dziembowski, W. A.; Marquette, W.
Bibcode: 1999AAS...194.4205G
Altcode: 1999BAAS...31..882G
Beneath the photosphere, on average the Sun is almost a perfect
sphere. Historically, the main photospheric asymmetry studied has been
the magnetic activity cycle as reflected in the Maunder butterfly
diagram. In recent years, more subtle signatures of asymmetries
varying with the solar cycle have been found. We have used temporal
averages of BBSO synoptic maps from the activity minimum which has just
ended to extract the low degree Legendre dependence of the data. We
present preliminary comparisons of the degree of correlation between
these Legendre coefficients varying through the solar cycle with the
temporally corresponding low degree Legendre coefficents from MDI
splitting data to enable us to garner another perspective, the role
of the magnetic field in the Sun's cycle dependent asphericity.
Title: Helioseismology and the solar age
Authors: Dziembowski, W. A.; Fiorentini, G.; Ricci, B.; Sienkiewicz, R.
Bibcode: 1999A&A...343..990D
Altcode: 1998astro.ph..9361D
The problem of measuring the solar age by means of helioseismology has
been recently revisited by Guenther & Demarque (1997) and by Weiss
& Schlattl (1998). Different best values for t_seis and different
assessment of the uncertainty resulted from these two works. We show
that depending on the way seismic data are used, one may obtain t_seis~
4.6 Gy close to the age of the oldest meteorites, t_met=4.57 Gy,
like in the first paper, or above 5 Gy like in the second paper. The
discrepancy in the seismic estimates of the solar age may be eliminated
by assuming higher than the standard metal abundance and/or an upward
revision of the opacities in the solar radiative interior. We argue
that the most accurate and robust seismic measure of the solar age
are the small frequency separations, D_{l,n}=nu_ {l,n}-nu_ {l+2,n-1},
for spherical harmonic degrees l=0,2 and radial orders n>>l. The
seismic age inferred by minimization of the sum of squared differences
between the model and the solar small separations is t_seis=4.66+/-0.11
Gy, a number consistent with meteoritic data. Our analysis supports
earlier suggestions of using small frequency separations as stellar
age indicators.
Title: Magnetic Effects on Stellar Oscillations
Authors: Bigot, L.; Provost, J.; Berthomieu, G.; Dziembowski, W. A.;
Goode, P. R.
Bibcode: 1999RoAJ....9S.129B
Altcode:
We calculate the effect of a strong dipole magnetic field (0.5-1.5 kG)
on stellar oscillations. To do this, we adopt a boundary layer approach,
by taking into account the dynamical effect of the magnetic field only
in a very thin layer, at the star surface. The magnetic field leads to
a damping of oscillations due to Alfvénic wave losses of energy. It
appears then an imaginary part of the frequency (~1-15 ?Hz) and a shift
of the real part (~1-15 ?Hz). The mode identification is complicated:
since the Lorentz force depends on the colatitude, one must represent
the oscillations by a linear combination of. We tackled non-axisymmetric
oscillations (m0) and then generalized the result of Dziembowski and
Goode (1996). These magnetic effects strongly depend on the geometric
nature of the mode (degree and azimuthal order m). We apply our
calculatio ns to roAp stars, whose oscillations appear essentially
as dipole modes ( = 1, m = 0) aligned with the magnetic axis. This
work does not explain this geometrical preference, since it does not
minimize Alfvénic losses of energy. However, it shows that one must
take into account the magnetic field to identify modes in roAp stars.
Title: Helioseismology, solar models and neutrino fluxes.
Authors: Castellani, V.; Degl'Innocenti, S.; Dziembowski, W. A.;
Fiorentini, G.; Ricci, B.
Bibcode: 1999NuPhS..70..301C
Altcode: 1997astro.ph.12174C
The authors present results concerning a systematical
analysis of helioseismic implications on solar structure and
neutrino production. They find Yph = 0.238 - 0.259,
Rb/Rsun = 0.708 - 0.714 and ρb
= (0.185 - 0.199) gr/cm3. In the interval 0.2 <
R/Rsun < 0.65, the quantity U = P/ρ is determined with an
accuracy of ±5o/oo or better. At the solar center
still one has remarkable accuracy, ΔU/U < 4%. The authors compare
the predictions of recent solar models (standard and non-standard)
with the helioseismic results. By constructing helioseismically
constrained solar models, the central solar temperature is found to
be T = 1.58×107K with a conservatively estimated accuracy
of 1.4%, so that the major uncertainty on neutrino fluxes is due to
nuclear cross section and not to solar inputs.
Title: MDI Signs of the Rise of Cycle 23
Authors: Goode, P. R.; Dziembowski, W. A.
Bibcode: 1999soho....9E..60G
Altcode:
We find sizeable, systematic changes in solar oscillation frequencies
implying significant changes in the spherically symmetric structure
of the Sun's outer layers, as well as in its asphericity through
a P18 Legendre distortion. We conducted a search for corresponding
asymmetries in Ca II K data from BBSO. We found tight temporal and
angular correlations of the respective asphericities up through
P10. This result emphasizes the direct role of the magnetic field in
producing the frequency changes. We carried out inversions of the
frequency differences and the splitting coefficients assuming that
the source of the evolving changes is a varying stochastic magnetic
field. With respect to the most recent activity minimum, we detected
a significant perturbation in the spherical part at a depth of 25-100
Mm which may be interpreted as being due to magnetic field changes of
about 40 KG and/or relative temperature perturbations of 6x105. Further,
we found somewhat less significant evidence for a shallower, asymmetric
perturbation.
Title: Asteroseismology of the beta Cephei stars. II. 12 (DD) Lacertae
Authors: Dziembowski, W. A.; Jerzykiewicz, M.
Bibcode: 1999A&A...341..480D
Altcode:
Five pulsation modes are simultaneously excited in this well-known beta
Cephei star. Three of them, including the one with the largest light and
radial-velocity amplitudes, form a triplet. The triplet is equidistant
in frequency to within the errors of measurement, that is, 0.0003 d(-1)
. Explaining why the triplet should be so nearly equidistant turns out
to be a real challenge to the theory. We investigate the following
three options: (1) rotational splitting, (2) an oblique magnetic
pulsator, and (3) nonlinear phase lock. Unfortunately, apart from
the frequencies, the data are meager. Photometric indices yield the
effective temperature and surface gravity of rather low accuracy. In
addition, the existing determinations of the spherical harmonic degree
of even the strongest observed mode are discrepant. Consequently,
the model parameters are not well constrained. We show that of the
three above-mentioned options, the oblique pulsator model is unlikely
because it would require excessively strong dipolar field or a special
field geometry. The rotational splitting is a possibility, but only for
an l = 2, p_0 mode in a model with specific values of the effective
temperature and surface gravity. Finally, we note that the nonlinear
phase lock may be the solution. However, verifying this depends on
the progress of nonlinear calculations.
Title: Solar Cycle Onset Seen in SOHO Michelson Doppler Imager
Seismic Data
Authors: Dziembowski, W. A.; Goode, P. R.; di Mauro, M. P.; Kosovichev,
A. G.; Schou, J.
Bibcode: 1998ApJ...509..456D
Altcode:
We have analyzed time changes in centroid frequencies and multiplet
frequency splittings of solar oscillations determined with the Michelson
Doppler Imager instrument (MDI) on SOHO. The data were divided into
five consecutive 72 day sets covering the period from 1996 May 1
through 1997 April 25. We have detected a significant trend in the
a4 and a6 frequency splitting coefficients,
which reflects a decrease in the P4 distortion (described by
the fourth-degree Legendre polynomial of colatitude) and an increase
in the P6 distortion. The rise of the latter distortion
seems to coincide precisely with the rise in the number of new cycle
sunspots. Such sharp and detailed clues to activity onset are new and do
not exist in splitting data from the rising phase of the last cycle. The
relative differences among the solar radii inferred from the f-mode
frequencies from the five sets (at most 6 × 10-6 or 4 km)
are formally significant, reaching a minimum during the observed period.
Title: Effects of magnetic activity on solar-like oscillations
Authors: Dziembowski, W. A.; Goupil, M. -J.
Bibcode: 1998mons.proc...69D
Altcode:
In solar-like stars, faster rotation implies higher magnetic
activity. We calculate p-mode oscillation spectra for stars rotating
much faster than the Sun, taking into account perturbations caused the
centrifugal force and by the magnetic field. The latter is described
by scaling up the perturbations determined in the Sun at two activity
stages. We argue that, in spite of disorderly looking oscillation
spectra, we still should be able to recover information about the
internal rotation. Measuring the magnetic perturbation is important
for interpretation of the helioseismic variations in the solar cycle.
Title: Oscillations in XX Pyx and FG VIR and the prospects for
asteroseismology of delta Scuti stars from space
Authors: Dziembowski, W. A.; Balona, L. A.; Goupil, M. -J.; Pamyatnykh,
A. A.
Bibcode: 1998mons.proc..127D
Altcode:
Analyses of the rich oscillation spectra obtained from these two
delta Scuti stars using ground-based photometry reveal problems which
are likely to be encountered in data from future space asteroseismic
missions. The main difficulty is mode identification. It is unlikely
that the richer spectra from space observations will alleviate this
problem. Space observations of delta Scuti stars, however, are expected
to increase our understanding of mode selection. This, in itself,
justifies the inclusion of delta Scuti stars in the target list for
future asteroseismic space missions.
Title: On the accuracy of helioseismic determination of solar helium
abundance
Authors: Richard, O.; Dziembowski, W. A.; Sienkiewicz, R.; Goode,
Philip R.
Bibcode: 1998A&A...338..756R
Altcode:
The Helium abundance in the solar envelope is one of the most important
seismic observables. We investigate the accuracy of its determination
taking into account uncertainties in the data and in the inversion
procedure. Our best value for the helium abundance in the photosphere is
Ysun=0.248. The estimated uncertainty of 0.002 is dominated
by uncertainties in the inversion. This does not account for possibly
larger inaccuracies in the thermodynamical data.
Title: Helioseismic Studies of Differential Rotation in the Solar
Envelope by the Solar Oscillations Investigation Using the Michelson
Doppler Imager
Authors: Schou, J.; Antia, H. M.; Basu, S.; Bogart, R. S.; Bush,
R. I.; Chitre, S. M.; Christensen-Dalsgaard, J.; Di Mauro, M. P.;
Dziembowski, W. A.; Eff-Darwich, A.; Gough, D. O.; Haber, D. A.;
Hoeksema, J. T.; Howe, R.; Korzennik, S. G.; Kosovichev, A. G.;
Larsen, R. M.; Pijpers, F. P.; Scherrer, P. H.; Sekii, T.; Tarbell,
T. D.; Title, A. M.; Thompson, M. J.; Toomre, J.
Bibcode: 1998ApJ...505..390S
Altcode:
The splitting of the frequencies of the global resonant acoustic modes
of the Sun by large-scale flows and rotation permits study of the
variation of angular velocity Ω with both radius and latitude within
the turbulent convection zone and the deeper radiative interior. The
nearly uninterrupted Doppler imaging observations, provided by the
Solar Oscillations Investigation (SOI) using the Michelson Doppler
Imager (MDI) on the Solar and Heliospheric Observatory (SOHO) spacecraft
positioned at the L1 Lagrangian point in continuous sunlight,
yield oscillation power spectra with very high signal-to-noise ratios
that allow frequency splittings to be determined with exceptional
accuracy. This paper reports on joint helioseismic analyses of
solar rotation in the convection zone and in the outer part of the
radiative core. Inversions have been obtained for a medium-l mode set
(involving modes of angular degree l extending to about 250) obtained
from the first 144 day interval of SOI-MDI observations in 1996. Drawing
inferences about the solar internal rotation from the splitting data
is a subtle process. By applying more than one inversion technique
to the data, we get some indication of what are the more robust
and less robust features of our inversion solutions. Here we have
used seven different inversion methods. To test the reliability and
sensitivity of these methods, we have performed a set of controlled
experiments utilizing artificial data. This gives us some confidence
in the inferences we can draw from the real solar data. The inversions
of SOI-MDI data have confirmed that the decrease of Ω with latitude
seen at the surface extends with little radial variation through much
of the convection zone, at the base of which is an adjustment layer,
called the tachocline, leading to nearly uniform rotation deeper
in the radiative interior. A prominent rotational shearing layer in
which Ω increases just below the surface is discernible at low to
mid latitudes. Using the new data, we have also been able to study the
solar rotation closer to the poles than has been achieved in previous
investigations. The data have revealed that the angular velocity
is distinctly lower at high latitudes than the values previously
extrapolated from measurements at lower latitudes based on surface
Doppler observations and helioseismology. Furthermore, we have found
some evidence near latitudes of 75° of a submerged polar jet which
is rotating more rapidly than its immediate surroundings. Superposed
on the relatively smooth latitudinal variation in Ω are alternating
zonal bands of slightly faster and slower rotation, each extending
some 10° to 15° in latitude. These relatively weak banded flows
have been followed by inversion to a depth of about 5% of the solar
radius and appear to coincide with the evolving pattern of ``torsional
oscillations'' reported from earlier surface Doppler studies.
Title: Observations of the solar interior.
Authors: Dziembowski, W.
Bibcode: 1998Urani..69..148D
Altcode:
No abstract at ADS
Title: Unstable non-radial modes in radial pulsators: theory and
an example
Authors: Van Hoolst, T.; Dziembowski, W. A.; Kawaler, S. D.
Bibcode: 1998MNRAS.297..536V
Altcode:
We study the possibility of the excitation of non-radial oscillations
in classical pulsating stars. The stability of an RR Lyrae model is
examined through non-adiabatic non-radial calculations. We also explore
stability in the presence of non-linear coupling between radial and
non-radial modes of nearly identical frequency. In our model, a large
number of unstable low-degree (l=1,2) modes have frequencies in the
vicinity of unstable radial mode frequencies. The growth rates of
such modes, however, are considerably smaller than those of the radial
modes. We also recover an earlier result that at higher degrees (l=6-12)
there are modes trapped in the envelope with growth rates similar to
those of radial modes. Subsequently, monomode radial pulsation of this
model is considered. The destabilizing effect of the 1:1 resonance
between the radial mode and nearby non-radial modes of low degrees is
studied, with the assumption that the excited radial mode saturates
the linear instability of all other modes. The instability depends on
the radial mode amplitude, the frequency difference, the damping rate
of the non-radial mode, and the strength of the non-linear coupling
between the modes considered. At the pulsation amplitudes typical for
RR Lyrae stars, the instability of the monomode radial pulsation and
the concomitant resonant excitation of some non-radial oscillation
modes is found to be very likely.
