Discovery of Three Pulsating, Mixed-atmosphere, Extremely Low-mass White Dwarf Precursors

TL;DR

This paper reports the first detection of pulsations in three mixed-atmosphere, extremely low-mass white dwarf precursors, confirming that He presence is essential for pulsations and opening new avenues for studying their internal structure.

Contribution

It provides the first empirical evidence linking He in atmospheres to pulsations in pre-ELM white dwarfs, supporting theoretical models.

Findings

Detected optical variability with periods 320-590 s in three pre-ELM WDs.

Confirmed pulsations occur only with significant He in the atmosphere.

Supports theoretical predictions of p-mode pulsations in mixed-atmosphere pre-ELM WDs.

Abstract

We report the discovery of pulsations in three mixed-atmosphere, extremely low-mass white dwarf (ELM WD, M $\leqslant$ 0.3 M$_{\odot}$) precursors. Following the recent discoveries of pulsations in both ELM and pre-ELM WDs, we targeted pre-ELM WDs with mixed H/He atmospheres with high-speed photometry. We find significant optical variability in all three observed targets with periods in the range 320--590 s, consistent in timescale with theoretical predictions of $p$-mode pulsations in mixed-atmosphere $\approx$ 0.18 M$_{\odot}$ He-core pre-ELM WDs. This represents the first empirical evidence that pulsations in pre-ELM WDs can only occur if a significant amount of He is present in the atmosphere. Future, more extensive, time-series photometry of the brightest of the three new pulsators offers an excellent opportunity to constrain the thickness of the surface H layer, which regulates the cooling timescales for ELM WDs.