97 research outputs found
The Extent and Cause of the Pre-White Dwarf Instability Strip
One of the least understood aspects of white dwarf evolution is the process
by which they are formed. We are aided, however, by the fact that many H- and
He-deficient pre-white dwarfs (PWDs) are multiperiodic g-mode pulsators.
Pulsations in PWDs provide a unique opportunity to probe their interiors, which
are otherwise inaccesible to direct observation. Until now, however, the nature
of the pulsation mechanism, the precise boundaries of the instability strip,
and the mass distribution of the PWDs were complete mysteries. These problems
must be addressed before we can apply knowledge of pulsating PWDs to improve
understanding of white dwarf formation. This paper lays the groundwork for
future theoretical investigations of these stars. In recent years, Whole Earth
Telescope observations led to determination of mass and luminosity for the
majority of the (non-central star) PWD pulsators. With these observations, we
identify the common properties and trends PWDs exhibit as a class. We find that
pulsators of low mass have higher luminosity, suggesting the range of
instability is highly mass-dependent. The observed trend of decreasing periods
with decreasing luminosity matches a decrease in the maximum (standing-wave)
g-mode period across the instability strip. We show that the red edge can be
caused by the lengthening of the driving timescale beyond the maximum
sustainable period. This result is general for ionization-based driving
mechanisms, and it explains the mass-dependence of the red edge. The observed
form of the mass-dependence provides a vital starting point for future
theoretical investigations of the driving mechanism. We also show that the blue
edge probably remains undetected because of selection effects arising from
rapid evolution.Comment: 40 pages, 6 figures, accepted by ApJ Oct 27, 199
Dynamic model atmospheres of cool giants
Cool giant stars are highly dynamical objects, and complex micro-physical
processes play an important role in their extended atmospheres and winds. The
interpretation of observations, and in particular of high-resolution IR
spectra, requires realistic self-consistent model atmospheres. Current
dynamical models include rather detailed micro-physics, and the resulting
synthetic spectra compare reasonably well with observations. A transition from
qualitative to quantitative modelling is taking place at present. We give an
overview of existing dynamical model atmospheres for AGB stars, discussing
recent advances and current trends in modelling, including 3D 'star-in-a-box'
models. When comparing synthetic spectra and other observable properties
resulting from dynamical models with observations we focus on the near- and
mid-IR wavelength range.Comment: 12 pages, 3 figures, to be published in Proc. of ESO Workshop on High
Resolution Infrared Spectroscopy in Astronomy, eds. Kaeufl H.U., Siebenmorgen
R., Moorwood A., ESO Astrophysics Symposia, Springe
Local Radiative Hydrodynamic and Magnetohydrodynamic Instabilities in Optically Thick Media
We examine the local conditions for radiative damping and driving of short
wavelength, propagating hydrodynamic and magnetohydrodynamic (MHD) waves in
static, optically thick, stratified equilibria. We show that so-called strange
modes in stellar oscillation theory and magnetic photon bubbles are intimately
related and are both fundamentally driven by the background radiation flux
acting on compressible waves. We identify the necessary criteria for unstable
driving of these waves, and show that this driving can exist in both gas and
radiation pressure dominated media, as well as pure Thomson scattering media in
the MHD case. The equilibrium flux acting on opacity fluctuations can drive
both hydrodynamic acoustic waves and magnetosonic waves unstable. In addition,
magnetosonic waves can be driven unstable by a combination of the equilibrium
flux acting on density fluctuations and changes in the background radiation
pressure along fluid displacements. We briefly describe the conditions under
which these instabilities might be manifested in both main sequence stellar
envelopes and accretion disks.Comment: 55 pages, revised version accepted for publication by ApJ. New
appendix added justifying WKB analysi
New SPB stars in the field of the young open cluster NGC 2244 discovered by the MOST photometric satellite
During two weeks of nearly continuous optical photometry of the young open
cluster NGC 2244 obtained by the MOST satellite, we discovered two new SPB
stars, GSC 00154-00785 and GSC 00154-01871. We present frequency analyses of
the MOST light curves of these stars, which reveal two oscillation frequencies
(0.61 and 0.71 c/d) in GSC 00154-00785 and two (0.40 and 0.51 c/d) in GSC
00154-01871. These frequency ranges are consistent with g-modes of excited in models of main-sequence or pre-main-sequence (PMS) stars of
masses 4.5 - 5 and solar composition .
Published proper motion measurements and radial velocities are insufficient to
establish unambiguously cluster membership for these two stars. However, the
PMS models which fit best their eigenspectra have ages consistent with NGC
2244. If cluster membership can be confirmed, these would be the first known
PMS SPB stars, and would open a new window on testing asteroseismically the
interior structures of PMS stars.Comment: accepted for publication in MNRA
Convective quenching of stellar pulsations
Context: we study the convection-pulsation coupling that occurs in cold
Cepheids close to the red edge of the classical instability strip. In these
stars, the surface convective zone is supposed to stabilise the radial
oscillations excited by the kappa-mechanism.
Aims: we study the influence of the convective motions onto the amplitude and
the nonlinear saturation of acoustic modes excited by kappa-mechanism. We are
interested in determining the physical conditions needed to lead to a quenching
of oscillations by convection.
