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 References of "Van Grootel, Valérie"      in Complete repository Arts & humanities   Archaeology   Art & art history   Classical & oriental studies   History   Languages & linguistics   Literature   Performing arts   Philosophy & ethics   Religion & theology   Multidisciplinary, general & others Business & economic sciences   Accounting & auditing   Production, distribution & supply chain management   Finance   General management & organizational theory   Human resources management   Management information systems   Marketing   Strategy & innovation   Quantitative methods in economics & management   General economics & history of economic thought   International economics   Macroeconomics & monetary economics   Microeconomics   Economic systems & public economics   Social economics   Special economic topics (health, labor, transportation…)   Multidisciplinary, general & others Engineering, computing & technology   Aerospace & aeronautics engineering   Architecture   Chemical engineering   Civil engineering   Computer science   Electrical & electronics engineering   Energy   Geological, petroleum & mining engineering   Materials science & engineering   Mechanical engineering   Multidisciplinary, general & others Human health sciences   Alternative medicine   Anesthesia & intensive care   Cardiovascular & respiratory systems   Dentistry & oral medicine   Dermatology   Endocrinology, metabolism & nutrition   Forensic medicine   Gastroenterology & hepatology   General & internal medicine   Geriatrics   Hematology   Immunology & infectious disease   Laboratory medicine & medical technology   Neurology   Oncology   Ophthalmology   Orthopedics, rehabilitation & sports medicine   Otolaryngology   Pediatrics   Pharmacy, pharmacology & toxicology   Psychiatry   Public health, health care sciences & services   Radiology, nuclear medicine & imaging   Reproductive medicine (gynecology, andrology, obstetrics)   Rheumatology   Surgery   Urology & nephrology   Multidisciplinary, general & others Law, criminology & political science   Civil law   Criminal law & procedure   Criminology   Economic & commercial law   European & international law   Judicial law   Metalaw, Roman law, history of law & comparative law   Political science, public administration & international relations   Public law   Social law   Tax law   Multidisciplinary, general & others Life sciences   Agriculture & agronomy   Anatomy (cytology, histology, embryology...) & physiology   Animal production & animal husbandry   Aquatic sciences & oceanology   Biochemistry, biophysics & molecular biology   Biotechnology   Entomology & pest control   Environmental sciences & ecology   Food science   Genetics & genetic processes   Microbiology   Phytobiology (plant sciences, forestry, mycology...)   Veterinary medicine & animal health   Zoology   Multidisciplinary, general & others Physical, chemical, mathematical & earth Sciences   Chemistry   Earth sciences & physical geography   Mathematics   Physics   Space science, astronomy & astrophysics   Multidisciplinary, general & others Social & behavioral sciences, psychology   Animal psychology, ethology & psychobiology   Anthropology   Communication & mass media   Education & instruction   Human geography & demography   Library & information sciences   Neurosciences & behavior   Regional & inter-regional studies   Social work & social policy   Sociology & social sciences   Social, industrial & organizational psychology   Theoretical & cognitive psychology   Treatment & clinical psychology   Multidisciplinary, general & others     Showing results 1 to 20 of 131 1 2 3 4 5 6     A large oxygen-dominated core from the seismic cartography of a pulsating white dwarfGiammichele, Noemi; Charpinet, Stéphane; Fontaine, Gilles et alin Nature (in press)White-dwarf stars are the end product of stellar evolution for most stars in the Universe1. Their interiors bear the imprint of fundamental mechanisms that occur during stellar evolution2,3. Moreover ... [more ▼]White-dwarf stars are the end product of stellar evolution for most stars in the Universe1. Their interiors bear the imprint of fundamental mechanisms that occur during stellar evolution2,3. Moreover, they are important chronometers for dating galactic stellar populations, and their mergers with other white dwarfs now appear to be responsible for producing the type Ia supernovae that are used as standard cosmological candles4. However, the internal structure of white-dwarf stars—in particular their oxygen content