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The accretion of heavy material from debris disk on the surface of hydrogen-rich white dwarfs induces a double diffusivity instability known as the fingering convection. It leads to an efficient extra mixing which brings the accreted material deeper in the star than by considering only mixing in the surface dynamical convection zone, in a time scale much shorter than that of gravitational settling. We performed numerical simulations of a continuous accretion of heavy material having a bulk Earth composition on the two well studied DAZ and ZZ Ceti pulsators GD 133 and G 29-38. We find that the existence of fingering convection implies much larger accretion rates to explain the observed abundances than previous estimates based on the standard mixing length theory and gravitational settling only., Instituto de Astrofísica de La Plata

An increasing number of white dwarf stars show atmospheric chemical composition polluted by heavy elements accreted from debris disk material. The existence of such debris disks 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 is a good candidate for searching for 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 a best fit model with $M$/$M_{\odot}$ = 0.630 $\pm$ 0.002, $T_{\rm eff}$ = 12400 K $\pm$ 70 K, log($M_{\rm He}/M$) = -2.00 $\pm$ 0.02, log($M_{\rm H}/M$) = -4.50 $\pm$ 0.02 and determining a rotation period of $\approx$ 7 days., 10 pages, 13 figures, accepted by MNRAS

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 hours 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 l=1 or l=2 modes. Hence, a rotation period of $5.52\pm 0.22$ days was deduced from the period spacing in the multiplets. Theoretical static models were built and a best fit model for KUV~08368+4026 was obtained with $0.692\pm0.002$ solar mass, $(2.92\pm0.02)\times 10^{-3}$ solar luminosity and the hydrogen mass fraction of $10^{-4}$ stellar mass., 9 pages, 7 figures

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., Instituto de Astrofísica de La Plata

A precision of order one percent is needed on the parameters of exoplanet-hosts stars in order to correctly characterize the planets themselves. This will be achieved by asteroseismology. It is important in this context to test the influence on the derived parameters of introducing atomic diffusion with radiative accelerations in the models. In this paper, we begin this study with the case of the star 94 Ceti A. We performed a complete asteroseismic analysis of the exoplanet-host F-type star 94 Ceti A, from the first radial-velocity observations with HARPS up to the final computed best models. This star is hot enough to suffer from important effects of atomic diffusion, including radiative accelerations. We tested the influence of such effects on the computed frequencies and on the determined stellar parameters. We also tested the effect of including a complete atmosphere in the stellar models. The radial velocity observations were done 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). 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 percents, whereas adding a complete atmosphere does not change the results by more than one percent., 7 pages, 6 figures, accepted for publication in A&A

GD 133 is a DAZ white dwarf with an atmosphere polluted by heavy elements accreted from a debris disk, which is formed by the disruption of rocky planetesimals with orbits bringing them at the white dwarf tidal radius. To reach such orbits implies the potential presence of a perturbing planet. GD 133 is a ZZ Ceti pulsator close to the blue edge of the instability strip. The presence of a planet could be revealed by the periodical variation of the observed pulsation periods induced by the orbital motion of the white dwarf around the barycenter of the system. We started a multi-site photometric follow-up aimed at detecting the signature of this potential planet. As a preliminary result of this work in progress, we give the parameters of the GD 133 best-fit model derived from asteroseismology.

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.

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.

PG 1351+489 is one of the 20 DBVs ― pulsating helium-atmosphere white dwarf stars ― known and has the simplest power spectrum for this class of star, making it a good candidate to study cooling rates. We report accurate period determinations for the main peak at 489.334 48 s and two other normal modes using data from the Whole Earth Telescope (WET) observations of 1995 and 2009. In 2009, we detected a new pulsation mode and the main pulsation mode exhibited substantial change in its amplitude compared to all previous observations. We were able to estimate the star's rotation period, of 8.9 h, and discuss a possible determination of the rate of period change of (2.0 ± 0.9) x 10 ―13 s s ―1 , the first such estimate for a DBV.

KPD 1930+2752 is a short-period pulsating subdwarf B (sdB) star. It is also an ellipsoidal variable with a known binary period just over two hours. The companion is most likely a white dwarf and the total mass of the system is close to the Chandresakhar limit. In this paper we report the results of Whole Earth Telescope (WET) photometric observations during 2003 and a smaller multisite campaign from 2002. From 355 hours of WET data, we detect 68 pulsation frequencies and suggest an additional 13 frequencies within a crowded and complex temporal spectrum between 3065 and 6343 $\mu$Hz (periods between 326 and 157 s). We examine pulsation properties including phase and amplitude stability in an attempt to understand the nature of the pulsation mechanism. We examine a stochastic mechanism by comparing amplitude variations with simulated stochastic data. We also use the binary nature of KPD 1930+2752 for identifying pulsation modes via multiplet structure and a tidally-induced pulsation geometry. Our results indicate a complicated pulsation structure that includes short-period ($\approx 16$ h) amplitude variability, rotationally split modes, tidally-induced modes, and some pulsations which are geometrically limited on the sdB star.

