An emerging and enigmatic spectral class of isolated DAe white dwarfs

TL;DR

This paper investigates two DAe white dwarfs with emission lines but no detectable magnetic fields, analyzing their variability and spectral features to understand their nature and relation to magnetic DAHe stars.

Contribution

It provides detailed spectroscopic and photometric analysis of DAe white dwarfs, highlighting their variability and potential connection to magnetic DAHe stars without confirmed magnetic fields.

Findings

Measured magnetic field upper limit of <0.05 MG for both stars.

Observed anti-phase variability between emission lines and photometry.

DAe stars cluster near DAHe stars in Gaia HR diagram.

Abstract

Two recently discovered white dwarfs, WDJ041246.84$+$754942.26 and WDJ165335.21$-$100116.33, exhibit H$\alpha$ and H$\beta$ Balmer line emission similar to stars in the emerging DAHe class, yet intriguingly have not been found to have detectable magnetic fields. These white dwarfs are assigned the spectral type DAe. We present detailed follow-up of the two known DAe stars using new time-domain spectroscopic observations and analysis of the latest photometric time-series data from TESS and ZTF. We measure the upper magnetic field strength limit of both stars as $B < 0.05$ MG. The DAe white dwarfs exhibit photometric and spectroscopic variability, where in the case of WDJ041246.84$+$754942.26 the strength of the H$\alpha$ and H$\beta$ emission cores varies in anti-phase with its photometric variability over the spin period, which is the same phase relationship seen in DAHe stars. The DAe white dwarfs closely cluster in one region of the Gaia Hertzsprung-Russell diagram together with the DAHe stars. We discuss current theories on non-magnetic and magnetic mechanisms which could explain the characteristics observed in DAe white dwarfs, but additional data are required to unambiguously determine the origin of these stars.