and the stratification of their cores—is still poorly known, because of remaining uncertainties in the physics involved in stellar modelling codes5,6. Here we report a measurement of the radial chemical stratification (of oxygen, carbon and helium) in the hydrogen- deficient white-dwarf star KIC08626021 (J192904.6+444708), independently of stellar-evolution calculations. We use archival data7,8 coupled with asteroseismic sounding techniques9,10 to determine the internal constitution of this star. We find that the oxygen content and extent of its core exceed the predictions of existing models of stellar evolution. The central homogeneous core has a mass of 0.45 solar masses, and is composed of about 86 per cent oxygen by mass. These values are respectively 40 per cent and 15 per cent greater than those expected from typical white-dwarf models. These findings challenge present theories of stellar evolution and their constitutive physics, and open up an avenue for calibrating white-dwarf cosmochronology11. [less ▲]Detailed reference viewed: 53 (4 ULiège) New observations and asteroseismic analysis of the subdwarf B pulsator PG 1219+534Van Grootel, Valérie ; Peters, Marie-Julie; Green, Elizabeth M. et alin Open Astronomy (in press)We present a new asteroseismic modeling of the hot B subdwarf (sdB) pulsator PG 1219+534, based on a 3-month campaign with the Mont4K/Kuiper combination at Mt Bigelow (Arizona) and on updated atmospheric ... [more ▼]We present a new asteroseismic modeling of the hot B subdwarf (sdB) pulsator PG 1219+534, based on a 3-month campaign with the Mont4K/Kuiper combination at Mt Bigelow (Arizona) and on updated atmospheric parameters from high S/N low and medium resolution spectroscopy. On the basis of the nine independent pulsation periods extracted from the photometric light curve, we carried out an astroseismic analysis by applying the forward modeling approach using our latest (third and fourth generation) sdB models. Atmospheric parameters (Teff = 34 258±170 K, log g = 5.838±0.030) were used as independent constraints, as well as partial mode identification based on observed multiplet structures we ascribed to stellar rotation. The optimal model found is remarkably consistent between various analyses with third and fourth generation of sdB models, and also with previously published analysis with second generation sdB models. It corresponds to a sdB with a canonical mass (0.46 ± 0.02 M⊙), rather thin H-He envelope (log q(envl)= −3.75 ± 0.12), and close to He-burning exhaustion (Xcore(C + O) = 0.86 ± 0.05). We also investigate the internal rotation of the star. We find that PG 1219+534 rotates very slowly (Prot=34.91 ± 0.84 days) and that solid-body rotation is reached at least down to ∼60% of the radius. [less ▲]Detailed reference viewed: 15 (1 ULiège) Stellar parameters for TRAPPIST-1Van Grootel, Valérie ; Silva Fernandes, Catarina ; Gillon, Michaël et alin Astrophysical Journal (in press)TRAPPIST-1 is an ultracool dwarf star transited by seven Earth-sized planets, for which thorough characterization of atmospheric properties, surface conditions encompassing habitability and internal ... [more ▼]TRAPPIST-1 is an ultracool dwarf star transited by seven Earth-sized planets, for which thorough characterization of atmospheric properties, surface conditions encompassing habitability and internal compositions is possible with current and next generation telescopes. Accurate modeling of the star is essential to achieve this goal. We aim to obtain updated stellar parameters for TRAPPIST- 1 based on new measurements and evolutionary models, compared to those used in discovery studies. We present a new measurement for the parallax of TRAPPIST-1, 82.4 $\pm$ 0.8 mas, based on 188 epochs of observations with the TRAPPIST and Liverpool Telescopes from 2013 to 2016. This revised parallax yields an updated luminosity of $L_*=(5.22\pm0.19)\times 10^{-4} L_{\odot}$, very close to the previous estimate but almost twice more precise. We next present an updated estimate for TRAPPIST-1 stellar mass, based on two approaches: mass from stellar evolution modeling, and empirical mass derived from dynamical masses of equivalently classified ultracool dwarfs in astrometric binaries. We combine them through a Monte-Carlo approach to derive a semi-empirical estimate for the mass of TRAPPIST-1. We also derive estimate for the radius by combining this mass with stellar density inferred from transits, as well as estimate for the effective temperature from our revised luminosity and radius. Our final results