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.

Aims. We present an asteroseismological study of PG 0122+200, the coolest known pulsating PG 1159 (GW Vir) star. Our results are based on an augmented set of the full PG 1159 evolutionary models recently presented by Miller Bertolami & Althaus (2006). Methods. We perform extensive computations of adiabatic g-mode pulsation periods on PG 1159 evolutionary models with stellar masses ranging from 0.530 to 0.741 M⊙. These models take into account the complete evolution of progenitor stars, through the thermally pulsing asymptotic giant branch phase and born-again episode. We constrain the stellar mass of PG 0122+200 by comparing the observed period spacing with the asymptotic period spacing and with the average of the computed period spacings. We also employ the individual observed periods to find a representative seismological model for PG 0122+200. Results. We derive a stellar mass of 0.626 M⊙ from a comparison between the observed period spacing and the computed asymptotic period spacing, and a stellar mass of 0.567 M⊙ by comparing the observed period spacing with the average of the computed period spacing. We also find, on the basis of a period-fit procedure, an asteroseismological model representative of PG 0122+200 that is able to reproduce the observed period pattern with an average of the period differences of δΠ̄i = 0.88 s and a root-mean-square residual of σδΠi = 1.27 s. The model has an effective temperature Teff = 81 500 K, a stellar mass M* = 0.556 M⊙, a surface gravity log g = 7.65, a stellar luminosity and radius of log(L*/L⊙) = 1.14 and log(R*/R ⊙) = -1.73, respectively, and a He-rich envelope thickness of Menv = 1.9 × 10-2 M⊙. We derive a seismic distance d ∼ 614 pc and a parallax π ∼ 1.6 mas. The results of the period-fit analysis carried out in this work suggest that the asteroseismological mass of PG 0122+200 could be ∼6-20% lower than hitherto thought, and in closer agreement (to within ∼5%) with the spectroscopic mass. This result suggests that a reasonable consistency between the stellar mass values obtained from spectroscopy and asteroseismology can be expected when detailed PG 1159 evolutionary models are considered., Facultad de Ciencias Astronómicas y Geofísicas, Instituto de Astrofísica de La Plata

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 evidences of nonlinear behaviors. Methods. We analyse nearly two years of uninterrupted short cadence data, concentrating in particular on identified triplets due to stellar rotation that show intriguing behaviors during the course of the observations. Results. We find clear signatures of nonlinear effects attributed to resonant mode coupling mechanisms. We find that a triplet at 4310 {\mu}Hz and this doublet at 3681 {\mu}Hz (most likely the two visible components of an incomplete triplet) have clear periodic frequency and amplitude modulations typical of the so-called intermediate regime of the resonance, with time scales consistent with theoretical expectations. Another triplet at 5073 {\mu}Hz 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 identified modulations are the first clear-cut signatures of nonlinear resonant couplings occurring in white dwarf stars. These should resonate as a warning to projects aiming 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 reliably such rates, unless they could be corrected beforehand. These results should motivate further theoretical work to develop nonlinear stellar pulsation theory., Comment: 14 pages, 13 Figures, 2 Tables, accepted to A&A

One of the brightest pulsating DA white dwarfs (ZZ Ceti stars), G 185-32, shows 19 periods in its power spectrum, from 71 s to 651 s. Its short periods are among the shortest ones observed in a ZZ Ceti star. The one at 141.9 s shows an amplitude which does not vary with wavelength, in contrast with what the linear theory predicts for $\ell=1$ or $\ell=2$ non-radial g -modes. The mass and T eff of G 185-32 are well constrained by spectroscopic observations and parallax measurement. The aim of the present paper is to take advantage of this rich observational background to derive the fundamental parameters of the star from asteroseismology. We computed a grid of realistic models for G 185-32 and the periods of their adiabatic $\ell=1$ and $\ell=2$ non-radial g -modes. We determined the model which fits best the observations. We identify 14 periods as real modes and determine their $\ell$, k , and m values. We show that the remaining 5 periods are linear combinations.We find that G 185-32 oscillates mostly according to $\ell=2$ modes. We present a new hypothesis to account for the peculiar behaviour of the 141.9 s mode's amplitude and suggest that it is a true mode interfering with higher $\ell$ degree modes, involving a possible triplet resonance. We determine the main parameters of G 185-32: its total mass $M_{*}=0.638$ ( ± 0.007) $M_{\odot}$, its hydrogen mass fraction $M_{\rm H}= 1.70$ ( ± 0.10) $\times~10^{-4}~M_{*}$, which implies a helium mass fraction of ≈ $10^{-2}~M{*}$, its $T_{\rm eff}= 12\,280$ ( ± )80 K, and estimate its rotation period to be 14.5 h.

