Context. A large fraction of white dwarfs show photospheric chemical composition that is polluted by heavy elements accreted from a debris disk. Such debris disks result from the tidal disruption of ...rocky planetesimals that have survived to whole stellar evolution from the main sequence to the final white dwarf stage. Determining the accretion rate of this material is an important step toward estimating the mass of the planetesimals and understanding the ultimate fate of the planetary systems. Aims. The accretion of heavy material with a mean molecular weight, μ, higher than the mean molecular weight of the white dwarf outer layers, induces a double-diffusive instability producing the fingering convection and an extra-mixing. As a result, the accreted material is diluted deep into the star. We explore the effect of this extra-mixing on the abundance evolution of Mg, O, Ca, Fe and Si in the cases of the two well-studied polluted DAZ white dwarfs: GD 133 and G 29-38. Methods. We performed numerical simulations of the accretion of material that has a chemical composition similar to the bulk Earth composition. We assumed a continuous and uniform accretion and considered a range of accretion rates from 104 g/s to 1010 g/s. Two cases are simulated, one using the standard mixing length theory (MLT) and one including the double-diffusive instability (fingering convection). Results. The double-diffusive instability develops on a very short timescale. The surface abundance rapidly reaches a stationary value while the depth of the zone mixed by the fingering convection increases. In the case of GD 133, the accretion rate needed to reproduce the observed abundances exceeds by more than two orders of magnitude the rate estimated by neglecting the fingering convection. In the case of G 29-38 the needed accretion rate is increased by approximately 1.7 dex. Conclusions. Our numerical simulations of the accretion of heavy elements on the hydrogen-rich white dwarf GD 133 and G 29-38 show that fingering convection is an efficient mechanism to mix the accreted material deeply. We find that when fingering convection is taken into account, accretion rates higher by 1.7 to 2 dex than those inferred from the standard MLT are needed to reproduce the abundances observed in G 29-38 and GD 133.
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. 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. 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. 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. The results should motivate further theoretical work to develop the nonlinear stellar pulsation theory.
Context. A precision of the order of one percent is needed on the parameters of exoplanet-hosts stars to correctly characterise the planets themselves. This can be achieved by asteroseismology. It is ...important in this context to test the influence of introducing atomic diffusion with radiative accelerations in the models upon the derived parameters. In this paper, we begin this study with the case of the star 94 Ceti A. Aims. We aim to perform a complete asteroseismic analysis of the exoplanet-host F-type star 94 Ceti A, from the first radial-velocity observations with the High Accuracy Radial velocity Planet Searcher (HARPS) up to the final computed best models. We also aim to test the influence of atomic diffusion, including radiative accelerations, upon the computed frequencies and on the determined stellar parameters. 94 Ceti A is hot enough to suffer these effects. We also aim to test the effect of including a complete atmosphere in the stellar models. Methods. The radial velocity observations were performed with HARPS in 2007. The low degree modes were derived and identified using classical methods and compared with the results obtained from stellar models computed with the Toulouse Geneva Evolution Code (TGEC). Results. We obtained precise parameters for the star 94 Ceti A. We showed that including atomic diffusion with radiative accelerations can modify the age by a few percent, whereas adding a complete atmosphere does not change the results by more than one percent. Conclusions. Atomic diffusion including radiative accelerations should be taken into account in all the computations of stellar models with masses larger than 1.3 M⊙ in order to determine accurate parameters for observed stars.
After the initial discoveries fifteen years ago, over 200 extrasolar planets have now been detected. Most of them orbit main-sequence stars similar to our Sun, although a few planets orbiting red ...giant stars have been recently found. When the hydrogen in their cores runs out, main-sequence stars undergo an expansion into red-giant stars. This expansion can modify the orbits of planets and can easily reach and engulf the inner planets. The same will happen to the planets of our Solar System in about five billion years and the fate of the Earth is matter of debate. Here we report the discovery of a planetary-mass body (Msini = 3.2MJupiter) orbiting the star V 391 Pegasi at a distance of about 1.7 astronomical units (au), with a period of 3.2 years. This star is on the extreme horizontal branch of the Hertzsprung-Russell diagram, burning helium in its core and pulsating. The maximum radius of the red-giant precursor of V 391 Pegasi may have reached 0.7 au, while the orbital distance of the planet during the stellar main-sequence phase is estimated to be about 1 au. This detection of a planet orbiting a post-red-giant star demonstrates that planets with orbital distances of less than 2 au can survive the red-giant expansion of their parent stars.
