Showing papers by "Gilles Fontaine published in 2013"

The Newly Discovered Pulsating Low-mass White Dwarfs: An Extension of the ZZ Ceti Instability Strip

Abstract: In light of the exciting discovery of g-mode pulsations in extremely low-mass, He-core DA white dwarfs, we report on the results of a detailed stability survey aimed at explaining the existence of these new pulsators as well as their location in the spectroscopic Hertzsprung-Russell diagram. To this aim, we calculated some 28 evolutionary sequences of DA models with various masses and chemical layering. These models are characterized by the so-called ML2/α = 1.0 convective efficiency and take into account the important feedback effect of convection on the atmospheric structure. We pulsated the models with the nonadiabatic code MAD, which incorporates a detailed treatment of time-dependent convection. On the other hand, given the failure of all nonadiabatic codes, including MAD, to account properly for the red edge of the strip, we resurrect the idea that the red edge is due to energy leakage through the atmosphere. We thus estimated the location of that edge by requiring that the thermal timescale in the driving region—located at the base of the H convection zone—be equal to the critical period beyond which l = 1 g-modes cease to exist. Using this approach, we find that our theoretical ZZ Ceti instability strip accounts remarkably well for the boundaries of the empirical strip, including the low-gravity, low-temperature regime where the three new pulsators are found. We also account for the relatively long periods observed in these stars, and thus conclude that they are true ZZ Ceti stars, but with low masses.

Third generation stellar models for asteroseismology of hot B subdwarf stars - A test of accuracy with the pulsating eclipsing binary PG 1336–018

Abstract: Context. Asteroseismic determinations of structural parameters of hot B subdwarfs (sdB) have been carried out for more than a decade now. These analyses rely on stellar models whose reliability for the required task needs to be evaluated critically. Aims. We present new models of the so-called third generation (3G) dedicated to the asteroseismology of sdB stars, in particular to long-period pulsators observed from space. These parameterized models are complete static structures suitable for analyzing both p -a ndg-mode pulsators, contrary to the former second generation (2G) models that were limited to p-modes. While the reliability of the 2G models has been successfully verified in the past, this important test still has to be conducted on the 3G structures. Methods. The close eclipsing binary PG 1336−018 provides a unique opportunity to test the reliability of hot B subdwarf models. We compared the structural parameters of the sdB component in PG 1336−018 obtained from asteroseismology based on the 3G models, with those derived independently from the modeling of the reflection/irradiation effect and the eclipses observed in the light curve. Results. The stellar parameters inferred from asteroseismology using the 3G models are found to be remarkably consistent with both the preferred orbital solution obtained from the binary light curve modeling and the updated spectroscopic estimates for the surface gravity of the star. The seismology gives M∗ = 0.471 ± 0.006 M� , R∗ = 0.1474 ± 0.0009 R� ,a nd logg = 5.775 ± 0.007, while orbital modeling leads to M∗ = 0.466 ± 0.006 M� , R∗ = 0.15 ± 0.01 R� ,l ogg = 5.77 ± 0.06, and spectroscopy yields log g = 5.771 ± 0.015. In comparison, seismology from a former analysis based on the 2G models gave very similar results with M∗ = 0.459 ± 0.005 M� , R∗ = 0.151 ± 0.001 R� ,a nd logg = 5.739 ± 0.002. We also show that the uncertainties on the input physics included in stellar models have, at the current level of accuracy, no noticeable impact on the structural parameters derived by asteroseismology. Conclusions. The stellar models (both of second and third generation) presently used to carry out quantitative seismic analyses of sdB stars are reliable for the task. The stellar parameters inferred by this technique, at least for those that could be tested (M∗, R, and log g), appear to be both very precise and accurate, as no significant systematic effect has been found.

