$\textit{Gaia}$ white dwarfs within 40 pc I: spectroscopic observations of new candidates

TL;DR

This study conducts a comprehensive spectroscopic survey of 230 Gaia-selected white dwarf candidates within 40 parsecs, confirming 191 as white dwarfs and revealing diverse spectral types, compositions, and potential planetary debris, thus enriching the local white dwarf census.

Contribution

First large-scale spectroscopic follow-up of Gaia DR2 white dwarf candidates within 40 pc, providing detailed classifications and identifying rare and interesting objects.

Findings

Confirmed 191 white dwarfs and identified 39 contaminants.

Discovered diverse spectral types including magnetic, metal-rich, and ultra-cool white dwarfs.

Detected potential planetary debris signatures and stellar magnetic phenomena.

Abstract

We present a spectroscopic survey of 230 white dwarf candidates within 40 pc of the Sun from the William Herschel Telescope and Gran Telescopio Canarias. All candidates were selected from $\textit{Gaia}$ Data Release 2 (DR2) and in almost all cases had no prior spectroscopic classifications. We find a total of 191 confirmed white dwarfs and 39 main-sequence star contaminants. The majority of stellar remnants in the sample are relatively cool ($\langle T_{\rm eff} \rangle$ = 6200 K), showing either hydrogen Balmer lines or a featureless spectrum, corresponding to 89 DA and 76 DC white dwarfs, respectively. We also recover two DBA white dwarfs and 9--10 magnetic remnants. We find two carbon-bearing DQ stars and 14 new metal-rich white dwarfs. This includes the possible detection of the first ultra-cool white dwarf with metal lines. We describe three DZ stars for which we find at least four different metal species, including one which is strongly Fe- and Ni-rich, indicative of the accretion of a planetesimal with core-Earth composition. We find one extremely massive (1.31 $\pm$ 0.01 M$_{\odot}$) DA white dwarf showing weak Balmer lines, possibly indicating stellar magnetism. Another white dwarf shows strong Balmer line emission but no infrared excess, suggesting a low-mass sub-stellar companion. High spectroscopic completeness ($>$99%) has now been reached for $\textit{Gaia}$ DR2 sources within 40 pc sample, in the northern hemisphere ($\delta >$ 0 deg) and located on the white dwarf cooling track in the Hertzsprung-Russell diagram. A statistical study of the full northern sample is presented in a companion paper.