Références de "Van Grootel, Valérie"
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Voir détailBasic Principles of White Dwarf Asteroseismology
Fontaine, Gilles; Brassard, Pierre; Charpinet, Stéphane et al

in Napiwotzki, R.; Burleigh, M. (Eds.) The White Dwarfs Stars (in press)

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Voir détailA seven-planet resonant chain in TRAPPIST-1
Luger, Rodrigo; Sestovic, Marko; Kruse, Ethan et al

in Nature Astronomy (2017), 1

The TRAPPIST-1 system is the first transiting planet system found orbiting an ultracool dwarf star[SUP] 1 [/SUP]. At least seven planets similar in radius to Earth were previously found to transit this ... [plus ▼]

The TRAPPIST-1 system is the first transiting planet system found orbiting an ultracool dwarf star[SUP] 1 [/SUP]. At least seven planets similar in radius to Earth were previously found to transit this host star[SUP] 2 [/SUP]. Subsequently, TRAPPIST-1 was observed as part of the K2 mission and, with these new data, we report the measurement of an 18.77 day orbital period for the outermost transiting planet, TRAPPIST-1 h, which was previously unconstrained. This value matches our theoretical expectations based on Laplace relations[SUP] 3 [/SUP] and places TRAPPIST-1 h as the seventh member of a complex chain, with three-body resonances linking every member. We find that TRAPPIST-1 h has a radius of 0.752 R [SUB]⊕[/SUB] and an equilibrium temperature of 173 K. We have also measured the rotational period of the star to be 3.3 days and detected a number of flares consistent with a low-activity, middle-aged, late M dwarf. [moins ▲]

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Voir détailTwo massive rocky planets transiting a K-dwarf 6.5 parsecs away
Gillon, Michaël ULg; Demory, Brice-Olivier; Van Grootel, Valérie ULg et al

in Nature Astronomy (2017), 1

HD 219134 is a K-dwarf star at a distance of 6.5 parsecs around which several low-mass planets were recently discovered[SUP]1,2[/SUP]. The Spitzer Space Telescope detected a transit of the innermost of ... [plus ▼]

HD 219134 is a K-dwarf star at a distance of 6.5 parsecs around which several low-mass planets were recently discovered[SUP]1,2[/SUP]. The Spitzer Space Telescope detected a transit of the innermost of these planets, HD 219134 b, whose mass and radius (4.5 M[SUB]⊕[/SUB] and 1.6 R[SUB]⊕[/SUB] respectively) are consistent with a rocky composition[SUP]1[/SUP]. Here, we report new high-precision time-series photometry of the star acquired with Spitzer revealing that the second innermost planet of the system, HD 219134c, is also transiting. A global analysis of the Spitzer transit light curves and the most up-to-date HARPS-N velocity data set yields mass and radius estimations of 4.74 ± 0.19 M[SUB]⊕[/SUB] and 1.602 ± 0.055 R[SUB]⊕[/SUB] for HD 219134 b, and of 4.36 ± 0.22 M[SUB]⊕[/SUB] and 1.511 ± 0.047 R[SUB]⊕[/SUB] for HD 219134 c. These values suggest rocky compositions for both planets. Thanks to the proximity and the small size of their host star (0.778 ± 0.005 R[SUB]⊙[/SUB])[SUP]3[/SUP], these two transiting exoplanets — the nearest to the Earth yet found — are well suited for a detailed characterization (for example, precision of a few per cent on mass and radius, and constraints on the atmospheric properties) that could give important constraints on the nature and formation mechanism of the ubiquitous short-period planets of a few Earth masses. [moins ▲]

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Voir détailSeven temperate terrestrial planets around the nearby ultracool dwarf star
Gillon, Michaël ULg; Triaud, Amaury; Demory, Brice-Olivier et al

in Nature (2017), 542

One focus of modern astronomy is to detect temperate terrestrial exoplanets well-suited for atmospheric characterisation. A milestone was recently achieved with the detection of three Earth-sized planets ... [plus ▼]