International audience; This paper analyses the Whole Earth Telescope observations of HL Tau 76, the first discovered pulsating DA white dwarf. The star was observed during two Whole Earth Telescope campaigns. It was a second priority target during the XCOV13 campaign in 1996 and the first priority one during the XCOV18 campaign in 1999. The 1999 campaign reached 66.5% duty cycle. With a total duration of 18 days, the frequency resolution achieved is 0.68 ?Hz. With such a frequency resolution, we were able to find as many as 78 significant frequencies in the power spectrum, of which 34 are independent frequencies after removal of all linear combinations. In taking into account other frequencies present during the 1996 WET campaign and those present in earlier data, which do not show up in the 1999 data set, we find a total of 43 independent frequencies. This makes HL Tau 76 the richest ZZ Ceti star in terms of number of observed pulsation modes. We use those pulsation frequencies to determine as much as possible of the internal structure of HL Tau 76. The pulsations in HL Tau 76 cover a wide range of periods between 380 s and 1390 s. We propose an identification for 39 of those 43 frequencies in terms of ?=1 and ?=2 non-radial g-modes split by rotation. We derive an average rotation period of 2.2 days. The period distribution of HL Tau 76 is best reproduced if the star has a moderately "thick" hydrogen mass fraction log qH ? -7.0. The results presented in this paper constitute a starting point for a detailed comparison of the observed periods with the periods calculated for models as representative as possible of HL Tau 76.

International audience; The old nova DQ Herculis was the Whole Earth Telescope Northern Hemisphere target for the 1997 July campaign and was observed on four nights with the SARA 0.9 m telescope during 2003 June. We present updated ephemerides for the eclipse and 71 s timings. The Fourier transform displays power at the presumed white dwarf spin period of 71.0655 s, but no significant power at either 142 or 35.5 s. The mean pulsed light curve is obtained by folding on the orbital period modulus the mean ephemeris of the 71 s period, and from this we calculate an O-C phase diagram and amplitude versus orbital phase diagrams. In addition to the phase variations during eclipse ingress and egress, the WET data reveal significant phase variations outside of eclipse. These must result from the self-eclipse of a nonaxisymmetric disk. We simulated the disk in DQ Her using smoothed particle hydrodynamics. We improve our effective spatial signal-to-noise ratio by combining 250 snapshots of the N=20,000 phase space solution over time to obtain a 5 million particle ensemble disk. From the surface shape of the ensemble disk, the radius and vertical height above the midplane of the rim of the reprocessing region can be derived as a function of azimuthal angle. From this profile we can calculate the O-C phase and amplitude diagrams as a function of inclination angle. The calculated O-C diagrams are a remarkably good match to the observed phase and amplitude variations of the 71 s signal. The best match is for inclination angle 89.7d.

We report observations of the planetary nebula nucleus Abell 43, obtained at the 2.5m Nordic Optical Telescope, which shows that it is a pulsator. Abell 43, a "hybrid-PG 1159" type star, is the second pulsator of this class, after HS 2324+3944. From the limited data set aquired, we find that Abell 43 exhibits at least two periods of 2600s and 3035s, the longest ones observed up to now in PG 1159 and "hybrid-PG 1159" pulsators. This strongly suggests that the variations are due to non-radial g-mode pulsations and cannot be a consequence of binarity. This discovery raises puzzling questions regarding the exitation mechanism in this H rich, C and O poor "hybrid-PG 1159" since the C and O abundances are too low to trigger the instability through the $\kappa$-mechanism invoked to explain the instability in the PG 1159 stars and in the previously known "hybrid-PG 1159" pulsator HS 2324+3944., Comment: 6 pages, 4 figures Accepted for publication in Astronomy and Astrophysics

A brief description of the various types of variable stars occuring in the advanced stages of stellar evolution is given. This includes the Planetary Nebulae Nuclei variables (PNNV), the PG 1159 variables, the sdB pulsators and the DB and DA white dwarf pulsators. In this review, the emphasis is put on the importance of diffusion for either the excitation mechanism or the properties of the oscillations.

