Vauclair, G.
Fu, J.-N.
Solheim, J.-E.
Kim, S.-L.
Chevreton, M.
Dolez, N.
Chen, L.
Wood, M. A.
Silver, I. M.
The pre-white dwarf pulsators of PC 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.
Dolez, N.
Vauclair, G.
Kleinman, S. J.
Chevreton, M.
Fu, J. N.
Solheim, J.-E.
González Perez, J. M.
Ulla, A.
Fraga, L.
Kanaan, A.
Reed, M.
Kawaler, S.
O'Brien, M. S.
Metcalfe, T. S.
Nather, R. E.
Sanwal, D.
Klumpe, E. W.
Mukadam, A.
Wood, M. A.
Ahrens, T. J.
Silvestri, N.
Sullivan, D.
Sullivan, T.
Jiang, X. J.
Xu, D. W.
Ashoka, B. N.
Leibowitz, E.
Ibbetson, P.
Ofek, E.
Kilkenny, D.
Meištas, E. G.
Alisauskas, D.
Janulis, R.
Kalytis, R.
Moskalik, P.
Zola, S.
Krzesinski, J.
Ogloza, W.
Handler, G.
Silvotti, R.
Bernabei, S.
Fu, J.-N.
Vauclair, G.
Solheim, J.-E.
Chevreton, M.
Dolez, N.
O'Brien, M. S.
Kim, S.-L.
Park, B.-G.
Handler, G.
Medupe, R.
Wood, M.
Gonzalez Perez, J.
Hashimoto, O.
Kinugasa, K.
Taguchi, H.
Kambe, E.
Provencal, J.
Dreizler, S.
Schuh, S.
Leibowitz, E.
Lipkin, Y.
Zhang, X.-B.
Paparo, M.
Szeidl, B.
Virághalmy, G.
Zsuffa, D.
Dolez, N.
Vauclair, S.
Michel, E.
Hui-Bon-Hoa, A.
Vauclair, G.
Le Contel, D.
Mathias, P.
Poretti, E.
Amado, P. J.
Rainer, M.
Samadi, R.
Baglin, A.
Catala, C.
Auvergne, M.
Uytterhoeven, K.
Valtier, J. -C.
Context. We present an analysis of the observations of HD 51106 and HD 50747 by the satellite CoRoT, obtained during its initial run, and of the spectroscopic preparatory observations. Aims. We complete an analysis of the light curve, extract the main frequencies observed, and discuss some preliminary interpretations about the stars. Methods. We used standard Fourier transform and pre-whitening methods to extract information about the periodicities of the stars. Results. HD 51106 is an ellipsoidal binary, the light curve of which can be completely explained by the tidal deformation of the star and smaller secondary effects. HD 50747 is a triple system containing a variable star, which exhibits many modes of oscillation with periods in the range of a few hours. On the basis of this period range and the analysis of the physical parameters of the star, we conclude that HD 50747 is gamma-Doradus star.
Charpinet, S.
Green, E. M.
Baglin, A.
van Grootel, V.
Fontaine, G.
Vauclair, G.
Chaintreuil, S.
Weiss, W. W.
Michel, E.
Auvergne, M.
Catala, C.
Samadi, R.
Baudin, F.
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 similar to 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 similar to 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 mu Hz frequency range with a dominant mode at 205.29 mu Hz (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. 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 mu Hz, a quintuplet at 5287 mu Hz and a (l > 2) multiplet at 5412 mu Hz, 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 mu Hz and a doublet at 394 mu Hz show modulated amplitude and constant frequency which can be associated with a narrow transitory regime of the resonance. Another triplet at 519 mu Hz appears to be in a frequency-locked regime where both frequency and amplitude are constant. Additionally, three linear combinations of frequencies near 6076 mu Hz also show amplitude and frequency modulations, which are likely related to a three-mode direct resonance of the type nu(0) similar to nu(1) + nu(2). 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.