One focus of modern astronomy is to detect temperate terrestrial exoplanets well-suited for atmospheric characterisation. A milestone was recently achieved with the detection of three Earth-sized planets transiting (i.e. passing in front of) a star just 8% the mass of the Sun 12 parsecs away. Indeed, the transiting configuration of these planets combined with the Jupiter-like size of their host star - named TRAPPIST-1 - makes possible indepth studies of their atmospheric properties with current and future astronomical facilities. Here we report the results of an intensive photometric monitoring campaign of that star from the ground and with the Spitzer Space Telescope. Our observations reveal that at least seven planets with sizes and masses similar to the Earth revolve around TRAPPIST-1. The six inner planets form a near-resonant chain such that their orbital periods (1.51, 2.42, 4.04, 6.06, 9.21, 12.35 days) are near ratios of small integers. This architecture suggests that the planets formed farther from the star and migrated inward. The seven planets have equilibrium temperatures low enough to make possible liquid water on their surfaces. [moins ▲]

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Voir détailMaking Sense Out of Pulsating Pre-ELM and ELM White Dwarfs
Fontaine, Gilles; Istrate, Alina; Gianninas, Alexandros et al

in Astronomical Society of the Pacific Conference Series (2017), 509

We present a unified view of pulsations in both pre-ELM and ELM white dwarfs within the framework of state-of-the-art binary evolution calculations that take into account the combined effects of diffusion ... [plus ▼]

We present a unified view of pulsations in both pre-ELM and ELM white dwarfs within the framework of state-of-the-art binary evolution calculations that take into account the combined effects of diffusion and rotational mixing. We find that rotational mixing is able to maintain against settling a sufficient amount of helium in the envelope in order to fuel pulsations through He II-He III ionization on the pre-ELM branch of the evolutionary track in the spectroscopic HR diagram. By the time such a low-mass white dwarf enters the ZZ Ceti instability strip on the cooling branch, settling has taken over rotational mixing and produced a pure H envelope. Such a star then pulsates again, but, this time, as a DA white dwarf of the ZZ Ceti type. [moins ▲]

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Voir détailThe theoretical instability strip of V777 Her white dwarfs
Van Grootel, Valérie ULg; Fontaine, Gilles; Brassard, Pierre et al

in Astronomical Society of the Pacific Conference Series (2017), 509

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Voir détailDiscovery of temperate Earth-sized planets transiting a nearby ultracool dwarf star
Jehin, Emmanuel ULg; Gillon, Michaël ULg; Lederer, Susan M. et al

in AAS/Division for Planetary Sciences Meeting Abstracts (2016, October 01)

We report the discovery of three short-period Earth-sized planets transiting a nearby ultracool dwarf star using data collected by the Liège TRAPPIST telescope, located in la Silla (Chile). TRAPPIST-1 is ... [plus ▼]

We report the discovery of three short-period Earth-sized planets transiting a nearby ultracool dwarf star using data collected by the Liège TRAPPIST telescope, located in la Silla (Chile). TRAPPIST-1 is an isolated M8.0±0.5-type dwarf star at a distance of 12.0±0.4 parsecs as measured by its trigonometric parallax, with an age constrained to be > 500 Myr, and with a luminosity, mass, and radius of 0.05%, 8% and 11.5% those of the Sun, respectively. The small size of the host star, only slightly larger than Jupiter, translates into Earth-like radii for the three discovered planets, as deduced from their transit depths. The inner two planets receive four and two times the irradiation of Earth, respectively, placing them close to the inner edge of the habitable zone of the star. Several orbits remain possible for the third planet based on our current data. The infrared brightness of the host star combined with its Jupiter-like size offer the possibility of thoroughly characterizing the components of this nearby planetary system. [moins ▲]

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Voir détailA combined transmission spectrum of the Earth-sized exoplanets TRAPPIST-1 b and c
de Wit, Julien; Wakeford, Hannah R.; Gillon, Michaël ULg et al

in Nature (2016), 537

Three Earth-sized exoplanets were recently discovered close to the habitable zone of the nearby ultracool dwarf star TRAPPIST-1 (ref. 3). The nature of these planets has yet to be determined, as their ... [plus ▼]

