Might carbon-atmosphere white dwarfs harbour a new type of pulsating star?

Abstract

Aims. In the light of the recent and unexpected discovery of a new type of white dwarfs, those with carbon-dominated atmospheres, we examine the asteroseismological potential of such stars. The motivation behind this is based on the observation that past models of carbon-atmosphere white dwarfs have partially ionized outer layers that bear a strong resemblance to those responsible for mode excitation in models of pulsating DB (helium-atmosphere) and pulsating DA (hydrogen-atmosphere) white dwarfs. Methods. We carry out a stability survey of models of carbon-atmosphere white dwarfs following a full nonadiabatic approach. To connect with previously known pulsating white dwarfs, we specifically search for excited g-modes with � = 1 in the period window 80−1200 s. We concentrate on models with Teff ≤ 30 000 K, a limit below which the real carbon-atmosphere stars are found, and investigate the effects of changing the surface gravity, the composition of the envelope, and the treatment of the convective efficiency. Results. Our main result is that carbon-atmosphere white dwarfs may indeed drive low-order g-modes in certain regions of parameter space. For instance, log g = 8.0 models characterized by an envelope composition X(C) = X(He) = 0.5 and ML2 convection, show a broad instability strip in the range 18 400−12 600 K. This is shifted to 20 800−17 200 K for log g = 9.0 models. In this interval of surface gravity, the excited periods are found between ∼100 s and ∼700 s, and the shorter periods are excited in the higher-gravity models. Adding carbon in the envelope mixture tends to extinguish pulsational driving. Conclusions. Given the right location in parameter space, some carbon-atmosphere white dwarfs are predicted to show pulsational instability against gravity modes. We are eagerly awaiting the results of observational searches for luminosity variations in these stars.