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Astron. Astrophys. 342, 745-755 (1999)

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1. Introduction

1.1. Pulsating and nonpulsating PG 1159 stars

The stars of the PG 1159 spectral class (31 members) constitute the intermediate evolutionary phase between the end of the constant luminosity phase - at the tip of the asymptotic giant branch (AGB) - and the beginning of the white dwarf (WD) cooling phase. Probing their interior structure provides direct constraints on both classes of stars and may help to understand better the transition from AGB to WDs and the nuclear burning turn off process. A powerful method to probe the interior structure of the pre-WD stars and to determine some of their basic stellar parameters is given by asteroseismology. This is possible because 15 PG 1159 stars and central stars of planetary nebulae (CSPN) of type [WC], called GW Vir stars from the prototype (PG 1159-035), show multiperiodic luminosity variations which have been interpreted as nonradial g-mode pulsations. Among them, ten are CSPN, while five appear not to be surrounded by a nebula (Bradley 1998). The nature of the luminosity variations of the GW Vir stars was first proven to be stellar pulsation in the case of PG 1159-035 itself (Winget et al. 1991). Nevertheless, and despite the successful results from adiabatic models to which we will refer to below, the pulsation mechanism of the GW Vir stars is still not well understood. Although almost all authors agree that the pulsations should be driven by the [FORMULA]-[FORMULA] mechanism, based on the C/O cyclic ionization (Starrfield et al. 1984), a good agreement between spectroscopic abundances and observed pulsation periods has not been found yet (Bradley & Dziembowski 1996). A new element in this picture was recently added by Dreizler & Heber (1998): their results suggest that the GW Vir pulsations could be related to the nitrogen abundance. On the other hand, in the adiabatic pulsation field, theory may explain several observed phenomena as frequency splitting due to rotation and/or magnetic fields, period spacing of successive overtones, and variations around the average period spacing caused by mode trapping in the outer layers of the star (Kawaler & Bradley 1994 and references therein). However, the measurement of frequency and period spacing, which leads to accurate determination of rotation, weak magnetic fields, stellar mass, external layer masses, and even luminosity and distance, needs power spectra with low noise and high frequency resolution. For these reasons it is necessary to obtain long and nearly continuous data sets, such as those obtained by the Whole Earth Telescope network (Nather et al. 1990).

1.2. HS 2324+3944

The star HS 2324+3944 (hereafter HS 2324) is one out of four peculiar members of the PG 1159 spectral class showing strong H Balmer absorption in their spectra (Dreizler et al. 1996), called "hybrid PG 1159 stars" (Napiwotzki & Schönberner 1991) or lgEH PG 1159, following the notation scheme of Werner (1992). It has an effective temperature of (130 000 [FORMULA] 10 000) K and a surface gravity [FORMULA]=6.2 [FORMULA] 0.2 (Dreizler et al. 1996). Recent new analysis of the HST-GHRS spectrum of HS 2324 show that the C/He and O/He ratio (0.4 and 0.04 by number) is as high as in ordinary PG 1159 stars (Dreizler 1998). Therefore only the hydrogen abundance (H/He=2 by number) makes it unusual. HS 2324 does not show direct signs of on-going mass loss from P Cygni shaped line profiles, like several other luminous PG 1159 stars (Koesterke et al. 1998). However, detailed line profiles from high resolution Keck spectroscopy show evidence of mass loss in the order of roughly [FORMULA] (Dreizler et al., in preparation). Regarding effective temperature and gravity, it belongs to the subgroup of luminous PG 1159 stars which are in general Central Stars of Planetary Nebulae. However, differently from all the other known hybrid PG 1159 stars, no nebula is detected around HS 2324 (Werner et al. 1997).

HS 2324 was discovered to be variable by Silvotti (1996). Handler et al. (1997), with more extensive observations, showed that at least four different frequencies were active and therefore that the GW Vir hypothesis was the most likely. For other two hybrid PG 1159 stars, the nuclei of A 43 and NGC 7094, periodic light variations are only suspected (Ciardullo & Bond 1996). The interest for the variability of HS 2324 is enhanced by its hydrogen abundance. The presence of H was generally considered as a inhibitor of pulsations (Stanghellini et al. 1991). First steps to test the effects of the presence of H in the driving regions have been undertaken by Saio (1996) and Gautschy (1997). The models of Saio (1996) do pulsate with 3[FORMULA] of H mass fraction. The models of Gautschy (1997) are able to reproduce the observed periods of HS 2324, with a very similar H abundance of 20[FORMULA] by mass.

For all the reasons stated above, HS 2324 is a very interesting star: the analysis of its photometric behaviour at high frequency resolution may give important results not only for a detailed study of the star itself, but also for more general questions regarding the GW Vir pulsation phenomenon. Therefore we decided to carry out a multisite photometric campaign on HS 2324, which may be considered as a first step for successive more extensive campaigns.

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© European Southern Observatory (ESO) 1999

Online publication: February 23, 1999