92 research outputs found
Recent Advances in Modeling Stellar Interiors
Advances in stellar interior modeling are being driven by new data from
large-scale surveys and high-precision photometric and spectroscopic
observations. Here we focus on single stars in normal evolutionary phases; we
will not discuss the many advances in modeling star formation, interacting
binaries, supernovae, or neutron stars. We review briefly: 1) updates to input
physics of stellar models; 2) progress in two and three-dimensional evolution
and hydrodynamic models; 3) insights from oscillation data used to infer
stellar interior structure and validate model predictions (asteroseismology).
We close by highlighting a few outstanding problems, e.g., the driving
mechanisms for hybrid gamma Dor/delta Sct star pulsations, the cause of giant
eruptions seen in luminous blue variables such as eta Car and P Cyg, and the
solar abundance problem.Comment: Proceedings for invited talk at conference High Energy Density
Laboratory Astrophysics 2010, Caltech, March 2010, submitted for special
issue of Astrophysics and Space Science; 7 pages; 5 figure
Toward a New Kind of Asteroseismic Grid Fitting
Recent developments in instrumentation (e.g., in particular the Kepler and
CoRoT satellites) provide a new opportunity to improve the models of stellar
pulsations. Surface layers, rotation, and magnetic fields imprint erratic
frequency shifts, trends, and other non-random behavior in the frequency
spectra. As our observational uncertainties become smaller, these are
increasingly important and difficult to deal with using standard fitting
techniques. To improve the models, new ways to compare their predictions with
observations need to be conceived. In this paper we present a completely
probabilistic (Bayesian) approach to asteroseismic model fitting. It allows for
varying degrees of prior mode identification, corrections for the discrete
nature of the grid, and most importantly implements a treatment of systematic
errors, such as the "surface effects." It removes the need to apply semi-
empirical corrections to the observations prior to fitting them to the models
and results in a consistent set of probabilities with which the model physics
can be probed and compared. As an example, we show a detailed asteroseismic
analysis of the Sun. We find a most probable solar age, including a 35 +- 5
million year pre-main sequence phase, of 4.591 billion years, and initial
element mass fractions of X_0 = 0.72, Y_0 = 0.264, Z_0 = 0.016, consistent with
recent asteroseismic and non-asteroseismic studies.Comment: 15 pages, 5 figures, accepted for publication in The Astrophysical
Journal; v2 contains minor changes made in the proofs (updated references &
corrected typos
Pulsation modes in rapidly rotating stellar models based on the Self-Consistent Field method
Context: New observational means such as the space missions CoRoT and Kepler
and ground-based networks are and will be collecting stellar pulsation data
with unprecedented accuracy. A significant fraction of the stars in which
pulsations are observed are rotating rapidly.
Aims: Our aim is to characterise pulsation modes in rapidly rotating stellar
models so as to be able to interpret asteroseismic data from such stars.
Methods: The pulsation code developed in Ligni\`eres et al. (2006) and Reese
et al. (2006) is applied to stellar models based on the self-consistent field
(SCF) method (Jackson et al. 2004, 2005, MacGregor et al. 2007).
Results: Pulsation modes in SCF models follow a similar behaviour to those in
uniformly rotating polytropic models, provided that the rotation profile is not
too differential. Pulsation modes fall into different categories, the three
main ones being island, chaotic, and whispering gallery modes, which are
rotating counterparts to modes with low, medium, and high l-|m| values,
respectively. The frequencies of the island modes follow an asymptotic pattern
quite similar to what was found for polytropic models. Extending this
asymptotic formula to higher azimuthal orders reveals more subtle behaviour as
a function of m and provides a first estimate of the average advection of
pulsation modes by rotation. Further calculations based on a variational
principle confirm this estimate and provide rotation kernels that could be used
in inversion methods. When the rotation profile becomes highly differential, it
becomes more and more difficult to find island and whispering gallery modes at
low azimuthal orders. At high azimuthal orders, whispering gallery modes, and
in some cases island modes, reappear.Comment: 16 pages, 11 figures, accepted for publication in A&
High resolution optical spectroscopy of IRAS 09425-6040 (=GLMP 260)
We present high resolution optical spectroscopic observations of IRAS
09425-6040, a peculiar, extremely red, C-rich AGB star showing prominent O-rich
dust features in its ISO infrared spectrum attributed to crystalline silicates.
Our analysis shows that IRAS 09425-6040 is indeed a C-rich star slightly
enriched in lithium (log (Li/H) + 12 ~ 0.7) with a low 12C/13C = 15+-6 ratio.
