A connection between the instability strips of ZZ Ceti and V777 Herculis white dwarfs

Abstract

Aims. We aim to determine the theoretical instability strips of white dwarfs with diverse H and He content in their atmospheres, from a solar composition to a H-depleted atmosphere. Pulsators with mixed H-He atmospheres are indeed known to exist, and these are the white dwarfs in cataclysmic accreting systems of the GW Lib type. We thus also aim to determine the range of periods of excited pulsation modes, and to qualitatively compare these to the observed periods in GW Lib white dwarf pulsators. Methods. In the first full nonadiabatic stability analysis of pulsators of this kind, we applied a time-dependent convection treatment and an energy leakage argument to compute, for cooling models of white dwarfs with various masses and envelope compositions, the location of the blue and the red edges, as well as the properties of pulsation modes. Results. We find that our derived instability strips form a true continuum in the log g-T eff plane and that their individual location depends uniquely on the assumed atmospheric composition, from the solar composition models at low effective temperatures to the H-depleted models at much higher temperatures. Taking into account our previous results from the ZZ Ceti (pure H atmosphere) and V777 Her (pure He atmosphere) white dwarf pulsators, this implies that all of these instability domains are connected via the same fundamental driving mechanism. Applying our results to the case of white dwarf pulsators of the GW Lib type, we find that our theoretical instability strips can qualitatively account for all of the known cases. The computed range of periods of excited modes also compares qualitatively very well to the observed ones. Conclusions. The GW Lib pulsators are very similar in nature to ZZ Ceti and V777 Her white dwarfs. It is the diverse chemical compositions in their atmosphere and envelope that defines their specific pulsation properties. Beyond GW Lib pulsators, white dwarfs can sometimes exhibit mixed H-He atmospheres, such as in the recently found proto-He white dwarf pulsators. Our results open the way towards quantitative asteroseismology of these various kinds of white dwarfs.

Pulsating white dwarfs: new insights

“…The location of the instability strip of the class of GW Lib stars is affected by accretion, which changes the temperature structure of the envelope and therefore the driving region. Theoretical models indicate excitation of g modes due to the H/HeI ionization zone, but enhanced He abundance due to accretion also can drive pulsations as a result of the subsurface HeII partial ionization zone (Arras et al, 2006;Van Grootel et al, 2015). On the other hand, Saio (2019) proposes that toroidal r modes are also detected -analyzing the substructure of the pulsation peaks-and concludes the outburst in the star GW Librae increased the rotation rate of the H envelope significantly.…”

A Model of the Pulsating Extremely Low-mass White Dwarf Precursor WASP 0247–25B

“…It is known that He is the essential ingredient, through He II-He III ionization, for driving pulsation modes in the regime of effective temperature where the known pulsating ELM proto-WDs are found, i.e., in a region where H is totally ionized and cannot contribute to pulsational driving (Jeffery & Saio 2013;Van Grootel, V. and Fontaine, G. and Brassard, P. and Dupret, M.-A. 2015;Córsico et al 2016;Gianninas et al 2016).…”

NGTS and HST insights into the long-period modulation in GW Librae

“…The H/He ratio affects the driving of pulsations, and has hence to be considered as a third dimension, effectively establishing an instability volume rather than the two-dimensional strips in eff and log . Consequently, an additional HeII partial ionisation zone may form that can drive pulsations at higher temperatures compared to the ZZ Ceti stars (Townsley et al 2004;Arras et al 2006;Van Grootel et al 2015). In fact, the known accreting white dwarf pulsators span a wide range of effective temperatures from 10500 K to above 15000 K (Szkody et al 2010).…”