The age and stellar parameters of the procyon binary system

Abstract: The Procyon AB binary system (orbital period 40.838 yr, a newly refined determination) is near and bright enough that the component radii, effective temperatures, and luminosities are very well determined, although more than one possible solution to the masses has limited the claimed accuracy. Preliminary mass determinations for each component are available from Hubble Space Telescope imaging, supported by ground-based astrometry and an excellent Hipparcos parallax; we use these for our preferred solution for the binary system. Other values for the masses are also considered. We have employed the TYCHO stellar evolution code to match the radius and luminosity of the F5 IV-V primary star to determine the system's most likely age as 1.87 ± 0.13 Gyr. Since prior studies of Procyon A found its abundance indistinguishable from solar, the solar composition of Asplund, Grevesse, and Sauval (Z = 0.014) is assumed for the Hertzsprung-Russell diagram fitting. An unsuccessful attempt to fit using the older solar abundance scale of Grevesse & Sauval (Z = 0.019) is also reported. For Procyon B, 11 new sequences for the cooling of non-DA white dwarfs have been calculated to investigate the dependences of the cooling age on (1) the mass, (2) core composition, (3) helium layer mass, and (4) heavy-element opacities in the helium envelope. Our calculations indicate a cooling age of 1.19 ± 0.11 Gyr, which implies that the progenitor mass of Procyon B was 2.59 M ☉. In a plot of initial versus final mass of white dwarfs in astrometric binaries or star clusters (all with age determinations), the Procyon B final mass lies several σ below a straight line fit.

A NLTE analysis of the hot subdwarf O star Bd+28 4211. I. The UV spectrum

Abstract: We present a detailed analysis of the UV spectrum of the calibration star Bd+28 4211 using high-quality spectra obtained with the HST and FUSE satellites. To this aim, we compare quantitatively the observed data with model spectra obtained from state-of-the-art NLTE metal line-blanketed model atmospheres and synthetic spectra calculated with TLUSTY and SYNSPEC. We thus determine in a self-consistent way the abundances of eleven elements with well-defined lines in the UV, namely those of C, N, O, F, Mg, Si, P, S, Ar, Fe, and Ni. The derived abundances range from about solar to 1/10 solar. We find that the overall quality of the derived spectral fits is very satisfying. Our spectral analysis can be used to constrain rather tigthly the effective temperature of Bd+28 to a value of teff = 82,000 +/- 5000 K. We also estimate conservatively that its surface gravity falls in the range log g = 6.2 -0.1/+0.3. Assuming that the Hipparcos measurement for Bd+28 is fully reliable and that our model atmospheres are reasonably realistic, we can reconcile our spectroscopic constraints with the available parallax measurement only if the mass of Bd+28 is significantly less than the canonical value of 0.5 msun for a representative post-EHB star.

A non-lte analysis of the hot subdwarf o star bd+28°4211. i. the uv spectrum

Abstract: We present a detailed analysis of the UV spectrum of the calibration star BD+28°4211 using high-quality spectra obtained with the Hubble Space Telescope and Far-Ultraviolet Spectroscopic Explorer satellites. To this aim, we compare quantitatively the observed data with model spectra obtained from state-of-the-art non-LTE metal line-blanketed model atmospheres and synthetic spectra calculated with TLUSTY and SYNSPEC. We thus determine in a self-consistent way the abundances of 11 elements with well-defined lines in the UV, namely those of C, N, O, F, Mg, Si, P, S, Ar, Fe, and Ni. The derived abundances range from about solar to 1/10 solar. We find that the overall quality of the derived spectral fits is very satisfying. Our spectral analysis can be used to constrain rather tightly the effective temperature of BD+28°4211 to a value of T eff = 82, 000 ± 5000 K. We also estimate conservatively that its surface gravity falls in the range log g = 6.2. Assuming that the Hipparcos measurement for BD+28°4211 is fully reliable and that our model atmospheres are reasonably realistic, we can reconcile our spectroscopic constraints with the available parallax measurement only if the mass of BD+28°4211 is significantly less than the canonical value of 0.5 M ☉ for a representative post-extended horizontal branch star.

Asteroseismology of hot B subdwarf stars

Abstract: Non-radial pulsations in Extreme Horizontal Branch stars (also known as hot B subdwarfs or sdB stars) offer strong opportunities to study, through asteroseismology, the structure and internal dynamics of stars in this intermediate stage of stellar evolution. Most sdB stars directly descend from former red giants and are expected to evolve straight into white dwarfs after core helium exhaustion. They thus represent the most direct link between these two stages. Their properties should therefore reflect both the outcome of the core evolution of red giant stars and the initial state for a fraction of the white dwarfs. We review the status of this field after a decade of efforts to exploit both p-mode and g-mode pulsating sdB stars as asteroseismic laboratories. From the discoveries of these two classes of pulsators in 1997 and 2003, respectively, up to the current epoch of data gathering of unprecedented quality from space, a lot of progress has been made in this area and prospects for future achievements look very promising.