We also found some evidence that it may be enriched in s-elements. Combining
our results with other observational data taken from the literature we conclude
that the star is possibly an intermediate-mass TP-AGB star (M > 3 M_sun) close
to the end of its AGB evolution which may have only very recently experienced a
radical change in its chemistry, turning into a carbon-rich AGB star.Comment: 5 pages, 2 figures, accepted for publication in A&
Vertical abundance stratification in the blue horizontal branch star HD135485
It is commonly believed that the observed overabundances of many chemical
species relative to the expected cluster metallicity in blue horizontal branch
(BHB) stars appear as a result of atomic diffusion in the photosphere. The slow
rotation of BHB stars (with T_eff > 11,500K), typically v sin{i} < 10 km/s, is
consistent with this idea. In this work we search for observational evidence of
vertical chemical stratification in the atmosphere of HD135485. If this
evidence exists, it will demonstrate the importance of atomic diffusion
processes in the atmospheres of BHB stars. We undertake an extensive abundance
stratification analysis of the atmosphere of HD135485, based on recently
acquired high resolution and S/N CFHT ESPaDOnS spectra and a McDonald-CE
spectrum. Our numerical simulations show that nitrogen and sulfur reveal
signatures of vertical abundance stratification in the stellar atmosphere. It
appears that the abundances of these elements increase toward the upper
atmosphere. This fact cannot be explained by the influence of microturbulent
velocity, because oxygen, carbon, neon, argon, titanium and chromium do not
show similar behavior and their abundances remain constant throughout the
atmosphere. It seems that the iron abundance may increase marginally toward the
lower atmosphere. This is the first demonstration of vertical abundance
stratification of metals in a BHB star.Comment: 8 pages, 5 figures, accepted to A&
An asteroseismic study of the beta Cephei star 12 Lacertae: multisite spectroscopic observations, mode identification and seismic modelling
We present the results of a spectroscopic multisite campaign for the beta
Cephei star 12 (DD) Lacertae. Our study is based on more than thousand
high-resolution high S/N spectra gathered with 8 different telescopes in a time
span of 11 months. In addition we make use of numerous archival spectroscopic
measurements. We confirm 10 independent frequencies recently discovered from
photometry, as well as harmonics and combination frequencies. In particular,
the SPB-like g-mode with frequency 0.3428 1/d reported before is detected in
our spectroscopy. We identify the four main modes as (l1,m1) = (1, 1), (l2,m2)
= (0, 0), (l3,m3) = (1, 0) and (l4,m4) = (2, 1) for f1 = 5.178964 1/d, f2 =
5.334224 1/d, f3 = 5.066316 1/d and f4 = 5.490133 1/d, respectively. Our
seismic modelling shows that f2 is likely the radial first overtone and that
the core overshooting parameter alpha_ov is lower than 0.4 local pressure scale
heights.Comment: 16 pages, 11 figures, accepted in MNRA
Presupernova Structure of Massive Stars
Issues concerning the structure and evolution of core collapse progenitor
stars are discussed with an emphasis on interior evolution. We describe a
program designed to investigate the transport and mixing processes associated
with stellar turbulence, arguably the greatest source of uncertainty in
progenitor structure, besides mass loss, at the time of core collapse. An
effort to use precision observations of stellar parameters to constrain
theoretical modeling is also described.Comment: Proceedings for invited talk at High Energy Density Laboratory
Astrophysics conference, Caltech, March 2010. Special issue of Astrophysics
and Space Science, submitted for peer review: 7 pages, 3 figure
Interaction Between Convection and Pulsation
This article reviews our current understanding of modelling convection
dynamics in stars. Several semi-analytical time-dependent convection models
have been proposed for pulsating one-dimensional stellar structures with
different formulations for how the convective turbulent velocity field couples
with the global stellar oscillations. In this review we put emphasis on two,
widely used, time-dependent convection formulations for estimating pulsation
properties in one-dimensional stellar models. Applications to pulsating stars
are presented with results for oscillation properties, such as the effects of
convection dynamics on the oscillation frequencies, or the stability of
pulsation modes, in classical pulsators and in stars supporting solar-type
oscillations.Comment: Invited review article for Living Reviews in Solar Physics. 88 pages,
14 figure
Catalog of Galactic Beta Cephei Stars
We present an extensive and up-to-date catalog of Galactic Beta Cephei stars.
This catalog is intended to give a comprehensive overview of observational
characteristics of all known Beta Cephei stars. 93 stars could be confirmed to
be Beta Cephei stars. For some stars we re-analyzed published data or conducted
our own analyses. 61 stars were rejected from the final Beta Cephei list, and
77 stars are suspected to be Beta Cephei stars. A list of critically selected
pulsation frequencies for confirmed Beta Cephei stars is also presented. We
analyze the Beta Cephei stars as a group, such as the distributions of their
spectral types, projected rotational velocities, radial velocities, pulsation
periods, and Galactic coordinates. We confirm that the majority of these stars
are multiperiodic pulsators. We show that, besides two exceptions, the Beta
Cephei stars with high pulsation amplitudes are slow rotators. We construct a
theoretical HR diagram that suggests that almost all 93 Beta Cephei stars are
MS objects. We discuss the observational boundaries of Beta Cephei pulsation
and their physical parameters. We corroborate that the excited pulsation modes
are near to the radial fundamental mode in frequency and we show that the mass
distribution of the stars peaks at 12 solar masses. We point out that the
theoretical instability strip of the Beta Cephei stars is filled neither at the
cool nor at the hot end and attempt to explain this observation
Horizontal Branch Stars: The Interplay between Observations and Theory, and Insights into the Formation of the Galaxy
We review HB stars in a broad astrophysical context, including both variable
and non-variable stars. A reassessment of the Oosterhoff dichotomy is
presented, which provides unprecedented detail regarding its origin and
systematics. We show that the Oosterhoff dichotomy and the distribution of
globular clusters (GCs) in the HB morphology-metallicity plane both exclude,
with high statistical significance, the possibility that the Galactic halo may
have formed from the accretion of dwarf galaxies resembling present-day Milky
Way satellites such as Fornax, Sagittarius, and the LMC. A rediscussion of the
second-parameter problem is presented. A technique is proposed to estimate the
HB types of extragalactic GCs on the basis of integrated far-UV photometry. The
relationship between the absolute V magnitude of the HB at the RR Lyrae level
and metallicity, as obtained on the basis of trigonometric parallax
measurements for the star RR Lyrae, is also revisited, giving a distance
modulus to the LMC of (m-M)_0 = 18.44+/-0.11. RR Lyrae period change rates are
studied. Finally, the conductive opacities used in evolutionary calculations of
low-mass stars are investigated. [ABRIDGED]Comment: 56 pages, 22 figures. Invited review, to appear in Astrophysics and
Space Scienc
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