Gravity modes on rapidly rotating accreting white dwarfs and their variation after dwarf novae

Abstract

Accreting white dwarfs in Cataclysmic variables (CVs) show short-period (tens of minutes) brightness variations that are consistent with non-radial oscillations similar to gravity (g) modes observed in isolated white dwarfs (WDs). GW Librae, a dwarf nova, was the first CV in which non-radial oscillations were observed and continues to be the best studied accreting WD displaying these pulsations. Unlike isolated WDs, accreting WDs rotate rapidly, with spin periods comparable to or shorter than typical low-order oscillation periods. Accreting WDs also have a different relationship between their interior temperature and surface temperature. The surface temperature of an accreting WD varies on a months to year timescale between dwarf novae accretion events, allowing study of how this temperature change effects g-mode behavior. Here we show results from adiabatic seismological calculations for accreting WDs, focusing on low-order ($\ell =1$) modes. We demonstrate how g-modes vary in response to temperature changes in the subsurface layers due to a dwarf nova accretion event. These calculations include rotation non-perturbatively, required by the high spin rate. We discuss the thermal history of these accreting WDs, and compare the seismological properties with and without rotation. Comparison of $g$-mode frequencies to observed objects may allow inference of features of the structure of the WD such as mass, surface abundance, accretion history, and more. The variation of mode frequencies during cooling after an outburst provides a novel method of identifying modes.

arXiv comment18 pages, 13 figures, Accepted to ApJ

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