Pulsating low-mass white dwarfs in the frame of new evolutionary sequences

Abstract

Context. Some low-mass white-dwarf (WD) stars with H atmospheres currently being detected in our galaxy, show long-period g(gravity)-mode pulsations, and comprise the class of pulsating WDs called extremely low-mass variable (ELMV) stars. At present, it is generally believed that these stars have thick H envelopes. However, from stellar evolution considerations, the existence of low-mass WDs with thin H envelopes is also possible. Aims. We present a thorough asteroseismological analysis of ELMV stars on the basis of a complete set of fully evolutionary models that represents low-mass He-core WD stars harboring a range of H envelope thicknesses. Although there are currently nine ELMVs, here we only focus on those that exhibit more than three periods and whose periods do not show significant uncertainties. Methods. We considered g-mode adiabatic pulsation periods for low-mass He-core WD models with stellar masses in the range [0.1554–0.4352] M⊙, effective temperatures in the range [6000–10 000] K, and H envelope thicknesses in the interval −5.8 ≲ log(MH/M⋆)≲ −1.7. We explore the effects of employing different H-envelope thicknesses on the adiabatic pulsation properties of low-mass He-core WD models, and perform period-to-period fits to ELMV stars to search for a representative asteroseismological model. Results. We found that the mode-trapping effects of g modes depend sensitively on the value of MH, with the trapping cycle and trapping amplitude larger for thinner H envelopes. We also found that the asymptotic period spacing, ΔΠa, is longer for thinner H envelopes. Finally, we found asteroseismological models (when possible) for the stars under analysis, characterized by canonical (thick) and by thin H envelope. The effective temperature and stellar mass of these models are in agreement with the spectroscopic determinations. Conclusions. The fact that we have found asteroseismological solutions with H envelopes thinner than canonical gives a suggestion of the possible scenario of formation of these stars. Indeed, in the light of our results, some of these stars could have been formed by binary evolution through unstable mass loss.

UVIT Open Cluster Study. I. Detection of a White Dwarf Companion to a Blue Straggler in M67: Evidence of Formation through Mass Transfer

“…This makes the secondary companion belong to the class of low mass WDs, which are formed only in close binaries, as single star evolution prohibits the formation of WDs with mass less than 0.4 M within the Hubble time (Brown et al 2010;Istrate et al 2016). The accepted mechanism for the formation of low-mass He-core WDs is either through unstable mass loss via common-envelope episodes or stable mass loss via Roche-lobe overflow in close binaries (Istrate et al 2016;Calcaferro et al 2018).…”

Discovery, TESS Characterization, and Modeling of Pulsations in the Extremely Low-mass White Dwarf GD 278

“…We further analyzed the identified periods observed in GD 278 using the low-mass white dwarf models of Calcaferro et al (2018). In short, the models expand upon the ELM white dwarf models described in Althaus et al (2013) generated using the LPCODE stellar evolution code.…”

“…Adiabatic pulsation periods for non-radial dipole (ℓ = 1) and quadrupole (ℓ = 2) g-modes were computed employing the adiabatic version of the LP-PUL pulsation code (see, Córsico & Althaus 2006, for details). The models of lowmass white dwarfs we employ in this work include not just canonically thick hydrogen layers, but also include new evolutionary sequences of thinner envelopes (Calcaferro et al 2018).…”

A revisit of PSR J1909$-$3744 with 15-year high-precision timing

“…Fortunately, the effective temperature and the surface gravity of the WD companion of PSR J1909−3744 place it closely to the red edge of the instability strip of extremely-low mass white dwarf variable stars (ELMVs) (Córsico et al 2012;Van Grootel et al 2013;Kilic et al 2018). Additional constraints on the thickness of the hydrogen envelope (and therefore cooling age) could be obtained if one would investigate the asteroseismological properties for the possible evolutionary solutions and (ideally) compare it with observed pulsation periods (Calcaferro et al 2018). This is beyond the scope of this paper and it will be addressed in a future work.…”