References of "Green, Elizabeth M"
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See detailModeling the hot subdwarf PB 8783 by asteroseismology
Van Grootel, Valérie ULg; Charpinet, Stephane; Fontaine, Gilles et al

in Astronomical Society of the Pacific Conference Series (2014), 481

We present the preliminary seismic modeling of one of the hottest and most compact subdwarf pulsators, PB 8783 (EO Ceti). This is a well observed hot subdwarf star, including a 78 d campaign in white ... [more ▼]

We present the preliminary seismic modeling of one of the hottest and most compact subdwarf pulsators, PB 8783 (EO Ceti). This is a well observed hot subdwarf star, including a 78 d campaign in white light photometry that we carried out at Mount Bigelow, Arizona, during the fall 2007. PB 8783 has also been observed at length in spectroscopy, revealing a spectrum highly contaminated by a main sequence companion. It is extremely difficult to disentangle the contribution of the two components and, as a consequence of this, the exact nature of the hot subdwarf (sdB or sdO star) is undetermined. We propose here to test the two hypotheses by asteroseismology. Although the sdB possibility cannot be excluded, the pulsation modes observed in PB 8783 are much better accommodated in the case of an sdO star. [less ▲]

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See detailThe Mass Distribution of sdB Stars Derived by Asteroseismology and Other Means: Implications for Stellar Evolution Theory
Van Grootel, Valérie ULg; Charpinet, Stéphane; Fontaine, Gilles et al

in Astronomical Society of the Pacific Conference Series (2014), 481

Understanding the formation of sdB stars is one of the remaining challenges of stellar evolution theory. Competing scenarios have been proposed to account for the existence of such evolved objects. They ... [more ▼]

Understanding the formation of sdB stars is one of the remaining challenges of stellar evolution theory. Competing scenarios have been proposed to account for the existence of such evolved objects. They give quite different mass distributions for resulting sdB stars. Detailed asteroseismic analyses, including mass estimates, of 15 pulsating hot B subdwarfs have been published in the past decade. Masses have also been reliably determined by light curve modeling and spectroscopy for 7 sdB components of eclipsing or reflection binaries. We present here the empirical mass distribution of sdB stars on the basis of these samples. Implications are also briefly discussed. [less ▲]

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See detailPrecision and Accuracy of Asteroseismology Applied to sdB stars Using the Forward Modeling Method
Charpinet, Stéphane; Van Grootel, Valérie ULg; Brassard, Pierre et al

in Astronomical Society of the Pacific Conference Series (2014), 481

Detailed seismic studies of hot B subdwarf (sdB) stars using the forward modeling approach provide measurements of their fundamental parameters at very interesting precisions. For instance, masses, radii ... [more ▼]

Detailed seismic studies of hot B subdwarf (sdB) stars using the forward modeling approach provide measurements of their fundamental parameters at very interesting precisions. For instance, masses, radii, and log g values derived this way are typically claimed at ∼ 1 − 2%, ∼ 0.5%, and ∼ 0.1 % precision, respectively. However, this method relies on still imperfect stellar models that contains various uncertainties associated with their inner structure and the underlying microphysics. A signature of these imperfections is the inability of current best-fit seismic models to reproduce all the observed oscillation frequencies at the precision of the observations. Therefore, the question of the accuracy (as opposed to the precision) of the derived parameters obtained from this approach is legitimate. Here, we revisit the question of precision and accuracy based on new, third generation, complete static models of sdB stars developed for asteroseismology and applied to the case of the eclipsing system PG 1336-018. This allows us to evaluate the reliability of the method and quantify the impact of various uncertainties in the stellar models on the derived stellar parameters. Finally, we discuss the intrinsic potential of asteroseismology for precise measurements of stellar parameters and show that we are far from having fully exploited this technique in terms of precision that can, in principle, be achieved. [less ▲]

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See detailObservational Asteroseismology of Hot Subdwarf Stars with the Mont4K/Kuiper Combination at the Steward Observatory Mount Bigelow Station
Fontaine, Gilles; Green, Elizabeth M.; Charpinet, Stéphane et al

in Astronomical Society of the Pacific Conference Series (2014), 481

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 ... [more ▼]

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 Montre ́al. 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. [less ▲]

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See detail6th Conference on Hot Subdwarf Stars and Related Objects
Van Grootel, Valérie ULg; Green, Elizabeth M.; Fontaine, Gilles et al

Book published by Astronomical Society of the Pacific Conference Series (2014)

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See detailReaching the 1% accuracy level on stellar mass and radius determinations from asteroseismology
Van Grootel, Valérie ULg; Charpinet, Stephane; Fontaine, Gilles et al

in Proceedings of the International Astronomical Union (2014), 301

Asteroseismic modeling of subdwarf B (sdB) stars provides measurements of their fundamental parameters with a very good precision; in particular, the masses and radii deter- mined from asteroseismology ... [more ▼]

Asteroseismic modeling of subdwarf B (sdB) stars provides measurements of their fundamental parameters with a very good precision; in particular, the masses and radii deter- mined 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. [less ▲]

