Search

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, T eff = 12 400 ± 70 K, log(M He/ M) = −2.00 ± 0.02, log(M H/ M) = −4.50 ± 0.02, and determining a rotation period of ≈7 d. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 M0 in order to determine accurate parameters for observed stars. [ABSTRACT FROM AUTHOR]

Asteroseismology is a unique tool to explore the internal structure of stars through both observational and theoretical research. The internal structure of pulsating hydrogen shell white dwarfs (ZZ Ceti stars) detected by asteroseismology is regarded as the representative of all DA white dwarfs. Observations for KUV 08368+4026, which locates in the middle of the ZZ Ceti instability strip, have been carried out in 1999 and from 2009 to 2012 with either single-site runs or multisite campaigns. Time series photometric data of about 300 h were collected in total. Through data reduction and analysis, 30 frequencies were extracted, including four triplets, two doublets, one single mode and further signals. The independent modes are identified as either 1 = 1 or 1 = 2 modes. Hence, a rotation period of 5.52 ± 0.22 d was deduced from the period spacing in the multiplets. Theoretical static models were built and a best-fitting model for KUV 08368+4026 was obtained with 0.692 ± 0.002M☉, (2.92 ± 0.02) × 10-3 L☉ and the hydrogen mass fraction of 10-4 stellar mass. [ABSTRACT FROM AUTHOR]

Heavy elements are observed in the atmospheres of many DA and DB white dwarfs, and their presence is attributed to the accretion of matter coming from debris disks. Several authors have deduced accretion rates from the observed abundances, taking into account the mixing induced by the convective zones and the gravitational settling. The obtained values are different for DA and DB white dwarfs. Here we show that an important process was forgotten in all these computations: thermohaline mixing, induced by the inverse μ-gradient built during the accretion process. Taking this mixing into account leads to an increase of the derived accretion rates, specially for DA white dwarfs, and modifies the conclusions. [ABSTRACT FROM AUTHOR]

In the last two decades, considerable efforts have been devoted to the study of white dwarfs pulsators. Owing to ground-based multi-sites observational campaigns, and more recently to the long time-based high-precision photometric observations with the Kepler satellite, a large number of pulsating white dwarfs have been studied through the asteroseismology method. I emphasize here the accuracy that this method achieves on the determination of the total mass, one of the fundamental stellar parameter that can be derived from asteroseismology, among others. Then I discuss the issue of the age determination, emphasizing the contribution of the neutrinos cooling and the importance of the outer layer mass fraction. I show that some observations throw some doubts on the assumption that the cooling is the only cause of the observed frequency variations. Finally I summarize the results demonstrating that the entire stellar angular momentum is lost during the evolutionary phases prior to the white dwarf stage. While the asteroseismology of white dwarf stars has given a rich harvest of results on their internal structure and evolution, there are still some challenges to face. Determining accurate mass of the outer layers, specifically for the DA white dwarfs, is one of them. An increasing number of white dwarf pulsators are now known to exhibit frequency and amplitude temporal variations. The physical mechanism producing such variabilities has to be identified. [ABSTRACT FROM AUTHOR]

Context. Recent observations of a large number of DA and DB white dwarfs show evidence of debris disks, which are the remnants of old planetary systems. The infrared excess detected with Spitzer and the lines of heavy elements observed in their atmospheres with high-resolution spectroscopy converge on the idea that planetary material accretes onto these stars. Accretion rates have been derived by several authors with the assumption of a steady state between accretion and gravitational settling. The results are unrealistically different for DA and DB white dwarfs. Aims. When heavy matter is accreted onto stars, it induces an inverse μ-gradient that leads to fingering (thermohaline) convection. The aim of this letter is to study the impact of this specific process on the derived accretion rates in white dwarfs and on the difference between DA and DB. Methods. We solve the diffusion equation for the accreted heavy elements with a time-dependent method. The models we use have been obtained both with the IRAP code, which computes static models, and the La Plata code, which computes evolutionary sequences. Computations with pure gravitational settling are compared with computations that include fingering convection. Results. The most important result is that fingering convection has very important effects on DAs but is inefficient in DBs.When only gravitational settling is taken into account, the time-dependent computations lead to a steady state, as postulated by previous authors. When fingering convection is added, this steady state occurs much later. Conclusions. The surprising difference found in the past for the accretion rates derived for DA and DB white dwarfs disappears. The derived accretion rates for DAs are increased when fingering convection is taken into account, whereas those for DBs are not modified. More precise and developed results will be given in a forthcoming paper. [ABSTRACT FROM AUTHOR]

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 ℓ = 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 MH/M* = 10−8.5. [ABSTRACT FROM AUTHOR]

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 FROM AUTHOR]

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 FROM AUTHOR]