We present photometric results for four new variable stars discovered in the vicinity of the ZZ Ceti-type pulsating white dwarf BR Cam. Observations were performed on 5 nights in November 2003 using the 1.8m telescope at Bohyunsan Optical Astronomy Observatory with no filter, on 3 nights in December 2003 using the 0.61m telescope at Sobaeksan Optical Astronomy Observatory with V, I filters, and on 3 nights in October 2004 using the 1.0m telescope at Mt. Lemmon Optical Astronomy Observatory with V, I filters. We estimated their periods from the phase-match technique for one eclipsing binary and the multiple frequency analysis for three pulsating stars. By considering the light curve shape, period and amplitude difference between two passbands, we classified the objects by their variability types as follows: V1 (USNO-A2.0 1425-05691757) is a W UMa-type eclipsing binary with an orbital period of ; V2 (USNO-A2.0 1425-05703335) is a multi-periodic Set-type pulsating star with a dominant period of ; V3 (USNO-A2.0 1425-05699659) is also a Set-type pulsating star with a period of ; and V 4 (USNO-A2.0 1425-05707705) is a RR Lyr-type pulsating star with a period of .

International audience; We report on the analysis of multisite time-series photometry of the pulsating pre-white dwarf (GW Vir star) PG 1707+427, obtained by the Whole Earth Telescope collaboration. This is the last of the known GW Vir stars without surrounding nebulae to be resolved by multisite data. Successful resolution of the pulsation spectrum resulted from the combination of high signal-to-noise observations with a large telescope and wide coverage in longitude with smaller telescopes. We find a series of 8 pulsation frequencies (along with two nonlinear combination frequencies), and identify 7 of them as part of a sequence of l=1 modes, with a common period spacing of 23.0 s. This spacing implies that the mass of PG 1707+427 is 0.57 M&sun;. Preliminary model fits suggest that the mass determined via asteroseismology is consistent with the mass determined from spectroscopy combined with evolutionary tracks.

We report our analysis of the stability of pulsation periods in the DAV star (pulsating hydrogen atmosphere white dwarf) ZZ Ceti, also called R548. On the basis of observations that span 31 years, we conclude that the period 213.13 s observed in ZZ Ceti drifts at a rate dP/dt 5 (5.5 plus or minus 1.9) x 10(exp -15) ss(sup -1), after correcting for proper motion. Our results are consistent with previous P values for this mode and an improvement over them because of the larger time base. The characteristic stability timescale implied for the pulsation period is |P||P(raised dot)|greater than or equal to 1.2 Gyr, comparable to the theoretical cooling timescale for the star. Our current stability limit for the period 213.13 s is only slightly less than the present measurement for another DAV, G117-B15A, for the period 215.2 s, establishing this mode in ZZ Ceti as the second most stable optical clock known, comparable to atomic clocks and more stable than most pulsars. Constraining the cooling rate of ZZ Ceti aids theoretical evolutionary models and white dwarf cosmochronology. The drift rate of this clock is small enough that we can set interesting limits on reflex motion due to planetary companions.

We report on the 323 hours of nearly uninterrupted time series photometric observations of the DBV star GD 358 acquired with the Whole Earth Telescope (WET) during May 23 to June 8, 2000. We acquired more than 232 000 independent measurements and detected the non-radial g-modes consistent with degree ℓ = 1 and radial order 8 to 20 and their linear combinations up to 6th order. We also detect, for the first time, a high amplitude ℓ = 2 mode, with a period of 796 s. In the 2000 WET data, the largest amplitude modes are similar to those detected with the WET observations of 1990 and 1994, but the highest combination order previously detected was 4th order.