Three Earth-sized exoplanets were recently discovered close to the habitable zone of the nearby ultracool dwarf star TRAPPIST-1 (ref. 3). The nature of these planets has yet to be determined, as their masses remain unmeasured and no observational constraint is available for the planetary population surrounding ultracool dwarfs, of which the TRAPPIST-1 planets are the first transiting example. Theoretical predictions span the entire atmospheric range, from depleted to extended hydrogen-dominated atmospheres. Here we report observations of the combined transmission spectrum of the two inner planets during their simultaneous transits on 4 May 2016. The lack of features in the combined spectrum rules out cloud-free hydrogen-dominated atmospheres for each planet at ≥10σ levels; TRAPPIST-1 b and c are therefore unlikely to have an extended gas envelope as they occupy a region of parameter space in which high-altitude cloud/haze formation is not expected to be significant for hydrogen-dominated atmospheres. Many denser atmospheres remain consistent with the featureless transmission spectrum—from a cloud-free water-vapour atmosphere to a Venus-like one. [moins ▲]

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Voir détailThe theoretical instability strip of V777 Her white dwarfs
Van Grootel, Valérie ULg; Fontaine, Gilles; Brassard, Pierre et al

Communication orale (2016, July)

We present a new theoretical investigation of the instability strip of V777 Her (DBV) white dwarfs. We apply a time-dependent convection (TDC) treatment to cooling models of DB and DBA white dwarfs. Using ... [plus ▼]

We present a new theoretical investigation of the instability strip of V777 Her (DBV) white dwarfs. We apply a time-dependent convection (TDC) treatment to cooling models of DB and DBA white dwarfs. Using the spectroscopic calibration for the convective efficiency, ML2/alpha=1.25, we find a wide strip covering the range of effective temperature from 30,000 K down to about 22,000 K at log g = 8.0. This accounts very well for the empirical instability strip derived from a new accurate and homogenous spectroscopic analysis of known pulsators. Our approach leads to an exact description of the blue edge and to a correct understanding of the onset and development of pulsational instabilities, similarly to our results of TDC applied to ZZ Ceti white dwarfs in the recent past. We propose that, contrarily to what is generally believed, there is practically no fuzziness on the boundaries of the V777 Her instability strip due to traces of hydrogen in the atmospheres of some of these helium-dominated-atmosphere stars. Contrary to the blue edge, the red edge provided by TDC computations is far too cool compared to the empirical one. A similar situation was observed for the ZZ Ceti stars as well. We hence test the energy leakage argument (i.e., the red edge occurs when the thermal timescale in the driving region becomes equal to the critical period beyond which gravity modes cease to exist), which was successful to correctly reproduce the red edge of ZZ Ceti white dwarfs. Based on this argument, the red edge is qualitatively well reproduced as indicated above. However, upon close inspection, it may be about 1000 K too cool compared to the empirical one, although the latter relies on a few objects only. We also test the hypothesis of including turbulent pressure in our TDC computations in order to provide an alternate physical mechanism to account for the red edge. First promising results are presented. [moins ▲]

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Voir détailPulsating Hot Subdwarfs in Omega Centauri
Randall, S. K.; Calamida, A.; Fontaine, G. et al

in The Messenger (2016), 164

We recently discovered the first globular cluster hot subdwarf pulsators in Omega Centauri (ω Cen). These stars were initially thought to belong to the class of rapidly pulsating subdwarf B stars, which ... [plus ▼]

We recently discovered the first globular cluster hot subdwarf pulsators in Omega Centauri (ω Cen). These stars were initially thought to belong to the class of rapidly pulsating subdwarf B stars, which are well established among the field star population and have become showcases for asteroseismology. However, our spectroscopic analysis revealed the ω Cen variables to be significantly hotter than expected, indicating that they form a new class of subdwarf O pulsators clustered around 50 000 K, not known among the field star population. Non-adiabatic pulsation modelling suggests that the driver for the pulsations occurs via the same iron opacity mechanism that is at work in the rapidly pulsating subdwarf B stars. [moins ▲]

