Gaia white dwarfs within 40 pc - I. Spectroscopic observations of new candidates
Kepler, S. O.; Hermes, J. J.; Gänsicke, B. T.; de Martino, D.; Jordan, S.; Tremblay, P. -E.; Marsh, T. R.; Hollands, M. A.; Koester, D.; Rebassa-Mansergas, A.; Raddi, R.; Torres, S.; Cunningham, T.; Parsons, S. G.; Toloza, O.; Schreiber, M. R.; Pelisoli, I.; Charpinet, S.; Silvotti, R.; McCleery, J.; Brown, W. R.; Gentile Fusillo, N. P.; Cukanovaite, E.; Farihi, J.; Kleinman, S. J.; Manser, C. J.; Nitta, A.; Toonen, S.; Izquierdo, P.; Ren, J. -J.; Giammichele, N.; van Grootel, V.; Hoskin, M. J.
United Kingdom, Germany, Spain, United States, France, Belgium, Brazil, Italy, China, Chile
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 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 (<Teff> = 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 that is strongly Fe- and Ni-rich, indicative of the accretion of a planetesimal with core-Earth composition. We find one extremely massive (1.31 ± 0.01 M⊙) 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. A high spectroscopic completeness (>99 per cent) has now been reached for Gaia DR2 sources within 40-pc sample, in the Northern hemisphere (δ > 0°) 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.