New nearby white dwarfs from Gaia DR1 TGAS and UCAC5/URAT

Using an accurate Gaia TGAS 25pc sample, nearly complete for GK stars, and selecting common proper motion (CPM) candidates from UCAC5, we search for new white dwarf (WD) companions around nearby stars with relatively small proper motions. For investigating known CPM systems in TGAS and for selecting CPM candidates in TGAS+UCAC5, we took into account the expected effect of orbital motion on the proper motion as well as the proper motion catalogue errors. Colour-magnitude diagrams (CMDs) $M_J/J-K_s$ and $M_G/G-J$ were used to verify CPM candidates from UCAC5. Assuming their common distance with a given TGAS star, we searched for candidates that occupied similar regions in the CMDs as the few known nearby WDs (4 in TGAS) and WD companions (3 in TGAS+UCAC5). CPM candidates with colours and absolute magnitudes corresponding neither to the main sequence nor to the WD sequence were considered as doubtful or subdwarf candidates. With a minimum proper motion of 60mas/yr, we selected three WD companion candidates, two of which are also confirmed by their significant parallaxes measured in URAT data, whereas the third may also be a chance alignment of a distant halo star with a nearby TGAS star (angular separation of about 465arcsec). One additional nearby WD candidate was found from its URAT parallax and $GJK_s$ photometry. With HD 166435 B orbiting a well-known G1 star at ~24.6pc with a projected physical separation of ~700AU, we discovered one of the hottest WDs, classified by us as DA2.0$\pm$0.2, in the solar neighbourhood. We also found TYC 3980-1081-1 B, a strong cool WD companion candidate around a recently identified new solar neighbour with a TGAS parallax corresponding to a distance of ~8.3pc and our photometric classification as ~M2 dwarf. This raises the question whether previous assumptions on the completeness of the WD sample to a distance of 13pc were correct.

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