Title: Effects of moderate rotation on stellar pulsation. I. Third
order perturbation formalism
Authors: Soufi, F.; Goupil, M. J.; Dziembowski, W. A.
Bibcode: 1998A&A...334..911S
Altcode:
Interpretation of the available ground-based and forthcoming
space observations of multiperiodic variable stars require accurate
computations of oscillation frequencies. Typical rotational equatorial
velocities of upper main sequence stars range between 50 and 200 km/s
and the effect of rotation on the oscillation frequencies of these
stars must be included. The rotation can still be considered as a
perturbation, provided the expansion used in the perturbation method
is carried out far enough to provide frequencies accurate enough to
match that of the observations. For that purpose, we develop here
a perturbation formalism for adiabatic oscillations of moderately
rotating stars which is consistently valid up to third order in the
rotation rate. The formalism includes the case of near-degeneracy
i.e. when the frequencies of two (or three) oscillation modes happen
to be close to each other. This occurs systematically for l=0 and
l=2 p-modes. Near-degeneracy leads to spherical harmonic mixing and
affects frequencies. Consequences for mode identification are discussed.
Title: Shortcomings of the Standard Solar Model
Authors: Dziembowski, W. A.
Bibcode: 1998SSRv...85...37D
Altcode:
The SSM, invented in early nineteen sixties, remains a useful
construction. There are now much larger number of its predictions
that may be compared with observations than when it was first
introduced. Seismic sounding based on oscillations frequencies
provides the best test of the physical input for modelling stellar
evolution. The results of the test must be viewed as a support for the
standard theory of stellar evolution. However, significant differences
in the sound-speed, photospheric He abundance, and other parameters
between the Sun and the current models remain. Shortcomings in the EOS
and in treatment of convection have been revealed. The differences in
the sound-speed in the radiative interior may be explained by small
opacity errors but other explanations are possible. Results of seismic
sounding support the idea that the element mixing in the outer part
of the radiative interior occurred during a significant fraction of
the Sun's life. Such mixing is considered as a possible explanation
of the deficit of lithium. The shortcomings of SSM cannot explain the
deficits of measured neutrino fluxes.
Title: Towards a seismic model of the delta Scuti star XX Pyxidis
Authors: Pamyatnykh, A. A.; Dziembowski, W. A.; Handler, G.; Pikall, H.
Bibcode: 1998A&A...333..141P
Altcode: 1998astro.ph..1264P
Frequencies of 13 oscillation modes in the star XX Pyxidis
(CD-24 7599) are accurately measured but for none of the modes
the spherical harmonic degree (l) is known. We present results
of an attempt to construct the model whose low-l mode frequencies
reproduce possibly close the observations. Models are constrained by
the mean photometric and spectroscopic data for the star. However,
the strongest constraint on the effective temperture is from the
requirement that the modes excited in the star fall into the range
of the modes driven by the opacity mechanism. Our models are built
with the standard stellar evolution code allowing no overshooting
from the convective core. Effects of rotation are taken into account
both in stellar evolution and in linear nonadiabatic oscillation
calculations. Uniform rotation rate and conservation of the global
angular momentum during evolution are assumed. We find several distinct
mode identifications and associated stellar models leading to frequency
fits of similar quality. Determination of the l values for some of
the modes could remove the ambiguity. None of the fits is satifactory:
the mean departures exceed the mean observational frequency error by at
least one order of magnitude. The fits could be improved by means of
adjusting model parameters that were kept fixed. However, such effort
will be meaningful only after improving accuracy in calculation of
the effects of rotation in oscillation frequencies.
Title: On Some Observational Consequences of Nonlinearities in
Stellar Pulsations
Authors: Goupil, M. J.; Dziembowski, W. A.; Fontaine, G.
Bibcode: 1998BaltA...7...21G
Altcode: 1998OAst....7...21G
Numerical experiments, which take into account a resonant coupling
due to rotation, provide synthetic power spectra for triplets of the
l=1 modes split by rotation for two variable white dwarfs, GD 358 and
G 29-38 and a beta Cephei star DD Lac. The coupling coefficients are
unknown and are set arbitrarily. Nevertheless, the results show that
conditions are easily met in white dwarfs for the existence of various
types of temporal behavior similar to those observed. Triplets with
constant amplitudes and nonlinearly forced equal splits can exist next
to triplets with time dependent amplitudes and frequencies. It is found
that mode trapping, through its effect on growth rates, can influence
finite amplitude behavior. For the fast rotating star DD Lac, it is
more difficult to obtain nonlinear equal splits, which may indicate
that its resonant coupling coefficients are less nonadiabatic than in
the case of white dwarfs.
Title: Asteroseismology of the beta Cephei star 12(DD)Lacertae
Authors: Dziembowski, W. A.; Jerzykiewicz, M.
Bibcode: 1998IAUS..185..379D
Altcode:
Five pulsation modes have been detected in this well-known beta
Cephei star. Three of them, including the strongest one, form an
equidistant frequency triplet. We consider identifications of the
observed pulsation frequencies with computed eigenfrequencies of low
degree modes (l <= 2) in a series of stellar models covering the
range of the effective temperature and surface gravity consistent
with best available data. We show that the existing determinations
of the degree of even the strongest observed mode are discrepant and
therefore do little to constrain the problem. Finally, we discuss the
difficulties posed by the observed equidistant frequency triplet.
Title: Solar models: constraints from helioseismology and neutrino
production
Authors: degl'Innocenti, S.; Castellani, V.; Dziembowski, W. A.;
Fiorentini, G.; Ricci, B.
Bibcode: 1998MmSAI..69..539D
Altcode:
No abstract at ADS
Title: Solar Asymmetries from SOHO/MDI Splitting Data
Authors: Goode, P. R.; Dziembowski, W. A.; DiMauro, M. P.; Kosovichev,
A. G.; Schou, J.
Bibcode: 1998ESASP.418..887G
Altcode: 1998soho....6..887G
No abstract at ADS
Title: Differential rotation of the solar interior: new helioseismic
results by inversion of the SOI-MDI/SOHO data
Authors: Di Mauro, M. P.; Dziembowski, W. A.
Bibcode: 1998MmSAI..69..559D
Altcode:
No abstract at ADS
Title: Nonradial Mode Instability in Static and Radially Pulsating
Models of RR Lyrae Stars
Authors: Van Hoolst, T.; Dziembowski, W. A.; Kawaler, S. D.
Bibcode: 1998ASPC..135..232V
Altcode: 1998hcsp.conf..232V
Anomalous behaviours such as amplitude modulation or two mode excitation
among classical pulsating stars remains unexplained. The possible role
of nonradial mode excitation has already been suggested (Kovacs 1993,
Van Hoolst & Waelkens 1995) but no calculations for realistic
models have been done. As a first step toward clarifying the problem,
we investigate linear nonadiabatic properties of nonradial modes in
centrally condensed stars. We begin with an outline of the method
of calculation of the nonadiabatic oscillations in full and realistic
models of such stars. The method is then applied to a static model which
corresponds to a RR Lyrae star in the mid of the instability strip. We
find a large number of unstable low degree modes in the vicinity of
unstable radial modes. The growth rates of such modes, however, are
considerably lower than those of the radial. We also recover an old
result (Dziembowski 1977, Osaki 1977) that at higher degrees there
are modes trapped in the envelope with growth rates similar to that
of radial modes. Subsequently, we consider monomode radial pulsation
of this model. We assume that the excited mode saturates the linear
instability of all modes and we study the destabilizing effect of the
1:1 resonance between the radial mode and nearby nonradial modes of
low degrees. The instability depends on the radial mode amplitude, the
frequency difference, and the damping rate of the nonradial mode. We
find that at the pulsation amplitudes typical for RR Lyrae stars the
resonant excitation of some nonradial oscillation is almost certain
and that the excitation of the l=1 mode is most likely. References:
Dziembowski, W.D., 1977, Acta Astron. 27, 95 Kovacs, G., 1993, in:
Annals of the New York Academy of Sciences, eds.: J.R. Buchler and
H.E. Kanrup, p.70 Osaki, Y., 1977, Publ. Astron. Soc. Japan 29, 235
Van Hoolst, T, Waelkens, C, 1995, Astron. Astrophys. 295, 361
Title: Rotation of the Solar Interior: New Results by Helioseismic
Data Inversions
Authors: Di Mauro, M. P.; Dziembowski, W. A.; Paternó, L.
Bibcode: 1998ESASP.418..759D
Altcode: 1998soho....6..759D
We determine the variation of the Sun's angular velocity with latitude
and radius by means of an helioseismic inversion of more than 30,000
of p-mode splitting coefficients. These data were obtained from the
first set of uninterrupted Doppler images from SOI-MDI (on board of
the SOHO satellite) in 1996, which yield splittings of great accuracy,
never obtained in previous sets of data. The degree ell ranges from 1 to
250, and the frequency from 954 μ Hz to 4556 μ Hz. In order to invert
the data, we used the localized averaging kernel method (Backus &
Gilbert 1970) in the form as derived by Pijpers and Thompson (1992),
known as SOLA (Subtractive Optimally Localized Averaging) in 1.5
dimensions. It means that the 2-dimensional inverse problem has been
decomposed into series of 1-dimensional SOLA independent inversions
in the radial direction. This has allowed us to probe rotation in the
convection zone and the outer part of the radiative core, and obtain
more reliable results closer to the poles (Schou et al. 1998). We are
also able to study rotational details in the shear layer (tachocline)
located near the base of the convection zone and determine the
behaviour of the angular velocity in the solar core. Unfortunately,
the data uncertainties for low ell degrees result in a rotational
profile in the deepest layers of low significance. In order to infer
accurately the rotation of the deep interior, we also invert some
sets of data with more accurate splittings of the lowest degree modes
(ell ~1-4). Currently, the best data sets for this purpose still come
from groundbased instruments such as LOWL, BISON and IRIS (Tomczyk et
al. 1995; Elsworth et al. 1994; Lazreck et al. 1996). However these
inversions give conflicting results in the core. Solution of this
problem may await the accurate low-ell data from the GOLF instrument
on SOHO.
Title: Spherical and aspherical structure of the sun: First year of
SOHO/MDI observations
Authors: Kosovichev, A. G.; Schou, J.; Scherrer, P. H.; Goode, P. H.;
Dziembowski, W. A.; Rhodes, E. J., Jr.; SOI Structure Inversion Team
Bibcode: 1998IAUS..185..157K
Altcode:
We report the results of one year of continuous observations of the
Sun's internal structure using data from the Medium-l Program of
the Michelson Doppler Imager (MDI) on board SOHO. The data provide
continuous coverage of p modes of angular degree l from 0 to 250,
and the f mode from l=100 to 250. The striking stability of solar
Dopplergrams measured by MDI, without an intervening atmosphere,
substantially decreases the noise in the solar oscillations power
spectrum compared with ground-based observations. This permits
detection of lower amplitude oscillations, extending the range and
precision of measured normal mode frequencies. We present new inversion
results for the radial and latitudinal seismic solar structures with
particular attention to zonal asphericity inferred with the high
angular resolution from the data. Using f-mode frequency splitting
we estimate the large-scale structure of the subsurface magnetic
fields. The variations of the solar structure observed during the
first year of MDI observations are also discussed.
Title: B star pulsation - theory and seismological prospects
Authors: Dziembowski, W. A.
Bibcode: 1998IAUS..185..355D
Altcode:
Two distinct types of oscillations are encountered among Main Sequence
B stars. In beta Cep stars, occurring in the B0 - B2 range and varying
with periods of few hours, the excited modes are p- and/or g-modes
of low radial degrees. In SPB stars, which occur in the B4-B8 range,
high-order g-modes having periods from one to four days are excited. All
these stars owe their oscillations to the opacity driving mechanism
acting in the metal-opacity-bump zone. We understand how the detected
modes are excited but we do not understand yet the large diversity
in the form of their pulsation. For that an essential progress in
the nonlinear theory and modeling of multimodal oscillations is
needed. The modes excited in these stars probe deep stellar interior
and therefore they are of potential interest for testing theory of
stellar evolution and rotation. However, attempts made to construct
models for two multiperiodic beta Cep revealed primarily problems that
must be solved before this application could be implemented. An easier,
though still not fully unexplored, application of this type of stars
is probing ages and metal abundances of the host stellar systems.
Title: Solar Asymmetries from SOHO/MDI Splitting Data
Authors: Dziembowski, W. A.; Goode, P. R.; Di Mauro, M. P.; Kosovichev,
A. G.; Schou, J.
Bibcode: 1998ESASP.418..887D
Altcode: 1998soho....6..887D
Systematic changes in p-mode frequencies through the solar cycle
have been discovered during the previous high activity phase. Most
significant changes were found in the even-a coefficients of
the fine structure in the oscillation spectra (Kuhn, 1988;
and Libbrecht and Woodard, 1990). We analyzed time changes in
frequencies determined with the SOHO/MDI instrument. The data were
divided into five 72-day sets covering (1) 5/1/96-7/11/96, (2)
7/12/96-9/21/96, (3) 9/22/96-12/2/96, (4) 12/3/96-2/12/97, and (5)
2/13/97-4/25/97. The splitting coefficients ak are defined by
nuvlosell,n,m-bar nuell,n = sum{k = 1}
ak {cal P}kell(m), where {cal P} are
are orthogonal polynomials (see Ritzwoller and Lavely 1991 and Schou,
et al. 1994). We analyzed behavior of the even order coefficients,
a2k, which arise from the respective, P2k
(cos θ), distortion of the Sun's structure. We found a significant
trend in behavior of the a4 and a6 coefficients,
which reflects a decrease of the P4 and an increase of the
P6 distortions. This trend is the same as seen in the BBSO
data (Libbrecht and Woodard, 1990) between 1986 and 1988 i.e. at the
onset of the previous activity phase. The trend in a2 is
not so apparent. The centroid frequencies, bar nuell,n, as
already reported by Kosovichev et al. (1998), exhibit small nonmonotonic
variations. The relative differences in solar radius inferred from the
f-mode frequencies in the five sets (at most 5 times 10-6)
are formally significant, but again there is no trend.