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Voir détailWASP-121 b: a hot Jupiter close to tidal disruption transiting an active F star
Delrez, Laetitia ULg; Santerne, A.; Almenara, J.-M. et al

in Monthly Notices of the Royal Astronomical Society (2016), 458(4), 4025-4043

We present the discovery by the WASP-South survey of WASP-121 b, a new remarkable short-period transiting hot Jupiter. The planet has a mass of 1.183_{-0.062}^{+0.064} MJup, a radius of 1.865 ± 0.044 RJup ... [plus ▼]

We present the discovery by the WASP-South survey of WASP-121 b, a new remarkable short-period transiting hot Jupiter. The planet has a mass of 1.183_{-0.062}^{+0.064} MJup, a radius of 1.865 ± 0.044 RJup, and transits every 1.274 9255_{-0.000 0025}^{+0.000 0020} days an active F6-type main-sequence star (V = 10.4, 1.353_{-0.079}^{+0.080} M⊙, 1.458 ± 0.030 R⊙, Teff = 6460 ± 140 K). A notable property of WASP-121 b is that its orbital semimajor axis is only ˜1.15 times larger than its Roche limit, which suggests that the planet is close to tidal disruption. Furthermore, its large size and extreme irradiation (˜7.1 109 erg s-1 cm-2) make it an excellent target for atmospheric studies via secondary eclipse observations. Using the TRAnsiting Planets and PlanetesImals Small Telescope, we indeed detect its emission in the z'-band at better than ˜4σ, the measured occultation depth being 603 ± 130 ppm. Finally, from a measurement of the Rossiter-McLaughlin effect with the CORALIE spectrograph, we infer a sky-projected spin-orbit angle of 257.8°_{-5.5°}^{+5.3°}. This result may suggest a significant misalignment between the spin axis of the host star and the orbital plane of the planet. If confirmed, this high misalignment would favour a migration of the planet involving strong dynamical events with a third body. [moins ▲]

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Voir détailPulsating hot O subdwarfs in ω Centauri: mapping a unique instability strip on the extreme horizontal branch
Randall, S. K.; Calamida, A.; Fontaine, G. et al

in Astronomy and Astrophysics (2016), 589

We present the results of an extensive survey for rapid pulsators among Extreme Horizontal Branch (EHB) stars in ω Cen. The observations performed consist of nearly 100 h of time-series photometry for ... [plus ▼]

We present the results of an extensive survey for rapid pulsators among Extreme Horizontal Branch (EHB) stars in ω Cen. The observations performed consist of nearly 100 h of time-series photometry for several off-centre fields of the cluster, as well as low-resolution spectroscopy for a partially overlapping sample. We obtained photometry for some 300 EHB stars, for around half of which we are able to recover light curves of sufficient quality to either detect or place meaningful non-detection limits for rapid pulsations. Based on the spectroscopy, we derive reliable values of log g, T[SUB]eff[/SUB] and log N(He) /N(H) for 38 targets, as well as good estimates of the effective temperature for another nine targets, whose spectra are slightly polluted by a close neighbour in the image. The survey uncovered a total of five rapid variables with multi-periodic oscillations between 85 and 125 s. Spectroscopically, they form a homogeneous group of hydrogen-rich subdwarf O stars clustered between 48 000 and 54 000 K. For each of the variables we are able to measure between two and three significant pulsations believed to constitute independent harmonic oscillations. However, the interpretation of the Fourier spectra is not straightforward due to significant fine structure attributed to strong amplitude variations. In addition to the rapid variables, we found an EHB star with an apparently periodic luminosity variation of ~2700 s, which we tentatively suggest may be caused by ellipsoidal variations in a close binary. Using the overlapping photometry and spectroscopy sample we are able to map an empirical ω Cen instability strip in log g - T[SUB]eff[/SUB] space. This can be directly compared to the pulsation driving predicted from the Montréal "second-generation" models regularly used to interpret the pulsations in hot B subdwarfs. Extending the parameter range of these models to higher temperatures, we find that the region where p-mode excitation occurs is in fact bifurcated, and the well-known instability strip between 29 000-36 000 K where the rapid subdwarf B pulsators are found is complemented by a second one above 50 000 K in the models. While significant challenges remain at the quantitative level, we believe that the same κ-mechanism that drives the pulsations in hot B subdwarfs is also responsible for the excitation of the rapid oscillations observed in the ω Cen variables. Intriguingly, the ω Cen variables appear to form a unique class. No direct counterparts have so far been found either in the Galactic field, nor in other globular clusters, despite dedicated searches. Conversely, our survey revealed no ω Cen representatives of the rapidly pulsating hot B subdwarfs found among the field population, though their presence cannot be excluded from the limited sample. Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere, Chile (proposal IDs 083.D-0833, 386.D-0669, 087.D-0216 and 091.D-0791).The reduced spectra are only available at the CDS via anonymous ftp to <A href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A> (ftp://130.79.128.5) or via <A href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/589/A1">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/589/A1</A> [moins ▲]

