Download Scientific Justification

Scientific Justification
As white dwarf stars cool over time, from hot planetary nebula nuclei down to the coolest temperatures possible given the finite age of the Galaxy, there are three narrow ranges of temperature
where they can begin to pulsate. The interval where a given white dwarf will pulsate is determined
by its spectral type. The hydrogen-atmosphere variables (DAVs), clustered near T eff ∼ 12, 000 K,
are the coolest class of pulsating white dwarfs. Just as earthquakes allow seismologists to study the
interior of the Earth, the global pulsations in white dwarfs send gravity-driven seismic waves deep
through the interior and bring information to the surface in the form of brightness variations. We
can use high-speed photometry to observe these variations in brightness over time, and then match
the observations with a theoretical model to determine the internal composition and structure.
When a white dwarf gets cool enough, the C/O core eventually undergoes a phase transition from
liquid to solid—it crystallizes, from the center outward. This is important because the crystallization process releases latent heat, which delays the gradual cooling of the star. Since we can use
the coolest white dwarfs in any stellar population (the Galactic disk and halo, open and globular
clusters) to determine the age, it is crucial that we calibrate this cooling delay. Otherwise, we will
underestimate the true ages. A typical white dwarf with a mass of 0.6 M¯ will begin to crystallize
when it reaches Teff ∼ 6000-8000 K, depending on the core composition. More massive white dwarfs
have higher central pressures, so they can begin to crystallize at higher temperatures.
Prior to the Sloan Digital Sky Survey (SDSS), only one white dwarf was known (BPM 37093)
that was massive enough (1.1 M¯ ) to be crystallizing while still hot enough to be pulsating. The
pulsations do not penetrate the solid core, so the inner boundary for each mode is located at the
edge of the crystallized core rather than at the center of the star. This changes the period of each
mode, compared to what it would have been in the absence of crystallization. As a consequence,
we can calibrate the theory of crystallization empirically by modeling these pulsations. The initial
theoretical study of this star was performed by Montgomery & Winget (1999). When attempting to
match the average period spacing with models, they found a troubling degeneracy between changes
to the crystallized mass fraction and changes to the thickness of the surface hydrogen layer, leading
to ambiguous results. By exploiting the information contained in the individual periods, Metcalfe
et al. (2004, 2005) were recently able to lift this degeneracy and determine a unique best-fit model.
To ensure that our structural models provide an adequate description of the stellar interior, it is
essential that we apply this method to additional white dwarf stars. Imposing the requirement
that our model must be able to explain two independent sets of observations—from stars operating
under slightly different physical conditions—is a very powerful constraint. Spectra of the newlydiscovered star WD0923+0120 indicate that it is the most massive DAV star in the current SDSS
sample (1.06 M¯ , Mukadam et al. 2004). Just like BPM 37093, it exhibits the very low-amplitude
pulsations that are characteristic of a crystallizing star (see Figure 1). The discovery observations
were limited to two short (∼2 hour) runs to confirm the star’s variability, but these data are
insufficient for asteroseismic model-fitting. We propose one week of time-series photometry as part
of a multi-site campaign to resolve the pulsation spectrum of this star. Detailed modeling of these
observations will provide an independent test of crystallization theory in stellar interiors.
REFERENCES
Metcalfe, T. S., Montgomery, M. H., Kanaan, A. 2004, Testing White Dwarf Crystallization Theory
with Asteroseismology of the Massive Pulsating DA Star BPM 37093, ApJ, 605, L133.
Metcalfe, T. S., Montgomery, M. H., Kanaan, A. 2005, The Crystal Method: Asteroseismology of
BPM 37093, ASP Conf., 334, 465.
Montgomery, M. H. & Winget, D. E. 1999, The Effect of Crystallization on the Pulsations of White
Dwarf Stars, ApJ, 526, 976
Mukadam, A. S., et al. 2004, Thirty-Five New Pulsating DA White Dwarf Stars, ApJ, 607, 982
Figure 1—The Fourier Transform (FT) of the discovery observations for WD0923+0120 from a
2-hour light curve (top, Mukadam et al. 2004), compared to a similar FT of the previously known
crystallized pulsator BPM 37093 (bottom, Metcalfe et al. 2004). There are 8 independent pulsation
modes in BPM 37093 with frequencies between 1500 and 2000 µHz, but only two clear peaks appear
in the FT from the 2-hour observation. WD0923+0120 exhibits an unknown number of pulsation
modes in this same frequency range with a larger total power than BPM 37093, but only one peak
is clearly visible in the discovery observations. We are proposing a week-long multi-site campaign
to resolve the detailed frequency structure in this star to provide an independent test of stellar
crystallization theory.