On the Challenging Variability of LS IV-14°116: Pulsational Instabilities Excited by the Ε-Mechanism (original) (raw)
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Astrophysical Journal Supplement Series, 2000
We present the Ðrst results of a large, systematic adiabatic survey of the pulsation properties of models of subdwarf B (sdB) stars. This survey is aimed at providing the most basic theoretical data with which to analyze the asteroseismological properties of the recently discovered class of pulsating sdB stars (the EC 14026 stars). Such a theoretical framework has been lacking up to now. In this paper, the Ðrst of a series of three, an adiabatic pulsation code is used to compute, in the 80È1500 s period window, the radial (l \ 0) and nonradial (from l \ 1 up to l \ 3) oscillation modes for a representative evolutionary model of subdwarf B stars. Quantities such as the periods, kinetic energies, Ðrst-order rotational splitting coefficients, eigenfunctions, and weight functions are given by the code, providing a complete set of very useful diagnostic tools with which to study the mode properties. The main goal is to determine how these quantities relate to the internal structure of B subdwarfs, a crucial and necessary step if one wants to eventually apply the tools of asteroseismology to EC 14026 stars. All modes (p, f, and g) were considered in order to build the most complete picture we can have on pulsations in these stars. In that context, we show that g-modes are essentially deep interior modes oscillating mainly in the radiative helium-rich core (but not in the convective nucleus), while p-modes are shallower envelope modes. We demonstrate that g-modes respond to a trapping/conÐnement phenomenon induced mainly by the He/H chemical transition between the H-rich envelope and the He-rich core of subdwarf B stars. This phenomenon is very similar in nature to the g-mode trapping and conÐnement mechanisms observed in pulsating white dwarf models. We emphasize that p-modes may also experience distortions of their period distribution due to this He/H transition, although these are not as pronounced as in the g-mode case. These phenomena are of great interest as they can potentially provide powerful tools for probing the internal structure of these objects, in particular, with respect to constraining the mass of their H-rich envelope. The results given in this Ðrst paper form the minimal background on pulsation mode characteristics in sdB stars. Upcoming discussions on additional mode properties in subdwarf B star models (Paper II and Paper III of this series) will strongly rely on these basic results since they provide essential guidance in understanding mode period behaviors as functions of B subdwarf stellar parameters and/or evolution. Subject headings : stars : interiors È stars : oscillations È subdwarfs 223 224 CHARPINET ET AL.
Astrophysical Journal Supplement Series, 2002
We present the Ðrst results of a large, systematic adiabatic survey of the pulsation properties of models of subdwarf B (sdB) stars. This survey is aimed at providing the most basic theoretical data with which to analyze the asteroseismological properties of the recently discovered class of pulsating sdB stars (the EC 14026 stars). Such a theoretical framework has been lacking up to now. In this paper, the Ðrst of a series of three, an adiabatic pulsation code is used to compute, in the 80È1500 s period window, the radial (l \ 0) and nonradial (from l \ 1 up to l \ 3) oscillation modes for a representative evolutionary model of subdwarf B stars. Quantities such as the periods, kinetic energies, Ðrst-order rotational splitting coefficients, eigenfunctions, and weight functions are given by the code, providing a complete set of very useful diagnostic tools with which to study the mode properties. The main goal is to determine how these quantities relate to the internal structure of B subdwarfs, a crucial and necessary step if one wants to eventually apply the tools of asteroseismology to EC 14026 stars. All modes (p, f, and g) were considered in order to build the most complete picture we can have on pulsations in these stars. In that context, we show that g-modes are essentially deep interior modes oscillating mainly in the radiative helium-rich core (but not in the convective nucleus), while p-modes are shallower envelope modes. We demonstrate that g-modes respond to a trapping/conÐnement phenomenon induced mainly by the He/H chemical transition between the H-rich envelope and the He-rich core of subdwarf B stars. This phenomenon is very similar in nature to the g-mode trapping and conÐnement mechanisms observed in pulsating white dwarf models. We emphasize that p-modes may also experience distortions of their period distribution due to this He/H transition, although these are not as pronounced as in the g-mode case. These phenomena are of great interest as they can potentially provide powerful tools for probing the internal structure of these objects, in particular, with respect to constraining the mass of their H-rich envelope. The results given in this Ðrst paper form the minimal background on pulsation mode characteristics in sdB stars. Upcoming discussions on additional mode properties in subdwarf B star models (Paper II and Paper III of this series) will strongly rely on these basic results since they provide essential guidance in understanding mode period behaviors as functions of B subdwarf stellar parameters and/or evolution. Subject headings : stars : interiors È stars : oscillations È subdwarfs 223 224 CHARPINET ET AL.