The pre-white dwarf pulsators of PG 1159 type, or GW Virginis variable stars, are in a phase of rapid evolution towards the white dwarf cooling sequence. The rate of change of their nonradial g-mode frequencies can be measured on a reasonably short time scale. From a theoretical point of view, it was expected that one could derive the rate of cooling of the stellar core from such measurements. At the cool end of the GW Virginis instability strip, it is predicted that the neutrinos flux dominates the cooling. PG 0122+200 which defines the red edge of the instability strip is in principle a good candidate to check this prediction. It has been followed-up through multisite photometric campaigns for about fifteen years. We report here the first determination of the rate of change of its 7 largest amplitude frequencies. We find that the amplitudes of the frequency variations are one to two orders of magnitude larger than predicted by theoretical models based on the assumption that these variations are uniquely caused by cooling. The time scales of the variations are much shorter than the ones expected from a neutrino dominated core cooling. These results point to the existence of other mechanisms responsible for the frequency variability. We discuss the role of nonlinearities as one possible mechanism. [ABSTRACT FROM PUBLISHER]

HR 1217 is one of the best-studied rapidly oscillating Ap (roAp) stars, with a frequency spectrum of alternating even- and odd-ℓ modes that are distorted by the presence of a strong, global magnetic field. Several recent theoretical studies have found that within the observable atmospheres of roAp stars the pulsation modes are magneto-acoustic with significant frequency perturbations that are cyclic with increasing frequency. To test these theories a Whole Earth Telescope extended coverage campaign obtained 342 h of JohnsonBdata at 10-s time resolution for the roAp star HR 1217 over 35 d with a 36 per cent duty cycle in 2000 November–December. The precision of the derived amplitudes is 14μmag, making this one of the highest precision ground-based photometric studies ever undertaken. Substantial support has been found for the new theories of the interaction of pulsation with the strong magnetic field. In particular, the frequency jump expected as the magnetic and acoustic components cycle through 2π rad in phase has been found. Additionally, comparison of the new 2000 data with an earlier 1986 multisite study shows clear amplitude modulation for some modes between 1986 and 2000. The unique geometry of the roAp stars allows their pulsation modes to be viewed from varying aspect with rotation, yielding mode identification information in the rotational sidelobes that is available for no other type of pulsating star. Those rotational sidelobes in HR 1217 confirm that two of the modes are dipolar, or close to dipolar; based on the frequency spacings andHipparcosparallax, three other modes must be eitherormodes, either distorted by the magnetic field, or a mix ofm-modes of givenℓ where the mixture is the result of magnetic and rotational effects. A study of all high-speed photometric JohnsonBdata from 1981 to 2000 gives a rotation period, as found in previous pulsation and photometric studies, but inconsistent with a different rotation period found in magnetic studies. We suggest that this rotation period is correct and that zero-point shifts between magnetic data sets determined from different spectral lines are the probable cause of the controversy over the rotation period. This WET data set is likely to stand as the definitive ground-based study of HR 1217. It will be the baseline for comparison for future space studies of HR 1217, particularly theMOSTsatellite observations. [ABSTRACT FROM AUTHOR]

We have acquired new time series photometry of the two pulsating DB white dwarf stars KUV 05134+2605 and PG 1654+160 with the Whole Earth Telescope. Additional single-site photometry is also presented. We use all these data plus all available archival measurements to study the temporal behaviour of the pulsational amplitudes and frequencies of these stars for the first time. We demonstrate that both KUV 05134+2605 and PG 1654+160 pulsate in many modes, the amplitudes of which are variable in time; some frequency variability of PG 1654+160 is also indicated. Beating of multiple pulsation modes cannot explain our observations; the amplitude variability must therefore be intrinsic. We cannot find stable modes to be used for determinations of the evolutionary period changes of the stars. Some of the modes of PG 1654+160 appear at the same periods whenever detected. The mean spacing of these periods (≈40 s) suggests that they are probably caused by non-radial gravity-mode pulsations of spherical degree l = 1. If so, PG 1654+160 has a mass around 0.6 M[sub ⊙]. The time-scales of the amplitude variability of both stars (down to two weeks) are consistent with theoretical predictions of resonant mode coupling, a conclusion which might however be affected by the temporal distribution of our data. [ABSTRACT FROM AUTHOR]

We present a detailed mode identification performed on the 1994 Whole Earth Telescope (WET) run on GD 358. The results are compared with that obtained for the same star from the 1990 WET data. The two temporal spectra show very few qualitative differences, although amplitude changes are seen in most modes, including the disappearance of the mode identified as k=14 in the 1990 data. The excellent coverage and signal-to-noise ratio obtained during the 1994 run lead to the secure identification of combination frequencies up to fourth order, i.e. peaks that are sums or differences of up to four parent frequencies, including a virtually complete set of second-order frequencies, as expected from harmonic distortion. We show how the third-order frequencies are expected to affect the triplet structure of the normal modes by back-interacting with them. Finally, a search for ℓ=2 modes was unsuccessful, not verifying the suspicion that such modes had been uncovered in the 1990 data set. [ABSTRACT FROM AUTHOR]

Reports the discovery of two variables from five candidates selected observed from the Kiso survey, with the 1.93-m telescope at Observatoire de Haute-Provence, France, on February, 1996. How much the observation precision was estimated at; Data reduction and analysis; Result of multiple frequency analysis for KUV 08368 + 4026 and KUV 11370 + 4222; Concluding remarks.

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. [ABSTRACT FROM AUTHOR]

A preface to the journal "EAS Publication Series" is presented.