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 We report the discovery of low-amplitude oscillations in the star HD 92277 from long, continuous observations in the r and g bands using the CSTAR telescopes in Antarctica. A total of more ...than 1950 hours of high-quality light curves were used to categorize HD 92277 as a new member of the δ Scuti class. We have detected 21 (20 frequencies are independent and one is the linear combination) and 14 (13 frequencies are independent and one is the linear combination) pulsation frequencies in the r and g bands, respectively, indicating a multi-periodic pulsation behavior. The primary frequency f1 = 10.810 days−1 corresponds to a period of 0.0925 days and is an l = 1 mode. We estimate a B − V index of 0.39 and derive an effective temperature of 6800 K for HD 92277. We conclude that long, continuous and uninterrupted time-series photometry can be performed from Dome A, Antarctica, and that this is especially valuable for asteroseismology where multi-color observations (often not available from space-based telescopes) assist with mode identification.
Context. The unprecedented photometric quality and time coverage offered by the Kepler spacecraft has opened up new opportunities to search for signatures of nonlinear effects that affect oscillation ...modes in pulsating stars. Aims. The data accumulated on the pulsating hot B subdwarf KIC 10139564 are used to explore in detail the stability of its oscillation modes, focusing in particular on evidences of nonlinear behaviors. Methods. We analyzed 38 months of contiguous short-cadence data, concentrating on mode multiplets induced by the star rotation and on frequencies forming linear combinations that show intriguing behaviors during the course of the observations. Results. We find clear signatures that point toward nonlinear effects predicted by resonant mode coupling mechanisms. These couplings can induce various mode behaviors for the components of multiplets and for frequencies related by linear relationships. We find that a triplet at 5760 μHz, a quintuplet at 5287 μHz and a (ℓ > 2) multiplet at 5412 μHz, all induced by rotation, show clear frequency and amplitude modulations which are typical of the so-called intermediate regime of a resonance between the components. One triplet at 316 μHz and a doublet at 394 μHz show modulated amplitude and constant frequency which can be associated with a narrow transitory regime of the resonance. Another triplet at 519 μHz appears to be in a frequency-locked regime where both frequency and amplitude are constant. Additionally, three linear combinations of frequencies near 6076 μHz also show amplitude and frequency modulations, which are likely related to a three-mode direct resonance of the type ν0 ~ ν1 + ν2. Conclusions. The identified frequency and amplitude modulations are the first clear-cut signatures of nonlinear resonant couplings occurring in pulsating hot B subdwarf stars. However, the observed behaviors suggest that the resonances occurring in these stars usually follow more complicated patterns than the simple predictions from current nonlinear theoretical frameworks. These results should therefore motivate further work to develop the theory of nonlinear stellar pulsations, considering that stars such as KIC 10139564 now offer remarkable testbeds to do so.
Context. The asteroseismic exploitation of long period, g-mode hot B subdwarf (sdB) pulsators has been a long sought objective undermined, thus far, by the difficulty of obtaining sufficiently ...precise and continuous time series data from the ground. Aims: Fast photometry from space appears to be the only means of gathering the appropriate asteroseismic data for this type of star. We explore this possibility with the CoRoT (COnvection, ROtation, and planetary Transits) satellite. Methods: We obtained ~24 days of high quality, nearly continuous photometric data with CoRoT during a short run (SRa03) dedicated to the long period sdB pulsator KPD 0629-0016. We analysed the frequency (period) content of the CoRoT time series by combining Fourier analysis, nonlinear least squares fitting, and prewhitening techniques. Results: Our study has led to the detection of a large number of g-mode pulsations in KPD 0629-0016, with 17 frequencies clearly identified in addition to 7 possible (although more uncertain) peaks emerging above the mean noise level (estimated at ~57 ppm). This is more than is typically detected for sdB stars from the ground and, more importantly, the frequencies of all uncovered g-modes are, for the first time, reliably measured. This paves the way for a detailed asteroseismic analysis of this star. The oscillations are found in the 90-400 muHz frequency range with a dominant mode at 205.29 muHz (P = 1.353 h) of amplitude 0.246% of the mean brightness, i.e., typical of mid-radial order g-mode pulsations. Conclusions: These photometric observations of KPD 0629-0016 demonstrate that g-mode sdB pulsators have rich oscillation spectra that are accessible to current space-based facilities. CoRoT opens up a new era in asteroseismology of hot B subdwarf stars.
Context.
A non-negligible fraction of white dwarf stars show the presence of heavy elements in their atmospheres. The most accepted explanation for this contamination is the accretion of material ...coming from tidally disrupted planetesimals, which forms a debris disk around the star.
Aims.
We provide a grid of models for hydrogen-rich white dwarfs accreting heavy material. We sweep a 3D parameter space that has different effective temperatures, envelope hydrogen contents, and accretion rates. The grid is appropriate for determining accretion rates in white dwarfs that show the presence of heavy elements.
Methods.
Full evolutionary calculations of accreting white dwarfs were computed including all relevant physical processes, particularly the fingering (thermohaline) convection, a process neglected in most previous works, which has to be considered to obtain realistic estimations. Accretion is treated as a continuous process, and bulk-Earth composition is assumed for the accreted material.
Results.
We obtain final (stationary or near-stationary) and reliable abundances for a grid of models that represent hydrogen-rich white dwarfs of different effective temperatures and hydrogen contents, which we apply to various accretion rates.
Conclusions.
Our results provide estimates of accretion rates, accounting for thermohaline mixing, to be used for further studies on evolved planetary systems.
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. GD133 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. We started a multi-site photometric follow-up aimed at detecting the signature of this potential planet. As a partial result of this work in progress, we give the parameters of a preliminary best-fit model derived from asteroseismology.
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 l = 1 or l = 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 ± 0.002 M⊙, (2.92 ± 0.02) × 10−3 L⊙ and the hydrogen mass fraction of 10−4 stellar mass.
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.
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 Teff 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.
Context. The PG 1159 pre-white dwarf stars experiment a rapidly cooling phase with a time scale of a few 106 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 × 104 years, which is significantly shorter than the predicted evolutionary time scale of 8 × 106 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.
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