Relativistic Corrections in White Dwarf Asteroseismology

Abstract

With the precision now afforded by modern space-based photometric observations from the retired K2 and current TESS missions, the effects of general relativity (GR) may be detectable in the light curves of pulsating white dwarfs (WDs). Almost all WD models are calculated using a Newtonian description of gravity and hydrodynamics. To determine if the inclusion of GR leads to observable effects, we used idealized models of compact stars and made side-by-side comparisons of mode periods computed using a: (i) Newtonian and (ii) GR description of the equilibrium structure and nonradial pulsations. For application to WDs, it is only necessary to include the first post-Newtonian (1 pn ) approximation to GR. The mathematical nature of the linear nonradial pulsation problem is then qualitatively unchanged and the GR corrections can be written as extensions of the classic Dziembowski equations. As such, GR effects might easily be included in existing asteroseismology codes. The idealized stellar models are (i) 1 pn relativistic polytropes and (ii) stars with a cold degenerate electron equation of state featuring a near-surface chemical transition from μ _e = 2 to μ _e = 1, simulating a surface hydrogen layer. A comparison of Newtonian and 1 pn normal mode periods reveals fractional differences in the order of the surface gravitational redshift z . For a typical WD, this fractional difference is ∼10 ^−4 and is greater than the period uncertainty σ _Π /Π of many WD pulsation modes observed by TESS. Consistent theoretical modeling of periods observed in these stars should, in principle, include GR effects to 1 pn order.

Keywords

• Asteroseismology
• Gravitation
• General relativity
• White dwarf stars