Aims. We show that the exoplanet-host star iota Horologii, alias HD 17051, which belongs to the so-called Hyades stream, was formed within the primordial Hyades stellar cluster and has evaporated ...towards its present location, 40 pc away. Methods. This result has been obtained unambiguously by studying the acoustic oscillations of this star, using the HARPS spectrometer in La Silla Observatory (ESO, Chili). Results. Besides the fact that ι Hor belongs to the Hyades stream, we give evidence that it has the same metallicity, helium abundance, and age as the other stars of the Hyades cluster. They were formed together, at the same time, in the same primordial cloud. Conclusions. This result has strong implications for theories of stellar formation. It also indicates that the observed overmetallicity of this exoplanet-host star, about twice that of the Sun, is original and not caused by planet accretion during the formation of the planetary system.
Aims. This paper presents a detailed and precise study of the characteristics of the exoplanet host star and CoRoT main target HD 52265, derived from asteroseismic studies. We compare our results ...with previous estimates, and provide a comprehensive summary and discussion. Methods. Our basic method is similar to that previously used by the Toulouse group for solar-type stars. Models are computed with various initial chemical compositions and the computed p-mode frequencies are compared with the observed ones. All models include atomic diffusion and we discuss the importance of radiative accelerations. Several tests are used, including the usual frequency combinations and the fits to the échelle diagrams. Possible surface effects are introduced and discussed. Automatic codes are also used to identify the best-fit model for this star (SEEK and AMP) and their results are compared with those obtained with the detailed method. Results. We find precise results for the mass, radius, and age of this star, as well as its effective temperature and luminosity. We also estimate the initial helium abundance. These results are important for the characterization of the star-planet system.
ABSTRACT
An increasing number of white dwarf stars shows atmospheric chemical composition polluted by heavy elements accreted from debris disc material. The existence of such debris discs strongly ...suggests the presence of one or more planet(s) whose gravitational interaction with rocky planetesimals is responsible for their disruption by tidal effect. The ZZ Ceti pulsator and polluted DAZ white dwarf GD 133 are good candidates for searching such a potential planet. We started in 2011 a photometric follow-up of its pulsations. As a result of this work in progress, we used the data gathered from 2011 to 2015 to make an asteroseismological analysis of GD 133, providing the star parameters from the best-fitting model with M/M⊙ = 0.630 ± 0.002, Teff = 12 400 ± 70 K, log(MHe/M) = −2.00 ± 0.02, log(MH/M) = −4.50 ± 0.02, and determining a rotation period of ≈7 d.
Aims.HD 52265 is the only known exoplanet-host star selected as a main target for the seismology programme of the CoRoT satellite. As such, it will be observed continuously during five months, which ...is of particular interest when studying planetary systems. This star was misclassified as a giant in the Bright Star Catalog, while it is more probably on the main-sequence or at the beginning of the subgiant branch. We performed an extensive analysis of this star, showing how asteroseismology may lead to a precise determination of its external parameters and internal structure. Methods.We first reviewed the observational constraints on the metallicity, the gravity, and the effective temperature derived from the spectroscopic observations of HD 52265. We also derived its luminosity using the Hipparcos parallax. We computed the evolutionary tracks for models of various metallicities that cross the relevant observational error boxes in the gravity-effective temperature plane. We selected eight different stellar models that satisfy the observational constraints, computed their p-modes frequencies, and analysed specific seismic tests. Results.The possible models for HD 52265, which satisfy the constraints derived from the spectroscopic observations, are different in both their external and internal parameters. They lie either on the main sequence or at the beginning of the subgiant branch. The differences in the models lead to quite different properties of their oscillation frequencies. We give evidence of an interesting specific behaviour of these frequencies in the case of helium-rich cores: the “small separations” may become negative and give constraints on the size of the core. We expect that the observations of this star by the CoRoT satellite will allow a choice between these possible models.
The pulsating DA white dwarfs (ZZ Ceti stars) are g-mode non-radial pulsators. Asteroseismology provides strong constraints on their global parameters and internal structure. Since all the DA white ...dwarfs falling in the ZZ Ceti instability strip do pulsate, the internal structure derived from asteroseismology brings knowledge for the DA white dwarfs as a whole group. HS 0507+0434B is one of the ZZ Ceti stars which lies approximately in the middle of the instability strip for which we have undertaken a detailed asteroseismological study. We carried out multisite observation campaigns in 2007 and from 2009 December to 2010 January. In total, 206 h of photometric time series have been collected. They have been analysed by means of Fourier analysis and simultaneous multifrequency sine wave fitting. In total, 39 frequency values are resolved including six triplets and a number of linear combinations. We identify the triplets as = 1 g modes split by rotation. We derived the period spacing, the rotational splitting and the rotation rate. From the comparison of the observed periods with the theoretical periods of a series of models, we estimate the fundamental parameters of the star: its total mass M
*/M = 0.675, its luminosity L/L = 3.5 × 10−3, and its hydrogen mass fraction M
H/M
* = 10−8.5.
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