Title: Precise Determination of the Solar Helium Abundance by
Helioseismology
Authors: Richard, O.; Dziembowski, W. A.; Sienkiewicz, R.; Goode,
Philip R.
Bibcode: 1998ESASP.418..517R
Altcode: 1998soho....6..517R
Helium abundance in the solar envelope is one of the most important
seismic observables. We investigate the accuracy of its determination
taking into account uncertainties in data and in the inversion
procedure. Our best value for the helium abundance in the photosphere
is Yodot = 0.248. The estimated uncertainty of 0.002 is
dominated by uncertainties in the inversion. This does not account
for possibly larger inaccuracies in thermodynamical data.
Title: Observations Versus Theory: The Delta Scuti Star CD-24 7599
Authors: Pikall, H.; Handler, G.; Pamyatnykh, A. A.; Dziembowski, W.
Bibcode: 1998ASPC..135..486P
Altcode: 1998hcsp.conf..486P
This newly discovered d Scuti star shows 13 frequncies, for which no
idenification of the harmonic degrees are available. Spectroscopy
was done, but is only accurate enough for deriving (v sin i). The
model parameters were constrained from ubvy photometry. We show that
stability consideration as well as patterns in the periodogram are
useful for further constraining the parameters. An attempt has been
made to accurately reproduce the measured frequencies with the model
ones considering various mode identifications. A moderate success
has been achieved. To reproduce observed frequencies at the level of
10-3, we need to postulate that at least 2 of the excited modes have
l greater than 2.
Title: Shortcomings of the Standard Solar Model
Authors: Dziembowski, W. A.
Bibcode: 1998sce..conf...37D
Altcode:
No abstract at ADS
Title: Helio- and Asteroseismology
Authors: Dziembowski, W. A.
Bibcode: 1998ASPC..135..427D
Altcode: 1998hcsp.conf..427D
I will review the history of using the data on Sun and other star
oscillations to answer unsolved problems of stellar structure
theory. This will not only be a succes story. I will also identify
problems that have prevented a wider application of stellar seismology.
Title: Determination of the Sun's Seismic Radius from the SOHO
Michelson Doppler Imager
Authors: Schou, J.; Kosovichev, A. G.; Goode, P. R.; Dziembowski, W. A.
Bibcode: 1997ApJ...489L.197S
Altcode:
Dopplergrams from the Michelson Doppler Imager (MDI) instrument on board
the SOHO spacecraft have been used to accurately measure frequencies of
the Sun's fundamental (f) mode in the medium angular degree range, l =
88--250. The comparison of these frequencies with the corresponding
frequencies of the standard solar models suggests that the apparent
photospheric solar radius (695.99 Mm) used to calibrate the models
should be reduced by approximately 0.3 Mm. The precise value of the
seismologically determined solar radius depends on the description
of the subsurface layer of superadiabatic convection. The discrepancy
between the "seismic" and apparent photospheric radii is not explained
by the known systematic errors in the helioseismic and photospheric
measurements. If confirmed, this discrepancy represents an interesting
new challenge to theories of solar convection and solar modeling.
Title: Helioseismic measure of solar activity-meaning and
applications.
Authors: Dziembowski, W. A.; Philip, R. Goode; Schou, J.; Tomczyk,
Steve
Bibcode: 1997A&A...323..231D
Altcode:
We analyze the antisymmetric part of the fine structure in the LOWL
data, and find a remarkable agreement with the BBSO data taken during
the 1986 activity minimum. For both, the P_4_(cosθ) component of the
Sun's asphericity is dominant. We discuss the importance of measuring
this part of the fine structure as a global probe of the Sun's varying
magnetic activity. The asphericity affects oscillation frequencies in
a way that corrupts any inversion for the radial structure of the deep
solar interior. The results of inversion of the original and cleansed
data show that at the current minimal level of solar activity, the
effect is within the errors. However, this is not true in the case of
measurements taken in years of high activity. We mimic such measurements
by adding in appropriate frequency shifts evaluated from 1989 BBSO data.
Title: The structure of the instability strip and mode identification
for beta CEP stars in three young open clusters
Authors: Balona, L. A.; Dziembowski, W. A.; Pamyatnykh, A.
Bibcode: 1997MNRAS.289...25B
Altcode:
We use a grid of stellar models to calculate isochrones. These are
used to determine the ages of NGC 3293, NGC 4755 and NGC 6231. Using
a non-adiabatic pulsation code, we compute the instability strip for
beta Cep variables. We find that there is a correlation between the
frequencies of the excited modes and the mass for stars of a given
age. A calibration is presented which allows age determination of beta
Cep stars in clusters with greater accuracy than isochrone fitting. The
strong dependence of the frequency-mass relationship on age explains
the temperature difference between the instability strips in NGC 3293
and NGC 4755. The second part of the paper examines the question of
mode identification for beta Cep stars in NGC 3293 and NGC 4755. We
present new uvby observations for these stars and use the dependence
of the amplitude on wavelength to estimate the modes. We are able to
isolate a few radial pulsators in the two clusters and to deduce their
masses, effective temperatures and luminosities.
Title: Helioseismology and standard solar models
Authors: Degl'Innoccenti, S.; Dziembowski, W. A.; Fiorentini, G.;
Ricci, B.
Bibcode: 1997APh.....7...77D
Altcode: 1996astro.ph.12053D
We present a systematical analysis of uncertainties in the
helioseismological determination of quantities characterizing the
solar structure. We discuss the effect of errors on the measured
frequencies, the residual solar model dependence and the uncertainties
of the inversion method. We find Y ph = 0.238 - 0.259,
R b/R ⊙ = 0.708 - 0.714 and ϱb =
(0.185 - 0.199) gr/cm 3 (the index b refers to the bottom of
the convective envelope). In the interval 0.2 < R/R ⊙
< 0.65 , the quantity U = P/ϱ is determined with and accuracy
of ±5‰ or better. The predictions of a few recent solar model
calculations are compared with helioseismological results.
Title: New Whole Earth Telescope observations of CD-24 7599: steps
towards delta Scuti star seismology
Authors: Handler, G.; Pikall, H.; O'Donoghue, D.; Buckley, D. A. H.;
Vauclair, G.; Chevreton, M.; Giovannini, O.; Kepler, S. O.; Goode,
P. R.; Provencal, J. L.; Wood, M. A.; Clemens, J. C.; O'Brien, M. S.;
Nather, R. E.; Winget, D. E.; Kleinman, S. J.; Kanaan, A.; Watson,
T. K.; Nitta, A.; Montgomery, M. H.; Klumpe, E. W.; Bradley, P. A.;
Sullivan, D. J.; Wu, K.; Marar, T. M. K.; Seetha, S.; Ashoka, B. N.;
Mahra, H. S.; Bhat, B. C.; Babu, V. C.; Leibowitz, E. M.; Hemar,
S.; Ibbetson, P. A.; Mashal, E.; Meistas, E. G.; Dziembowski, W. A.;
Pamyatnykh, A. A.; Moskalik, P.; Zola, S.; Pajdosz, G.; Krzesinski,
J.; Solheim, J. E.; Bard, S.; Massacand, C. M.; Breger, M.; Gelbmann,
M. J.; Paunzen, E.; North, P.
Bibcode: 1997MNRAS.286..303H
Altcode:
92h of new Whole Earth Telescope observations have been acquired for the
delta Scuti star CD-24 7599. All the seven pulsation modes reported by
Handler et al. are confirmed. However, significant amplitude variations
which are not caused by beating of closely spaced frequencies occurred
within two years. Analysing the combined data of both WET runs,
we detect six further pulsation modes, bringing the total number
up to 13. We also examine our data for high-frequency pulsations
similar to those exhibited by rapidly oscillating Ap stars, but we
do not find convincing evidence for variability in this frequency
domain. From new colour photometry and spectroscopy we infer that
CD-24 7599 is a hot main-sequence delta Scuti star with approximately
solar metallicity and vsini=52& plusmn2kms^-1. We cannot yet
propose a definite pulsation mode identification, but we report the
detection of a characteristic frequency spacing between the different
modes. We ascribe it to the simultaneous presence of l=1 and l=2
modes of consecutive radial order. A comparison of this frequency
spacing with frequencies of solar-metallicity models, as well as
stability analysis, allows us to constrain tightly the evolutionary
state of CD-24 7599. It is in the first half of its main-sequence
evolution, and has a mass of 1.85 +/-0.05 M_solar and a mean density of
rho^-=0.246+/-0.020rho^-_solar. This yields a seismological distance
of 650 +/- 70 pc, which is as accurate as distance determinations
for delta Scuti stars observed in clusters. Most of the pulsation
modes are pure p modes of radial order k=4-6, but the g_1 mode of
l=2 is likely to be excited and observed as well. Since a significant
contribution to this mode's kinetic energy comes from the outer part
of the convective core, CD-24 7599 becomes particularly interesting
for testing convective overshooting theories.
Title: Helioseismic constraints to the central solar temperature
and neutrino fluxes
Authors: Ricci, B.; Berezinsky, V.; degl'Innocenti, S.; Dziembowski,
W. A.; Fiorentini, G.
Bibcode: 1997PhLB..407..155R
Altcode: 1997astro.ph..5164R
The central solar temperature T and its uncertainties are calculated in
helioseismologically-constrained solar models. From the best fit to the
convective radius, density at the convective radius and seismically
determined helium abundance the central temperature is found to be
T = 1.58 × 107 K, in excellent agreement with Standard
Solar Models. Conservatively, we estimate that the accuracy of this
determination is ΔT/T = 1.4%, better than that in SSM. Neutrino fluxes
are calculated. The lower limit to the boron neutrino flux, obtained
with maximum reduction factors from all sources of uncertainties,
is 2σ higher than the flux measured recently by SuperKamiokande.
Title: Seismic sounding of the solar core: purging the corruption
from the Sun's magnetic activity.
Authors: Dziembowski, W. A.; Goode, P. R.
Bibcode: 1997A&A...317..919D
Altcode:
Probing the structure and rotation of the solar core is one of the
greatest challenges to helioseismology. We show that the seismic
information in the observed low degree solar oscillations which probe
the core is severely contaminated. This contamination arises from the
Sun's near surface magnetic activity. The effect on the oscillation
frequencies varies with the solar cycle-vanishing at solar minimum
and growing with increasing surface activity. We demonstrate that this
contamination can be quantified and removed after determining the fine
structure of the entire oscillation spectrum.
Title: Sounding stellar interiors
Authors: Dziembowski, W.
Bibcode: 1997IAUS..181..317D
Altcode:
No abstract at ADS
Title: New solar models including helioseismological constraints
and light-element depletion.
Authors: Richard, O.; Vauclair, S.; Charbonnel, C.; Dziembowski, W. A.
Bibcode: 1996A&A...312.1000R
Altcode: 1996astro.ph..1136R
We have computed new solar models using the same stellar evolution
code as described in Charbonnel, Vauclair and Zahn (1992). This code,
originating from Geneva, now includes the computation of element
segregation for helium and 12 heavier isotopes. It may also include
any type of mixing of the stellar gas, provided this mixing can be
parametrized with an effective diffusion coefficient as a function
of radius. Here we introduced rotation-induced mixing as prescribed
by Zahn (1992). We present five solar models: 1) the standard model,
computed with heavy element abundances as given by Grevesse (1991);
2) a model including pure element segregation (no mixing outside
the convective zone) with Grevesse (1991) as initial abundances;
3) same model as (2), but iterated so that the final abundances are
those of Grevesse (1991); 4) a model with both element segregation and
rotation-induced mixing, leading to lithium and beryllium depletion
consistent with the observations, with Grevesse (1991) as initial
abundances; 5) same model as (4) but iterated to obtain Grevesse (1991)
as final abundances. This model (5) now represents our best new solar
model consistent with the observations. The u=P/ρ function computed
as a function of radius in these new solar models are compared to
the helioseismological results obtained for the same function by
Dziembowski et al (1994). Improving the physics of the models leads
to a better consistency with helioseismology. In our best model (5),
which includes both segregation and mixing, the relative difference
in the u function between the model and the helioseismological results
is smaller than 0.5 per cent at all radii except at the center and the
surface. Meanwhile lithium is depleted by a factor 155 and beryllium by
a factor 2.9, which is consistent with the observations. The bottom of
the convective zone lies at a fractional radius of 0.716, consistent
with helioseismology. The neutrino fluxes are not decreased in any
of these models. The models including the computations of element
segregation lead to a present surface helium abundance of: Y_surf_
between 0.248 and 0.258, which is in satisfactory agreement with the
value derived from helioseismology.
Title: Seismic probing of the solar core
Authors: Dziembowski, W. A.
Bibcode: 1996BASI...24..133D
Altcode:
No abstract at ADS
Title: The Current State of Solar Modeling
Authors: Christensen-Dalsgaard, J.; Dappen, W.; Ajukov, S. V.;
Anderson, E. R.; Antia, H. M.; Basu, S.; Baturin, V. A.; Berthomieu,
G.; Chaboyer, B.; Chitre, S. M.; Cox, A. N.; Demarque, P.; Donatowicz,
J.; Dziembowski, W. A.; Gabriel, M.; Gough, D. O.; Guenther, D. B.;
Guzik, J. A.; Harvey, J. W.; Hill, F.; Houdek, G.; Iglesias, C. A.;
Kosovichev, A. G.; Leibacher, J. W.; Morel, P.; Proffitt, C. R.;
Provost, J.; Reiter, J.; Rhodes, E. J., Jr.; Rogers, F. J.; Roxburgh,
I. W.; Thompson, M. J.; Ulrich, R. K.
Bibcode: 1996Sci...272.1286C
Altcode:
Data from the Global Oscillation Network Group (GONG) project and
other helioseismic experiments provide a test for models of stellar
interiors and for the thermodynamic and radiative properties, on which
the models depend, of matter under the extreme conditions found in the
sun. Current models are in agreement with the helioseismic inferences,
which suggests, for example, that the disagreement between the predicted
and observed fluxes of neutrinos from the sun is not caused by errors in
the models. However, the GONG data reveal subtle errors in the models,
such as an excess in sound speed just beneath the convection zone. These
discrepancies indicate effects that have so far not been correctly
accounted for; for example, it is plausible that the sound-speed
differences reflect weak mixing in stellar interiors, of potential
importance to the overall evolution of stars and ultimately to estimates
of the age of the galaxy based on stellar evolution calculations.