A Driving Mechanism for the Newly Discovered Long-Period Pulsating Subdwarf B Stars
Astrophysical Journal, 2003
We present the results of a stability survey carried out for a sequence of representative models of subdwarf B stars spanning the range of effective temperature 22; 000 K T eff 38; 000 K. We show that long-period, high-order g-modes are excited in the cooler models through the same -mechanism that successfully explains the presence of short-period, low-order p-modes in the hotter EC 14026 pulsators. This is analogous to the case of the Cep/slowly pulsating B stars on the main sequence. We stress that radiative levitation is needed to boost the iron abundance in the driving region for both types of pulsating subdwarf B stars. And indeed, we find that pulsation modes cannot be excited in B subdwarf models if the metallicity is assumed to be uniform and solar. On the basis of our current models, we propose that the pulsation modes detected in longperiod pulsating subdwarf B stars have values of the degree index l ¼ 3 and/or 4, not the canonical values l ¼ 1; 2, a suggestion that is, in principle, testable through multicolor photometry or time-resolved spectroscopy. In this way, we are able to explain quite well, at least at the qualitative level, the main observed characteristics of these pulsators. On the first account, the excited high-order g-modes with l ¼ 3 and 4 in our models have periods that overlap with the range of quasi-periods observed in these stars. On the second account, if the observable modes in these pulsators have indeed such '' high '' values of l as we suggest, we find a natural explanation for the fact that their amplitudes are distinctly and systematically smaller than the amplitudes observed in EC 14026 stars. Finally, our results are also consistent with the observed fact that the long-period pulsators appear systematically cooler than the short-period EC 14026 stars. We point out, however, that our analysis suggests effective temperatures for the long-period B subdwarf pulsators that are somewhat lower than current spectroscopic estimates. The solution to this problem may come from future improvements in the models, the establishment of an effective temperature scale for subdwarf B stars that is free of systematic effects, or both.
Adiabatic Survey of Subdwarf B Star Oscillations. II. Effects of Model Parameters on Pulsation Modes
Astrophysical Journal Supplement Series, 2002
We present the Ðrst results of a large, systematic adiabatic survey of the pulsation properties of models of subdwarf B (sdB) stars. This survey is aimed at providing the most basic theoretical data with which to analyze the asteroseismological properties of the recently discovered class of pulsating sdB stars (the EC 14026 stars). Such a theoretical framework has been lacking up to now. In this paper, the Ðrst of a series of three, an adiabatic pulsation code is used to compute, in the 80È1500 s period window, the radial (l \ 0) and nonradial (from l \ 1 up to l \ 3) oscillation modes for a representative evolutionary model of subdwarf B stars. Quantities such as the periods, kinetic energies, Ðrst-order rotational splitting coefficients, eigenfunctions, and weight functions are given by the code, providing a complete set of very useful diagnostic tools with which to study the mode properties. The main goal is to determine how these quantities relate to the internal structure of B subdwarfs, a crucial and necessary step if one wants to eventually apply the tools of asteroseismology to EC 14026 stars. All modes (p, f, and g) were considered in order to build the most complete picture we can have on pulsations in these stars. In that context, we show that g-modes are essentially deep interior modes oscillating mainly in the radiative helium-rich core (but not in the convective nucleus), while p-modes are shallower envelope modes. We demonstrate that g-modes respond to a trapping/conÐnement phenomenon induced mainly by the He/H chemical transition between the H-rich envelope and the He-rich core of subdwarf B stars. This phenomenon is very similar in nature to the g-mode trapping and conÐnement mechanisms observed in pulsating white dwarf models. We emphasize that p-modes may also experience distortions of their period distribution due to this He/H transition, although these are not as pronounced as in the g-mode case. These phenomena are of great interest as they can potentially provide powerful tools for probing the internal structure of these objects, in particular, with respect to constraining the mass of their H-rich envelope. The results given in this Ðrst paper form the minimal background on pulsation mode characteristics in sdB stars. Upcoming discussions on additional mode properties in subdwarf B star models (Paper II and Paper III of this series) will strongly rely on these basic results since they provide essential guidance in understanding mode period behaviors as functions of B subdwarf stellar parameters and/or evolution.
