Asteroseismology of hot B subdwarf stars

in EPJ Web of Conferences (2013, March), 43

Nonradial 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 ... [more ▼]

Nonradial 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. [less ▲]

A preliminary look at the empirical mass distribution of hot B subdwarf stars

in Astronomy and Astrophysics (2012), 539

We present the results of about a decade of efforts toward building an empirical mass distribution for hot B subdwarf stars on the basis of asteroseismology. So far, our group has published detailed ... [more ▼]

We present the results of about a decade of efforts toward building an empirical mass distribution for hot B subdwarf stars on the basis of asteroseismology. So far, our group has published detailed analyses pertaining to 16 pulsating B subdwarfs, including estimates of the masses of these pulsators. Given that measurements of the masses of B subdwarfs through more classical methods (such as full orbital solutions in binary stars) have remained far and few, asteroseismology has proven a tool of choice in this endeavor. On the basis of a first sample of 15 pulsators, we find a relatively sharp mass distribution with a mean mass of 0.470 M⊙, a median value of 0.470 M⊙, and a narrow range 0.441−0.499 M⊙ containing some 68.3% of the stars. We augmented our sample with the addition of seven stars (components of eclipsing binaries) with masses reliably established through light curve modeling and spectroscopy. The new distribution is very similar to the former one with a mean mass of 0.470 M⊙, a median value of 0.471 M⊙, and a slightly wider range 0.439−0.501 M⊙ containing some 68.3% of the stars. Although still based on small-number statistics, our derived empirical mass distribution compares qualitatively very well with the expectations of stellar evolution theory. [less ▲]

The empirical mass distributions of hot B subdwarfs derived by asteroseismology

Conference (2011, October)

Detailed asteroseismic analyses of 15 pulsating B subdwarfs have been published since a decade, including estimates of the masses of these stars. We present in this talk the empirical mass distribution ... [more ▼]

Detailed asteroseismic analyses of 15 pulsating B subdwarfs have been published since a decade, including estimates of the masses of these stars. We present in this talk the empirical mass distribution for hot B subdwarfs on the basis of this sample. We find a sharp mass distribution with a mean mass of 0.470 Msun, a median value of 0.471 Msun, and 68.3% of the stars fall in the narrow range of mass 0.441-0.499 Msun. In a second experiment, we augment our sample with the addition of 5 hot B subdwarfs components of eclipsing binaries, with masses reliably determined by light curve modeling and spectroscopy. The new mass distribution is very similar to the former one with a mean mass of 0.469 Msun, a median value of 0.471 Msun, and a range 0.436-0.501 Msun containing 68.3% of the stars. We also discuss in this talk how these empirical mass distributions, although still based on small-number statistics, compare with the expectations of stellar evolution theory. [less ▲]

Sounding the cores of stars by gravity-mode asteroseismology

Conference (2011, February)

Asteroseismology is a recent branch of astrophysics that studies the interiors of stars by the interpretation of their pulsation spectra. A wide variety of stars exhibit pulsations, including gravity ... [more ▼]

Asteroseismology is a recent branch of astrophysics that studies the interiors of stars by the interpretation of their pulsation spectra. A wide variety of stars exhibit pulsations, including gravity-modes (driven by buoyancy) that usually penetrate deep inside the stars. By probing these deep layers unreachable from classical observations, the g-mode oscillations bring invaluable information for stellar evolution and astrophysics in general. I will illustrate in my talk the power of g-mode asteroseismology by the example of Extreme Horizontal Branch stars, that are on an intermediate stage of evolution, and show how g-modes allow us to determine the properties of the cores in these stars, including their convective characteristics, size and composition. [less ▲]

Structural and core parameters of the hot B subdwarf KPD 1943+4058 as inferred from g-mode oscillations

Conference (2010, June)

We present a seismic analysis of the pulsating hot B subdwarf KPD 1943+4058 (KIC 005807616) on the basis of the long-period, gravity-mode pulsations recently uncovered by Kepler. This is the first time ... [more ▼]

