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The masses, radii and luminosities of the components of U Geminorum
We present a phase-resolved spectroscopic study of the secondary star inthe cataclysmic variable (CV) U Gem. We use our data to measure theradial velocity semi-amplitude, systemic velocity and rotationalvelocity of the secondary star. Combining this with literature dataallows us to determine masses and radii for both the secondary star andwhite dwarf, which are independent of any assumptions about theirstructure. We use these to compare their properties to those of fieldstars and find that both components follow field mass-radiusrelationships. The secondary star has the mass, radius, luminosity andphotometric temperature of an M2 star, but a spectroscopic temperatureof M4. The latter may well be due to a high metallicity. There is atroubling inconsistency between the radius of the white dwarf inferredfrom its gravitational redshift and inclination and that inferred fromits temperature, flux and astrometric distance.We find that there are two fundamental limits to the accuracy of theparameters we can derive. First, the radial velocity curve of thesecondary star deviates from a sinusoid, in part because of itsasphericity (which can be modelled) and in part because the line flux isnot evenly distributed over its surface. Secondly, we cannot be certainwhich spectral type is the best match for the lines of the secondarystar, and the derived rotational velocity is a function of the spectraltype of the template star used.
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Observation and Astrometry data
| Constellation: | Gémeaux |
| Right ascension: | 06h51m13.91s |
| Declination: | +25°34'06.8" |
| Apparent magnitude: | 9.723 |
| Proper motion RA: | -9.2 |
| Proper motion Dec: | -10.2 |
| B-T magnitude: | 11.305 |
| V-T magnitude: | 9.854 |
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