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

¹ Lund Observatory, Box 43, 22100 Lund, Sweden
² Astrophysics Group, Imperial College London, Blackett Laboratory, Prince Consort Road, London, SW7 2BZ, UK
³ Astronomical Institute, Utrecht University, PO Box 80000, 3508 TA Utrecht, The Netherlands
⁴ Department of Astronomy and Astrophysics, University of Toronto, 60 St George Street, Toronto, Ontario M5S 3H8, Canada e-mail: mhvk@astro.utoronto.ca
⁵ Department of Physics and Astronomy, University of North Carolina, Chapel Hill, NC 27599-3255, USA e-mail: clemens@physics.unc.edu

Corresponding author: R. Kotak, rubina@ic.ac.uk

Received: 30 June 2003
Accepted: 19 September 2003

Abstract

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 . 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.