DAHe white dwarfs from the DESI Survey

Abstract

A new class of white dwarfs, dubbed DAHe, that present Zeeman-split Balmer lines in emission has recently emerged. However, the physical origin of these emission lines remains unclear. We present here a sample of 21 newly identified DAHe systems and determine magnetic field strengths and (for a subset) periods which span the ranges of ≃ 6.5–147 MG and ≃ 0.4–36 h respectively. All but four of these systems were identified from the Dark Energy Spectroscopic Instrument (DESI) survey sample of more than 47 000 white dwarf candidates observed during its first year of observations. We present detailed analysis of the new DAHe WD J161634.36+541011.51 with a spin period of 95.3 min, which exhibits an anti-correlation between broadband flux and Balmer line strength that is typically observed for this class of systems. All DAHe systems cluster closely on the Gaia Hertzsprung-Russell diagram where they represent ≃ 1 per cent of white dwarfs within that region. This grouping further solidifies their unexplained emergence at relatively late cooling times and we discuss this in context of current formation theories. Nine of the new DAHe systems are identifiable from SDSS spectra of white dwarfs that had been previously classified as featureless DC-type systems. We suggest high S/N, unbiased observations of DCs as a possible route for discovering additional DAHe systems.

Measuring White Dwarf Variability from Sparsely Sampled Gaia DR3 Multi-epoch Photometry

“…the Gaia DR3 light curves independently, and identify 17 new likely spotted white dwarfs. The majority of the newly identified spotted white dwarfs (12/17) have effective temperatures between 6500 and 10,000 K, placing them inside the range of known DAHe strongly magnetic white dwarfs with Balmer-line emission (Greenstein & McCarthy 1985;Gänsicke et al 2020;Reding et al 2020Reding et al , 2023Walters et al 2021;Manser et al 2023), motivating future time-resolved spectroscopic follow-up. Our sample contains several known DAHe (see Table 2).…”

An emerging and enigmatic spectral class of isolated DAe white dwarfs

“…We investigated the possibility that the period measured for WD J0412+7549 is actually half the period of variability, due to there being potentially two poles or emission spots (Manser et al 2023;Reding et al 2023). We inspected the phase-folded light curves on both 𝑃 = 2.2891144(16) h and 2𝑃, but no additional structures were evident on twice the period.…”

“…Walters et al (2021) conducted recent work on GD 356 with new time-resolved data and also confirmed spectroscopic and photometric variability in this white dwarf. Survey data and targeted spectroscopic follow-up of DAHe candidates, selected due to their photometric variability from hundreds of thousands of white dwarf ★ Contact e-mail: Abbigail.Elms@warwick.ac.uk candidates first identified from the spacecraft Gaia (Gaia Collaboration et al 2023), have led to the discovery of 26 DAHe stars to date (Manser et al 2023;Reding et al 2023).…”

“…There are 1066 white dwarfs in the 40 pc volume-limited sample of white dwarfs (McCleery et al 2020;O'Brien et al 2023) of which 655 are classified as DA, so the identification of four DA white dwarfs with Balmer emission lines to date within this volume attests to a fraction of 0.61 per cent. The two DAe and 26 DAHe stars closely cluster in one region on the Gaia Hertzsprung-Russell diagram (HRD; Gänsicke et al 2020;Walters et al 2021;Manser et al 2023) and have a remarkable homogeneity in atmospheric parameters, with effective temperatures 7400 K ≲ 𝑇 eff ≲ 8500 K and white dwarf masses 0.5 M ⊙ ≲ M WD ≲ 0.8 M ⊙ . The DAe and DAHe stars with reliable time-resolved data have measured variability periods of ≃ 0.08 − 36 h, which is plausibly linked to the rotation period.…”

“…DAe and DAHe white dwarfs are encapsulated into one DA(H)e class in this work based on our initial hypothesis that these objects have a similar origin but differ in terms of magnetic field strength and possibly other properties. The magnetic field strengths observed at the surface of DAHe stars range from ≃ 5 − 147 MG (Greenstein & McCarthy 1985;Gänsicke et al 2020;Reding et al 2020;Walters et al 2021;Manser et al 2023;Reding et al 2023), resulting in a range of physical effects acting upon the white dwarfs including altered or suppressed surface convection and possibly altered hydrostatic structure of atmospheric layers (Landstreet 1987;Tremblay et al 2015;Ferrario et al 2015;Fuller & Mathis 2023). However, 86± 7 10 per cent of magnetic white dwarfs within the same region of the Gaia HRD do not show Balmer line emission (Manser et al 2023), suggesting that magnetic field strength is not the only physical parameter defining the DA(H)e class.…”