Discovery of very high energy γ-ray emission from the BLLacertae object PKS0301243 with H.E.S.S.

Abstract: The active galactic nucleus PKS 0301−243 (z = 0.266) is a high-synchrotron-peaked BL Lac object that is detected at high energies (HE, 100 MeV 100 GeV) by the High Energy Stereoscopic System (H.E.S.S.) from observations between September 2009 and December 2011 for a total live time of 34.9 h. Gamma rays above 200 GeV are detected at a significance of 9.4σ. A hint of variability at the 2.5σ level is found. An integral flux I(E > 200 GeV) = (3.3 ± 1.1stat ± 0.7syst) × 10 −12 ph cm −2 s −1 and a photon index Γ= 4.6 ± 0.7stat ± 0.2syst are measured. Multi-wavelength light curves in HE, X-ray and optical bands show strong variability, and a minimal variability timescale of eight days is estimated from the optical light curve. A single-zone leptonic synchrotron self-Compton scenario satisfactorily reproduces the multi-wavelength data. In this model, the emitting region is out of equipartition and the jet is particle dominated. Because of its high redshift compared to other sources observed at TeV energies, the very high energy emission from PKS 0301−243 is attenuated by the extragalactic background light (EBL) and the measured spectrum is used to derive an upper limit on the opacity of the EBL.

Accretion and activity on the post-common-envelope binary RR Caeli

Abstract: Context. Current scenarios for the evolution of interacting close binaries – such as cataclysmic variables (CVs) – rely mainly on our understanding of low-mass star angular momentum loss (AML) mechanisms. The coupling of stellar wind with its magnetic field, i.e., magnetic braking, is the most promising mechanism believed to drive AML in these stars. There are basically two properties thought to drive magnetic braking: the stellar magnetic field and the stellar wind. Understanding the mechanisms that drive AML therefore requires a comprehensive understanding of these two properties as well.Aims. RR Cae is a well-known nearby (d = 20 pc) eclipsing DA+M binary with an orbital period of P = 7.29 h. The system harbors a metal-rich cool DA white dwarf (WD) and a highly active M-dwarf locked in synchronous rotation. The metallicity of the WD suggests that wind accretion is taking place, which provides a good opportunity to obtain the mass-loss rate of the M-dwarf component. We aim to reach a better understanding of the AML mechanisms in close binaries by characterizing the relevant properties of the M-dwarf component of this system.Methods. We analyzed multi-epoch time-resolved high-resolution spectra of RR Cae in search for traces of magnetic activity and accretion. We selected a number of well-known chromospheric activity indicators and studied their phase-dependence and long-term behavior. Indirect-imaging tomographic techniques were also applied to provide the surface brightness distribution of the magnetically active M-dwarf. The blue part of the spectrum was modeled using a state-of-the-art atmosphere model to constrain the WD properties and its metal enrichment. The latter was used to improve the determination of the mass-accretion rate from the M-dwarf wind.Results. Doppler imaging of the M-dwarf component of RR Cae reveals a polar feature similar to those observed in fast-rotating solar-type stars. Analysis of tomographic reconstruction of the Hα emission line reveals two components, one traces the motion of the M dwarf and is generated by chromospheric activity, while the other clearly follows the motion of the WD. The presence of metals in the WD spectrum suggests that this component arises from accretion of the M-dwarf wind. A model fit to the WD spectrum provides T eff = (7260 ± 250) K and log g = (7.8 ± 0.1) dex with a metallicity of ⟨log [X /X ⊙ ]⟩ = (−2.8 ± 0.1) dex. This maps into a mass-accretion rate of Ṁ acc = (7 ± 2) × 10-16 M ⊙ yr-1 onto the surface of the WD.

