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

TL;DR

This study analyzes pulsation modes in the white dwarf G 117-B15A, confirming some modes and revealing puzzling wavelength-dependent amplitude behavior that challenges existing models, especially for the 272s periodicity.

Contribution

The paper provides detailed spectrophotometric analysis of pulsation modes, confirming mode identification and highlighting unexplained behavior in amplitude variations across wavelengths.

Findings

Confirmed the l=1 identification for the 215s mode.

Discovered anomalous wavelength dependence of the 272s mode.

Found that existing models cannot explain the 272s amplitude behavior.

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.