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Astron. Astrophys. 329, L13-L16 (1998)

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4. Discussion

From our reported photometric observations we conclude that BPM 24754 is a pulsator. It has the second longest period ([FORMULA]  min) but its amplitude, between 0.007 and 0.025 mag, is smaller than some other ZZ Ceti stars. Therefore, BPM 24754 does not fit the period-amplitude relation - the longer the mean period, the larger the mean amplitude (Fontaine & Winget 1987; Clemens 1993). As BPM 24754 is probably multiperiodic, additional time series photometry are needed to get more information on its amplitude spectrum. Clemens (1993) also showed that both period and amplitude increase monotonically with decreasing temperature, consistent with theory. Then, where is BPM 24754 in relation to the instability strip? To answer this question, we determine its atmospheric parameters [ [FORMULA], [FORMULA] ] through synthetic spectra produced by the ML1/ [FORMULA] model atmospheres of Koester et al. (1994). The atmospheric parameters for BPM 24754 were determined using the same fitting technique as described by Bergeron et al. (1992) that compare only the Balmer line profiles between observed and synthetic spectra. In this case we used six lines: H [FORMULA] to H9 and the best synthetic spectrum fit to BPM 24754 is shown in Fig. 2. The values derived for BPM 24754 are: [FORMULA]  K and [FORMULA]. With these atmospheric parameters, we interpolate the white dwarf evolutionary models of Wood (1995), which have "thick" hydrogen layers of [FORMULA], and derive a mass of [FORMULA]. This effective temperature places it outside the ZZ Ceti instability strip limits as determined by Bergeron et al. (1995) who used both optical and UV spectra and ML2/ [FORMULA] models1. We also recall that [FORMULA] determination is very uncertain for DAV stars. Koester & Vauclair (1997) show that even for well-studied DAV stars, e.g. GD 165, the determination of [FORMULA] from recent years can differ by 2600 K. Moreover, Bergeron et al. (1995) and Koester & Vauclair (1997) show that one can only get consistent solutions by fitting both optical and UV spectra simultaneously. At present, we have only the optical spectra at hand and therefore our conclusions may change if UV spectra become available.

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© European Southern Observatory (ESO) 1998

Online publication: November 24, 1997