Title: The Seismic Structure of the Sun
Authors: Gough, D. O.; Kosovichev, A. G.; Toomre, J.; Anderson,
E.; Antia, H. M.; Basu, S.; Chaboyer, B.; Chitre, S. M.;
Christensen-Dalsgaard, J.; Dziembowski, W. A.; Eff-Darwich, A.;
Elliott, J. R.; Giles, P. M.; Goode, P. R.; Guzik, J. A.; Harvey,
J. W.; Hill, F.; Leibacher, J. W.; Monteiro, M. J. P. F. G.; Richard,
O.; Sekii, T.; Shibahashi, H.; Takata, M.; Thompson, M. J.; Vauclair,
S.; Vorontsov, S. V.
Bibcode: 1996Sci...272.1296G
Altcode:
Global Oscillation Network Group data reveal that the internal
structure of the sun can be well represented by a calibrated standard
model. However, immediately beneath the convection zone and at the
edge of the energy-generating core, the sound-speed variation is
somewhat smoother in the sun than it is in the model. This could be a
consequence of chemical inhomogeneity that is too severe in the model,
perhaps owing to inaccurate modeling of gravitational settling or to
neglected macroscopic motion that may be present in the sun. Accurate
knowledge of the sun's structure enables inferences to be made about
the physics that controls the sun; for example, through the opacity,
the equation of state, or wave motion. Those inferences can then be
used elsewhere in astrophysics.
Title: A Theoretical Analysis of Pulsation Driving in PG 1159 Stars
Authors: Bradley, P. A.; Dziembowski, W. A.
Bibcode: 1996ApJ...462..376B
Altcode:
Our understanding of stars of the PG 1159 spectral type
is not yet satisfactory, in spite of the recent success of
asteroseismology. Kawaler and coworkers match the observed pulsation
frequencies of PG 1159-035 and PG 2131+066 quite well with evolutionary
models, but they fail to identify the mechanism exciting their
pulsations. Stanghellini, Cox, & Starrfield show that the classical
Κ, γ mechanism acting in the C/O partial ionization zone can excite
certain g-modes but requires compositions that seem unrealistic. Here
we study the impact of the new OPAL opacities on the conditions
required to drive the modes observed in the PG 1159 spectral class
stars. To this end, we present the nonadiabatic pulsation results of
a parametric survey of quasi-evolutionary models of PG 1159 pre-white
dwarfs. We examine the effect of varying the chemical composition of the
driving region, the stellar radius, and stellar mass on the location
of the instability strip and the maximum unstable period. Changes in
the oxygen mass fraction of the driving region and the stellar radius
have a strong effect on the predicted spectrum of unstable modes. We do not find unstable modes with periods longer than 150 s unless
the driving region, located near 10-9 M*, has at
least 50% oxygen. The maximum unstable period increases by factors of
2-3 when we increase the radius of our models by 40%-50%. Decreasing
the stellar mass also increases the radius, and the maximum unstable
period increases from ∼300-400 s at 0.65 Msun to ∼800 s
at 0.50 Msun for models with 50:50 C/O cores. Based on these
results, we suggest that no pulsating PG 1159 star has a driving region
with photospheric abundances; rather they are probably oxygen-rich. In
addition, we believe PG 1159-035 and PG 1707+427 probably have larger
radii than the seismological models of Kawaler & Bradley predict,
because our evolutionary models with pure oxygen cores fail to predict
unstable modes with periods up to the ∼1000 s we observe. Models with
larger radii also have rates of period change closer to that observed
for the 516 5 mode of PG 1159-035. In contrast, our present 50:50 C/O
evolutionary models are able to duplicate the observed maximum unstable
periods of the two coolest pulsating PG 1159 stars, PG 2131+066 and
PG 0122+200. This suggests that the last two stars have radii close
to that predicted by our models, and that their driving regions are
less oxygen-rich than in the hotter pulsating PG 1159 stars.
Title: GONG Data: Implications for the Sun's Interior and Near
Surface Magnetic Field
Authors: Goode, P. R.; Dziembowski, W. A.; Rhodes, E. J., Jr.; Tomczyk,
S.; Schou, J.; GONG Magnetic Effects Team
Bibcode: 1996AAS...188.5307G
Altcode: 1996BAAS...28..904G
The solar oscillation spectrum and the fine structure in it from the
first complete month of GONG data have been used to place a limit
on the Sun's internal magnetic field. The limit is consistent with
the magnetic pressure being no more than 1/1000 of the gas pressure
between the Sun's deep interior and its surface. This conclusion is
consistent with earlier results. The GONG data are from a time near
magnetic activity minimum. The effect of the near surface magnetic
field on the fine structure in the oscillation spectrum reflects
a perturbation of quadrupole toroidal symmetry. This geometry also
dominated at the last activity minimum. The meaning of this result is
discussed. The near surface magnetic perturbation is not spherically
symmetric. This corrupts the results of inversions designed to probe
the Sun's deep interior. The solution to this problem is presented.
Title: Magnetic Effects on Oscillations in roAp Stars
Authors: Dziembowski, W. A.; Goode, Phillip R.
Bibcode: 1996ApJ...458..338D
Altcode:
We calculate the effect of a simple dipole magnetic field on high-order
p-mode oscillations. The stellar models, oscillation modes, and range
of field intensity were selected to correspond to the data on roAp
stars. We did not account for the field in the static models. Some of
the modes we calculate exhibit a strong driving due to the κ-mechanism
acting in the hydrogen ionization zone. This driving is only somewhat
smaller than the radiative damping occurring beneath. We argue that the
situation is likely to reverse after needed improvements are made in
model calculations. The effect of the field is very significant. At
KG photospheric intensity, the mode frequencies are shifted by about
10-20 μHz from their nonmagnetic values. Such shifts are comparable to
the small separations. Damping rates due to Alfvénic wave losses are in
the 2-10 μHz range and are comparable to nonadiabatic damping rates. Surface amplitudes significantly depart from pure, single spherical
harmonic dependence, which severely complicates mode identification
and observational determination of large separations. Thus, taking
into account the effects of the magnetic field is a prerequisite to
any meaningful roAp star asteroseismology and to understanding mode
selection in these objects.
Title: Asteroseismology of the β Cephei stars. I. 16 (EN) Lacertae
Authors: Dziembowski, W. A.; Jerzykiewicz, M.
Bibcode: 1996A&A...306..436D
Altcode:
16 (EN) Lacertae is a single-lined spectroscopic binary and an eclipsing
variable. It consists of the well-known β Cephei star and an invisible
secondary. Four pulsation modes, including a radial one, are found
to be simultaneously present in the β Cephei primary. We consider
all possible identifications of the observed pulsation frequencies
with the computed ones for low-degree modes (l<=2) in a series of
stellar models covering the range of T_eff_ and mean density consistent
with the best available data. Only models allowing no overshooting are
taken into account and with this restriction the conclusion that the
rotation rate increases inward is unavoidable. There are ambiguities
in the identification of the modes. We recommend observations that
should enable a unique identification and, as a result, yield precise
model parameters and better constraints on the differential rotation.
Title: Can we measure the rotation rate inside stars ?
Authors: Goupil, M. -J.; Dziembowski, W. A.; Goode, P. R.; Michel, E.
Bibcode: 1996A&A...305..487G
Altcode:
We examine the possibility of obtaining localized information on the
rotation rate inside stars through asteroseismic observations from
space. Attention is focused on δ Scuti stars where both opacity-driven
and solar-like turbulence-driven modes may be detected. Plausible sets
of modes with attendant rotational splitting data are deduced from
the results of linear stability calculations, effects of amplitude
averaging for modes with higher l's, and information gathered from
ground-based photometry. For such sets of modes, optimally localized
averaging kernels are constructed, and we show that fairly detailed
information about the behavior of the rotation rate can be obtained only
if opacity-driven modes like those anticipated are indeed detected. The
turbulence-driven modes are essentially irrelevant for probing rotation.
Title: Updated Seismic Solar Model
Authors: Dziembowski, W. A.; Goode, Philip R.; Pamyatnykh, A. A.;
Sienkiewicz, R.
Bibcode: 1995ApJ...445..509D
Altcode:
Recently released low-l solar oscillation data from the BISON network
are combined with BBSO data to obtain an updated solar seismic model
of the Sun's interior. For the core, the solar seismic model from the
new data is more consistent with the current standard solar models
than our earlier seismic model. An astrophysical solution to the solar
neutrino problem fades away.
Title: The δ Scuti star FG Virginis. I. Multiple pulsation
frequencies determined with a combined DSN/WET campaign.
Authors: Breger, M.; Handler, G.; Nather, R. E.; Winget, D. E.;
Kleinman, S. J.; Sullivan, D. J.; Li, Z. -P.; Solheim, J. E.; Jiang,
S. -Y.; Liu, Z. -L.; Wood, M. A.; Watson, T. K.; Dziembowski, W. A.;
Serkowitsch, E.; Mendelson, H.; Clemens, J. C.; Krzesinski, J.;
Pajdosz, G.
Bibcode: 1995A&A...297..473B
Altcode:
A coordinated photometric campaign of FG Vir at nine observatories
covering 170 hours was undertaken by DSN (Delta Scuti Network) and
WET (Whole Earth Telescope). Two different observing techniques were
adopted for the two telescope networks in order to optimize different
frequency ranges. Ten pulsation frequencies between 9.19 and 34.12c/d
(112 and 395μHz) were detected with amplitudes ranging from 0.8 to
22mmag. Pulsational instability is observed only in specific frequency
regions. Additional frequencies of pulsation within these regions
probably exist, but do not reach the significance criterion of amplitude
signal/noise adopted by us. Comparisons with previously obtained data
show that the amplitudes of the main frequencies are stable over a year
or longer. A preliminary identification of the ten dominant frequencies
is proposed in a stellar model with 1.8Msun_ in advanced
main-sequence phase of evolution. The frequencies correspond to low
order p and g modes with l<=2 and radial order 1 to 6. According
to the linear nonadiabatic calculations, the identified modes are
driven by the opacity mechanism along with many other modes. For
asteroseismology of δ Scuti stars, FG Vir is an extremely important
candidate, especially because of the probable presence of g modes.
Title: Asteroseismology of the Beta Cephei Variable 16 (EN) Lacertae
Authors: Dziembowski, W. A.; Jerzykiewicz, M.
Bibcode: 1995ASPC...83..289D
Altcode: 1995IAUCo.155..289D; 1995aasp.conf..289D
No abstract at ADS
Title: Pulsation in Hot Stars
Authors: Dziembowski, W. A.
Bibcode: 1995ASPC...78..275D
Altcode: 1995aapn.conf..275D
No abstract at ADS
Title: Uncertainties in the Position of the Beta Cephei Instability
Strip in the HR Diagram
Authors: Pamyatnykh, A. A.; Dziembowski, W. A.; Moskalik, P.
Bibcode: 1995ASPC...83..291P
Altcode: 1995IAUCo.155..291P; 1995aasp.conf..291P
No abstract at ADS
Title: Consequences of Rapid Rotation on Mode Identification
Authors: Soufi, F.; Goupil, M. J.; Dziembowski, W. A.; Sienkiewicz, H.
Bibcode: 1995ASPC...83..321S
Altcode: 1995IAUCo.155..321S; 1995aasp.conf..321S
No abstract at ADS
Title: Seismic Solar Model
Authors: Dziembowski, W. A.; Goode, P. R.; Pamyatnykh, A. A.;
Sienkiewicz, R.
Bibcode: 1995ASPC...76..124D
Altcode: 1995gong.conf..124D
No abstract at ADS
Title: On Expected Constraints on Stellar Transport Processes from
Space Seismic Missions: EVRIS, COROT
Authors: Goupil, M. J.; Michel, E.; Cassisi, S.; Dziembowski, W. A.;
Lebreton, Y.; Baglin, A.; Auvergne, M.; Catala, C.
Bibcode: 1995ASPC...83..453G
Altcode: 1995IAUCo.155..453G; 1995aasp.conf..453G
No abstract at ADS
Title: The Opacity Driven Pulsators
Authors: Dziembowski, W. A.
Bibcode: 1995ASPC...76..586D
Altcode: 1995gong.conf..586D
No abstract at ADS
Title: The opacity driven pulsators
Authors: Dziembowski, W. A.
Bibcode: 1995HiA....10..584D
Altcode:
No abstract at ADS
Title: Nonadiabatic observables in β Cephei models.
Authors: Cugier, H.; Dziembowski, W. A.; Pamyatnykh, A. A.
Bibcode: 1994A&A...291..143C
Altcode:
Using results of linear nonadiabatic calculations for oscillations of
β Cephei star models, we calculate amplitudes and phases for light,
colour and radial velocity for the unstable modes of low harmonic
degrees, l. The nonadiabatic observables are the amplitude ratios and
the phase differences for various oscillating parameters. We construct
theoretical diagrams involving these observables as well as pulsation
periods and compare them with the stellar data. Balona & Stobie
(1979) showed that the diagrams based on two-colour photometric data
may be used to determine the l-value of an observed mode. Our use of
results of nonadiabatic calculations improves their method. We show,
in paricular, that the diagrams employing the satellite ultraviolet
measurements are the best for discrimination between the l=0, 1 and 2
cases. The clearest separation of the domains for the three l-values
occurs in the diagram making use of both photometric and the radial
velocity data. The observational points fall into three theoretical
domains and an assignment of the l-value is unambiguous. A comparison
of the theory with the observations is also made using the Period
versus Amplitude ratios diagrams in various photometric systems. The
agreement is very satisfactory and, in most cases, the l-value can
be determined. The nonadiabatic observables are useful not only to
determine l but also the radial order of the observed modes as well
as for constrain mean stellar parameters. As an example we consider
the case of δ Ceti - a single mode β Cephei star. Our results point
to the significant multicolour photometric and spectroscopic data
for asteroseismology.