The empirical mass distribution of hot B subdwarfs: implications for stellar evolution theory

Abstract: Subdwarf B (sdB) stars are hot, compact, and evolved objects that form the very hot end of the horizontal branch, the so-called Extreme Horizontal Branch (EHB). Understanding the formation of sdB stars is one of the remaining challenges of stellar evolution theory. Several scenarios have been proposed to account for the existence of such objects, made of He-burning core surrounded by very thin H-rich envelope. They give quite different theoretical mass distributions for the resulting sdB stars. Detailed asteroseismic analyses, including mass estimates, of 15 pulsating hot B subdwarfs have been published since a decade. The masses have also been reliably determined by light curve modeling and spectroscopy for 7 sdB components of eclipsing and/or reflection effect binaries. These empirical mass distributions, although based on small-number statistics, can be compared with the expectations of stellar evolution theory. In particular, the two He white dwarfs merger scenario does not seem to be the dominant channel to form isolated sdB stars, while the post-red giant branch scenario is reinforced. This opens new questions on extreme mass loss of red giants to form EHB stars, possibly in connection with the recently discovered close substellar companions and planets orbiting sdB stars.

The angular momentum of isolated white dwarfs

Abstract: This is a very brief report on an ongoing program aimed at mapping the internal rotation profiles of stars through asteroseismology Three years ago, we developed and applied successfully a new technique to the pulsating GW Vir white dwarf PG 1159−035, and were able to infer that it rotates very slowly and rigidly over some 99% of its mass We applied the same approach to the three other GW Vir pulsators with available rotational splitting data, and found similar results We discuss the implications of these findings on the question of the angular momentum of white dwarfs resulting from single star evolution

An overview of white dwarf stars

Abstract: We present a brief summary of what is currently known about white dwarf stars, with an emphasis on their evolutionary and internal properties. As is well known, white dwarfs represent the end products of stellar evolution for the vast majority of stars and, as such, bear the signatures of past events (such as mass loss, mixing phases, loss and redistribution of angular momentum, and thermonuclear burning) that are of essential importance in the evolution of stars in general. In addition, white dwarf stars represent ideal testbeds for our understanding of matter under extreme conditions, and work on their constitutive physics (neutrino production rates, conductive and radiative opacities, interior liquid/solid equations of state, partially ionized and partially degenerate envelope equations of state, diffusion coefficients, line broadening mechanisms) is still being actively pursued. Given a set of constitutive physics, cooling white dwarfs can be used advantageously as cosmochronometers. Moreover, the field has been blessed by the existence of four distinct families of pulsating white dwarfs, each mapping a different evolutionary phase, and this allows the application of the asteroseismological method to probe and test their internal structure and evolutionary state. We set the stage for the reviews that follow on cooling white dwarfs as cosmochronometers and physics laboratories, as well as on the properties of pulsating white dwarfs and the asteroseismological results that can be inferred. 1. BASIC FACTS ABOUT WHITE DWARFS • White dwarfs are the end products of stellar evolution for the vast (95%) majority of stars. They have run out of thermonuclear fuel, and most of them have burned H and He in their interiors. • Most observable white dwarfs are isolated or part of non-interacting binaries. They are believed to have C-O cores and to descend from main sequence stars with masses in the range from slightly less than 1Mto ∼8M� . This maps into a narrow range offinal masses centered around ∼0.6M� . This implies important mass loss in previous evolutionary (red giant) phases. • White dwarfs have a stratified structure (see Fig. 1). Most have a C-O core (containing ∼99% of the total mass M) surrounded by a thin He mantle (∼1% M at most), itself surrounded by a thinner but opaque H envelope (∼0.01% M at most). • White dwarfs are compact cooling bodies in hydrostatic equilibrium; gravity is balanced by degenerate electron pressure. This implies an evolution at almost constant radius. From a pulsation point of view, white dwarfs have a mechanical structure radically different from those of non- degenerate stars (see Fig. 2).

Quantitative Asteroseismology: Determination of the Core Composition and Layering of the ZZ Ceti Star R 548. Part II

Abstract: We present a detailed asteroseismological study of the pulsating white dwarf R548 based on fits to newly detected periods and the use of the forward method. In this first part, we concentrate on the frequency extraction analysis. Based on an unexploited CFHT/LAPOUNE broadband photometric campaign of high S/N, we are able to obtain a finer frequency spectrum and uncover two triplets, two doublets, and two singlets. The low-amplitude and “simple” pulsator R548 is an ideal candidate for carrying on a complete asteroseismological analysis within the framework of the adiabatic approximation to obtain optimized structural parameters, including the envelope layering and the bulk composition of the core.

