Uncovering the chemical structure of the pulsating low-mass white dwarf SDSS J115219.99+024814.4

Abstract

Pulsating low-mass white dwarf (WD) stars are WDs with stellar masses between 0.30 and 0.45 M⊙ that show photometric variability due to gravity-mode pulsations. Within this mass range, they can harbour both a helium core and hybrid core, depending if the progenitor experienced helium-core burning during the pre-WD evolution. SDSS J115219.99+024814.4 is an eclipsing binary system where both components are low-mass WDs, with stellar masses of 0.362 ± 0.014 M⊙ and 0.325 ± 0.013 M⊙. In particular, the less-massive component is a pulsating star, showing at least three pulsation periods of ∼1314, ∼1069, and ∼582.9 s. This opens the way to use asteroseismology as a tool to uncover its inner chemical structure, in combination with the information obtained using the light-curve modelling of the eclipses. To this end, using binary evolutionary models leading to helium- and hybrid-core WDs, we compute adiabatic pulsations for ℓ = 1 and ℓ = 2 gravity modes with Gyre. We found that the pulsating component of the SDSS J115219.99+024814.4 system must have a hydrogen envelope thinner than the value obtained from binary evolution computations, independently of the inner composition. Finally, from our asteroseismological study, we find a best-fitting model characterized by Teff=10917 K, M = 0.338 M⊙, and MH = 10−6 M⊙ with the inner composition of a hybrid WD.