Title: A Seismic Model of the Sun's Interior
Authors: Dziembowski, W. A.; Goode, Philip R.; Pamyatnykh, A. A.;
Sienkiewicz, R.
Bibcode: 1994ApJ...432..417D
Altcode:
We advance the method of frequency inversion revealing a more
accurate seismic sounding of the solar core. We show that with the
quoted observational errors, it is possible to achieve a precision of
approximately 10-3 in the sound speed determination through
most of the sun's interior. Only for r less than 0.05 solar radius is
the precision approximately 10-2. The accuracy of the density
and pressure determinations is only slightly worse. Such restrictions
impose significant constraints on the microscopic physical data, i.e.,
opacities, nuclear-reaction cross sections, and diffusion coefficients,
as well as on the solar age. The helioseimic age is consistent with
that from meteorites. The currently available data for low-degree
p-mode frequencies exhibit a scatter that is larger than the quoted
errors, and therefore the actual precision of seismic inferences is
less than what we report, especially for the solar core. We invert
p-mode data to obtain a solar seismic model. Comparisons of the solar
seismic model with current theoretical models shows a need for some
refinements within the framework of the standard solar model. Only
in the innermost part of the core (r less than 0.05 solar radius)
do we see a feature in the seismic sound speed that cannot easily be
accounted for by refinements of the model. But the reality of the
feature is by no means certain. We find no evidence supporting an
astrophysical solution to the solar neutrino problem.
Title: Internal Rotation of the Sun
Authors: Duvall, T. L., Jr.; Dziembowski, W.; Goode, P. R.; Gough,
D. O.; Harvey, J. W.; Leibacher, J. W.
Bibcode: 1994snft.book..414D
Altcode:
No abstract at ADS
Title: The New Opacities and B-Star Pulsations
Authors: Dziembowski, W. A.
Bibcode: 1994IAUS..162...55D
Altcode:
No abstract at ADS
Title: G-Mode instability in the main sequence B-type stars
Authors: Dziembowski, W. A.; Moskalik, P.; Pamyatnykh, A. A.
Bibcode: 1994IAUS..162...69D
Altcode:
No abstract at ADS
Title: Nonadiabatic observables in Beta Cephei star models
Authors: Cugier, H.; Dziembowski, W. A.; Pamyatnykh, A. A.
Bibcode: 1994IAUS..162...15C
Altcode:
No abstract at ADS
Title: The structure of the solar core
Authors: Dziembowski, W. A.
Bibcode: 1994LNP...432...29D
Altcode: 1994LNPM...11...29D
The efforts to resolve the solar neutrino problem resulted in a
considerable progress in reliability and accuracy in modelling the
Sun's interior. Implications from helioseismic sounding supports the
standard picture of the solar evolution. A comparison of neutrino fluxes
measured in various experiments seems to rule out a non-standard solar
core as the solution of the neutrino problem.
Title: OP versus OPAL opacities: consequences for B star oscillations
Authors: Pamyatnykh, A. A.; Dziembowski, W. A.; Moskalik, P.; Seaton,
M. J.
Bibcode: 1994IAUS..162...70P
Altcode:
No abstract at ADS
Title: The opacity mechanism in B-type stars - II. Excitation of
high-order g-modes in main-sequence stars.
Authors: Dziembowski, W. A.; Moskalik, P.; Pamyatnykh, A. A.
Bibcode: 1993MNRAS.265..588D
Altcode:
We show that the OPAL opacities, in addition to explaining the origin
of the pulsations of β Cep stars, also predict the existence of a large
region in the main-sequence band at lower luminosities, where high-order
g-modes of low harmonic degree l are unstable. The excitation mechanism
remains the same, and is due to the usual w-effect acting in the metal
opacity bump (T ≍ 2 × 105 K). The new instability domain
nearly bridges the gap in spectral types between δ Sct and β Cep
stars. The periods of the unstable modes are in the range 0.4-3.5
d for l=1 and 2. We propose that this excitation mechanism causes
photometric variability in the slowly pulsating B-type stars (SPB
stars), and perhaps in other B stars for which variability in the
same period range has been reported. Typically, a large number
of modes are simultaneously unstable in one model. Most of them have
l>2. Such modes are not likely to be detected photometrically,
but may be visible in line profile changes. The excitation of many
high-l modes in a star may also cause a spurious contribution to
the rotational υ sin i values. Sequences of unstable modes at
each value of l exhibit a periodically varying departure from equal
spacing in period. This feature, first noted in white dwarf g-mode
spectra (calculated and measured), is in the present case a probe of
the region left behind the shrinking core (the μ-gradient zone). We
discuss prospects for and difficulties of SPB-star asteroseismology.
Title: The opacity mechanism in B-type stars - I. Unstable modes in
beta Cephei star models.
Authors: Dziembowski, W. A.; Pamiatnykh, A. A.
Bibcode: 1993MNRAS.262..204D
Altcode:
A stability survey for models of 7- to 16-solar-mass stars in the
main-sequence (MS) and early post-MS evolutionary phases is presented
on the basis of an improved version of the OPAL capacities, with the
metal abundance parameter Z having values of 0.02 and 0.03. For most of
the Beta Cep objects, a value of Z = 0.02 suffices to account for the
pulsational instability. The fundamental radial mode, p1, is unstable
in all sequences of stellar models considered, except the one with M =
7 solar masses and Z = 0.02. Instability of the overtones, up to p3 for
Z = 0.03, appears with mass increase. Nonradial modes of low degrees are
unstable in a wider frequency range encompassing low-order g-modes. All
of these modes may be associated with the Beta Cep phenomenon. For
higher spherical harmonic degrees, there is a domain of high-order
g-mode instability. Excitation of such modes may be responsible for
long-period variability of line profiles observed in some B-type stars.
Title: The Sun's Internal Angular Momentum from Seismology
Authors: Dziembowski, W. A.; Goode, P. R.
Bibcode: 1993ASPC...42..225D
Altcode: 1993gong.conf..225D
No abstract at ADS
Title: The Sun's Internal Rotation during and after the 1986 Activity
Minimum
Authors: Goode, P. R.; Dziembowski, W. A.
Bibcode: 1993ASPC...42..217G
Altcode: 1993gong.conf..217G
No abstract at ADS
Title: Radial and nonradial mode instability in B-type stars
Authors: Dziembowski, W. A.; Pamyatnykh, A. A.
Bibcode: 1993ASPC...40..721D
Altcode: 1993IAUCo.137..721D; 1993ist..proc..721D
No abstract at ADS
Title: Mode selection and other nonlinear phenomena in stellar
oscillations.
Authors: Dziembowski, W. A.
Bibcode: 1993ASPC...40..521D
Altcode: 1993ist..proc..521D; 1993IAUCo.137..521D
Nonlinear effects determining the amplitudes of unstable modes of
oscillations in stellar modes, are reviewed. The two basic processes
- saturation of the driving mechanism and resonant mode coupling
- are discussed within the framework of the amplitude equation
formalism. There are difficult problems in the theory of multiple mode
interaction that must be solved to make a prediction of amplitude
spectra possible. The observed spectra for δ Scu stars and other
multiperiodic variables, exhibit no simple pattern that would suggest
a solution of the theoretical problems.
Title: Probing the convective-radiative zone interface by means of
helio- and asteroseismology (invited paper)
Authors: Dziembowski, W.
Bibcode: 1993MmSAI..64..223D
Altcode:
No abstract at ADS
Title: Seismic Limits on the Sun's Internal Toroidal Field
Authors: Goode, P. R.; Dziembowski, W. A.
Bibcode: 1993ASPC...42..229G
Altcode: 1993gong.conf..229G
No abstract at ADS
Title: Sources of uncertainty in direct seismological measurements
of the solar helium abundance
Authors: Kosovichev, A. G.; Christensen-Dalsgaard, J.; Daeppen, W.;
Dziembowski, W. A.; Gough, D. O.; Thompson, M. J.
Bibcode: 1992MNRAS.259..536K
Altcode:
The methods by which Dappen et al. (1988, 1990, 1991) and Dziembowski
et al. (1990, 1991, 1992) recently obtained discrepant estimates of
the helium abundance in the solar convection zone are compared. The
aim of the investigation reported in this paper is to identify the
main source of the discrepancy. Using as proxy data eigenfrequencies
of a set of modes of a theoretical solar model, computed with the
same physics as were the frequencies of a reference model with which
these data are compared, the two methods yield similar results. Thus
we ascertain that the principal source of the discrepancy is not in
the inversions themselves, which yield essentially a measure of the
variation of the adiabatic exponent gamma of the material in the He II
ionization zone. Instead it is in the approximations adopted in the
treatment of heavy elements in the equation of state used to relate
the variation of gamma to chemical composition. We obtain acceptably
consistent results when inverting solar data by two methods using the
same equation of state. We attempt to identify the likely residual
sources of uncertainty.
Title: Effects of Differential Rotation on Stellar Oscillations:
A Second-Order Theory
Authors: Dziembowski, W. A.; Goode, Philip R.
Bibcode: 1992ApJ...394..670D
Altcode:
A complete formalism, valid through second order in differential
rotation, is developed and applied to calculate the frequencies of
stellar oscillations. The derivation is improved and the asymptotic
formulas for g-mode splittings are generalized. In application to
solar oscillations, it is found that the second-order effects are
dominated by distortion for l less than 500. Further, these effects
are sufficiently large that they must be accounted for in any effort to
seismically determine the sun's internal magnetic field. In the solar
oscillation spectrum, accidental degeneracies happen but cannot lead
to large frequency shifts. For evolved delta Scuti stars, calculated
spectra are dense, and, under the perturbing effect of rotation, members
of neighboring multiplets may overlap. The seismic potential of modes
of mixed p-mode and g-mode character is emphasized for these stars.
Title: New opacities and the origin of the β Cephei pulsation.
Authors: Moskalik, P.; Dziembowski, W. A.
Bibcode: 1992A&A...256L...5M
Altcode:
New opacities of Iglesias and Rogers were used in a pulsation stability
survey of β Cephei envelope models. It was found that the fundamental
radial mode is unstable and that the instability should persist in
nonradial modes of similar and lower frequencies. The driving is caused
by the usual κ-mechanism acting in a zone with temperatures near
2×105K where there is a bump in opacity. The instability
arises as a result of a slight surplus of this driving over the damping
which takes place in the rest of the star interior. Its occurrence
depends in a sensitive way on the heavy element content. The theoretical
instability strip on the H-R diagram agrees well with the observational
one. The theoretical strip widens in the supergiant region, which
suggests that the same mechanism may be responsible for oscillations
in the Luminous Blue Variables.
Title: Testing Stellar Evolution Theory with Oscillation Frequency
Data.
Authors: Dziembowski, W.
Bibcode: 1992RvMA....5..143D
Altcode:
No abstract at ADS
Title: Seismological Tests of Standard Solar Models Calculated with
New Opacities
Authors: Dziembowski, W. A.; Pamiatnykh, A. A.; Sienkiewicz, R.
Bibcode: 1992AcA....42....5D
Altcode:
We calculated models of the Sun adopting opacity data of Iglesias and
Rogers (1991) for three heavy element mixtures. Calculations were made
with a standard stellar evolution code ignoring effects of gravitational
settling and the convective overshooting. Using nearly 2300 measured
frequencies of solar p-modes we determined corrections to the sound
speed and density distribution in these models. The corrections were
found to be significantly smaller than those in models calculated with
earlier opacities. The model calculated with the Anders and Grevesse
(1989) mixture of heavy elements shows a remarkable agreement with
the helioseismic data. There is, however, a contradiction between this
consistency and a sizable correction to the surface helium abundance,
Delta Y approx -0.04. We argue that this large value may be a spurious
result caused by inadequacies in the MHD (Mihalas, Dappen and Hummer
1988) thermodynamics used in the models. We found new evidence for
such inadequacies in relatively large corrections to the sound speed
in the fractional radius range 0.85-0.95.
Title: Testing stellar evolution theory with oscillation frequency
data.
Authors: Dziembowski, W.
Bibcode: 1992RvMA....5..142D
Altcode:
Contents: Introduction. Mixing in stellar interiors - an unresolved
problem. Methods of asteroseismology. Mixing in the Sun's core? On the
interface between convective envelope and radiative interior. What did
we learn from helioseismology about the angular momentum evolution? Can
δ Scuti star observations help us to solve the overshooting problem?
Title: A potential asteroseismological test for convective
overshooting theories.
Authors: Dziembowski, W. A.; Pamyatnykh, A. A.
Bibcode: 1991A&A...248L..11D
Altcode:
Attention is given to that mode excited by the opacity mechanism in
Delta Scuti stars which is trapped in the region containing the outer
part of the convective core and the chemically inhomogeneous zone
behind the shrinking core. This mode evolves toward higher frequencies
that exhibit the 'avoided crossing' with constitutive p-modes. It is
suggested that this mode frequency's determination could furnish a
useful indication of the extent of the overshooting from the convective
core. Spaceborne observations yielding detailed periodograms for Delta
Scuti variables are recommended.
Title: Seismology for the Fine Structure in the Sun's Oscillations
Varying with Its Activity Cycle
Authors: Dziembowski, W. A.; Goode, Philip R.
Bibcode: 1991ApJ...376..782D
Altcode:
The symmetric part of the fine structure in the 1986 and 1988 solar
oscillation data of Libbrecht and Woodard (1990) is inverted to find
statistically significant evidence for a steady megagauss toroidal
field at the bottom of the convective envelope. The sizable amplitude
of a cycle-dependent near-surface perturbations, which is argued to
have its origin in the fibril field, is confirmed.
Title: Helium content in the solar convective envelope from
helioseismology
Authors: Dziembowski, W. A.; Pamiatnykh, A. A.; Sienkiewicz, R. .
Bibcode: 1991MNRAS.249..602D
Altcode:
Using measured frequencies of the solar p-modes, the fractional mass
content of helium in the sun's envelope is determined. It is found
that its value lies between 0.23 and 0.24. This result appears to be
unaffected by the uncertainties in the treatment of the atmosphere
and the outer convective zone, but it relies in a sensitive way on
calculation of thermodynamical quantities.