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.
Vauclair, G.
Fu, J. -N.
Solheim, J. -E.
Kim, S. -L.
Dolez, N.
Chevreton, M.
Chen, L.
Wood, M. A.
Silver, I. M.
Bognar, Zs.
Paparo, M.
Corsico, A. 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 mu Hz (400 s), its variations are best fitted by a combination of two terms: one long term with a time scale of 5.4 x 10(4) years, which is significantly shorter than the predicted evolutionary time scale of 8 x 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.
Corsico, A. H.
Bertolami, M. M. Miller
Althaus, L. G.
Vauclair, G.
Werner, K.
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-circle dot. 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-circle dot from a comparison between the observed period spacing and the computed asymptotic period spacing, and a stellar mass of 0.567 M-circle dot 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 (delta Pi(i)) over bar = 0.88 s and a root-mean-square residual of sigma(delta Pi i) = 1.27 s. The model has an effective temperature T-eff = 81 500 K, a stellar mass M-* = 0.556 M-circle dot, a surface gravity log g = 7.65, a stellar luminosity and radius of log(L-*/L-circle dot) = 1.14 and log(R-*/R-circle dot) = -1.73, respectively, and a He-rich envelope thickness of M-env = 1.9 x 10(-2) M-circle dot. We derive a seismic distance d similar to 614 pc and a parallax pi similar to 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 similar to 6-20% lower than hitherto thought, and in closer agreement (to within similar to 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.
Fu, J.-N.
Vauclair, G.
Solheim, J.-E.
Chevreton, M.
Dolez, N.
O'Brien, M.S.
Kim, S.-L.
Park, B.-G.
Handler, G.
Medupe, R.
Wood, M.
Perez, J.G.
Hashimoto, O.
Kinugasa, K.
Taguchi, H.
Kambe, E.
Provencal, J.
Dreizler, S.
Schuh, S.
Leibowitz, E.
Lipkin, Y.
Zhang, X.-B.
Paparo, M.
Szeidl, B.
Viraghalmy, G.
Zsuffa, D.
The variable pre-white dwarf PG 1159 stars (GW Vir) are g-mode non-radial pulsators. Asteroseismology puts strong constraints on their global parameters and internal structure. PG 0122+200 defines the red edge of the instability strip and its evolutionary timescale is predicted to be dominated by neutrino emission. Its study offers the opportunity to better understand the instability mechanism and to validate the physics of the neutrino production in dense plasma. To achieve such a goal requires determining precisely its fundamental parameters. This is the goal of this paper. We present new multi-site photometric observations obtained in 2001 and 2002. Together with previous data, they allow us to detect 23 frequencies, composed of 7 triplets and 2 single frequencies, which are used to constrain its internal structure and derive its fundamental parameters. All the observed frequencies correspond to l = 1 g-modes. The period distribution shows a signature of mode trapping from which we constrain the He-rich envelope mass fraction to be -6.0 les log(q y) les -5.3. The comparison of the mode trapping amplitudes among GW Vir stars suggests that the mass-loss efficiency must decrease significantly below T eff les 140 kK. We measure an average period spacing of 22.9 s from which we derive a mass of 0.59 plusmn 0.02 M odot. From the triplets we measure a mean rotational splitting of 3.74 muHz and a rotational period of 1.55 days. We derive an upper limit to the magnetic field of B les 4 times 10 3 G. The luminosity (log L/L odot = 1.3 plusmn 0.5) and the distance (D = 0.7 -0.4+1.0 kpc) are only weakly constrained due to the large uncertainty on the spectroscopically derived surface gravity and the absence of a measured parallax. From the asteroseismic mass, the ratio of the neutrino luminosity on the photon luminosity is 1.6 plusmn 0.2 confirming that the PG 0122+200 evolutionary time scale should be dominated by neutrino cooling. A measurement of P for the largest amplitude untrapped modes should verify this prediction.