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See detailg-Mode Oscillations in Hot B Subdwarf Stars
Charpinet, Stéphane; Van Grootel, Valérie ULg; Fontaine, Gilles et al

in Astronomical Society of the Pacific Conference Series (2013, December), 479

Observations from space with CoRoT and Kepler have revealed the rich pulsation spectra that characterize the long period hot B subdwarf (sdB) pulsators of the V1093 Her type. These evolved and compact ... [more ▼]

Observations from space with CoRoT and Kepler have revealed the rich pulsation spectra that characterize the long period hot B subdwarf (sdB) pulsators of the V1093 Her type. These evolved and compact helium core burning stars can develop low amplitude oscillations with periods in the range ∼ 1 − 4 hours that correspond to low degree, mid to high order g-modes. These modes have the property to penetrate deep inside the star, down to the boundary of the central convective core. They are consequently particularly interesting as asteroseismic probes. Here we briefly review how the g-modes behave in sdB stars, focusing in particular on the seismic signatures coming from the core itself. We also briefly summarize the first seismic results obtained from space data in pioneering efforts to exploit the potential of long period sdB pulsators as deep probes of the core structure. [less ▲]

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See detailReaching the 1% accuracy level on stellar mass and radius determinations from asteroseismology. The case of hot B subdwarfs
Van Grootel, Valérie ULg; Charpinet, Stephane; Fontaine, Gilles et al

Conference (2013, August)

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See detailReaching the 1% accuracy level on stellar mass and radius determinations from asteroseismology. The case of hot B subdwarfs
Van Grootel, Valérie ULg; Charpinet, Stéphane; Fontaine, Gilles et al

Conference (2013, July)

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See detailModeling the hot subdwarf PB 8783 by asteroseismology
Van Grootel, Valérie ULg; Charpinet, Stéphane; Fontaine, Gilles et al

Conference (2013, May)

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See detailThird generation stellar models for asteroseismology of hot B subdwarf stars. A test of accuracy with the pulsating eclipsing binary PG 1336–018
Van Grootel, Valérie ULg; Charpinet, Stéphane; Brassard, Pierre et al

in Astronomy and Astrophysics (2013), 553

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 ... [more ▼]

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- and g-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 compare 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⊙ , and log g = 5.775 ± 0.007, while orbitology leads to M∗ = 0.466 ± 0.006 M⊙ , R∗ = 0.15 ± 0.01 R⊙ , log g = 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⊙, and log g = 5.739±0.002. We also show that the uncertainties on the input physics included in stellar models have no noticeable impact, at the current level of accuracy, 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. [less ▲]

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See detailWhat asteroseismology can teach us about stellar evolution: the case of subdwarf B stars
Van Grootel, Valérie ULg; Fontaine, Gilles; Charpinet, Stephane et al

Conference (2013, April)

Subdwarfs B (sdB) stars are hot (Teff=20,000-40,000 K) and compact (log g= 5.0-6.2) evolved objects that form the very hot end of the horizontal branch, the so-called Extreme Horizontal Branch (EHB ... [more ▼]

Subdwarfs B (sdB) stars are hot (Teff=20,000-40,000 K) and compact (log g= 5.0-6.2) 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. Competing scenarios have been proposed to account for the existence of such evolved objects, and give quite different mass distributions for 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 or reflection binaries. I will present in the talk the empirical mass distributions of sdB stars on the basis of these samples. I will discuss how these empirical mass distributions, although still based on small-number statistics, compare 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 the extreme mass loss of red giants to form extreme horizontal branch stars, possibly in connection with the recently discovered close planets orbiting sdB stars. [less ▲]

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See detailThe empirical mass distribution of hot B subdwarfs: implications for stellar evolution theory
Van Grootel, Valérie ULg; Fontaine, Gilles; Charpinet, Stéphane et al

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

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 ... [more ▼]

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 evo- lution 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 astero- seismic 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 iso- lated 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. [less ▲]

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See detailAsteroseismology of hot B subdwarf stars
Charpinet, Stéphane; Van Grootel, Valérie ULg; Brassard, Pierre et al

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 ▲]

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See detailA preliminary look at the empirical mass distribution of hot B subdwarf stars
Fontaine, Gilles; Brassard, Pierre; Charpinet, Stéphane et al

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 ▲]

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See detailA compact system of small planets around a former red giant star
Charpinet, Stéphane; Fontaine, Gilles; Brassard, Pierre et al

in Nature (2011), 480

Planets that orbit their parent star at less than about one astronomical unit (1AU is the Earth-Sun distance) are expected to be engulfed when the star becomes a red giant. Previous observations have ... [more ▼]