Title: What We Know about the Sun's Internal Rotation from Solar
Oscillations
Authors: Goode, Philip R.; Dziembowski, W. A.; Korzennik, S. G.;
Rhodes, E. J., Jr.
Bibcode: 1991ApJ...367..649G
Altcode:
In this paper, a uniform approach of inversion was used to determine
the internal rotation rate of the sun from each of the six available
sets of solar oscillation data, which included the data of Duvall et
al. (1986), Rhodes et al. (1987, 1990), Tomczyk (1988), Brown and Morrow
(1987), and Libbrecht (1989). The technique chosen for inverting the
solar oscillation data was the discretized least-squares technique. The
results indicate that the rotation rate of the sun in the equatorial
plane declines going inward between the surface and 0.6 of the radius
and that the polar rate increases going inward (i.e., the surfacelike
differential rotation decreases with depth).
Title: Solar-cycle dependence of the Sun's deep internal rotation
shown by helioseismology
Authors: Goode, Philip R.; Dziembowski, W. A.
Bibcode: 1991Natur.349..223G
Altcode:
HELIOSEISMOLOGY, the study of solar oscillations, yields information on
the Sun's internal rotation and magnetism which is of great importance
in understanding the 22-year solar cycle. We show here that helioseismic
data suggest that the Sun's internal rotation rate, at depths greater
than half the solar radius, has changed systematically during the most
recent cycle. There is no variation, however, in the rotation over
a range of intermediate solar radii covering the upper part of the
Sun's radiative interior and the lower part of the convective zone;
this intermediate region is where, according to the same helioseismic
data, an abrupt change in rotation rate with depth accompanies the
transition from convective to radiative structure. We suggest that the
modulation of the rotation rate in the Sun's interior could be caused
by a torsional oscillation, provided that a poloidal magnetic field
of kilogauss strength exists in the radiative interior.
Title: The internal rotation and magnetism of the Sun from its
oscillations.
Authors: Dziembowski, W. A.; Goode, Philip R.
Bibcode: 1991sia..book..501D
Altcode:
The study of solar oscillations has revealed knowledge of the internal
rotation of the Sun and something of its internal magnetic field. The
authors present the formalism needed to determine the internal rotation
from oscillation data. Equations are developed that describe centrifugal
distortion and results are given. They sketch the formalism required
to treat poloidal and toroidal magnetic fields inside the Sun. Results
are presented for a toroidal field concentrated near the base of the
convection zone and for assumed relic poloidal and toroidal fields in
the deep interior.
Title: Solar model from helioseismology and the neutrino flux problem
Authors: Dziembowski, W. A.; Pamyatnykh, A. A.; Sienkiewicz, R.
Bibcode: 1990MNRAS.244..542D
Altcode:
The pressure and density in the sun's interior is determined using
observed frequencies of the solar p-modes for spherical harmonic degrees
less than 100. A nonasymptotic method, enabling accurate probing
of the energy-producing core, was developed. It is found that most
of the differences between standard solar models and the model from
helioseismology can be explained if the opacity in the sun's interior
is higher than assumed. There is an anomaly in the innermost part of
the interior which may be interpreted as evidence that the sun has a
small convective core. A search for the minimum value of the neutrino
flux is conducted, taking the pressure and density distributions from
helioseismology and considering a class of plausible profiles for
the hydrogen content. It is found that this minimum value exceeds
10 SNU. The results show that the solar p-mode data exacerbate the
neutrino flux problem, and suggest that the solution lies in the domain
of particle physics.
Title: Toward Seismology of δ Scuti Stars
Authors: Dziembowski, W.
Bibcode: 1990LNP...367..359D
Altcode: 1990LNP...367..357D; 1990psss.conf..359D
Many among Scuti variables were shown to be multiperiodic and all
of them probably are. Being in various stages of early evolutionary
phase these stars seem excellent candidates for seismic testing the
basic assumptions and physics of the stellar evolution theory. What
hinders any major progress in the field is an unresolved problem of
connecting sparse spectra of the observed oscillation frequencies
to dense spectra calculated for the models. Determination of the
rotation rate in the interior is probably the most important goal of
asteroseismology. In the case of Scuti stars there is a complication
following from the high density of the spectrum that the standard
formula for the rotational splitting may not be valid. To understand
mode selection mechanism we have to go beyond the linear adiabatic
theory of stellar oscillations. There are still uncertainties in
determining mode stability, but the real difficulty lies in prediction
which of many unstable models may reach detectable amplitudes. Trapping
in the envelope is a possible mechanism of mode selection, but it is not
clear yet whether it finds support in observational data. Observational
information on spherical harmonics associated with the observed
periodicities is crucial for mode identification. This was obtained
with use of both photometric and spectroscopic data for number of Scuti
stars. There are, however, uncertainties which must be clarified with
new data and improved methods of their analysis. Efforts in obtaining
periodograms for unevolved objects having simple theoretical frequency
spectra are encouraged.
Title: On themechanism of mode selection in delta Scuti stars.
Authors: Dziembowski, W.; Krolikowska, M.
Bibcode: 1990AcA....40...19D
Altcode:
All well studied variables of the Delta Sct type are evolved
objects whose theoretical frequency spectra for nonradial modes
are very dense. The phenomenon of trapping in the acoustic cavity
is investigated as a possible mode selection mechanism. It is shown
that, in the relevant frequency range, a partial trapping occurs for
the spherical harmonic degree l = 1 but it does not for l = 2 and
3. Consequences of this finding for the astroseismology are discussed.
Title: Has the sun's internal rotation changed through this activity
cycle ?
Authors: Goode, P. R.; Dziembowski, W. A.; Rhodes, E. J., Jr.;
Korzennik, S.
Bibcode: 1990LNP...367..349G
Altcode: 1990psss.conf..349G
The internal rotation of the Sun is determined from each of the
six available sets of solar oscillation splitting data. These data
span this activity cycle and best sample the region near the base of
the convection zone. Going inwards through the convection zone into
the outer radiative interior, the robust results are a decrease in
the rotation rate in the equatorial plane and a trend away from the
surface-like differential rotation toward solid body rotation. In
the equatorial plane of the radiative interior, the rotation rate
seems to systematically increase through the solar cycle. If true,
this suggests that the interior has a role in the activity cycle.
Title: Erratum: "On the mechanism of mode selection in δ Scuti stars"
[Acta Astron., Vol. 40, No. 1/2, p. 19 - 26 (1990)].
Authors: Dziembowski, W.; Królikowska, M.
Bibcode: 1990AcA....40..348D
Altcode:
No abstract at ADS
Title: The Toroidal Magnetic Field inside the Sun
Authors: Dziembowski, W. A.; Goode, Philip R.
Bibcode: 1989ApJ...347..540D
Altcode:
The seismology is developed which is needed to determine the internal
toroidal magnetic field of the sun from its oscillations. Applying
this seismology to the oscillation data of Libbrecht (1989). Evidence
was found for an axisymmetric quadrupole toroidal field of 2 + or -
1 MG centered near the base of the convection zone. This field has the
symmetry and location expected for the field fed by the solar dynamo
at its seat.
Title: Inferring the Sun's Internal Angular Velocity from Observed
p-Mode Frequency Splittings
Authors: Brown, Timothy M.; Christensen-Dalsgaard, Jorgen; Dziembowski,
Wojciech A.; Goode, Philip; Gough, Douglas O.; Morrow, Cherilynn A.
Bibcode: 1989ApJ...343..526B
Altcode:
The sun's internal solar velocity Omega is studied as a function of
latitude and radius using the solar oscillation data of Brown and
Morrow (1987). An attempt is made to separate robust inferences about
the sun from artifacts of the analysis. It is found that a latitudinal
variation of Omega similar to that observed at the solar surface exists
throughout the sun's convection zone and that the variation of Omega
with latitude persists to some extent even beneath the convection zone.
Title: The Radial Gradient in the Sun's Rotation
Authors: Dziembowski, W. A.; Goode, Philip R.; Libbrecht, K. G.
Bibcode: 1989ApJ...337L..53D
Altcode:
The solar oscillation data of Libbrecht (1989) are inverted, and it
is found that there is a sharp radial gradient in the sun's rotation
at the base of the convection zone. The existence of a sharp radial
gradient there may be used to suggest that it is the site of the dynamo
which drives the sunspot cycle.
Title: Using oscillation data to probe the internal rotation and
magnetism of the Sun.
Authors: Dziembowski, Wojciech
Bibcode: 1988ESASP.286..259D
Altcode: 1988ssls.rept..259D
Integral equations relating the rotation rate and the toroidal field
intensity to the parameters describing fine structure in the frequency
spectra are discussed. Solutions of the inverse problem for the rotation
are presented. Features which may be relevant to the magnetic activity
were found in the radial dependence of the rotation rate. Some evidence
exists for changes with the solar cycle. There is no clear signature
of direct effect of magnetism in the observed fine structure.
Title: How comparison between observed and calculated p-mode
eigenfrequencies can give information on the internal structure of
the sun
Authors: Dziembowski, W. A.; Paterno, L.; Ventura, R.
Bibcode: 1988A&A...200..213D
Altcode:
The frequencies of some 400 identified solar p-modes of degree 0 ≤
l ≤ 14 have been compared with the corresponding eigenfrequencies
deduced from two standard solar models in order to investigate on the
frequency difference δν between observation and theory as a function
of frequency ν and degree l. The behaviour of δν(ν, l) permits
to have information on the correctness of the internal structure of
the Sun. The results of this preliminary analysis show that errors in
the computed solar structure mainly arise from the treatment of the
surface layers, though small errors in the innermost layers cannot be
ruled out. A subsequent comparison between the eigenfrequencies of
the two models clarifies the method of this analysis and points out
that few per cent changes in sound speed in the deep interior affect
the eigenfrequencies by few μHz.
Title: The Magnetic Field Inside the Sun
Authors: Dziembowski, W.; Goode, P. R.
Bibcode: 1988IAUS..123..171D
Altcode:
Duvall, Harvey and Pomerantz (1986) reported the existence of a
"structural asymmetry" inside the Sun. The authors show here that
this asymmetry is not a consequence of the Sun's rotation. They
attribute the asymmetry, rather, to a toroidal magnetic field inside
the Sun. Consistency requires a field of about one megagauss located in
the lower part of the convection zone. Accounting for such a field would
wreak havoc on our understanding of the solar dynamo and convection.
Title: Asteroseismology - Results and Prospects
Authors: Dappen, W.; Dziembowski, W. A.; Sienkiewicz, R.
Bibcode: 1988IAUS..123..233D
Altcode:
The main effects of stellar mass and evolution on oscillation
frequencies are discussed with the help of simplified wave-propagation
diagrams. Frequency separations resulting from asymptotic
expressions are compared with the corresponding results from numerical
computations. The seismological issues of solar-like stars and Ap stars
are discussed in some detail, and a progress report on the equation of
state for stellar interiors is given. The review ends with a summary
of properties and important physical problems for selected classes of
variable stars.
Title: Nonlinear Effects in Low Amplitude Variables
Authors: Dziembowski, W.
Bibcode: 1988msp..conf..127D
Altcode:
No abstract at ADS
Title: Nonlinear mode coupling in oscillating stars. III. Amplitude
limitingeffect of the rotation in the Delta Scuti stars.
Authors: Dziembowski, W.; Krolikowska, M.; Kosovichev, A.
Bibcode: 1988AcA....38...61D
Altcode:
The authors derive the amplitude equations describing the three-mode
coupling in the presence of rotation. The formalism is applied to
calculate the amplitudes of the low order acoustic modes at the onset
of the parametric instability to the growth of gravity modes. Numerical
calculations made for a ZAMS star model show that significant lowering
of these amplitudes occurs for Vrot > 20 km/s. This
may explain observed absence of high amplitude pulsators in rapidly
rotating stars.
Title: Inertial Modes Trapped in the Solar Convective Envelope
Authors: Dziembowski, W.; Kosovichev, A.; Kozlowski, M.
Bibcode: 1988IAUS..123..117D
Altcode:
No abstract at ADS
Title: Solar rotation.
Authors: Dziembowski, W.
Bibcode: 1988PoAst..36....3D
Altcode:
Sunspot observations made by Johannes Hevelius in 1642 - 1644 are
the first ones providing significant information about the solar
differential rotation. In modern astronomy the determination of the
rotation rate is done in a routine way by measuring positions of various
structures on the solar surface as well as by studying the Doppler
shifts of spectral lines. In recent years a progress in helioseismology
enabled determination of the rotation rate in the layers inaccessible
for direct observations. There are still uncertainties concerning,
especially, the temporal variations of the rotation rate and its
behaviour in the radiative interior. We are far from understanding the
observations. Theoretical works have not yet resulted in a satisfactory
model for the angular momentum transport in the convective zone.
Title: In memory J. S. Stodółkiewicz.
Authors: Dziembowski, W.
Bibcode: 1988PoAst..36..147D
Altcode:
No abstract at ADS
Title: Limiting Amplitude Effect of the Parametric Resonance in
Rotating Main Sequence Stars
Authors: Królikowska, M.; Dziembowski, W.
Bibcode: 1988msp..conf..141K
Altcode:
No abstract at ADS
Title: Low frequency oscillations in slowly rotating stars. I. General
properties.
Authors: Dziembowski, W.; Kosovichev, A.
Bibcode: 1987AcA....37..313D
Altcode:
The case of general nonuniform rotation is considered. Some of the
results, however, are applicable only to spherical rotation. Partial
differential equations for adiabatic oscillations are reduced to a
system of ordinary equations by means of a truncated spherical-harmonic
expansion. Asymptotic solutions are obtained and used in the discussion
of mode properties and classification. These solutions are also
employed in the numerical method of a quantitative treatment of the
problem. It is pointed out that approximations used in previous studies
of quasi-toroidal modes are not generally valid in whole stellar models,
and this refers also to the case of the uniform rotation.
Title: Low Frequency Oscillations in Slowly Rotating Stars - Part
Two - Inertial Modes in the Solar Convective Envelope
Authors: Dziembowski, W.; Kosovichev, A.; Kozlowski, M.