We present the first results of an ensemble and systematic survey of oscillation mode variability in pulsating hot B subdwarf (sdB) and white dwarf stars observed with the original Kepler mission. The satellite provides uninterrupted high-quality photometric data with a time baseline that can reach up to 4 yr collected on pulsating stars. This is a unique opportunity to characterize long-term behaviors of oscillation modes. A mode modulation in amplitude and frequency can be independently inferred by its fine structure in the Fourier spectrum, from the sLSP, or with prewhitening methods applied to various parts of the light curve. We apply all these techniques to the sdB star KIC 3527751, a long-period-dominated hybrid pulsator. We find that all the detected modes with sufficiently large amplitudes to be thoroughly studied show amplitude and/or frequency variations. Components of three identified quintuplets around 92, 114, and 253 mu Hz show signatures that can be linked to nonlinear interactions according to the resonant mode coupling theory. This interpretation is further supported by the fact that many oscillation modes are found to have amplitudes and frequencies showing correlated or anticorrelated variations, a behavior that can be linked to the amplitude equation formalism, where nonlinear frequency corrections are determined by their amplitude variations. Our results suggest that oscillation modes varying with diverse patterns are a very common phenomenon in pulsating sdB stars. Close structures around main frequencies therefore need to be carefully interpreted in light of this finding to secure a robust identification of real eigenfrequencies, which is crucial for seismic modeling. The various modulation patterns uncovered should encourage further developments in the field of nonlinear stellar oscillation theory. It also raises a warning to any long-term project aiming at measuring the rate of period change of pulsations caused by stellar evolution, or at discovering stellar (planetary) companions around pulsating stars using timing methods, as both require very stable pulsation modes.

The Kepler spacecraft provides new opportinuties to observe long term frequency and amplitude modulations of oscillation modes in pulsating stars. We analyzed more than three years of uninterrupted data obtained with this instrument on the hot B subdwarf (sdB) star KIC 10139564 and found clear signatures of nonlinear resonant mode coupling affecting several multiplets. The observed periodic frequency and amplitude modulations may allow for new asteroseismic diagnostics, providing in particular ways to measure linear growth rates of pulsation modes in hot subdwarf stars for the first time.

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.

This report focuses on a few subjects where substantial progress has been achieved during the past three years. Several colleagues were asked to describe what they consider as the highlights in their field, and we are very grateful to them for their prompt and competent response. They covered the following topics: massive stars (A. Maeder), intermediate mass stars (P. Demarque), low mass objects (G. Chabrier), stellar convection (H. Spruit), atomic diffusion (G. Michaud), rotational mixing (J.-P. Zahn), helioseismology (S. Basu) and solar neutrinos (E. Schatzman). J.-P. Zahn was responsible for collecting and editing their contributions.

In this paper, we publish the Chinese-French bi-site high-accuracy photoelectric observations of the DA type pulsating white dwarf G 255-2 made in 1995 and 1997. After careful data reduction, we make a multi-frequency analysis, and determine its eigen frequencies and modes. In connection with the theoretical model of white dwarfs, we make a preliminary astro-seismological analysis.

The white dwarfs are promising laboratories for the study of cosmochronology and stellar evolution. Through observations of the pulsating white dwarfs, we can measure their internal structures and compositions, critical to understanding post-main-sequence evolution, along with their cooling rates, which will allow us to calibrate their ages directly. The most important set of white dwarf variables to measure are the oldest of the pulsators, the cool DA variables (DAVs), which have not been explored previously through asteroseismology due to their complexity and instability. Through a time-series photometry data set spanning 10 yr, we explore the pulsation spectrum of the cool DAV, G29-38 and find an underlying structure of 19 (not including multiplet components) normal-mode, probably l=1 pulsations amidst an abundance of time variability and linear combination modes. Modeling results are incomplete, but we suggest possible starting directions and discuss probable values for the stellar mass and hydrogen layer size. For the first time, we have made sense out of the complicated power spectra of a large-amplitude DA pulsator. We have shown that its seemingly erratic set of observed frequencies can be understood in terms of a recurring set of normal-mode pulsations and their linear combinations. With this result, we have opened the interior secrets of the DAVs to future asteroseismological modeling, thereby joining the rest of the known white dwarf pulsators.

Observation of g-mode pulsations in the variable pre-white dwarf (GW Virginis) stars provides a unique means to probe their interiors and to study the late stages of stellar evolution. Multisite campaigns have in several cases proved highly successful in decoding pre-white dwarf light curves. Three previous attempts to untangle the pulsation spectrum of the coolest GW Virginis star, PG 0122+200, confirmed the existence of multiple g-modes but left the fundamental period spacing and therefore the star's mass and luminosity in doubt. We present an analysis based on new observations of PG 0122+200 obtained during a Whole Earth Telescope (WET) campaign conducted in the fall of 1996. Although our coverage was, because of bad weather, far poorer than in previous WET campaigns, we confirm the previous result that PG 0122+200 rotates once in 1.6 ± 0.1 days. The most likely period spacing supported by the data implies a mass of 0.69±0.03 M☉. Based on the best seismology we can currently do, the cooling of PG 0122+200 is dominated by neutrino losses. This is not true for all pre-white dwarf stars and makes PG 0122+200 the prime candidate for learning useful physics. Constraints placed on the cooling rate of PG 0122+200 by future measurement of dΠ/dt could provide a unique test of the standard theory of lepton interactions in the (experimentally unexplored) region of phase-space appropriate to pre-white dwarf interiors.