A survey of hot subdwarf pulsators in ω Cen

Abstract: We recently discovered an apparently new class of pulsating Extreme Horizontal Branch (EHB) star in ω Cen. Tightly clustered around ∼50,000 K, these H-rich sdO stars exhibit rapid, multi-periodic oscillations on a timescale of 100 s. While four such objects have been detected in ω Cen, no counterparts have yet been found among the field population. Conversely, the rapid sdB pulsators around 31,000 K that are well-studied in the Galactic field have yet to be found in a globular cluster. We discuss the implications of this and also report the discovery of a fifth EHB pulsator in ω Cen.

Accretion and activity on the post-common-envelope binary RR~Cae

Abstract: Current scenarios for the evolution of interacting close binaries - such as cataclysmic variables (CVs) - rely mainly on our understanding of low-mass star angular momentum loss (AML) mechanisms. The coupling of stellar wind with its magnetic field, i.e., magnetic braking, is the most promising mechanism to drive AML in these stars. There are basically two properties driving magnetic braking: the stellar magnetic field and the stellar wind. Understanding the mechanisms that drive AML therefore requires a comprehensive understanding of these two properties. RRCae is a well-known nearby (d=20pc) eclipsing DA+M binary with an orbital period of P=7.29h. The system harbors a metal-rich cool white dwarf (WD) and a highly active M-dwarf locked in synchronous rotation. The metallicity of the WD suggests that wind accretion is taking place, which provides a good opportunity to obtain the mass-loss rate of the M-dwarf component. We analyzed multi-epoch time-resolved high-resolution spectra of RRCae in search for traces of magnetic activity and accretion. We selected a number of well-known activity indicators and studied their short and long-term behavior. Indirect-imaging tomographic techniques were also applied to provide the surface brightness distribution of the magnetically active M-dwarf, and reveals a polar feature similar to those observed in fast-rotating solar-type stars. The blue part of the spectrum was modeled using a atmosphere model to constrain the WD properties and its metal enrichment. The latter was used to improve the determination of the mass-accretion rate from the M-dwarf wind. The presence of metals in the WD spectrum suggests that this component arises from accretion of the M-dwarf wind. A model fit to the WD gives Teff=(7260+/-250)K and logg=(7.8+/-0.1) dex with a metallicity of =(-2.8+/-0.1)dex, and a mass-accretion rate of dotMacc=(7+/-2)x1e-16Msun/yr.

The Instability Strip of ZZ Ceti White Dwarfs and Its Extension to the Extremely Low Mass Pulsators

Abstract: Aims. The determination of the location of the theoretical ZZ Ceti instability strip in the log g−Teff diagram has remained a challenge over the years due to the lack of a suitable treatment for convection in these stars. For the first time, a full nonadiabatic approach including time-dependent convection is applied to ZZ Ceti pulsators, and we provide the appropriate details related to the inner workings of the driving mechanism. Methods. We used the nonadiabatic pulsation code MAD with a representative evolutionary sequence of a 0.6 M DA white dwarf. This sequence is made of state-of-the-art models that include a detailed modeling of the feedback of convection on the atmospheric structure. The assumed convective efficiency in these models is the so-called ML2/α = 1.0 version. We also carried out, for comparison purposes, nonadiabatic computations within the frozen convection approximation, as well as calculations based on models with standard grey atmospheres. Results. We find that pulsational driving in ZZ Ceti stars is concentrated at the base of the superficial H convection zone, but at depths, near the blue edge of the instability strip, somewhat larger than those obtained with the frozen convection approach. Despite the fact that this approach is formally invalid in such stars, particularly near the blue edge of the instability strip, the predicted boundaries are not dramatically different in both cases. The revised blue edge for a 0.6 M model is found to be around Teff = 11 970 K, some 240 K hotter than the value predicted within the frozen convection approximation, in rather good agreement with the empirical value. On the other hand, our predicted red edge temperature for the same stellar mass is only about 5600 K (80 K hotter than with the frozen convection approach), much lower than the observed value. Conclusions. We correctly understand the development of pulsational instabilities of a white dwarf as it cools at the blue edge of the ZZ Ceti instability strip. Our current implementation of time-dependent convection however still lacks important ingredients to fully account for the observed red edge of the strip. We will explore a number of possibilities in the future papers of this series.

