EDP Sciences
Free access
Volume 388, Number 1, June II 2002
Page(s) 219 - 234
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20020474

A&A 388, 219-234 (2002)
DOI: 10.1051/0004-6361:20020474

Time-resolved optical spectroscopy of the pulsating DA white dwarf HS 0507+0434B

New constraints on mode identification and pulsation properties
R. Kotak1, M. H. van Kerkwijk2 and J. C. Clemens3

1  Lund Observatory, Box 43, 22100 Lund, Sweden
2  Astronomical Institute, Utrecht University, PO Box 80000, 3508 TA Utrecht, The Netherlands
    e-mail: M.H.vanKerkwijk@astro.uu.nl
3  Department of Physics and Astronomy, University of North Carolina, Chapel Hill, NC 27599-3255, USA
    e-mail: clemens@physics.unc.edu

(Received 11 September 2001 / Accepted 25 March 2002 )

We present a detailed analysis of time-resolved optical spectra of the ZZ Ceti white dwarf, HS 0507+0434B. Using the wavelength dependence of observed mode amplitudes, we deduce the spherical degree, $\ell$, of the modes, most of which have $\ell=1$. The presence of a large number of combination frequencies (linear sums or differences of the real modes) enabled us not only to test theoretical predictions but also to indirectly infer spherical and azimuthal degrees of real modes that had no observed splittings. In addition to the above, we measure line-of-sight velocities from our spectra. We find only marginal evidence for periodic modulation associated with the pulsation modes: at the frequency of the strongest mode in the lightcurve, we measure an amplitude of $2.6\pm1.0$ km s -1, which has a probability of 2% of being due to chance; for the other modes, we find lower values. Our velocity amplitudes and upper limits are smaller by a factor of two compared to the amplitudes found in ZZ Psc. We find that this is consistent with expectations based on the position of HS 0507+0434B in the instability strip. Combining all the available information from data such as ours is a first step towards constraining atmospheric properties in a convectionally unstable environment from an observational perspective.

Key words: stars: individual: HS 0507+0434, ZZ Psc -- white dwarfs -- oscillations -- convection

Offprint request: R. Kotak, rubina@astro.lu.se

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