The masses, radii and luminosities of the components of U Geminorum
Monthly Notices of the Royal Astronomical Society
Oxford University Press
We present a phase-resolved spectroscopic study of the secondary star in the cataclysmic variable (CV) U Gem. We use our data to measure the radial velocity semi-amplitude, systemic velocity and rotational velocity of the secondary star. Combining this with literature data allows us to determine masses and radii for both the secondary star and white dwarf, which are independent of any assumptions about their structure. We use these to compare their properties to those of field stars and find that both components follow field mass-radius relationships. The secondary star has the mass, radius, luminosity and photometric temperature of an M2 star, but a spectroscopic temperature of M4. The latter may well be due to a high metallicity. There is a troubling inconsistency between the radius of the white dwarf inferred from its gravitational redshift and inclination and that inferred from its temperature, flux and astrometric distance. We find that there are two fundamental limits to the accuracy of the parameters we can derive. First, the radial velocity curve of the secondary star deviates from a sinusoid, in part because of its asphericity (which can be modelled) and in part because the line flux is not evenly distributed over its surface. Secondly, we cannot be certain which spectral type is the best match for the lines of the secondary star, and the derived rotational velocity is a function of the spectral type of the template star used.
The Isaac Newton Telescope is operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. We thank Stuart Littlefair for useful discussions, and the referee Robert Smith for a careful reading and useful suggestions. Computing was performed on the Exeter node of the Starlink network, funded by PPARC.
This is the author accepted manuscript. The final version is available from [publisher] via http://dx.doi.org/10.1111/j.1365-2966.2005.09262.x
Monthly Notices of the Royal Astronomical Society, 2005, Vol. 361 (3), pp. 1091 - 1101