The Age and Stellar Parameters of the Procyon Binary System

Abstract: The Procyon AB binary system (orbital period 40.838 years, a newly-refined determination), is near and bright enough that the component radii, effective temperatures, and luminosities are very well determined, although more than one possible solution to the masses has limited the claimed accuracy. Preliminary mass determinations for each component are available from HST imaging, supported by ground-based astrometry and an excellent Hipparcos parallax; we use these for our preferred solution for the binary system. Other values for the masses are also considered. We have employed the TYCHO stellar evolution code to match the radius and luminosity of the F5 IV-V primary star to determine the system's most likely age as 1.87 +/- 0.13 Gyr. Since prior studies of Procyon A found its abundance indistinguishable from solar, the solar composition of Asplund, Grevesse & Sauval (Z=0.014) is assumed for the HR Diagram flitting. An unsuccessful attempt to fit using the older solar abundance scale of Grevesse & Sauval (Z=0.019) is also reported. For Procyon B, eleven new sequences for the cooling of non-DA white dwarfs have been calculated, to investigate the dependence of the cooling age on (1) the mass, (2) the core composition, (3) the helium layer mass, and (4) heavy-element opacities in the helium envelope. Our calculations indicate a cooling age of 1.19+/-0.11 Gyr, which implies that the progenitor mass of Procyon B was 2.59(+0.44,-0.26) Msun. In a plot of initial vs final mass of white dwarfs in astrometric binaries or star clusters (all with age determinations), the Procyon B final mass lies several sigma below a straight line fit.

The NectarCAM camera project

Abstract: In the framework of the next generation of Cherenkov telescopes, the Cherenkov Telescope Array (CTA), NectarCAM is a camera designed for the medium size telescopes covering the central energy range of 100 GeV to 30 TeV. NectarCAM will be finely pixelated (~ 1800 pixels for a 8 degree field of view, FoV) in order to image atmospheric Cherenkov showers by measuring the charge deposited within a few nanoseconds time-window. It will have additional features like the capacity to record the full waveform with GHz sampling for every pixel and to measure event times with nanosecond accuracy. An array of a few tens of medium size telescopes, equipped with NectarCAMs, will achieve up to a factor of ten improvement in sensitivity over existing instruments in the energy range of 100 GeV to 10 TeV. The camera is made of roughly 250 independent read-out modules, each composed of seven photo-multipliers, with their associated high voltage base and control, a read-out board and a multi-service backplane board. The read-out boards use NECTAr (New Electronics for the Cherenkov Telescope Array) ASICs which have the dual functionality of analogue memories and Analogue to Digital Converter (ADC). The camera trigger to be used will be flexible so as to minimize the read-out dead-time of the NECTAr chips. We present the camera concept and the design and tests of the various subcomponents. The design includes the mechanical parts, the cooling of the electronics, the readout, the data acquisition, the trigger, the monitoring and services.

g-mode trapping and period spacings in hot B subdwarf stars

Abstract: Hot B subdwarfs (sdB) are hot and compact helium core burning stars of nearly half a solar mass that can develop pulsational instabilities driving acoustic and/or gravity modes. These evolved stars are expected to be chemically stratified with an almost pure hydrogen envelope surrounding a helium mantle on top of a carbon/oxygen enriched core. However, the sdB stars pulsating in g-modes show regularities in their observed period distributions that, surprisingly (at first sight), are typical of the behavior of high order g-modes in chemically homogeneous (i.e., non-stratified) stars. This led to a claim that hot B subdwarfs could be much less chemically stratified than previously thought. Here, we reinvestigate trapping effects affecting g-modes in sdB stars. We show that standard stratified models of such stars can also produce nearly constant period spacings in the low frequency range similar to those found in g-mode spectra of sdB stars monitored with Kepler.

