Principle Investigators : Dolez, Vauclair, Kleinman
The hydrogen-atmosphere DA white dwarfs are the most common subclass of the white dwarfs. The details of their structure and composition hold important keys to unraveling mysteries of stellar evolution and galactic cosmochronology. The Whole Earth Telescope was invented to exploit the nonradial pulsations inherent in some white dwarfs to reveal their interiors to us and has been quite successful with members of the DO and DB classes of white dwarfs (amongst other classes of objects). HL Tau 76 we now hope will expand these successes to the DAs. We thus intend to observe this star with the WET to determine key quantities such as its H-layer mass, He-layer mass, and core composition.
One of the most important unanswered questions about the DAs is a quantity that asteroseismology can measure: the mass of the surface H layer. Depending on the mechanisms of late stellar evolution mass loss, this layer can be anywhere between that determined by the remains of H shell burning ($\approx 10^{-4}\Msun$) to the thinnest that would still allow the star to appear as a surface, hydrogen-rich DA ($\approx 10^{-15}\Msun$). Recently discussed similarities in many of the DAVs (including HL Tau 76) suggest a value closer to the former; past work on the chemical and spectral evolution of DAs prefers valuescloser to the latter. Getting a definitive measurement of the H layer on HL Tau 76 will be a big step in solving this problem. In addition, the current uncertainty in the DA H layer mass adds about an extra giga-year uncertainty to the age of the local galactic disk as measured by the coolest known white dwarfs.
HL Tau 76 is the first discovered DAV$^1$. It has been observed by several groups$^{2,3,4}$ in the early seventies. Since 1989, a renewed effort has been made to observe this star in several mono-site or bi-site campaigns$^{5,6}$, and in 1996, as second priority in a WET campaign, resulting in about 150 hours of observations$^7$. Despite the large amount of observations, however, none of them have sufficient coverage to allow us to disentangle possible multiplets from the aliasing effect of the observing windows. Thus it is at this point still impossible to give a firm asteroseismological analysis of this star: the information is not sufficient to extract a unique white dwarf model which fits the pattern of the observed modes. The single- and bi-site observations we already have are about as good as is possible to get; we must go to more complete coverage, ie. the WET, to begin to resolve the modes present in this star. What our current observations have shown us, however, is that we have every reason to believe we will be handsomely rewarded for our efforts.
A successful WET run on HL Tau 76 will allow us take advantage of the large archive of past observations. Once the modes are resolved by the WET,we can re-analyze the earlier data, knowing what unresolved modes may be present in them. Combining all the available data together with a good WET run will give valuable information on the mid- to long-term evolution of the star. We can then explore mode phase stability (similar to the ongoing analysis of another DAV, G117--B15A), testing current theories of white dwarf cooling and possibly probing many non-linear aspects of DAV pulsations.
We believe HL Tau 76 is the most promising DAV for successful asteroseismological measurements: it has many stable pulsation modes, increasing the number of interior probes available to us; many of these modes appear to include unresolved multiplets which the proposed observations will help resolve, providing key information for mode identification and analysis; and finally there is a large amount of archival data which we can make use of once we initially resolve the star's pulsations with the WET, thereby exploring the pulsational stability over a range of timescales, including days, years, and decades, and continuing on to the longest and most cosmologically interesting timescale, that of secular cooling. Indeed, HL Tau 76 has a lot of information sealed deep in its interior. These proposed observations will help extract it.
1. Landoldt, A., 1968 ApJ 153 151 2. Warner, B. & Nather, R.E., 1972, MNRAS, 156, 1 3. Pages, C.G., 1972, MNRAS, 159, 25 4. Fitch, W.S., 1973, ApJ, 181, L95 5. Kleinman, S.J., 1995, Ph.D. Dissertation, University of Texas, Austin 6. Dolez, N. & Kleinman, S.J., 1997, in "White Dwarfs: 10th European Workshop", eds. Isern, J., Hernanz, M., Garcia-Berro, E., Kluwer Academic Pub., 459 7. Dolez, N., 1997, Balt. Ast., 7, 153
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