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Voir détailTemperate Earth-sized planets transiting a nearby ultracool dwarf star
Gillon, Michaël ULg; Jehin, Emmanuel ULg; Lederer, Susan M. et al

in Nature (2016), 533

Star-like objects with effective temperatures of less than 2,700 kelvin are referred to as ‘ultracool dwarfs’. This heterogeneous group includes stars of extremely low mass as well as brown dwarfs ... [plus ▼]

Star-like objects with effective temperatures of less than 2,700 kelvin are referred to as ‘ultracool dwarfs’. This heterogeneous group includes stars of extremely low mass as well as brown dwarfs (substellar objects not massive enough to sustain hydrogen fusion), and represents about 15 per cent of the population of astronomical objects near the Sun. Core-accretion theory predicts that, given the small masses of these ultracool dwarfs, and the small sizes of their protoplanetary disks, there should be a large but hitherto undetected population of terrestrial planets orbiting them—ranging from metal-rich Mercury-sized planets to more hospitable volatile-rich Earth-sized planets. Here we report observations of three short-period Earth-sized planets transiting an ultracool dwarf star only 12 parsecs away. The inner two planets receive four times and two times the irradiation of Earth, respectively, placing them close to the inner edge of the habitable zone of the star. Our data suggest that 11 orbits remain possible for the third planet, the most likely resulting in irradiation significantly less than that received by Earth. The infrared brightness of the host star, combined with its Jupiter-like size, offers the possibility of thoroughly characterizing the components of this nearby planetary system. [moins ▲]

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Voir détailAmplitude and frequency variations of oscillation modes in the pulsating DB white dwarf star KIC~08626021. The likely signature of nonlinear resonant mode coupling
Zong, W.; Charpinet, S.; Vauclair, G. et al

in Astronomy and Astrophysics (2016), 585

Context. The signatures of nonlinear effects affecting stellar oscillations are difficult to observe from ground observatories because of the lack of continuous high-precision photometric data spanning ... [plus ▼]

