Publications of Valérie Van Grootel     Results 1-20 of 112. 1 2 3 4 5 6   Basic Principles of White Dwarf AsteroseismologyFontaine, Gilles; Brassard, Pierre; Charpinet, Stéphane et alin Napiwotzki, R.; Burleigh, M. (Eds.) The White Dwarfs Stars (in press)Detailed reference viewed: 55 (7 ULg) A combined transmission spectrum of the Earth-sized exoplanets TRAPPIST-1 b and cde Wit, Julien; Wakeford, Hannah R.; Gillon, Michaël et alin Nature (in press)Three Earth-sized exoplanets were recently discovered close to the habitable zone of the nearby ultracool dwarf star TRAPPIST-1. The nature of these planets has yet to be determined, since their masses ... [more ▼]Three Earth-sized exoplanets were recently discovered close to the habitable zone of the nearby ultracool dwarf star TRAPPIST-1. The nature of these planets has yet to be determined, since 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 a space-based measurement of the combined transmission spectrum of the two inner planets made possible by a favorable alignment resulting in their simultaneous transits on 04 May 2016. The lack of features in the combined spectrum rules out cloud-free hydrogen-dominated atmospheres for each planet at 10-$\sigma$ levels; TRAPPIST-1 b and c are hence unlikely to harbor an extended gas envelope as they lie in a region of parameter space where 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 atmosphere. [less ▲]Detailed reference viewed: 31 (8 ULg) WASP-121 b: a hot Jupiter close to tidal disruption transiting an active F starDelrez, Laetitia ; Santerne, A.; Almenara, J.-M. et alin Monthly Notices of the Royal Astronomical Society (2016), 458(4), 4025-4043We 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 ... [more ▼]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. [less ▲]Detailed reference viewed: 26 (2 ULg) Pulsating hot O subdwarfs in ω Centauri: mapping a unique instability strip on the extreme horizontal branchRandall, S. K.; Calamida, A.; Fontaine, G. et alin Astronomy and Astrophysics (2016), 589We 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 ... [more ▼]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 http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/589/A1 [less ▲]Detailed reference viewed: 13 (2 ULg) Temperate Earth-sized planets transiting a nearby ultracool dwarf starGillon, Michaël ; Jehin, Emmanuel ; Lederer, Susan M. et alin Nature (2016), 533Star-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 ... [more ▼]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. [less ▲]Detailed reference viewed: 35 (8 ULg) Amplitude and frequency variations of oscillation modes in the pulsating DB white dwarf star KIC~08626021. The likely signature of nonlinear resonant mode couplingZong, W.; Charpinet, S.; Vauclair, G. et alin Astronomy and Astrophysics (2016), 585Context. 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 ... [more ▼]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. [less ▲]Detailed reference viewed: 33 (3 ULg) HD 97658 and its super-EarthVan Grootel, Valérie ; Gillon, Michaël ; Valencia, D. et alin 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 ... [more ▼]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. [less ▲]Detailed reference viewed: 20 (5 ULg) Pulsations in hot subdwarf stars: recent advances and prospects for testing stellar physicsCharpinet, Stéphane; Van Grootel, Valérie ; Fontaine, Gilles et alConference (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 ... [more ▼]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. [less ▲]Detailed reference viewed: 10 (0 ULg) Close-in planets around sdB stars: A step toward constraining their masses ?Charpinet, S.; Grandjean, A. H. M. J.; Fontaine, G. et alPoster (2015, July)Detailed reference viewed: 9 (0 ULg) Driving Pulsation Modes in Models of the Two Pre-ELM Helium-Core White Dwarfs WASP 0247-25B and WASP 1628+10BVan Grootel, Valérie ; Fontaine, G.; Brassard, P. et alPoster (2015, July)Multi-periodic pulsations have recently been detected in two pre-extremely low mass (pre- ELM) helium-core white dwarfs as reported in Maxted et al. (2013, Nature, 498, 463) and Maxted et al. (2014, MNRAS ... [more ▼]Multi-periodic pulsations have recently been detected in two pre-extremely low mass (pre- ELM) helium-core white dwarfs as reported in Maxted et al. (2013, Nature, 498, 463) and Maxted et al. (2014, MNRAS, 444, 208). As such, they define a new class of pulsating stars in the HR diagram. Both objects are the secondary components in eclipsing close binaries, with solar-type main sequence stars as primaries. They are believed to be the remnants of former red giant stars stripped down of most of their mass through an active phase of binary evolution. The first of those is WASP 0247-25B, characterized by M = 0.186 ± 0.002 Mʘ, log g = 4.576 ± 0.011, and Teff = 11,380 ± 400 K (Maxted et al. 2013). Three pulsation modes have been detected so far, with periods of 381 s, 406 s, and 421 s. The second one, showing two modes with 669 s and 755 s, is WASP 1628+10B, with M = 0.135 ± 0.020 Mʘ, log g = 4.49 ± 0.05, and Teff = 9200 ± 600 K (Maxted et al. 2014). Adiabatic calculations using suitable evolutionary models have been presented in these publications, showing that the pulsation periods correspond to low-degree, mid-order p-mode oscillations (probably including radial modes). The question of the driving mechanism, however, has been left open, and we address that issue here. [less ▲]Detailed reference viewed: 13 (0 ULg) The Period-Effective Temperature Relation for DBV White DwarfsFontaine, G.; Van Grootel, Valérie ; Bergeron, P. et alPoster (2015, July)Detailed reference viewed: 6 (0 ULg) New asteroseismic analysis of the subdwarf B pulsator PG 1219+534Péters, M.