Astronomy & Astrophysics, 2006
Aims. We reexamine the theoretical instability domain of pulsating PG 1159 stars (GW Vir variables). Methods. We performed an extensive g-mode stability analysis on PG 1159 evolutionary models with stellar masses ranging from 0.530 to 0.741 M , for which the complete evolutionary stages of their progenitors from the ZAMS, through the thermally pulsing AGB and born-again phases to the domain of the PG 1159 stars have been considered. Results. We found that pulsations in PG 1159 stars are excited by the κ-mechanism due to partial ionization of carbon and oxygen, and that no composition gradients are needed between the surface layers and the driving region, much in agreement with previous studies. We show, for the first time, the existence of a red edge of the instability strip at high luminosities. We found that all of the GW Vir stars lay within our theoretical instability strip. Our results suggest a qualitative good agreement between the observed and the predicted ranges of unstable periods of individual stars. Finally, we found that generally the seismic masses (derived from the period spacing) of GW Vir stars are somewhat different from the masses suggested by evolutionary tracks coupled with spectroscopy. Improvements in the evolution during the thermally pulsing AGB phase and/or during the core helium burning stage and early AGB could help to alleviate the persisting discrepancies.
Pulsations in hot subdwarf stars: recent advances and prospects for testing stellar physics
Proceedings of the International Astronomical Union, 2015
The evolved, core helium burning, extreme horizontal branch stars (also known as hot B subdwarfs) host several classes of pulsators showing either p- or g-modes, or both. They offer particularly favorable conditions for probing with asteroseismology their internal structure, thus constituting arguably the most interesting seismic window for this intermediate stage of stellar evolution. G-modes in particular have the power to probe deep inside these stars, down to the convective He-burning core boundary where uncertain physics (convection, overshooting, semi-convection) is at work. Space data recently obtained with CoRoT and Kepler are offering us the possibility to probe these regions in detail and possibly shed new light on how these processes shape the core structure. In this short paper, we present the most recent advances that have taken place in this field and we provide hints of the foreseen future achievements of hot subdwarf asteroseismology.
Non-linear radial pulsation models for the early-type helium stars V652?Her and BX?Cir
Astronomy and Astrophysics, 2002
We report new non-linear pulsation models of the helium stars V652 Her and BX Cir. Linear theory has previously shown their pulsations to be due to iron-group bump instability. Recent high-resolution spectroscopic observations have provided high-precision measurements of their radial velocity curves and of their radii. Their masses remain less well determined. A hydrodynamic code including recent OPAL opacity data has been used to construct the models. These are compared with the observational data. In particular, we attempt to reproduce accurately the observed radial velocity and luminosity curves. The results impose additional constraints on those stellar dimensions, including mass, which remain poorly determined by observation. Final results show a model for V652 Her which reproduce the observed velocity and luminosity curves with 0.7 M and 23 400 K. For BX Cir, the mass must lie between 0.50 and 0.38 M if the temperature is in the range 22 400-24 000 K. However, the luminosity of the models is smaller than that measured directly by a factor of two.
