A puzzling periodicity in the pulsating DA white dwarf G 117-B15A

Rubina Kotak, M.H. Van Kerkwijk, J.C. Clemens

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We present time-resolved optical spectrophotometry of the pulsating hydrogen atmosphere (DA) white dwarf G 117-B15A. We find three periodicities in the pulsation spectrum (215 s, 272 s, and 304 s) all of which have been found in earlier studies. By comparing the fractional wavelength dependence of the pulsation amplitudes ( chromatic amplitudes) with models, we confirm a previous report that the strongest mode, at 215 s, has l = 1. The chromatic amplitude for the 272 s mode is very puzzling, showing an increase in fractional amplitude with wavelength that cannot be reproduced by the models for any ` at optical wavelengths. Based on archival HST data, we show that while the behaviour of the 215 s mode at ultra-violet wavelengths is as expected from models, the weird behaviour of the 272 s periodicity is not restricted to optical wavelengths in that it fails to show the expected increase in fractional amplitude towards shorter wavelengths. We discuss possible causes for the discrepancies found for the 272 s variation, but find that all are lacking, and conclude that the nature of this periodicity remains unclear.
Original languageEnglish
Pages (from-to)301-308
Number of pages8
JournalAstronomy and Astrophysics
Issue number1
Publication statusPublished - Jan 2004

ASJC Scopus subject areas

  • Space and Planetary Science

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