-J.; Van Grootel, Valérie ; Dupret, Marc-Antoine et alPoster (2015, July)Detailed reference viewed: 13 (0 ULg) A First Look at the Nonadiabatic Properties of Pulsating Accreting White Dwarfs of the GW Lib TypeVan Grootel, Valérie ; Fontaine, G.; Brassard, P. et alin Astronomical Society of the Pacific Conference Series (2015, June), 493We present results of a detailed stability survey of the pulsation properties of accreting white dwarfs of the GW Lib type. This is based on several state-of-the- art white dwarf evolutionary sequences ... [more ▼]We present results of a detailed stability survey of the pulsation properties of accreting white dwarfs of the GW Lib type. This is based on several state-of-the- art white dwarf evolutionary sequences with varying envelope compositions, from pure hydrogen to pure helium. Using the same tools as in Van Grootel et al. (2013), where we have presented the first consistent view of the ZZ Ceti instability strip, we have mapped the GW Lib instability strip over the effective temperature-surface gravity plane, and as a function of envelope composition. We find that the location of the GW Lib instability domain is a strong and continuous function of the assumed envelope composition. We can accomodate all of the known GW Lib pulsators in various strips according to their atmospheric compositions. [less ▲]Detailed reference viewed: 18 (1 ULg) The Instability Strip of ZZ Ceti White DwarfsVan Grootel, Valérie ; Fontaine, G.; Brassard, P. et alin Astronomical Society of the Pacific Conference Series (2015, June), 493The 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 ... [more ▼]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. We report here a detailed stability survey over the whole ZZ Ceti regime, including the low and extremely low masses. We computed to this aim 29 evolutionary sequences of DA models with various masses, chemical layering, and core compositions. 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 computed pulsation spectra for these models with the Liège nonadiabatic pulsation code MAD, which is the only one to conveniently incorporate a full time-dependent convection treatment and, thus, provides the best available description of the blue edge of the instability strip. On the other hand, given the failure of all nonadiabatic codes to account properly for the red edge of the strip, including MAD, we tested the idea that the red edge is due to energy leakage through the atmosphere. Using this approach, we found that our theoretical ZZ Ceti instability strip accounts remarkably well for the boundaries of the empirical strip. [less ▲]Detailed reference viewed: 15 (1 ULg) Method and Tools for an Objective Approach of White Dwarf AsteroseismologyCharpinet, S.; Giammichele, N.; Brassard, P. et alin Astronomical Society of the Pacific Conference Series (2015, June), 493Stellar oscillation modes propagate deep inside stars and carry on information potentially allowing us to reconstruct, through asteroseismology, parts of their internal structure and dynamics. We present ... [more ▼]Stellar oscillation modes propagate deep inside stars and carry on information potentially allowing us to reconstruct, through asteroseismology, parts of their internal structure and dynamics. We present and discuss here our strategy and tools developed to exploit objectively this potential for pulsating white dwarf stars. Our approach relies on a multimodal optimization technique aimed at identifying all potential seismic model solutions that best match the observed periods of a given star. This approach doesn't rely on model grids (hence avoiding their limitations) and makes use of static stellar structures of white dwarf stars as opposed to evolutionary models. The use of parameterized static models is a deliberate choice in this context and is aimed at approaching as closely as possible the real structure of the star as it is observed and probed by the detected oscillation modes without interferences from unavoidable model uncertainties propagated through billion years of evolution. We argue that this strategy is more likely to produce new tight constraints on the internal structure of white dwarf stars from which evolutionary models could be improved. [less ▲]Detailed reference viewed: 8 (0 ULg) High-speed Photometric Observations of ZZ Ceti White Dwarf CandidatesGreen, E.M.; Limoges, M.