are $M_*=0.089 \pm 0.006 M_{\odot}$, $R_* = 0.121 \pm 0.003 R_{\odot}$, and $T_{\rm eff} =$ 2516 $\pm$ 41 K. Considering the degree to which TRAPPIST-1 system will be scrutinized in coming years, these revised and more precise stellar parameters should be considered when assessing the properties of TRAPPIST-1 planets. [less ▲]Detailed reference viewed: 27 (1 ULiège) 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: 91 (10 ULiège) Early 2017 observations of TRAPPIST-1 with $\textit{Spitzer}$Delrez, Laetitia; Gillon, Michaël ; Triaud, Amaury H. M. J. et alE-print/Working paper (2018)The recently detected TRAPPIST-1 planetary system, with its seven planets transiting a nearby ultracool dwarf star, offers the first opportunity to perform comparative exoplanetology of temperate Earth ... [more ▼]The recently detected TRAPPIST-1 planetary system, with its seven planets transiting a nearby ultracool dwarf star, offers the first opportunity to perform comparative exoplanetology of temperate Earth-sized worlds. To further advance our understanding of these planets' compositions, energy budgets, and dynamics, we are carrying out an intensive photometric monitoring campaign of their transits with the $\textit{Spitzer Space Telescope}$. In this context, we present 60 new transits of the TRAPPIST-1 planets observed with $\textit{Spitzer}$/IRAC in February and March 2017. We combine these observations with previously published $\textit{Spitzer}$ transit photometry and perform a global analysis of the resulting extensive dataset. This analysis refines the transit parameters and provides revised values for the planets' physical parameters, notably their radii, using updated properties for the star. As part of our study, we also measure precise transit timings that will be used in a companion paper to refine the planets' masses and compositions using the transit timing variations method. TRAPPIST-1 shows a very low level of low-frequency variability in the IRAC 4.5-$\mu$m band, with a photometric RMS of only 0.11$\%$ at a 123-s cadence. We do not detect any evidence of a (quasi-)periodic signal related to stellar rotation. We also analyze the transit light curves individually, to search for possible variations in the transit parameters of each planet due to stellar variability, and find that the $\textit{Spitzer}$ transits of the planets are mostly immune to the effects of stellar variations. These results are encouraging for forthcoming transmission spectroscopy observations of the TRAPPIST-1 planets with the $\textit{James Webb Space Telescope}$. [less ▲]Detailed reference viewed: 18 (0 ULiège) Nonlinear asteroseismology: insight from amplitude and frequency modulations of oscillation modes in compact pulsators from Kepler photometryZong, Weikai; Charpinet, Stéphane; Vauclair, Gérard et alin EPJ Web of Conferences (2017, October), 160Nonlinear mode interactions are difficult to observe from ground-based telescopes as the typical periods of the modulations induced by those nonlinear phenomena are on timescales of weeks, months, even ... [more ▼]Nonlinear mode interactions are difficult to observe from ground-based telescopes as the typical periods of the modulations induced by those nonlinear phenomena are on timescales of weeks, months, even years. The launch of space telescopes, e.g., Kepler, has tremendously changed the situation and shredded new light on this research field. We present results from Kepler photometry showing evidence that nonlinear interactions between modes occur in the two compact pulsators KIC 8626021, a DB white dwarf, and KIC 10139564, a short period hot B subdwarf. KIC 8626021 and KIC 10139564 had been monitored by Kepler in short-cadence for nearly two years and more than three years without interruption, respectively. By analyzing these high-quality photometric data, we found that the modes within the triplets induced by rotation clearly reveal different behaviors: their frequencies and amplitudes may exhibit either periodic or irregular modulations, or remain constant. These various behaviors of the amplitude and of the frequency modulations of the oscillation modes observed in these two stars are in good agreement with those predicted within the amplitude equation formalism in the case of the nonlinear resonant mode coupling mechanism. [less ▲]Detailed reference viewed: 5 (0 ULiège) NANESSE: a Nanosatellite for Asteroseismology of the Nearest Stellar System with ExoplanetsSalmon, Sébastien ; Van Grootel, Valérie Scientific conference (2017, September 15)Detailed reference