We report the discovery of multiperiodic luminosity variations in the hot B subdwarf KPD 2109+4401. At least five periodicities are seen in the light curve, from 182.5 to 198.4 s. The largest oscillation has an amplitude 8.6 mmag in white light and a period 196.3 s. Our model atmosphere analysis of the time-averaged optical spectrum of KPD 2109+4401 indicates that this star has Teff 31,200 K and log g 5.84. A comparison with pulsation periods computed from stellar models having similar atmospheric parameters implies that the observed brightness variations must be identified with low-order radial and nonradial (p and f) pulsation modes. The overall similarity of periods and derived stellar parameters shows that KPD 2109+4401 is a genuine member of the EC 14026 class, the most recent family of pulsators uncovered in the field of asteroseismology. We note, however, that KPD 2109+4401 is slightly but significantly cooler than the previously known members of this class, thus widening significantly the empirical instability strip. Interestingly, theory predicts that a cooler EC 14026 pulsator (of a given surface gravity) should show larger excited periods, and this is indeed what KPD 2109+4401 shows, with periods reaching values as large as 198 s, not seen previously in other pulsators of the class. We also report on the current status of our ongoing survey to search for additional EC 14026 pulsators.

We report the discovery of multiperiodic luminosity variations in the hot B subdwarf PG 1047+003. At least five periodicities are seen in the light curve, from 104.2 s to 161.9 s, but others are also present at lower amplitudes in that interval. The largest oscillation has an amplitude of 9.2 millimag in white light and a period of 142.2 s. With atmospheric parameters Teff ~ 34,370 K and log g ~ 5.7 for PG 1047+003, these variations are identified with low-order radial and nonradial (p and f) pulsation modes. The similarity of periods and derived stellar parameters indicate that PG 1047+003 is a genuine member of the EC 14026 class, the latest and newest family of pulsators in the field of asteroseismology. However, it shows no evidence of a binary companion, implying that the mechanism for driving pulsations is internal to the star. We also report on the current status of our ongoing survey to search for additional sdB pulsators.

We present our analysis of 240 hr of white-light, high-speed photometry of the dwarf nova-like helium variable PG 1346+082 (CR Boo). We identify two frequencies in the low-state power spectrum, at 679.670 ± 0.004 μHz and 669.887 ± 0.008 μHz. The 679.670 μHz variation is coherent over at least a 2 week time span, the first demonstration of a phase-coherent photometric variation in any dwarf nova-like interacting binary white dwarf system. The high-state power spectrum contains a complex fundamental with a frequency similar, but not identical, to the low-state spectrum, and a series of harmonics not detected in low state. We also uncover an unexpected dependence of the high-frequency power's amplitude and frequency structure on overall system magnitude. We discuss these findings in light of the general AM CVn system model, particularly the implications of the high-order harmonics on future studies of disk structure, mass transfer, and disk viscosity.

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., Facultad de Ciencias Astronómicas y Geofísicas, Instituto de Astrofísica de La Plata

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.

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 December 2009 to January 2010. In total, 206 hours of photometric time-series have been collected. They have been analysed by means of Fourier analysis and simultaneous multi-frequency sine-wave fitting. In total, 39 frequency values are resolved including 6 triplets and a number of linear combinations. We identify the triplets as $\ell$=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$_{\odot}$ = 0.675, its luminosity L/L$_{\odot}$=3.5$\times 10^{-3}$, and its hydrogen mass fraction M$_{H}$/M$_{*}$= 10$^{-8.5}$., Comment: 12 pages,8 figures, accepted for publication in MNRAS

This paper presents the results of high speed photo­ metric observations of the cool variable DA white dwarf (DAV) GD 154 obtained with the Whole Earth Telescope. GD 154 is one of the c;oolest pulsating DA white dwarfs and its study is important for understanding the red edge of the ZZ Ceti insta­ bility strip. Its power spectrum is dominated by three indepen­ dent modes (P 1 = 1186.5s, Pz = 1088.6s and P3 = 402.6s), and their harmonics and linear combinations. None of the half-integer harmonics reported in previous observations were present during the WET campaign. We propose that the ob­ served modes are trapped in the thin outer hydrogen layers. From the resulting identification of the pulsation modes, one derives an estimate of the rotation period (2.3 days) and of the mass ofthe outer hydrogen layer (2 X w-IO M *).

