The Unusual Cataclysmic Binary Star RBS 0490 and the Space Density of Cataclysmic Variables11Based on observations obtained at the MDM Observatory, operated by Dartmouth College, Columbia University, Ohio State University, and the University of Michigan.

Abstract

ABSTRACTRBS (ROSAT Bright Source) 0490 is a cataclysmic variable star (CV) with unusually strong emission lines. The strength of the emission lines has led to a suggestion that the object is intrinsically faint and correspondingly nearby (∼33 pc), which, if true, would strongly affect estimates of the CV space density. Here we report astrometry, filter photometry, and time‐series spectroscopy of this object. The astrometry gives an absolute parallax πabs = 4.5 ± 1.5 mas and a relative proper motion of 102 mas yr−1. A Bayesian procedure gives a very uncertain distance estimate of d∼300 pc, and the small parallax alone implies d>133 pc (at 2 standard deviations). The mean V magnitude is 17.4, which implies MV = 10.9 - 5log [d/(200 pc)], neglecting extinction. At 200 pc, the space velocity would be over 90 km s−1 with respect to the local standard of rest. The time‐series spectroscopy shows a possible emission‐line radial velocity period near 46 minutes. This would be unusually short for an orbital period, and it may represent some other clock in the system.