Spectroscopic confirmation of UV-bright white dwarfs from the Sandage Two-Color Survey of the Galactic Plane

TL;DR

This study spectroscopically confirms the nature of UV-bright sources from the Sandage survey as various types of white dwarfs and hot subdwarfs, expanding the catalog of known hot stellar remnants.

Contribution

It provides the first spectroscopic confirmation and classification of UV-bright white dwarf candidates from the Sandage Two-Color Survey, including physical parameter estimates.

Findings

Confirmed 30 hydrogen white dwarfs (DA types)

Identified 5 helium white dwarfs (DB types)

Discovered 3 hot subdwarfs (sdOB, sdB)

Abstract

We present spectroscopic observations confirming the identification of hot white dwarfs among UV-bright sources from the Sandage Two-color Survey of the Galactic plane and listed in the Lanning (Lan) catalog of such sources. A subsample of 213 UV bright Lan sources have been identified as candidate white dwarfs based on the detection of a significant proper motion. Spectroscopic observations of 46 candidates with the KPNO 2.1m telescope confirm 30 sources to be hydrogen white dwarfs with subtypes in the DA1-DA6 range, and with one of the stars (Lan 161) having an unresolved M dwarf as a companion. Five more sources are confirmed to be helium white dwarfs, with subtypes from DB3 to DB6. One source (Lan 364) is identified as a DZ 3 white dwarf, with strong lines of calcium. Three more stars are found to have featureless spectra (to within detection limits), and are thus classified as DC white dwarfs. In addition, three sources are found to be hot subdwarfs: Lan 20 and Lan 480 are classified as sdOB, and Lan 432 is classified sdB. The remaining four objects are found to be field F star interlopers. Physical parameters of the DA and DB white dwarfs are derived from model fits.