Context. The signatures of nonlinear effects affecting stellar oscillations are difficult to observe from ground observatories because of the lack of continuous high-precision photometric data spanning extended enough time baselines. The unprecedented photometric quality and coverage provided by the Kepler spacecraft offers new opportunities to search for these phenomena. Aims: We use the Kepler data accumulated on the pulsating DB white dwarf KIC 08626021 to explore in detail the stability of its oscillation modes, searching, in particular, for evidence of nonlinear behaviors. Methods: We analyze nearly two years of uninterrupted short-cadence data, concentrating on identified triplets that are caused by stellar rotation and that show intriguing behaviors during the course of the observations. Results: We find clear signatures of nonlinear effects that could be attributed to resonant mode coupling mechanisms. These couplings occur between the components of the triplets and can induce different types of behaviors. We first notice that a structure at 3681 μHz, identified as a triplet in previous published studies, is in fact forming a doublet, with the third component being an independent mode. We find that a triplet at 4310 μHz and this doublet at 3681 μHz (most likely the two visible components of an incomplete triplet) have clear periodic frequency and amplitude modulations, which are typical of the so-called intermediate regime of the resonance, with timescales consistent with theoretical expectations. Another triplet at 5073 μHz is likely in a narrow transitory regime in which the amplitudes are modulated while the frequencies are locked. Using nonadiabatic pulsation calculations, based on a model representative of KIC 08626021 to evaluate the linear growth rates of the modes in the triplets, we also provide quantitative information that could be useful for future comparisons with numerical solutions of the amplitude equations. Conclusions: The observed modulations are the clearest hints of nonlinear resonant couplings occurring in white dwarf stars identified so far. These should resonate as a warning to projects that aim at measuring the evolutionary cooling rate of KIC 08626021, and of white dwarf stars in general. Nonlinear modulations of the frequencies can potentially jeopardize any attempt to measure such rates reliably, unless they can be corrected beforehand. These results should motivate further theoretical work to develop the nonlinear stellar pulsation theory. [moins ▲]

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Voir détailPulsations in hot subdwarf stars: recent advances and prospects for testing stellar physics
Charpinet, Stéphane; Van Grootel, Valérie ULg; Fontaine, G. et al

in Proceedings of the International Astronomical Union (2016), 29

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 ... [plus ▼]

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. [moins ▲]

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Voir détailHD 97658 and its super-Earth
Van Grootel, Valérie ULg; Gillon, Michaël ULg; Valencia, D. et al

in European Physical Journal Web of Conferences (2015, September 01)

Super-Earths transiting nearby bright stars are key objects that simultaneously allow for accurate measurements of both their mass and radius, providing essential constraints on their internal composition ... [plus ▼]

Super-Earths transiting nearby bright stars are key objects that simultaneously allow for accurate measurements of both their mass and radius, providing essential constraints on their internal composition. We present the confirmation, based on Spitzer observations, that the super-Earth HD 97658 b transits its host star. HD 97658 is a low-mass (M*=0.77+-0.05 Msun) K1 dwarf, as determined from the Hipparcos parallax and stellar evolution modeling. To constrain the planet parameters, we carry out Bayesian global analyses of Keck-HIRES radial velocities, and MOST and Spitzer photometry. HD 97658 b is a massive (Mp=7.55 +0.83,-0.79 Mearth) and large (Rp = 2.247 +0.098,-0.095 Rearth at 4.5 microns) super-Earth. We investigate the possible internal compositions for HD 97658 b. Our results indicate a large rocky component, by at least 60% by mass, and very little H-He components, at most 2% by mass. We also discuss how future asteroseismic observations can improve the knowledge of the HD 97658 system, in particular by constraining its age. [moins ▲]

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Voir détailPulsations in hot subdwarf stars: recent advances and prospects for testing stellar physics
Charpinet, Stéphane; Van Grootel, Valérie ULg; Fontaine, Gilles et al

Communication orale (2015, August 01)

The evolved, core helium burning, extreme horizontal branch stars (also known as hot sudwarfs) host several classes of pulsators showing either p- or g-modes, or both. They offer particularly favorable ... [plus ▼]

The evolved, core helium burning, extreme horizontal branch stars (also known as hot sudwarfs) 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 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 review, I will present the most recent advances that have taken place in this field and I will provide hints of the foreseen future achievements of hot subdwarf asteroseismology. [moins ▲]

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Voir détailClose-in planets around sdB stars: A step toward constraining their masses ?
Charpinet, S.; Grandjean, A. H. M. J.; Fontaine, G. et al

Poster (2015, July)

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Voir détailThe Period-Effective Temperature Relation for DBV White Dwarfs
Fontaine, G.; Van Grootel, Valérie ULg; Bergeron, P. et al

Poster (2015, July)

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