White dwarf stars represent the most common end point of stellar evolution. Understanding their interior promises to provide strong constraints on the prior evolution of their progenitors. The study of the pulsating white dwarf stars, which pulsate in normal modes, has already 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). The reason is that white dwarf stars are homogeneous and the pulsators are otherwise the same as the non-pulsators.
However, there are still several open questions. For instance, the pulsating DA white dwarfs (DAVs) were for a long time hard to understand, because they do not show many pulsation modes which we can use for asteroseismological anlysis, hence do not provide much information. These objects appear to separate into two groups, the hotter, low-amplitude, rapid (periods around 200 seconds), pulsators and the cooler, high-amplitude, slow (periods around 600 seconds), pulsators. In recent years, considerable insight into the pulsational behaviour of those stars has however been gained by the method of ensemble asteroseismology, i.e. by studying the group properties of pulsating DAVs (Clemens 1993, PhD thesis, University of Texas), or by making use of temporal changes in the pulsation spectra, revealing more and more modes with time (Kleinman et al. 1998, ApJ 495, 424). By performing ensemble studies, one can observe more normal modes to be used for asteroseismological analysis compared to single-star investigations.
In this respect, the situation for the pulsating DB white dwarf stars (DBVs) is not yet clear. Although the study of GD358 by Winget et al. (1994) has provided the most complete picture of a DBV to date, the other representatives of this group are often poorly observed. In addition, mostly high-amplitude DB pulsators have been found, with pulsation periods and amplitudes similar to that of the high-amplitude DAV stars. This situation has changed with the discovery of pulsations in EC 20058-5234 (Koen et al. 1995, MNRAS 277, 913), which turned out to be a the first short-period low-amplitude DB pulsator.
Very recently, a second short-period low-amplitude DBV has been discovered (Handler, unpublished). Just as EC 20058-5234, this star is also rather hot and it seems now that the pulsating DB white dwarfs separate into two groups like the DAVs. This is an excellent starting point to perform an exploration of the group properties of the DBVs. Several questions need to be addressed:
Regrettably, many DBVs are relatively faint and thus poorly studied. We have therefore started a project to obtain more observational data on these objects, and we have already collected the published and some unpublished observations. From a reanalysis of these data we concluded that we will need to reobserve four known DBVs (KUV 05134+2605, CBS 114, PG 1456+103 and PG 1654+160), for which mostly only small discovery data sets are available. We plan intense single-site studies or small multisite campaigns, including them as low-priority Whole Earth Telescope targets, if possible. The remaining DBVs have been studied with WET already; we will also make use of these data.
With all the data in hand, we will perform uniform period analyses and we will compare the behaviour of the different stars. We will then attempt to arrive at mode identifications for all DBVs, first by applying asteroseismology to the individual stars or attempt ensemble asteroseismology if possible. We are hopeful that this project will let us understand the interior structure (e.g. Helium layer masses) of the DBV white dwarfs as a whole and that of the individual objects and their origin as well.
The current proposal requests to include the DBV KUV 05134+2605 as a secondary target into XCOV 20. For this star, only the discovery data (Grauer et al. 1989, AJ 98, 2221) plus two unpublished runs are available, none of them being longer than 2.5 hours.
A frequency analysis of these data, although suffering from severe aliasing problems, results in four dominating frequencies (1414.9, 2829.7, 1504.7, 1429.5 or 1417.9 microHz [aliasing problem]). Prewhitening those, the two highest peaks in the residual amplitude spectrum appear at frequencies of 1287.6 and 1350.5 microHz. A comparison with the frequencies found for GD 358 (Winget et al. 1994) gives intriguing results:
KUV 05134+2605 GD 358
Freq. Period Ampl. Freq. Period k
(microHz) (s) (mma) (microHz) (s) (m=0)
1287.6 776.6 5.6 1297.6 770.7 17
1350.5 740.5 8.3 1361.9 734.3 16
1414.9 706.8 27.0 1427.3 700.6 15
1417.9 699.6 12.7
1504.7 664.6 13.0 1519.0 658.3 14
2829.7 353.4 7.7
Four of the six frequencies are very close to that of modes of GD 358: the periods are about 6 seconds longer, but the period spacing is the same within the errors. The other two frequencies we found can be interpreted as one rotationally split mode and one 2f-harmonic.
However, because of the severe aliasing, because of the likely presence of further modes, and obviously because of the probable similarity to GD 358 a more detailed study of KUV 05134+2605 is clearly needed; the Whole Earth Telescope is the ideal means to perform it. We will obtain single-site observations outside the WET run as well to increase the time base of the observations and to look for possible temporal variations in the power spectra.
Star RA (2000) Dec (2000) mag KUV 05134+2605 05 16 28 +26 08 36 16.3 (V)For observatories at intermediate Northern latitudes (e.g. McDonald) this star will be observable for approximately three hours after the first priority target has set and before the morning sky background becomes too bright.
The V magnitude given is uncertain and might imply that rather large telescope apertures are required. More practically, the discovery data, which were taken at a 1.5m telescope with a photomultiplier in unfiltered light showed a count rate of approximately 1700 counts/second above sky. We therefore estimate that KUV 05134+2605 can be observed with telescopes down to an aperture of 0.75m for useful data to result.
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