The Astrophysical Journal, 2009
We present a pulsational stability analysis of hot post-asymptotic giant branch (AGB) H-deficient pre-white dwarf stars with active He-burning shells. The stellar models employed are state-of-the-art equilibrium structures representative of PG1159 stars derived from the complete evolution of the progenitor stars, through the thermally pulsing AGB phase and born-again episode. On the basis of fully nonadiabatic pulsation computations, we confirmed theoretical evidence for the existence of a separate PG1159 instability strip in the log T eff-log g diagram characterized by short-period g-modes excited by the-mechanism. This instability strip partially overlaps the already known GW Vir instability strip of intermediate/long-period g-modes destabilized by the classical κ-mechanism acting on the partial ionization of C and/or O in the envelope of PG1159 stars. We found that PG1159 stars characterized by thick He-rich envelopes and located inside this overlapping region could exhibit both short and intermediate/ long periods simultaneously. As a natural application of our results, we study the particular case of VV 47, a pulsating planetary nebula nucleus (PG1159 type) that is particularly interesting because it has been reported to exhibit a rich and complex pulsation spectrum including a series of unusually short pulsation periods. We found that the long periods exhibited by VV 47 can be readily explained by the classical κ-mechanism, while the observed shortperiod branch below ≈300 s could correspond to modes triggered by the He-burning shell through the-mechanism, although more observational work is needed to confirm the reality of these short-period modes. Were the existence of short-period g-modes in this star convincingly confirmed by future observations, VV 47 could be the first known pulsating star in which both the κ-mechanism and the-mechanism of mode driving are simultaneously operating.
Pulsation in Intermediate-Mass Stars
Frontiers in Astronomy and Space Sciences
A new perspective of pulsation in stars within the δ Scuti instability strip has recently emerged as a result of Kepler observations. The majority of stars within the instability strip do not pulsate and practically all δ Scuti stars contain low frequencies. Because γ Doradus stars co-exist with δ Sct stars in the same region of the instability strip, it follows that γ Dor stars are driven by the same mechanism as δ Sct stars. The difference must be due to different mode selection processes. The search for an unknown damping factor which is missing from the models will be essential for further progress. Maia variables and hot γ Dor stars are briefly discussed. Luminosities of roAp stars obtained from Gaia DR2 parallaxes and spectroscopic effective temperatures show that the roAp stars are slightly evolved with temperatures in the range 6,300-8,300 K, considerably cooler than predicted by the models. The roAp stars and stars with solar-like oscillations share the same mass-temperature-luminosity relation, but with frequencies which are about 50 percent higher. This suggests that roAp frequencies are determined by the critical acoustic frequency, but this frequency is larger than in standard models, perhaps as a result of a temperature inversion in the atmosphere.
A pulsational approach to the luminosity of horizontal branch stellar structures
Monthly Notices of the Royal Astronomical Society, 1999
We discuss an alternative approach to constrain the absolute bolometric luminosity of Zero Age Horizontal Branch (ZAHB) structures by using the observational pulsational properties of ab type RR Lyrae stars and theoretical expectations concerning both the relation connecting the pulsational properties of these variables to their evolutionary ones, as luminosity, mass and effective temperature and, also the location in the H-R diagram for the fundamental pulsators instability strip boundaries. Since the main goal of this work is to obtain an evaluation of the ZAHB bolometric luminosity as much as possible independent on stellar evolution theory, we have minimized the use of evolutionary prescriptions, being the only adopted evolutionary input the allowed mass range for fundamental pulsators. Nevertheless, the effects on our final results related to the use of these evolutionary prescriptions have been carefully checked.