viewed: 31 (9 ULiège) Temporal Evolution of the High-energy Irradiation and Water Content of TRAPPIST-1 ExoplanetsBourrier, V.; de Wit, J.; Bolmont, E. et alin Astronomical Journal (The) (2017), 154The ultracool dwarf star TRAPPIST-1 hosts seven Earth-size transiting planets, some of which could harbor liquid water on their surfaces. Ultraviolet observations are essential to measuring their high ... [more ▼]The ultracool dwarf star TRAPPIST-1 hosts seven Earth-size transiting planets, some of which could harbor liquid water on their surfaces. Ultraviolet observations are essential to measuring their high-energy irradiation and searching for photodissociated water escaping from their putative atmospheres. Our new observations of the TRAPPIST-1 Lyα line during the transit of TRAPPIST-1c show an evolution of the star emission over three months, preventing us from assessing the presence of an extended hydrogen exosphere. Based on the current knowledge of the stellar irradiation, we investigated the likely history of water loss in the system. Planets b to d might still be in a runaway phase, and planets within the orbit of TRAPPIST-1g could have lost more than 20 Earth oceans after 8 Gyr of hydrodynamic escape. However, TRAPPIST-1e to h might have lost less than three Earth oceans if hydrodynamic escape stopped once they entered the habitable zone (HZ). We caution that these estimates remain limited by the large uncertainty on the planet masses. They likely represent upper limits on the actual water loss because our assumptions maximize the X-rays to ultraviolet-driven escape, while photodissociation in the upper atmospheres should be the limiting process. Late-stage outgassing could also have contributed significant amounts of water for the outer, more massive planets after they entered the HZ. While our results suggest that the outer planets are the best candidates to search for water with the JWST, they also highlight the need for theoretical studies and complementary observations in all wavelength domains to determine the nature of the TRAPPIST-1 planets and their potential habitability. [less ▲]Detailed reference viewed: 12 (1 ULiège) New observations and asteroseismic analysis of the subdwarf B pulsator PG 1219+534Van Grootel, Valérie ; Péters, Marie-Julie; Green, Elizabeth M. et alConference (2017, July)We present a new asteroseismic modeling of the hot B subdwarf (sdB) pulsator PG 1219+534, based on a 3-month campaign with the Mont4K/Kuiper combination at Mt Bigelow (Arizona) and on updated atmospheric ... [more ▼]We present a new asteroseismic modeling of the hot B subdwarf (sdB) pulsator PG 1219+534, based on a 3-month campaign with the Mont4K/Kuiper combination at Mt Bigelow (Arizona) and on updated atmospheric parameters from high S/N low and medium resolution spectroscopy. On the basis of the nine independent pulsation periods extracted from the photometric light curve, we carried out an astroseismic analysis by applying the forward modeling approach using our latest (third and fourth generation) sdB models. Atmospheric parameters (Teff = 34 258±170 K, log g = 5.838±0.030) were used as independent constraints, as well as partial mode identification based on observed multiplet structures we ascribed to stellar rotation. The optimal model found is remarkably consistent between various analyses with third and fourth generation of sdB models, and also with previously published analysis with second generation sdB models. It corresponds to a sdB with a canonical mass (0.46 ± 0.02 M⊙), rather thin H-He envelope (log q(envl)= −3.75 ± 0.12), and close to He-burning exhaustion (Xcore(C + O) = 0.86 ± 0.05). We also investigate the internal rotation of the star. We find that PG 1219+534 rotates very slowly (Prot=34.91 ± 0.84 days) and that solid-body rotation is reached at least down to ∼60% of the radius. [less ▲]Detailed reference viewed: 6 (0 ULiège) Planet remnants around evolved stars with CHEOPSVan Grootel, Valérie Conference (2017, July)I will talk about the opportunity to study planets/asteroids/debris discs around evolved stars with CHEOPS. Compact, post-red giant stars (subdwarf B stars and white dwarfs) will be part of the core ... [more ▼]I will talk about the opportunity to study planets/asteroids/debris discs around evolved stars with CHEOPS. Compact, post-red giant stars (subdwarf B stars and white dwarfs) will be part of the core program of TESS, which will provide, for the very first time, a census on the presence of planets/asteroids/debris discs around these stars. This will provide