Quantitative asteroseismology: Determination of the core composition and layering of the ZZ Ceti star R548. Part I

Abstract: We present a detailed asteroseismological study of the pulsating white dwarf R548 based on fits to newly detected periods and the use of the forward method. In this first part, we concentrate on the frequency extraction analysis. Based on an unexploited CFHT/LAPOUNE broadband photometric campaign of high S/N, we are able to obtain a finer frequency spectrum and uncover two triplets, two doublets, and two singlets. The low-amplitude and “simple” pulsator R548 is an ideal candidate for carrying on a complete asteroseismological analysis within the framework of the adiabatic approximation to obtain optimized structural parameters, including the envelope layering and the bulk composition of the core.

Improved determination of the atmospheric parameters of the pulsating sdB star Feige 48

Abstract: Given the importance of Feige 48 as an sdB pulsator, we sought to obtain the best possible estimates of its spectroscopic parameters with a grid of NLTE metal-blanketed model atmospheres constructed especially for that star. This small grid of 150 models includes 8 metallic elements whose abundances have been determined previously and reported in the literature. Our fitting procedure found the following parameters for Feige 48: T eff = 29 504 K, log g = 5.41 and log N (He)/N (H) = −2.90. These results are in very good agreement with previous spectroscopic estimates (which generally ignore either NLTE effects or metal blanketing), thus indicating that metal line-blanketing in NLTE – modeled for the first time here – is not a dominant factor in the atmosphere of Feige 48.

The Angular Momentum of Isolated White Dwarf Stars

Abstract: This is a very brief report on an ongoing program aimed at mapping the internal rotation profiles of stars through asteroseismology. Three years ago, we developed and applied successfully a new technique to the pulsating GW Vir white dwarf PG 1159−035, and were able to infer that it rotates very slowly and rigidly over some 99% of its mass. We applied the same approach to the three other GW Vir pulsators with available rotational splitting data, and found similar results. We discuss the implications of these findings on the question of the angular momentum of white dwarfs resulting from single star evolution. 1. ASTROPHYSICAL CONTEXT White dwarf stars represent the final phase of the evolution of some 95−97% of all stars. If stars were to keep their angular momentum throughout their evolution, their white dwarf descendants, due to their compact nature, would all rotate relatively rapidly, with periods of order of seconds. Observations of their photospheres show, however, that they rotate much slower (at least superficially), with periods (measured mostly through spectroscopy and polarimetry) ranging from a few hours to tens of years. While it is widely believed that most of the angular momentum of an isolated star is lost during important mass loss episodes as it goes through the red giant phases of its evolution, current observations have been unable to reveal how rotation varies with depth in a white dwarf. In particular, one could never exclude the possibility that the internal regions, inaccessible to direct observations, could spin quite rapidly and, hence, "hide" a substantial fraction of the original angular momentum. The reader will have realized here that we are referring to isolated objects that are the products of single star evolution. Asteroseismic inferences about the rotation state of pulsating white dwarfs have also been made under the assumption of solid body rotation, but nothing could be said about the extent of the stellar zone actually probed by the observed pulsation modes. In other words, those inferences did not reveal what specific region of a pulsating star (e.g., core or envelope) imprints most on the rotational signature left on the pulsation data. Using a novel method that we developed recently (1), we are now able to map or constrain the internal rotation profile of a pulsating white dwarf. Our method can most efficiently be used for GW Vir pulsators, for which the internal rotation profile can be mapped essentially over the full mass, thus inferring or constraining the total angular momentum. After having demonstrated that PG 1159−035 rotates slowly (33.67 ± 0.24h) and rigidly over some 99% of its mass (it should be noted that this mapping is more extensive than what has been achieved so far in the Sun through helioseismology, and is unique to asteroseismology; see Fig. 1), we have extended our approach to three other GW Vir pulsators. We briefly report on some of our results here.

Origin and pulsation of hot subdwarfs

Abstract: We briefly introduce hot subdwarfs and their evolutionary status before discussing the different types of known pulsators in more detail. Currently, at least six apparently distinct types of variable are known among hot subdwarfs, encompassing p- as well as g-mode pulsators and objects in the Galactic field as well as in globular clusters. Most of the oscillations detected can be explained in terms of an iron opacity mechanism, and quantitative asteroseismology has been very successful for some of the pulsators. In addition to helping constrain possible evolutionary scenarios, studies focussing on stellar pulsations have also been used to infer planets and characterize the rotation of the host star.