Planets that orbit their parent star at less than about one astronomical unit (1AU is the Earth-Sun distance) are expected to be engulfed when the star becomes a red giant. Previous observations have revealed the existence of post-red-giant host stars with giant planets orbiting as close as 0.116AU or with brown dwarf companions in tight orbits, showing that these bodies can survive engulfment. What has remained unclear is whether planets can be dragged deeper into the red-giant envelope without being disrupted and whether the evolution of the parent star itself could be affected. Here we report the presence of two nearly Earth-sized bodies orbiting the post-red-giant, hot B subdwarf star KIC 05807616 at distances of 0.0060 and 0.0076AU, with orbital periods of 5.7625 and 8.2293 hours, respectively. These bodies probably survived deep immersion in the former red-giant envelope. They may be the dense cores of evaporated giant planets that were transported closer to the star during the engulfment and triggered the mass loss necessary for the formation of the hot B subdwarf, which might also explain how some stars of this type did not form in binary systems. [less ▲]

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See detailThe empirical mass distributions of hot B subdwarfs derived by asteroseismology
Van Grootel, Valérie ULg; Fontaine, Gilles; Brassard, Pierre et al

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 ▲]

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See detailProbing the cores of Extreme Horizontal Branch stars by gravity-mode seismology
Van Grootel, Valérie ULg; Charpinet, Stéphane; Fontaine, Gilles et al

Conference (2011, June)

We present a seismic analysis of the pulsating subdwarf B star KPD 0629-0016 on the basis of the long-period, gravity-mode pulsations uncovered by CoRoT. Thanks to space- based facilities, g-mode ... [more ▼]

We present a seismic analysis of the pulsating subdwarf B star KPD 0629-0016 on the basis of the long-period, gravity-mode pulsations uncovered by CoRoT. Thanks to space- based facilities, g-mode seismology can now be exploited quantitatively for stars on the ex- treme horizontal branch, an objective undermined so far by the limitations of ground-based observations. The optimal seismic model offers an excellent fit, with a relative dispersion of 0.23%, to the 18 observed periods identified with theoretical modes of degrees l =1 and 2. The inferred structural parameters for KPD 0629–0016 include the total stellar mass, the thickness of the H-rich envelope, and, thanks to the sensitivity of g-modes, the size and the composition of the convective core. Our results suggest that extra mixing processes shape the helium-burning cores, that are representative of all horizontal branch stars in general, an intermediate and underrated stage of stellar evolution. [less ▲]

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See detailDeep asteroseismic sounding of the compact hot B subdwarf pulsator KIC02697388 from Kepler time series photometry
Charpinet, Stéphane; Van Grootel, Valérie ULg; Fontaine, Gilles et al

in Astronomy and Astrophysics (2011), 530

Context. Contemporary high precision photometry from space provided by the Kepler and CoRoT satellites generates significant breakthroughs in terms of exploiting the long-period, g-mode pulsating hot B ... [more ▼]

Context. Contemporary high precision photometry from space provided by the Kepler and CoRoT satellites generates significant breakthroughs in terms of exploiting the long-period, g-mode pulsating hot B subdwarf (sdBVs) stars with asteroseismology. Aims: We present a detailed asteroseismic study of the sdBVs star KIC02697388 monitored with Kepler, using the rich pulsation spectrum uncovered during the ~27-day-long exploratory run Q2.3. Methods: We analyse new high-S/N spectroscopy of KIC02697388 using appropriate NLTE model atmospheres to provide accurate atmospheric parameters for this star. We also reanalyse the Kepler light curve using standard prewhitening techniques. On this basis, we apply a forward modelling technique using our latest generation of sdB models. The simultaneous match of the independent periods observed in KIC02697388 with those of models leads objectively to the identification of the pulsation modes and, more importantly, to the determination of some of the parameters of the star. Results: The light curve analysis reveals 43 independent frequencies that can be associated with oscillation modes. All the modulations observed in this star correspond to g-mode pulsations except one high-frequency signal, which is typical of a p-mode oscillation. Although the presence of this p-mode is surprising considering the atmospheric parameters that we derive for this cool sdB star (Teff = 25 395 ± 227 K, log g = 5.500 ± 0.031 (cgs), and log N(He) /N(H) = -2.767 ± 0.122), we show that this mode can be accounted for particularly well by our optimal seismic models, both in terms of frequency match and nonadiabatic properties. The seismic analysis leads us to identify two model solutions that can both account for the observed pulsation properties of KIC02697388. Despite this remaining ambiguity, several key parameters of the star can be derived with stringent constraints, such as its mass, its H-rich envelope mass, its radius, and its luminosity. We derive the properties of the core proposing that it is a relatively young sdB star that has burnt less than ~34% (in mass) of its central helium and has a relatively large mixed He/C/O core. This latter measurement is in line with the trend already uncovered for two other g-mode sdB pulsators analysed with asteroseismology and suggests that extra mixing is occurring quite early in the evolution of He cores on the horizontal branch. Conclusions: Additional monitoring with Kepler of this particularly interesting sdB star should reveal the inner properties of KIC02697388 and provide important information about the mode driving mechanism and the helium core properties. [less ▲]

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See detailSounding the cores of stars by gravity-mode asteroseismology
Van Grootel, Valérie ULg; Charpinet, Stéphane; Fontaine, Gilles et al

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 ▲]

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