invaluable insights for the evolution of planetary systems and survival of close planets after the red giant phase. I will show how CHEOPS, with its larger collecting area and better sensitivity in the blue, could bring key added value to TESS for studying planets/asteroids/debris discs around evolved stars. [less ▲]Detailed reference viewed: 7 (0 ULiège) A seven-planet resonant chain in TRAPPIST-1Luger, Rodrigo; Sestovic, Marko; Kruse, Ethan et alin Nature Astronomy (2017), 1The 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 ... [more ▼]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. [less ▲]Detailed reference viewed: 118 (6 ULiège) Pulsations in white dwarf starsVan Grootel, Valérie ; Fontaine, Gilles; Brassard, Pierre et alConference (2017, June)I will present a description of the six distinct families of pulsating white dwarfs that are currently known. Pulsations are present at various stages of the evolution (from hot, pre-white dwarfs to cool ... [more ▼]I will present a description of the six distinct families of pulsating white dwarfs that are currently known. Pulsations are present at various stages of the evolution (from hot, pre-white dwarfs to cool white dwarfs), at various stellar masses, and for various atmospheric compositions. In all of them, a mechanism linked to opacity changes along the evolution drives the oscillations. The existence of these oscillations offers the opportunity to apply asteroseismology for constraining physics inside white dwarfs. The direct comparison between observed and theoretical pulsation frequencies yields the global parameters (e.g. mass and radius) and internal structure and composition (e.g. envelope layering, core composition) of the star. The deep understanding of the driving mechanism provides stringent constraints on the physical conditions at work where oscillations are excited. I will present the major achievements in the field of white dwarf asteroseismology, as well as the needed improvements for coming years in this field of research. [less ▲]Detailed reference viewed: 5 (0 ULiège) Two massive rocky planets transiting a K-dwarf 6.5 parsecs awayGillon, Michaël ; Demory, Brice-Olivier; Van Grootel, Valérie et alin Nature Astronomy (2017), 1HD 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 ... [more ▼]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. [less ▲]Detailed reference viewed: 51 (7 ULiège) Seven temperate terrestrial planets around the nearby ultracool dwarf starGillon, Michaël ; Triaud, Amaury; Demory, Brice-Olivier et alin Nature (2017), 542One 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 ... [more ▼]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. [less ▲]Detailed reference viewed: 170 (29 ULiège) Making Sense Out of Pulsating Pre-ELM and ELM White DwarfsFontaine, Gilles; Istrate, Alina; Gianninas, Alexandros et alin Astronomical Society of the Pacific Conference Series (2017), 509We 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 ... [more ▼]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. [less ▲]Detailed reference viewed: 13 (1 ULiège) The theoretical instability strip of V777 Her white dwarfsVan Grootel, Valérie ; Fontaine, Gilles; Brassard, Pierre et alin Astronomical Society of the Pacific Conference Series (2017), 509Detailed reference viewed: 16 (1 ULiège) Discovery of temperate Earth-sized planets transiting a nearby ultracool dwarf starJehin, Emmanuel ; Gillon, Michaël ; Lederer, Susan M. et alin 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 ... [more ▼]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. [less ▲]Detailed reference viewed: 75 (5 ULiège) A combined transmission spectrum of the Earth-sized exoplanets TRAPPIST-1 b and cde Wit, Julien; Wakeford, Hannah R.; Gillon, Michaël et alin Nature (2016), 537Three 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 ... [more ▼]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. [less ▲]Detailed reference viewed: 68 (12 ULiège) The theoretical instability strip of V777 Her white dwarfsVan Grootel, Valérie ; Fontaine, Gilles; Brassard, Pierre et alConference (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 ... [more ▼]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. [less ▲]Detailed reference viewed: 15 (1 ULiège) Pulsating Hot Subdwarfs in Omega CentauriRandall, S. K.; Calamida, A.; Fontaine, G. et alin The Messenger (2016), 164We 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 ... [more ▼]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. [less ▲]Detailed reference viewed: 8 (0 ULiège)