The sdA problem - III. New extremely low-mass white dwarfs and their precursors from Gaia astrometry

TL;DR

This study uses Gaia astrometry to identify and characterize extremely low-mass white dwarfs and their precursors within the sdA star population, significantly increasing known (pre-)ELMs.

Contribution

It introduces a new method leveraging Gaia data to distinguish (pre-)ELMs from other sdA stars, expanding the catalog of these objects by up to 50%.

Findings

90 sdA stars are inconsistent with the main sequence.

37 are near or within the white dwarf cooling sequence.

53 are likely (pre-)ELMs, increasing known numbers by 50%.

Abstract

The physical nature of the sdA stars---cool hydrogen-rich objects with spectroscopic surface gravities intermediate between main sequence and canonical mass white dwarfs---has been elusive since they were found in Sloan Digital Sky Survey Data Release 12 spectra. The population is likely dominated by metal-poor A/F stars in the halo with overestimated surface gravities, with a small contribution of extremely low-mass white dwarfs and their precursors, i.e., ELMs and pre-ELMs. In this work, we seek to identify (pre-)ELMs with radii smaller than is possible for main sequence stars, allowing even for very low metallicity. We analyse 3891 sdAs previously identified in the Sloan Digital Sky Survey using Gaia DR2 data. Our Monte Carlo analysis supports that 90 of these are inconsistent with the main sequence. 37 lie close to or within the canonical white dwarf cooling sequence, while the remaining 53 lie between the canonical white dwarfs and main sequence, which we interpret as likely (pre-)ELMs given their spectral class. Of these, 30 pass more conservative criteria that allow for higher systematic uncertainties on the parallax, as well as an approximate treatment of extinction. Our identifications increase the number of known (pre-)ELMs by up to 50 per cent, demonstrating how Gaia astrometry can reveal members of the compact (pre-)ELM subpopulation of the sdA spectral class.