First Results from Pan-STARRS1: Faint, High Proper Motion White Dwarfs in the Medium-Deep Fields

Abstract

The Pan-STARRS1 survey has obtained multi-epoch imaging in five bands (Pan-STARRS1 gP1, rP1, iP1, zP1, and yP1) on twelve “Medium Deep Fields”, each of which spans a 3.3 degree circle. For the period between Apr 2009 and Apr 2011 these fields were observed 50–200 times. Using a reduced proper motion diagram, we have extracted a list of 47 white dwarf (WD) candidates whose Pan-STARRS1 astrometry indicates a non-zero proper motion at the 6σ level, with a typical 1σ proper motion uncertainty of 10 mas/yr. We also used astrometry from SDSS (when available) and USNO-B to assess our proper motion fits. None of the WD candidates exhibits evidence of statistically significant parallaxes, with a typical 1σ uncertainty of 8 mas. Twelve of these candidates are known WDs, including the high proper motion (1.7′′ yr−1) WD LHS 291. We confirm three more objects as WDs through optical spectroscopy. Based on the Pan-STARRS1 colors, ten of the stars are likely to be cool WDs with 4170 K < Teff < 5000 K and cooling ages <9 Gyr. We classify these objects as likely thick disk WDs based on their kinematics. Our current sample represents only a small fraction of the Pan-STARRS1 data. With continued coverage from the Medium Deep Field Survey and the 3π survey, Pan-STARRS1 should find many more high proper motion WDs that are part of the old thick disk and halo.