Pulsations in white dwarf stars

Abstract: Abstract We first present a brief description of the six distinct families of pulsating white dwarfs that are now known. These are all opacity-driven pulsators showing low- to mid-order, low-degree gravity modes. We then discuss some recent highlights that have come up in the field of white dwarf asteroseismology.

Erratum to: A survey of hot subdwarf pulsators in ω Cen

Abstract: 1ESO, Karl-Schwarzschild-Str. 2, 85748 Garching bei Munchen, Germany 2Osservatorio Astronomico di Roma, Istituto Nazionale de Astrofisica, via Frascati 33, 0040 Monte Porzio Catone, Italy 3Departement de Physique, Universite de Montreal, CP. 6128, Succ. Centre-Ville, Montreal, QC H3C 3J7, Canada 4Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA 5Instituto de Astrofisica de Canarias, Calle Via Lactea s/n, 38205 La Laguna, Tenerife, Spain 6Departamento de Astrofisica, Universidad de La Laguna, Tenerife, Spain 7Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium 8Departamento de Astronomia y Astrofisica, Pontificia Universidad Catolica de Chile, Av. Vicuna Mackenna 4860, 782-0436 Macul, Santiago, Chile 9The Milky Way Millennium Nucleus, Av. Vicuna Mackenna 4860, 782-0436 Macul, Santiago, Chile 10Department of Physics, Universita di Roma “Tor Vergata”, via della Ricerca Scientifica 1, 00133 Rome, Italy 11Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK 12Department of Physics, University of Warwick, Coventry CV4 7AL, UK

Reaching the 1% accuracy level on stellar mass and radius determinations from asteroseismology

Abstract: Abstract Asteroseismic modeling of subdwarf B (sdB) stars provides measurements of their fundamental parameters with a very good precision; in particular, the masses and radii determined from asteroseismology are found to typically reach a precision of 1% containing various uncertainties associated with their inner structure and the underlying microphysics (composition and transition zones profiles, nuclear reaction rates, etc.). Therefore, the question of the accuracy of the stellar parameters derived by asteroseismology is legitimate. We present here the seismic modeling of the pulsating sdB star in the eclipsing binary PG 1336–018, for which the mass and the radius are independently and precisely known from the modeling of the reflection/irradiation effect and the eclipses observed in the light curve. This allows us to quantitatively evaluate the reliability of the seismic method and test the impact of uncertainties in our stellar models on the derived parameters. We conclude that the sdB star parameters inferred from asteroseismology are precise, accurate, and robust against model uncertainties.

Observational Asteroseismology of Hot Subdwarf Stars with the Mont4K/Kuiper Combination at the Steward Observatory Mount Bigelow Station

Abstract: In the last few years, we have carried out several extensive observational campaigns on pulsating hot subdwarf stars using the Mont4K CCD camera attached to the 1.55 m Kuiper Telescope on Mount Bigelow. The Mont4K is a joint partnership between the University of Arizona and Universite de Montreal. It was designed and built at Steward Observatory. Using the Mont4K/Kuiper combination, we have so far, and among others, gathered high-sensitivity broadband light curves for PG 1219+534, PB8783, HS 0702+6043, and Feige 48. We report very briefly on some of the most interesting observational results that came out of these campaigns.

The internal structure of ZZ Cet stars using quantitative asteroseismology: The case of R548

Abstract: Abstract We explore quantitatively the low but sufficient sensitivity of oscillation modes to probe both the core composition and the details of the chemical stratification of pulsating white dwarfs. Until recently, applications of asteroseismic methods to pulsating white dwarfs have been far and few, and have generally suffered from an insufficient exploration of parameter space. To remedy this situation, we apply to white dwarfs the same double-optimization technique that has been used quite successfully in the context of pulsating hot B subdwarfs. Based on the frequency spectrum of the pulsating white dwarf R548, we are able to unravel in a robust way the unique onion-like stratification and the chemical composition of the star. Independent confirmations from both spectroscopic analyses and detailed evolutionary calculations including diffusion provide crucial consistency checks and add to the credibility of the inferred seismic model. More importantly, these results boost our confidence in the reliability of the forward method for sounding white dwarf internal structure with asteroseismology.