Asteroseismology of the PG 1159 star PG 0122+200

Context: 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. Aims: To achieve such a goal requires determining precisely its fundamental parameters. This is the goal of this paper. Methods: 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. Results: 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? log(qy) ? -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? 140 kK. We measure an average period spacing of 22.9 s from which we derive a mass of 0.59±0.02 Mo.. From the triplets we measure a mean rotational splitting of 3.74 ?Hz and a rotational period of 1.55 days. We derive an upper limit to the magnetic field of B?4×103 G. The luminosity (log L/Lo. = 1.3±0.5) and the distance (D = 0.7^+1.0_-0.4 kpc) are only weakly constrained due to the large uncertainty on the spectroscopically derived surface gravity and the absence of a measured parallax. Conclusions: From the asteroseismic mass, the ratio of the neutrino luminosity on the photon luminosity is 1.6±0.2 confirming that the PG 0122+200 evolutionary time scale should be dominated by neutrino cooling. A measurement of dot{P} for the largest amplitude untrapped modes should verify this prediction

Whole Earth telescope observations of the ZZ Ceti star HL Tau 76

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 l = 1 and l = 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. ESO 2006