We present a seismic analysis of the pulsating hot B subdwarf KPD 1943+4058 (KIC 005807616) on the basis of the long-period, gravity-mode pulsations recently uncovered by Kepler. This is the first time that g-mode seismology can be exploited quantitatively for stars on the extreme horizontal branch, all previous successful seismic analyses having been confined so far to short-period, p-mode pulsators. We demonstrate that current models of hot B subdwarfs can explain quite well the observed g-mode periods, while being consistent with independent constraints provided by spectroscopy. We identify the 18 pulsations retained in our analysis as low-degree (l = 1 and 2), intermediate-order (k = −9 through −58) g-modes. The periods (frequencies) are recovered, on the average, at the 0.22% level, which is comparable to the best results obtained for p-mode pulsators. We infer the following structural and core parameters for KPD 1943+4058 : Teff = 28,050 ± 470 K, log g = 5.520 ± 0.029, M∗ = 0.4964 ± 0.0013 M⊙, log (Menv/M∗) = −2.552 ± 0.070, log (1−Mcore/M∗) = −0.366 ± 0.010, and Xcore(C+O) = 0.2612 ± 0.0080. We additionally derive the age of the star since the Zero-Age EHB 18.4 ± 1.0 Myr, the radius R = 0.2026 ± 0.0070 R⊙, the luminosity L = 22.92 ± 3.13 L⊙, the absolute magnitude MV = 4.21 ± 0.11, the reddening index E(B − V ) = 0.094 ± 0.017, and the distance d = 1183 ± 93 pc. [less ▲]

Progress in Sounding the Interior of Pulsating Hot Subdwarf Stars

in AIP Conference Proceedings (2009, September), 1170

Hot subdwarf stars, including helium core burning subdwarf B (sdB) stars and more evolved subdwarf O (sdO) stars, host three classes of nonradial pulsators. Two of them show rapid acoustic mode ... [more ▼]

Hot subdwarf stars, including helium core burning subdwarf B (sdB) stars and more evolved subdwarf O (sdO) stars, host three classes of nonradial pulsators. Two of them show rapid acoustic mode oscillations (with periods in the 60-600 s range), while the third class features slow gravity mode vibrations of period typically 1h. All three classes provide a strong potential for sounding the internal properties of stars in these evolved phases of stellar evolution through asteroseismology. This has so far been exploited mainly for the rapid p-mode sdB pulsators, but the long period g-mode sdB pulsators, with the advent of space observations, carry good promises for sounding even deeper regions inside these stars. In this review paper, we summarize the properties of pulsating hot subdwarf stars and we outline progress that have recently occured in the asteroseismic exploitation of the pulsations observed in sdB stars. [less ▲]

Testing the forward modeling approach in asteroseismology. I. Seismic solutions for the hot B subdwarf Balloon 090100001 with and without a priori mode identification

in Astronomy and Astrophysics (2008), 488

Context: Balloon 090100001, the brightest of the known pulsating hot B subdwarfs, exhibits simultaneoulsy both short- and long-period pulsation modes, and shows relatively large amplitudes for its ... [more ▼]

Context: Balloon 090100001, the brightest of the known pulsating hot B subdwarfs, exhibits simultaneoulsy both short- and long-period pulsation modes, and shows relatively large amplitudes for its dominant modes. For these reasons, it has been studied extensively over the past few years, including a successful experiment carried out at the Canada-France-Hawaii Telescope to pin down or constrain the value of the degree index ℓ of several pulsation modes through multicolor photometry. Aims: The primary goal of this paper is to take advantage of such partial mode identification to test the robustness of our standard approach to the asteroseismology of pulsating subdwarf B stars. The latter is based on the forward approach whereby a model that best matches the observed periods is searched for in parameter space with no a priori assumption about mode identification. When successful, this method leads to the determination of the global structural parameters of the pulsator. As a bonus, it also leads, after the fact, to complete mode identification. For the first time, with the availability of partial mode identification for Balloon 090100001, we are able to evaluate the sensitivity of the inferred seismic model to possible uncertainty in mode identification. Methods: We carry out a number of exercises based on the double optimization technique that we developed within the framework of the forward modeling approach in asteroseismology. We use the set of ten periods corresponding to the independent pulsation modes for which values of ℓ have been either formally identified or constrained through multicolor photometry in Balloon 090100001. These exercises differ in that they assume different a priori mode identification. Results: Our primary result is that the asteroseismic solution stands very robust, whether or not external constraints on the values of the degree ℓ are used. Although this may come as a small surprise, the test proves to be conclusive, and small differences in mode identification among the ten modes do not affect in any significant way, at the typical accuracy presently achieved, the final emergent seismic model. This is due to the structure of the p-mode pulsation spectra in sdB stars. In all cases, the inferred structural parameters of Balloon 090100001 remain practically unchanged. They correspond, and this constitutes our second important result, to a star beyond the TAEHB with T_eff = 28 000 ± 1 200 K, log g = 5.383 ± 0.004, M⋆/Msun = 0.432 ± 0.015, and log{M_env/M⋆} = -4.89 ± 0.14. Other structural parameters are also derived. [less ▲]