Bibcode: 1987AcA....37..331D
Altcode:
Due to their efficient trapping in the outer layers the inertial
oscillations may reach observable amplitudes even if they have energies
as low as those of the individual "five minute" modes. The authors
present results of eigenfrequencies and eigenvectors calculations that
should facilitate spectroscopic search for such oscillations.
Title: Low Frequency Oscillations in Slowly Rotating Stars - Part
Three - Kelvin-Helmholtz Instability
Authors: Dziembowski, W.; Kosovichev, A.
Bibcode: 1987AcA....37..341D
Altcode:
Adiabatic instabilities of stellar rotation to nonaxisymmetric
perturbations are investigated. Exact instability criteria are obtained
assuming various forms of the longitudinal differential rotation. It
is shown that the forms determined for the sun imply an instability,
but limitation of the formalism when applied to the convective zone
are pointed out.
Title: Asteroseismology for certain Ap stars.
Authors: Dziembowski, W.; Goode, Philip R.
Bibcode: 1986ASIC..169..441D
Altcode: 1986ssds.proc..441D
The discovery by Kurtz of phase coherent, rapid oscillations in certain
Ap stars holds great promise for the new field of asteroseismology. The
authors discuss a generalized oblique pulsator model for these stars
which allows to compare the effects of rotation and magnetism. A central
role for mode coupling and amplitude limitation in the observed period
doubling is suggested.
Title: Is the existence of a strong magnetic field in the Sun's
core plausible?
Authors: Dziembowski, W. A.; Paterno, L.; Ventura, R.
Bibcode: 1986ASIC..169..271D
Altcode: 1986ssds.proc..271D
The authors estimate the energies of the identified solar gravity
modes from the observed radial velocity amplitudes, and deduce that
the energies of some modes are of the same order of the total energy
of the convective motions in the solar envelope. This fact rules
out the possibility of direct or indirect excitation of these modes
by convection. It is suggested that the most plausible excitation
mechanism relies upon a magnetic torque caused by the presence of a
global magnetic field of the order of a megagauss in the Sun's core.
Title: Internal rotation of the Sun as a consequence of instabilities
in theradiative interior.
Authors: Dziembowski, W. A.; Paterno, L.; Ventura, R.
Bibcode: 1986ASIC..169..257D
Altcode: 1986ssds.proc..257D
The authors consider the effect of the molecular weight gradient in the
Sun's radiative interior on the Goldreich-Schubert-Fricke instability,
and deduce the marginal gradient of angular velocity which can be
tolerated. The same gradient is also calculated for the axisymmetric
baroclinic diffusive instability which has the lowest threshold of
the hydrodynamical instabilities. The curves obtained for the angular
velocity contradict those inferred from the observed splittings of
p-mode oscillations. The authors suggest that angular momentum can
be transferred from interior owing to some instability caused by the
presence of a global magnetic field in the Sun's core.
Title: Excitation of solar oscillation gravity modes by magnetic
torque
Authors: Dziembowski, W. A.; Paterno, L.; Ventura, R.
Bibcode: 1985A&A...151...47D
Altcode:
Observed radial velocity amplitudes are used to determine the energies
of the identified solar gravity modes. The energies of some modes are
of the same order of magnitude of the total energy of the convective
motions in the solar envelope. Therefore the authors argue that this
fact rules out the possibility of direct or indirect excitation of
these modes of convection. After having reviewed some possibilities
for a driving mechanism acting in the solar core, they suggest that the
most plausible driving mechanism is the effect of the magnetic torque
caused by the presence of a global magnetic field and mild turbulence
in the core.
Title: Frequency splitting in AP stars
Authors: Dziembowski, W.; Goode, P. R.
Bibcode: 1985ApJ...296L..27D
Altcode:
The oblique pulsator model by Kurtz was generalized to account for the
observed properties of rapidly oscillating Ap stars. In this model,
which includes advection and an oblique magnetic field, the modes are
represented, in general, by a superposition of all spherical harmonics
having the appropriate degree, l. It is predicted that an observer
will report a mode splitting into (2l + 1)-frequency components
with the spacing equal to the rotation frequency of the star. The
relative amplitudes at these frequencies are predicted following from
a postulated selective excitation mechanism due to the field, and the
diagnostic potential of the data on these Ap stars is discussed.
Title: Nonlinear mode coupling in oscillating stars. II - Limiting
amplitude effect of the parametric resonance in main sequence stars.
Authors: Dziembowski, W.; Krolikowska, M.
Bibcode: 1985AcA....35....5D
Altcode:
The authors study effects of the parametric resonance on the development
of acoustic mode instability in a model of ZAMS δ Scuti star. There
is a large number of gravity-mode pairs that may be excited already
at very low amplitudes of the acoustic mode in a consequence of the
parametric instability. A probability distribution of the minimum
amplitude at the onset of this instability (critical amplitude)
is determined for all acoustic modes with l ≤ 2 that are linearly
driven due to κ-mechanism. For none of these modes the mean critical
amplitude exceeds 0.02 mag. Equilibrium states involving three coupled
modes with constant amplitudes are studied.
Title: Fine structure of solar acoustic oscillations due to rotation.
Authors: Goode, P. R.; Dziembowski, W.
Bibcode: 1984sses.nasa..351G
Altcode: 1984sss..conf..351G
The nature of the fine structure of high order, low degree five minute
period solar oscillations following from various postulated forms of
spherical rotation is predicted. The first and second order effects
of rotation are included.
Title: Internal rotation of the Sun
Authors: Duvall, T. L., Jr.; Dziembowski, W. A.; Goode, P. R.; Gough,
D. O.; Harvey, J. W.; Leibacher, J. W.
Bibcode: 1984Natur.310...22D
Altcode:
The frequency difference between prograde and retrograde sectoral
solar oscillations is analysed to determine the rotation rate of
the solar interior, assuming no latitudinal dependence. Much of the
solar interior rotates slightly less rapidly than the surface, while
the innermost part apparently rotates more rapidly. The resulting
solar gravitational quadrupole moment is J2 = (1.7+/-0.4)
× 10-7 and provides a negligible contribution to current
planetary tests of Einstein's theory of general relativity.
Title: Simple asymptotic estimates of the fine structure in the
spectrum of solar oscillations due to rotation and magnetism
Authors: Dziembowski, W.; Goode, P. R.
Bibcode: 1984MmSAI..55..185D
Altcode:
Simple asymptotic formulae are presented and used to predict the effect
of rotation and magnetism on the fine structure in the spectrum of
solar oscillations. The authors compare the linear effect of rotation
on the fine structure to the quadratic effect of rotation. The
asymptotic formulae for the magnetically induced fine structure are
used to schematically determine the requisite field size to cause an
observable change in the structure and the nature of the change.
Title: On the role of resonances in double-mode pulsation
Authors: Dziembowski, W.; Kovacs, G.
Bibcode: 1984MNRAS.206..497D
Altcode:
Attention is given to the simultaneous effects of resonant coupling
and nonlinear saturation of the linear driving mechanism on the finite
amplitude solution of the multimodal pulsation problem and on its
stability. The calculations of both effects are in the lowest order of
approximation in terms of amplitudes. The 2:1 resonance between one
of the two linearly unstable modes and a higher frequency mode gives
rise to double-mode (fundamental and first overtone) pulsation. It is
noted that within a certain range of parameters, such as the frequency
mismatch, the linear growth, and damping rates, it is the only stable
solution to the problem. It is also shown that three-mode resonance
promotes a single-mode rather than a double-mode pulsation. Under
certain conditions, the nonresonant interaction occurring through the
collective saturation of the driving rates may lead to multimodal
variability. It is contended here, however, that this condition is
unlikely to be met in Cepheids and RR Lyrae stars.
Title: Introductory report : Vibrational Instability of Stars with
Rotation and/or Magnetic Field
Authors: Dziembowski, W.
Bibcode: 1984LIACo..25..346D
Altcode: 1984tpss.conf..346D; 1984trss.conf..346D
No abstract at ADS
Title: Theory and application of stellar seismology
Authors: Dziembowski, W.
Bibcode: 1984AdSpR...4h.143D
Altcode: 1984AdSpR...4..143D
Observation of multiperiodic variability in stars offers a powerful
tool to probe their internal structure, rotation and magnetism. This
follows from the possibility of identification of observed frequencies
with those calculated for realistic stellar models. In spherical
stars oscillations are described in terms of two-dimensional discrete
sets of acoustic- and gravity-modes. In realistic cases there is
no universal relation among the frequencies. Thus, each frequency
observed is an independent observable. In recent years comparisons
between observed and theoretical frequencies were made for the sun
and various variable stars. The most dramatic discrepancy occurs for
double-mode Cepheids. Magnetism and/or rotation induce fine structure in
the frequency spectrum. The width and structure of multiplets depend
on the size of these effects weighted in a specific way for each
mode. Recently, oscillation data were used to determine the behavior
of the angular velocity inside the sun. For stars only some average
rotation rates have been determined in this way.
Title: Limits on the Sun's core magnetism from solar oscillations
Authors: Dziembowski, W.; Goode, P. R.
Bibcode: 1983Natur.305...39D
Altcode:
Many years ago Cowling1 discussed the possibility that
the Sun has a significant relic field. This field would have poloidal
and toroidal components, with the toroidal component being driven by
dynamo action on the poloidal component. The toroidal field would be
quadrupole in nature having opposite senses in the upper and lower
hemispheres. Subsequently, Dicke2 proposed that the solar
quadrupole moment is caused by a strong, inclined toroidal field with
a magnitude of ~6×107 G. Ulrich and Rhodes3
suggested that a poloidal field with a magnitude of 3 × 108
G was required to account for some of the properties of the 5-min period
oscillation. Whereas Mestel and Moss4 claimed that such
fields may not be sufficiently stable to endure. Hill et al.5
argued that solar oscillation data imply that a simple poloidal field
is much weaker than 3 × 108 G and Gough6 has
suggested that the toroidal field may be much weaker than the 6 ×
107 G postulated by Dicke2. Magnetic fields,
like rotation, produce a fine structure in solar oscillations. Their
effects should be detectable provided the fields are sufficiently
intense. Here we perform an analysis of oscillation data due to Hill et
al.5 to show that limits of a few megagauss can be placed
on poloidal and toroidal magnetic fields inside the Sun. A limit can
thereby also be placed on the part of the quadrupole moment of the Sun
due to magnetism. These fields are too weak to induce a quadrupole
moment much larger than that which would result if the Sun rotated
rigidly at the observed surface equatorial rate.
Title: Resonant Coupling Between Solar Gravity Modes
Authors: Dziembowski, W.
Bibcode: 1983SoPh...82..259D
Altcode: 1983IAUCo..66..259D
It is shown that in consequence of the parametric resonance, g modes
of low spherical harmonic degree l are strongly coupled to the modes
of high degree. The coupling limits the growth of low l modes to very
small amplitudes. For g1, l = 1 mode, the final amplitude of
the radial velocity is of the order of 10 cm s-1. A mixing
of solar core as a result of a finite-amplitude development of linear
instability of this mode is thus highly unlikely.
Title: On the origin of the acoustic flux in Sirius B.
Authors: Dziembowski, W.; Gesicki, K.
Bibcode: 1983AcA....33..183D
Altcode:
An alternative hypothesis is presented to explain the soft X-ray flux
observed in Sirius B. The most popular explanation postulates the
existence of an acoustic flux of the order of 10 to the 29th ergs per
second generated in the subphotospheric convective zone. The present
alternative hypothesis of acoustic flux generation is based on the
authors' finding that the layered envelope models for this star are
unstable to short-period oscillations between 0.06 and 0.45 if the
hydrogen-rich layer is sufficiently shallow (mass less than about 5 x
10 to the -17th solar mass). Unstable p-modes convert 10 to the 29th
ergs/s of the radiative flux before becoming strongly nonlinear. It
is suggested that the acoustic flux is generated by the random-phase
resonant interaction between these modes and the traveling waves. Such
an interaction is expected to be the dominant nonlinear effect leading
to statistical equilibrium.
Title: Book-Review - Nonradial and Nonlinear Stellar Pulsation
Authors: Hill, H. A.; Dziembowski, W. A.; Deupree, R. G.
Bibcode: 1982ApL....22...79H
Altcode:
No abstract at ADS
Title: Nonlinear mode coupling in oscillating stars. I - Second
order theory of the coherent mode coupling.
Authors: Dziembowski, W.
Bibcode: 1982AcA....32..147D
Altcode:
The importance of mode coupling in the theory of stellar pulsation
has been recognized by several researchers. Dziembowski (1980) has
argued that differences concerning the behavior of dwarf and giant-type
pulsators are related to differences in amplitude limiting effects,
which in the case of dwarf pulsators is mode coupling. The present
investigation has the objective to provide a theoretical background for
the studies of mode interaction in various stars, taking into account
the sun and other main sequence variables, multimodal Cepheids and RR
Lyrae, and ZZ Ceti stars. It is assumed that only two or three modes
take part in the interaction. Attention is given to equations for
amplitude changes of the interacting modes, a suitable formula for the
coupling coefficient, the parametric resonance instability, the resonant
generation of a higher-frequency mode, and an equilibrium solution.
Title: Book-Review - Nonradial and Nonlinear Stellar Pulsation
Authors: Dziembowski, W. A.; Hill, H. A.; Fadeev, Y. A.
Bibcode: 1981SvA....25..751D
Altcode:
No abstract at ADS
Title: Excitation of gravity modes in white dwarfs with chemically
stratified envelopes
Authors: Dziembowski, W.; Koester, D.
Bibcode: 1981A&A....97...16D
Altcode:
The vibrational instability of white dwarf models is investigated,
taking into account effects of gravitational separation of elements. It
is pointed out that the models consist of a carbon core, helium zone,
and hydrogen outer layer separated by transition regions, the abundance
profile being determined by the diffusion equilibrium. It is found that
in the range of effective temperatures corresponding to the observed
range for ZZ Ceti stars, the models are unstable against gravity modes
when the total mass of hydrogen is less than approximately 1.2 x 10
to the -13 solar mass. The unstable modes are found to correspond to
low order spherical harmonics in the period range 100-1200 sec.