-M.; Gianninas, A. et alin Astronomical Society of the Pacific Conference Series (2015, June), 493We present high-speed photometric observations of ZZ Ceti white dwarf candidates drawn from the spectroscopic survey of bright DA stars from the Villanova White Dwarf Catalog by Gianninas et al., and from ... [more ▼]We present high-speed photometric observations of ZZ Ceti white dwarf candidates drawn from the spectroscopic survey of bright DA stars from the Villanova White Dwarf Catalog by Gianninas et al., and from the recent spectroscopic survey of white dwarfs within 40 parsecs of the Sun by Limoges et al. We report the discovery of six new ZZ Ceti pulsators from these surveys, and several photometrically constant DA white dwarfs, which we then use to refine the location of the ZZ Ceti instability strip. [less ▲]Detailed reference viewed: 7 (0 ULg) The CHEOPS mission: towards exoplanet characterisationVan Grootel, Valérie Scientific conference (2015, March)Detailed reference viewed: 10 (0 ULg) A connection between the instability strips of ZZ Ceti and V777 Her white dwarfsVan Grootel, Valérie ; Fontaine, Gilles; Brassard, Pierre et alin Astronomy and Astrophysics (2015), 575Aims. We aim to determine the theoretical instability strips of white dwarfs with diverse H and He content in their atmospheres, from a solar composition to a H-depleted atmosphere. Pulsators with mixed H ... [more ▼]Aims. We aim to determine the theoretical instability strips of white dwarfs with diverse H and He content in their atmospheres, from a solar composition to a H-depleted atmosphere. Pulsators with mixed H-He atmospheres are indeed known to exist, and these are the white dwarfs in cataclysmic accreting systems of the GW Lib type. We thus also aim to determine the range of periods of excited pulsation modes, and to qualitatively compare these to the observed periods in GW Lib white dwarf pulsators. Methods. In the first full nonadiabatic stability analysis of pulsators of this kind, we applied a time-dependent convection treatment and an energy leakage argument to compute, for cooling models of white dwarfs with various masses and envelope compositions, the location of the blue and the red edges, as well as the properties of pulsation modes. Results. We find that our derived instability strips form a true continuum in the log g-Teff plane and that their individual location depends uniquely on the assumed atmospheric composition, from the solar composition models at low effective temperatures to the H-depleted models at much higher temperatures. Taking into account our previous results from the ZZ Ceti (pure H atmosphere) and V777 Her (pure He atmosphere) white dwarf pulsators, this implies that all of these instability domains are connected via the same fundamental driving mechanism. Applying our results to the case of white dwarf pulsators of the GW Lib type, we find that our theoretical instability strips can qualitatively account for all of the known cases. The computed range of periods of excited modes also compares qualitatively very well to the observed ones. Conclusions. The GW Lib pulsators are very similar in nature to ZZ Ceti and V777 Her white dwarfs. It is the diverse chemical compositions in their atmosphere and envelope that defines their specific pulsation properties. Beyond GW Lib pulsators, white dwarfs can sometimes exhibit mixed H-He atmospheres, such as in the recently found proto-He white dwarf pulsators. Our results open the way towards quantitative asteroseismology of these various kinds of white dwarfs. [less ▲]Detailed reference viewed: 22 (1 ULg) The soundtrack of RR Lyrae in Omega Cen at high-frequencyCalamida, A.S.; Randall, S.K.; Monelli, M. et alin Memorie della Società Astronomica Italiana : Journal of the Italian Astronomical Society (2015), 86Detailed reference viewed: 67 (1 ULg) Spectral Analysis and Abundances of the Post-HB Star HD 76431Khalack, V.; Yameogo, B.; LeBlanc, F. et alin Monthly Notices of the Royal Astronomical Society (2014), 445HD76431 is a slow rotating post-HB star that shows an underabundance of helium by 0.5 dex relative to the solar value. These observational facts suggest that atomic diffusion could be active in its ... [more ▼]HD76431 is a slow rotating post-HB star that shows an underabundance of helium by 0.5 dex relative to the solar value. These observational facts suggest that atomic diffusion could be active in its atmosphere. We have used the MMT and Bok spectra to estimate the atmospheric parameters of the target star using the model atmospheres and synthetic spectra calculated with TLUSTY and SYNSPEC. The derived values of the effective temperature, surface gravity, helium abundance are consistent with those obtained by Ramspeck et al. (2001b). It appears that NLTE effect are not important for HD76431. We have used Stokes I spectra from ESPaDOnS at CFHT to perform an abundance analysis and a search for observational evidence of vertical stratification of the abundance of certain elements. The results of our abundance analysis are in good agreement with previously published data with respect to average abundances. Our numerical simulations show that carbon and nitrogen reveal signatures of vertical abundance stratification in the atmosphere of HD76431. It appears that the carbon abundance increases toward the deeper atmospheric layers. Nitrogen also shows a similar behaviour, but in deeper atmospheric layers we obtain a significant dispersion for the estimates of its abundance. To our knowledge, this is the first demonstration of vertical abundance stratification of metals in a post-HB star and up to now it is the hottest star to show such stratification features. We also report the detection of two SiIII and one TiIII emission lines in the spectra of HD76431 that were not detected in previous studies. [less ▲]Detailed reference viewed: 32 (1 ULg)