Asteroseismic Solutions for the Two Fast Subdwarf B Pulsators Balloon 090100001 and PG 1336-018

in Leaflet - Astronomical Society of the Pacific (2008, January), 392

We present two new asteroseismic studies of the rapid sdB pulsator PG 1336-018 and the hybrid sdB pulsator Balloon (BAL) 090100001. For PG 1336-018, the analysis is based on 25 periods extracted from the ... [more ▼]

We present two new asteroseismic studies of the rapid sdB pulsator PG 1336-018 and the hybrid sdB pulsator Balloon (BAL) 090100001. For PG 1336-018, the analysis is based on 25 periods extracted from the white-light photometry of the Xcov17 Whole Earth Telescope campaign (Kilkenny et al. 2003). For BAL 090100001, we use 10 periods of high amplitudes for which constraints on the ℓ-index have been derived from high S/N multicolour photometry. From asteroseismology, we estimate important structural parameters for these two stars, including the stellar mass and the H-rich envelope thickness. The analyses presented here are of particular interest since the asteroseismic results can be tested using independent constraints, which confirm the robustness of the solutions in both cases. [less ▲]

Observations and asteroseismic analysis of the rapidly pulsating hot B subdwarf PG 0911+456

in Astronomy and Astrophysics (2007), 476

Aims:The principal aim of this project is to determine the structural parameters of the rapidly pulsating subdwarf B star PG 0911+456 from asteroseismology. Our work forms part of an ongoing programme to ... [more ▼]

Aims:The principal aim of this project is to determine the structural parameters of the rapidly pulsating subdwarf B star PG 0911+456 from asteroseismology. Our work forms part of an ongoing programme to constrain the internal characteristics of hot B subdwarfs with the long-term goal of differentiating between the various formation scenarios proposed for these objects. So far, a detailed asteroseismic interpretation has been carried out for 6 such pulsators, with apparent success. First comparisons with evolutionary theory look promising, however it is clear that more targets are needed for meaningful statistics to be derived. Methods: The observational pulsation periods of PG 0911+456 were extracted from rapid time-series photometry using standard Fourier analysis techniques. Supplemented by spectroscopic estimates of the star's mean atmospheric parameters, they were used as a basis for the “forward modelling” approach in asteroseismology. The latter culminates in the identification of one or more “optimal” models that can accurately reproduce the observed period spectrum. This naturally leads to an identification of the oscillations detected in terms of degree ℓ and radial order k, and infers the structural parameters of the target. Results: The high S/N low- and medium resolution spectroscopy obtained led to a refinement of the atmospheric parameters for PG 0911+456, the derived values being T_eff = 31 940 ± 220 K, log g = 5.767 ± 0.029, and log He/H = -2.548 ± 0.058. From the photometry it was possible to extract 7 independent pulsation periods in the 150-200 s range with amplitudes between 0.05 and 0.8% of the star's mean brightness. There was no indication of fine frequency splitting over the 68-day time baseline, suggesting a very slow rotation rate. An asteroseismic search of parameter space identified several models that matched the observed properties of PG 0911+456 well, one of which was isolated as the “optimal” model on the basis of spectroscopic and mode identification considerations. All the observed pulsations are identified with low-order acoustic modes with degree indices ℓ = 0,1,2 and 4, and match the computed periods with a dispersion of only 0.26%, typical of the asteroseismological studies carried out to date for this type of star. The inferred structural parameters of PG 0911+456 are T_eff = 31 940 ± 220 K (from spectroscopy), log {g} = 5.777 ± 0.002, Mast/Msun = 0.39 ± 0.01, log{M_env/Mast} = -4.69 ± 0.07, R/Rsun = 0.133 ± 0.001 and L/Lsun = 16.4 ± 0.8. We also derive the absolute magnitude MV = 4.82 ± 0.04 and a distance d = 930.3 ± 27.4 pc. [less ▲]