Title: Book-Review - Nonradial and Nonlinear Stellar Pulsations -
Tucson - 1979MAR
Authors: Hill, H. A.; Dziembowski, W. A.; Wilcox, J. M.
Bibcode: 1981Sci...211.1339H
Altcode:
No abstract at ADS
Title: Book Review - Nonradial and Nonlinear Stellar Pulsations
Authors: Hill, H. A.; Dziembowski, W. A.; de Jager, C.
Bibcode: 1981SSRv...28Q.112H
Altcode:
No abstract at ADS
Title: On the Excitation Mechanism in Beta-Cephei Variables
Authors: Dziembowski, W.; Kubiak, M.
Bibcode: 1981AcA....31..153D
Altcode:
The driving effect of He^+ ionization edge suggested by Stellingwerf is
investigated for both radial and nonradial modes. For the nonradial
modes, corresponding to spherical harmonics of low l-values, the
driving effect is almost identical as for radial modes of the
same frequency. The effect as calculated on the basis from the
present opacity data is insufficient to cause instability, but the
assumption that it is actually responsible for excitation offers a
natural explanation of all major properties of beta Cephei stars. The
absence of observational data on variability among initial main-sequence
objects may be a consequence of high values of l of the excited modes.
Title: Book-Review - Nonradial and Nonlinear Stellar Pulsation -
Workshop - Tucson - 1979MAR12-16
Authors: Hill, H. A.; Dziembowski, W. A.; Heinzel, P.
Bibcode: 1981BAICz..32..254H
Altcode:
No abstract at ADS
Title: Book-Review - Nonradial and Nonlinear Stellar Pulsation
Authors: Hill, H. A.; Dziembowski, W. A.; Rudiger, G.
Bibcode: 1981AN....302..323H
Altcode:
No abstract at ADS
Title: On the Excitation Mechanism in Beta-Cephei Variables
Authors: Dziembowski, W.; Kubiak, M.; Groupe Etoiles Variables de
L'Observatoire de Nice
Bibcode: 1981pbs..work..357D
Altcode:
No abstract at ADS
Title: Book-Review - Nonradial and Nonlinear Stellar Pulsation
Authors: Hill, H. A.; Dziembowski, W. A.
Bibcode: 1981S&T....61R.342H
Altcode:
No abstract at ADS
Title: Delta Scuti variables - The link between giant- and dwarf-type
pulsators
Authors: Dziembowski, W.
Bibcode: 1980LNP...125...22D
Altcode: 1980nnsp.work...22D
An attempt is made to interpret the differences in pulsational behavior
between horizontal branch and main sequence (or early post-main
sequence) variables. Results of linear stability studies for delta
Scuti stars are reviewed and supplemented with new calculations. The
function of the opacity mechanism in generating instability to a
variety of radial and nonradial modes is discussed. The highest
excitation rates for these variables, though still much lower than
in the case of Cepheids, are found in fundamental modes with higher
spherical harmonic orders, l, and among higher overtones with low
values of l. It is argued that amplitudes in delta Scuti stars are
limited by nonlinear mode coupling, resulting in lower amplitudes and
multiperiodicity, rather than by saturation of the opacity mechanism
as is the case with horizontal branch variables.
Title: White Dwarf Pulsations
Authors: Dziembowski, W. A.
Bibcode: 1980HiA.....5..469D
Altcode:
No abstract at ADS
Title: Comments on Gravity Mode Excitation Mechanisms
Authors: Dziembowski, W.
Bibcode: 1980LNP...125..272D
Altcode: 1980nnsp.work..272D
No abstract at ADS
Title: How deep is the solar convection zone
Authors: Christensen-Dalsgaard, J.; Dziembowski, W.; Gough, D.
Bibcode: 1980LNP...125..313C
Altcode: 1980nnsp.work..313C
The interpretation by Hill and Caudell (1979) of some of their solar
oscillation data as being due to g modes of degree greater than 20 seems
to imply that the solar convection zone is much shallower than that in
standard solar models, probably representing only a few percent of the
radius. It is here attempted to match the observed periods in models
of this nature; the rather complicated spectrum of oscillations in
such models can be understood in terms of the asymptotic behavior of
modes of large degree. Possible excitation mechanisms for the modes
are briefly discussed.
Title: Nonradial and nonlinear stellar pulsation : proceedings
of a workshop held at the University of Arizona in Tucson, March
12-16, 1979
Authors: Hill, H. A.; Dziembowski, W. A.
Bibcode: 1980LNP...125.....H
Altcode: 1980nnsp.work.....H
These contributed and invited papers span theoretical problems and
provide new observational material on nonradial and nonlinear stellar
pulsations. Main groupings of papers deal with delta Scuti stars,
beta Cephei and other early-type variables, degenerate stars, and
the sun itself. The solar oscillations are covered by papers with
observational evidence, excitation mechanisms, and boundary conditions
used in pulsation theory.
Title: Vibrational Instabilities and Pulsational Properties of Cool
White Dwarfs
Authors: Dziembowski, W.
Bibcode: 1979wdvd.coll..359D
Altcode: 1979IAUCo..53..359D
Attention is focused on those aspects of the theory that may be
relevant in understanding the nature of ZZ Ceti-type variable white
dwarfs. Recent calculations show that the opacity mechanism can drive
a large variety of oscillation modes, including the ones that fit
observed periods. An estimate of nonlinear effects shows that resonant
mode coupling plays a dominant role in determining the finite amplitude
behavior of oscillations and is also probably responsible for rapid
amplitude changes observed in these variables.
Title: The Stability of Accreting White Dwarfs in Close Binary Systems
(Abstract)
Authors: Sienkiewicz, R.; Dziembowski, W.
Bibcode: 1978necb.conf..169S
Altcode:
No abstract at ADS
Title: On Solar Gravity Modes of High Order Spherical Harmonics
Authors: Dziembowski, W.; Pamjatnykh, A. A.
Bibcode: 1978pfsl.conf..135D
Altcode: 1978ESPM....2..135D
No abstract at ADS
Title: Nuclei of Planetary Nebulae in the Pulsation Instability Strip
Authors: Dziembowski, W.
Bibcode: 1977ivsw.conf..342D
Altcode: 1977IAUCo..42..342D
No abstract at ADS
Title: Light and radial velocity variations in a nonradially
oscillating star.
Authors: Dziembowski, W.
Bibcode: 1977AcA....27..203D
Altcode:
Formulae for light and radial velocity variations are derived in terms
of solution of the linear oscillations equations. A possibility of
observational identification of the oscillation mode is discussed. It
is shown that nonradial oscillations cannot be distinguished from the
radial ones on the basis of the amplitude ratios of radial velocity
and light variations. Instead, the use of the Baade-Weselink method
is recommended. For stars with known radii, this method permits to
determine the spherical harmonic order of the oscillation.
Title: Stability of Accreting White Dwarfs in Close Binary Systems
Authors: Sienkiewicz, R.; Dziembowski, W.
Bibcode: 1977ivsw.conf..327S
Altcode: 1977IAUCo..42..327S
No abstract at ADS
Title: Oscillating white dwarfs.
Authors: Dziembowski, W.
Bibcode: 1977AcA....27....1D
Altcode:
Vibrational instability toward linear nonradial nonadiabatic
oscillations is investigated for two 0.6-solar-mass white dwarf models
with effective temperatures around 10,000 K and containing hydrogen-
and helium-rich envelopes. Convective flux is ignored, but radiative
and molecular heat diffusion are taken into account. The atmospheric
velocity field is determined for each model along with variations in
emergent flux, luminosity, and radial velocity. The results show that
the basic vibrational-instability mechanism for variable white dwarfs
is the same as that for classical pulsating variables and that the
dwarfs may be considered as an extension of the Cepheid instability
strip to the highest effective temperatures and surface gravities. Some
differences in the oscillation properties of white dwarfs and giants
are noted which suggest that nonlinear coupling between self-excited
modes corresponding to high-order spherical harmonics is responsible
for the excitation of g-modes associated with low-order harmonics.
Title: Oscillations of giants and supergiants.
Authors: Dziembowski, W.
Bibcode: 1977AcA....27...95D
Altcode:
It is shown that nonradial oscillations of giant and supergiant stars
may be rapidly excited by a mechanism operating in outer layers. The
excited modes have a negligible amplitude in the deep interior and
with the use of proper boundary conditions they may be studied for
unfitted envelope models. It is further shown that the vibrational
instability due to radiative transfer in semiconvective zones located
in deep interior results in local progressive wave excitation. The
latter effect has no direct relevance to oscillations in outer layers
Title: The Mass and Evolutionary Status of AI Vel-Type Variables
Authors: Dziembowski, W.
Bibcode: 1975IAUS...67..251D
Altcode:
Proposed evolutionary models of AI Velorum variables are reviewed,
and further arguments are offered that these variables are about 0.2
solar mass stars evolving with a roughly constant luminosity toward
the white dwarf stage after having lost most of their mass during the
red giant phase. Such pre-white-dwarf configurations contain most of
their mass in a degenerate helium core, but their luminosity is due
almost exclusively to shell hydrogen burning.
Title: Excitation of Nonradial Oscillations by Partial Ionization
Zone Mechanism
Authors: Dziembowski, W. A.
Bibcode: 1975MSRSL...8..287D
Altcode: 1975phs..conf..287D
No abstract at ADS
Title: Low Mass Models for the AI Velorum Stars
Authors: Dziembowski, W.; Kozlowski, M.
Bibcode: 1974AcA....24..245D
Altcode:
Results of static model construction and linear nonadiabatic
calculations of pulsation properties for low mass (0.2-0.25 M_odot)
stars in the pulsation instability strip are presented. The models
have degenerate helium cores and represent pre-white dwarf evolution
still governed by hydrogen burning in the shell. A comparison with
observational data shows that such models are plausible for the AI
Velorum variables, though a difficulty with values of period ratios
determined for double mode pulsating objects is emphasized. Implications
for the mass problem are discussed.
Title: Vibrational Stability of 1 m_solar Star in the Phase of
Central Hydrogen Burning
Authors: Dziembowski, W. A.; Sienkiewicz, R.
Bibcode: 1973AcA....23..273D
Altcode:
Dilke and Gough (1972) have proposed a model of occasional mixing of
solar core to explain negative results of solar neutrino measurements
and the geological ice ages. In their model, mixing results from
nuclear-reaction driven vibrational instability dose not occur, as
the effect of nuclear reactions on the stability of relevant stellar
models is small.
Title: Nonradial Oscillations of Evolved Stars. I. Quasiadiabatic
Approximation
Authors: Dziembowski, W. A.
Bibcode: 1971AcA....21..289D
Altcode:
Linear oscillations of stars consisting of an extended envelope and
a highly condensed core are investigated. For such stars, even high
frequency modes become essentially internal gravity waves of short
wavelength in deep stellar interior. A suitable method of solution of
fourth order equation for adiabatic oscillations is described. Numerical
solution for l=0 (radial pulsation) and l=2 modes have been obtained
for a Cepheid model. Quasiadiabatic estimation of dissipation indicates
that the Cepheid-type stars are vibrationally stable against nonradial
perturbations.
Title: Nonradial Problems in the Theory of Stellar Structure, Part III
Authors: Dziembowski, W.
Bibcode: 1968PoAst..16...45D
Altcode:
The first chapter of the article deals with the theory of star
rotation. In particular the process of stabilization of angular
velocity distribution inside the star, as well as cases of stationary
distributions are discussed. The second chapter treats the flow in
binary systems. The non-adiabatic flows in quasi-static approximation
are taken into consideration.
Title: Nonradial Problems in the Theory of Stellar Structure, Part II
Authors: Dziembowski, W.
Bibcode: 1967PoAst..15..175D
Altcode:
This article concerns the meridional circulation in rigidly rotating
stars. The angular velocity changes caused by this circulation are
discussed.
Title: Non-radial Problems of the Theory of the Stellar
Structure. Part I
Authors: Dziembowski, W.
Bibcode: 1967PoAst..15...53D
Altcode:
This article opens a series of articles devoted to the problems
of the internal structure of stars for which the assumption of the
spherical symmetry has to be abandoned. The article gives a discussion
of the influence of rotation on the structure of the star. Particular
attention is given on the nonexistence of the thermal equilibrium when
an arbitrarily assumed distribution of the angular velocity inside
the star is considered; in particular the case of a rigid rotation
is considered.
Title: Internal structure and evolutionary changes in binaries:
Tidal friction in close binary systems
Authors: Dziembowski, W.
Bibcode: 1967oeds.conf..105D
Altcode:
No abstract at ADS
Title: Pulsation of Stars
Authors: Dziembowski, W.
Bibcode: 1966PoAst..14..149D
Altcode:
The article contains a review of new progress in the theory of star
pulsation. The main part of the paper deals with the problem of the
origin of pulsation. This is investigated by small perturbation method
of star models. The importance of a careful treatment of entropy
changes is emphasized.
Title: Application of the Tensor Virial Equation in Theory of
Rotating Masses
Authors: Dziembowski, W.
Bibcode: 1964PoAst..12..235D
Altcode:
An outline is given of the works of S. Chandrasekhar and
N.R. Lebovitz concerning stability and oscillations of the rotating
masses. Application of the tensor virial equation to the stability
problem of Maclaurin's spheroid and Jacobi's ellipsoid allows a
more general approach and results are obtained faster that with the
methods used previously. For the compressible masses this enables us to
examine approximately the influance of the rotation on the oscillation
frequencies, and in particular the influence of the rotation on star
pulsations in the politropic approximation.
Title: On the Equations of Internal Constitution of Components in
Close Binaries
Authors: Dziembowski, W.
Bibcode: 1963AcA....13..157D
Altcode:
In the present paper, equations of the internal constitution of stable
components in close binaries are deduced. The equation of equipotential
surface was assumed of the form r=overline r(1+sum_i Y_i), where Y_i
denotes the tesseral harmonics taken with respect to the centre of mass
of a component. Their squares and higher powers were neglected as well
as their changes within the period of revolution. The set of equations
obtained is of the same form as for single stars. The independent
variable is overline r - the mean distance of the equipotential surface
from the centre of mass of the component. Dependent variables are:
the pressure, temperature, flux of radiation and the mass of the
portion of the star limited by a level surface.