We report on analysis of 308.3 hrs of high speed photometry targeting the pulsating DA white dwarf EC14012-1446. The data were acquired with the Whole Earth Telescope (WET) during the 2008 international observing run XCOV26. The Fourier transform of the light curve contains 19 independent frequencies and numerous combination frequencies. The dominant peaks are 1633.907, 1887.404, and 2504.897 microHz. Our analysis of the combination amplitudes reveals that the parent frequencies are consistent with modes of spherical degree l=1. The combination amplitudes also provide m identifications for the largest amplitude parent frequencies. Our seismology analysis, which includes 2004--2007 archival data, confirms these identifications, provides constraints on additional frequencies, and finds an average period spacing of 41 s. Building on this foundation, we present nonlinear fits to high signal-to-noise light curves from the SOAR 4.1m, McDonald 2.1m, and KPNO 2m telescopes. The fits indicate a time-averaged convective response timescale of 99.4 +/- 17 s, a temperature exponent 85 +/- 6.2 and an inclination angle of 32.9 +/- 3.2 degrees. We present our current empirical map of the convective response timescale across the DA instability strip., Comment: Accepted for publication in the Astrophysical Journal

This paper presents a detailed and precise study of the characteristics of the Exoplanet Host Star and CoRoT main target HD 52265, as derived from asteroseismic studies. The results are compared with previous estimates, with a comprehensive summary and discussion. The 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 the importance of radiative accelerations is discussed. Several tests are used, including the usual frequency combinations and the fits of the \'echelle diagrams. The possible surface effects are introduced and discussed. Automatic codes are also used to find the best model for this star (SEEK, AMP) and their results are compared with that obtained with the detailed method. We find precise results for the mass, radius and age of this star, as well as its effective temperature and luminosity. We also give an estimate of the initial helium abundance. These results are important for the characterization of the star-planet system., Comment: 9 pages, 6 figures, 7 tables, to be published in Astronomy and Astrophysics

The star HD 52265 is a G0V metal-rich exoplanet-host star observed in the seismology field of the CoRoT space telescope from November 2008 to March 2009. The satellite collected 117 days of high-precision photometric data on this star, showing that it presents solar-like oscillations. HD 52265 was also observed in spectroscopy with the Narval spectrograph at the same epoch. We characterise HD 52265 using both spectroscopic and seismic data. The fundamental stellar parameters of HD 52265 were derived with the semi-automatic software VWA, and the projected rotational velocity was estimated by fitting synthetic profiles to isolated lines in the observed spectrum. The parameters of the observed p modes were determined with a maximum-likelihood estimation. We performed a global fit of the oscillation spectrum, over about ten radial orders, for degrees l=0 to 2. We also derived the properties of the granulation, and analysed a signature of the rotation induced by the photospheric magnetic activity. Precise determinations of fundamental parameters have been obtained: Teff = 6100 +- 60 K, log g = 4.35 +- 0.09, [M/H] = 0.19 +- 0.05, as well as vsini = 3.6 +0.3 -1.0 km/s. We have measured a mean rotation period P_rot = 12.3 +- 0.15 days, and find a signature of differential rotation. The frequencies of 31 modes are reported in the range 1500-2550 micro-Hz. The large separation exhibits a clear modulation around the mean value = 98.3 +- 0.1 micro-Hz. Mode widths vary with frequency along an S-shape with a clear local maximum around 1800 micro-Hz. We deduce lifetimes ranging between 0.5 and 3 days for these modes. Finally, we find a maximal bolometric amplitude of about 3.96 +- 0.24 ppm for radial modes., Comment: published in A&A, 11 pages, 14 figures, 6 tables

