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

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 (215s, 272s, and 304s) 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 215s, has l=1. The chromatic amplitude for the 272s mode is very puzzling, showing an increase in fractional amplitude with wavelength that cannot be reproduced by the models for any l at optical wavelengths. Based on archival HST data, we show that while the behaviour of the 215s mode at ultra-violet wavelengths is as expected from models, the weird behaviour of the 272s 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 272s variation, but find that all are lacking, and conclude that the nature of this periodicity remains unclear.

9 pages, 9 figures; accepted for publication in A&A

References in corpus (2)

• Time-resolved optical spectroscopy of the pulsating DA white dwarf HS 0507+0434B: New constraints on mode identification and pulsation properties
• A new look at the pulsating DB white dwarf GD 358:Line-of-sight velocity measurements and constraints on model atmospheres

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