Asteroseismology is the technique in which the interior structure of pulsating stars is deduced from the normal modes of their pulsations. We propose to observe a pulsating, helium-atmosphere DB white dwarf, PG 1456+103, to decipher its complicated pulsation spectrum. These observations will enable us to examine the DB stars as a group, to constrain the prior evolution of all DB white dwarf stars and to obtain an estimate of the $^{12}{\rm C}(\alpha,\gamma) ^{16}{\rm O}$ nuclear reaction rate.
The white dwarf stars represent the most common end point of stellar evolution. Considerable progress has been made in recent years in understanding their atmospheric structure, composition and evolution through optical and ultraviolet spectroscopy, and through detailed modelling. However, their interior structures and composition are difficult to probe observationally.
Asteroseismology is the only technique in which the interior structure of pulsating stars is deduced from the normal modes of their pulsations. Most white dwarfs do not pulsate except in certain zones of instability in their cooling curves. Since pulsation is merely a temperature effect in a short-lived phase in the evolution of white dwarf stars, the pulsating ones can be assumed to have the same internal structures as non-pulsators. The study of the pulsating white dwarf stars has already yielded spectacular results and opened new windows of understanding in the structure of white dwarfs in general (e.g. Winget et al. 1991, ApJ 378, 326; Winget et al. 1994, ApJ 430, 839).
Of particular interest are the pulsating helium-atmosphere (DBV) white dwarfs. Although the Whole Earth Telescope (WET, Nather et al. 1990, ApJ 361, 309) has enabled us to explore the interior structure of the prototype DBV GD 358 via asteroseismology, relatively little is known about other members of this class of pulsating star. The reason is that many of the other DBVs are poorly-observed, although the presence of rich pulsational mode spectra is suspected.
However, it is critical to constrain the interior structure and hence the prior evolution of more than just one DBV star; the question of whether DB white dwarf stars originate from DO white dwarfs (as GD 358 most probably has, Dehner \& Kawaler 1995, ApJ 445, L141) or from interacting binary white dwarfs or from both is by far not settled.
In addition, it has recently become clear that nuclear reaction rates can also be constrained via asteroseismology (Metcalfe et al. 2001, ApJ 557, 1021). The relative rates of the 3$\alpha$ and the $^{12}{\rm C}(\alpha,\gamma)^{16}{\rm O}$ reactions in a red giant star determine the final ratio of carbon to oxygen in the core of the resulting white dwarf. For the determination of the $^{12}{\rm C}(\alpha,\gamma) ^{16}{\rm O}$ rate, pulsating white dwarf stars are the best laboratories: asteroseismology provides the only way to measure the internal composition of a white dwarf. Hence, we expect a reduction the presently large uncertainties in our understanding of every astrophysical process that depends on this reaction, from supernovae explosions to galactic chemical evolution, through white dwarf asteroseismology.
Consequently, more DBVs with rich pulsation spectra need to be observed and their asteroseismological potential must be fully exploited. We have re-analysed all the time-series photometry of DBVs available to us and we have acquired and analysed fairly extensive observations of all other DBVs to provide a basis for DBV ensemble studies and to select the best candidates for WET observations. Using GD 358 as a template, we were able to unravel a frequency pattern (most likely of l=1 modes) which is followed by several DBVs, and we identified one star as the most promising WET target, PG 1456+103.
We have obtained two-site data of that star in the year 2001. These data revealed a rich mode spectrum, comparable to that of GD 358. Some of these modes follow the abovementioned general frequency pattern of the DBVs, whereas some do not. Rotational splitting is possible, but not yet detected, and small amplitude/phase instabilities could be present. The latter, together with the poor spectral window (despite the two-site coverage) make the detection of further low-amplitude modes, which are clearly discernible, presently impossible. The aliasing problem also prevents a safe frequency assignment to all but two of the high-amplitude modes. The WET is clearly needed for an unambiguous frequency analysis.
We therefore propose to observe PG 1456+103 with the WET. It is almost guaranteed that a rich mode spectrum allowing satisfying asteroseismological modeling will be revealed. The base for doing ensemble asteroseismology of all DBVs is already laid as well. Therefore, these observations will finally allow us to understand the DBVs as a group, and to constrain the prior evolution of all DB white dwarf stars as well as the nuclear reaction rates.
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