Context: The PG 1159 pre-white dwarf stars experiment a rapidly cooling phase with a time scale of a few 10 6 years. Theoretical models predict that the neutrinos produced in their core should play a dominant role in the cooling, mainly at the cool end of the PG 1159 sequence. Measuring the evolutionary time scale of the coolest PG 1159 stars could offer a unique opportunity to empirically constrain the neutrino emission rate. Aims. A subgroup of the PG 1159 stars are nonradial pulsators, the GW Vir type of variable stars. They exhibit g-mode pulsations with periods of a few hundred seconds. As the stars cool, the pulsation frequencies evolve according to the change in their internal structure. It was anticipated that the measurement of their rate of change would directly determine the evolution time scale and so constrain the neutrino emission rates. As PG 0122+200 (BB Psc) defines the red edge of the GW Vir instability strip, it is a good candidate for such a measurement. Methods. The pulsations of PG 0122+200 have been observed during 22 years from 1986 to 2008, through the fast photometry technique. We used those data to measure the rate of change of its frequencies and amplitudes. Results. Among the 24 identified ℓ = 1 modes, the frequency and amplitude variations have been obtained for the seven largest amplitude ones. We find changes of their frequency of much larger amplitudes and shorter time scales than the one predicted by theoretical models that assume that the cooling dominates the frequency variations. In the case of the largest amplitude mode at 2497 μHz (400 s), its variations are best fitted by a combination of two terms: one long term with a time scale of 5.4 × 10 4 years, which is significantly shorter than the predicted evolutionary time scale of 8 × 10 6 years; and one additional periodic term with a period of either 261 or 211 days. Some other mechanism(s) than the cooling must be responsible for such variations. We suggest that the resonant coupling induced within triplets by the star rotation could be such a mechanism. As a consequence, no useful constraints on the neutrino emission rate can presently be derived as long as the dominant mechanism is not properly understood. Conclusions. The temporal variations in the pulsation frequencies observed in PG 0122+200 cannot be simply attributed to the cooling of the star, regardless of the contribution of the neutrino losses. Our results suggest that the resonant coupling induced by the rotation plays a dominant role which must be further investigated., Facultad de Ciencias Astronómicas y Geofísicas

The ZZ Ceti stars form a class of variable white dwarfs: the hydrogen dominated atmosphere ones, which do pulsate in an instability strip in the effective temperature range 13000K-11500K. We know 22 such ZZ Ceti white dwarfs. Their variations are caused by nonradial g-mode pulsations with periods are in the range 100-1000 seconds.A subsample of the ZZ Ceti stars shows amplitude variations on time scales of the order of one month. These variations could be driven by nonlinear phenomena.

The ZZ Ceti HS 0507+0434B is of special interest because it lies in a common proper motion system with HS 0507+0434A, another DA white dwarf of Teff = 20000 K. It lies in the middle of the instability strip and is a potentially useful pulsator to study the convection‐pulsation interaction. Its pulsation spectrum has shown 10 independent frequencies and a number of linear combinations. In an effort to better constrain its internal structure, we have performed new observations of HS 0507+0434B. We describe these new data obtained from mono‐site as well as multi‐site campaigns between 2007 and 2010. The analysis of these data reveals a complex behavior of the pulsator, with both frequency and amplitude variations on this 1.5 year time scale. We identify 7 l = 1 multiplets. We computed a grid of models and their l = 1 g‐modes. We then applied a χ2 fit method to derive a preliminary best fit model with: M*/M⊙ = 0.70, L/L⊙ = 3.5×10−3, Teff = 12080 K, MH/M* = 10−4.4.The ZZ Ceti HS 0507+0434B is of special interest because it lies in a common proper motion system with HS 0507+0434A, another DA white dwarf of Teff = 20000 K. It lies in the middle of the instability strip and is a potentially useful pulsator to study the convection‐pulsation interaction. Its pulsation spectrum has shown 10 independent frequencies and a number of linear combinations. In an effort to better constrain its internal structure, we have performed new observations of HS 0507+0434B. We describe these new data obtained from mono‐site as well as multi‐site campaigns between 2007 and 2010. The analysis of these data reveals a complex behavior of the pulsator, with both frequency and amplitude variations on this 1.5 year time scale. We identify 7 l = 1 multiplets. We computed a grid of models and their l = 1 g‐modes. We then applied a χ2 fit method to derive a preliminary best fit model with: M*/M⊙ = 0.70, L/L⊙ = 3.5×10−3, Teff = 12080 K, MH/M* = 10−4.4.

KUV 08368+4026 was considered a member of ZZ Ceti stars with large amplitudes and frequencies around a narrow range of 1.6 mHz and 2.0 mHz. As the first effort of asteroseismological study for KUV 08368+4026 since the discovery of its variability in 1996, a single‐site run in February of 2009 and a 3‐site observation campaign from December of 2009 to January of 2010 for KUV 08368+4026 were carried out in China and Mexico. More than 15,000 frames on 32 nights were obtained. 19 frequencies were distinguished in the datasets with 8 frequencies identified as independent modes. These results are helpful in determining stellar parameters including mass, rotation period, hydrogen mass fraction, and in constructing practical stellar and oscillation models, which will bring new knowledge about the internal structures of ZZ Ceti stars.