Methods: we compute two-dimensional nonlinear simulations (DNS) of the
convection-pulsation coupling, in which the oscillations are sustained by a
continuous physical process: the kappa-mechanism. Thanks to both a frequential
analysis and a projection of the physical fields onto an acoustic subspace, we
study how the convective motions affect the unstable radial oscillations.
Results: depending on the initial physical conditions, two main behaviours
are obtained: (i) either the unstable fundamental acoustic mode has a large
amplitude, carries the bulk of the kinetic energy and shows a nonlinear
saturation similar to the purely radiative case; (ii) or the convective motions
affect significantly the mode amplitude that remains very weak. In this second
case, convection is quenching the acoustic oscillations. We interpret these
discrepancies in terms of the difference in density contrast: larger
stratification leads to smaller convective plumes that do not affect much the
purely radial modes, while large-scale vortices may quench the oscillations.Comment: 15 pages, 17 figures, 3 tables, accepted for publication in A&
The Potential of Asteroseismology for Hot, Subdwarf B Stars: A New Class of Pulsating Stars?
We present key sample results of a systematic survey of the pulsation
properties of models of hot B subdwarfs. We use equilibrium structures taken
from detailed evolutionary sequences of solar metallicity (Z = 0.02)
supplemented by grids of static envelope models of various metallicities (Z =
0.02, 0.04, 0.06, 0.08, and 0.10). We consider all pulsation modes with l = 0,
1, 2, and 3 in the 80--1500 s period window, the interval currently most
suitable for fast photometric detection techniques. We establish that
significant driving is often present in hot B subdwarfs and is due to an
opacity bump associated with heavy element ionization. We find that models with
Z >= 0.04 show low radial order unstable modes; both radial and nonradial (p,
f, and g) pulsations are excited. The unstable models have Teff > 30,000 K, and
log g > 5.7, depending somewhat on the metallicity. We emphasize that metal
enrichment needs only occur locally in the driving region. On this basis,
combined with the accepted view that local enrichments and depletions of metals
are common place in the envelopes of hot B subdwarfs, we predict that some of
these stars should show luminosity variations resulting from pulsational
instabilities.Comment: 9 pages, AAS LaTeX v4.0 + one postscript figure. Best with times.sty.
Submitted to Astrophysical Journal Letters. Postscript file also available at
http://shemesh.gsfc.nasa.gov/~dorman/Ben.htm
DNS of the kappa-mechanism. I. Radial modes in the purely radiative case
Context: Hydrodynamical model of the kappa-mechanism in a purely radiative
case. Aims: First, to determine the physical conditions propitious to
kappa-mechanism in a layer with a configurable conductivity hollow and second,
to perform the (nonlinear) direct numerical simulations (DNS) from the most
favourable setups. Methods: A linear stability analysis applied to radial modes
using a spectral solver and DNS thanks to a high-order finite difference code
are compared. Results: Changing the hollow properties (location and shape) lead
to well-defined instability strips. For a given position in the layer, the
amplitude and width of the hollow appear to be the key parameters to get
unstable modes driven by kappa-mechanism. The DNS achieved from these more
auspicious configurations confirm the growth rates as well as structures of
linearly unstable modes. The nonlinear saturation follows through intricate
couplings between the excited fundamental mode and higher damped overtones.Comment: 15 pages, 15 figures, 1 table, accepted for publication in A&
Most Detects G- and P-Modes in the B Supergiant HD 163899 (B2Ib/II)
The {\it Microvariability and Oscillations of Stars (MOST)} satellite
observed the B supergiant HD 163899 (B2 Ib/II) for 37 days as a guide star and
detected 48 frequencies \la 2.8 c d with amplitudes of a few
milli-magnitudes (mmag) and less. The frequency range embraces g- and p-mode
pulsations. It was generally thought that no g-modes are excited in less
luminous B supergiants because strong radiative damping is expected in the
core. Our theoretical models, however, show that such g-modes are excited in
massive post-main-sequence stars, in accordance with these observations. The
nonradial pulsations excited in models between at and at are roughly
consistent with the observed frequency range. Excitation by the Fe-bump in
opacity is possible because g-modes can be partially reflected at a convective
zone associated with the hydrogen-burning shell, which significantly reduces
radiative damping in the core. The {\it MOST} light curve of HD 163899 shows
that such a reflection of g-modes actually occurs, and reveals the existence of
a previously unrecognized type of variable, slowly pulsating B supergiants
(SPBsg) distinct from Cyg variables. Such g-modes have great potential
for asteroseismology.Comment: 24 pages, 9 figures, Astrophysical Journal in pres
MOST Detects g-Modes in the Late-Type be Star beta CMi (B8Ve)
The Microvariability and Oscillations of stars (MOST) satellite has detected
low-amplitude light variations (1 mmag) in the Be star
CMi (B8Ve). The observations lasted 41 days and the variations have typical
periods days. We demonstrate that the dominant frequencies are
consistent with prograde high-order g-modes of excited by the Fe-bump of
opacity in an intermediate-mass () star with a nearly
critical rotation period of 0.38 days. This is the first detection of nonradial
g-mode pulsations in a Be star later than B6 leading to the possibility that
pulsations are excited in all classical Be stars.Comment: 17 pages, 6 figures; Astrophysical Journal part 1 in pres
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