A precise HST parallax of the cataclysmic variable EX Hydrae, its system parameters, and accretion rate^*

¹ Universitäts-Sternwarte Göttingen, Geismarlandstr. 11, 37083 Göttingen, Germany e-mail: beuermann@uni-sw.gwdg.de
² New Mexico State University, Box 30001/MSC 4500, Las Cruces, NM 88003 USA e-mail: tharriso@nmsu.edu (Visiting Astronomer, Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation.)
³ McDonald Observatory, University of Texas, Austin, TX 78712 USA e-mail: mca@barney.as.utexas.edu; e-mail: fritz@astro.as.utexas.edu
⁴ Department of Physics and Astronomy, University of Southampton, Highfield, Southampton SO17 1BJ, UK e-mail: btg@astro.soton.ac.uk

Corresponding author: K. Beuermann, beuermann@uni-sw.gwdg.de

Received: 26 March 2003
Accepted: 15 September 2003

Abstract

Using the HST Fine Guidance Sensor, we have measured a high precision astrometric parallax of the cataclysmic variable EX Hydrae,  mas. From the wavelength-integrated accretion-induced energy flux, we derive a quiescent accretion luminosity for EX Hya of  erg s^-1. The quiescent accretion rate then is . The time-averaged accretion rate, which includes a small correction for the rare outbursts, is 6% higher. We discuss the system parameters of EX Hya and deduce  ,  , and , using recent radial velocity measurements of both components and restrictions imposed by other observational and theoretical constraints. We conclude that the secondary is undermassive, overluminous, and expanded over a ZAMS star of the same mass. Near the upper limit to M₁, the accretion rate of the white dwarf coincides with that due to near-equilibrium angular momentum loss by gravitational radiation and angular momentum transfer from the orbit into the spin-up of the white dwarf. Near the lower mass limit, the correspondingly higher accretion rate requires that either an additional angular momentum loss process is acting besides gravitational radiation or that accretion occurs on a near-adiabatic time scale. The latter possibility would imply that EX Hya is in a transient phase of high mass transfer and the associated spin-up of the white dwarf.