Absolute dimensions of eclipsing binaries. III. KM Hydrae : a detached AM system with unequal components.

Absolute dimensions of the components of the bright double-lined eclipsing Am binary KM Hya have been determined from new high-dispersion coude spectrograms and new, complete uvby light curves. The spectroscopic orbital elements are in excellent agreement with the photometric information on e and ω in the eccentric orbit and define the individual masses with an accuracy of 1-2 %. The photometric analysis takes advantage of the information provided by the phase of secondary minimum and the ratio between the durations of secondary and primary eclipses to fix e and ω. The photometric solution is then well- defined and the photometric elements are derived with a precision of about 1 %. The observed spectral line ratios are consistent with the luminosity ratio found from the light curves, and the rotational velocities correspond to synchronous rotation. The primary component of KM Hya has evolved into the upper half of the main-sequence band, while the secondary is still near the ZAMS. Theoretical stellar structure models by Hejlesen (1980) are found to fit the observations equally well for the following compositions: (X, Z) = (0.70, 0.04) at age 5 10⁸ yr, and (X, Z) = (0.80, 0.02) at age 8 1O⁸ yr, while most B and A-type systems studied fit the composition (X, Z) = (0.70, 0.02). The available evidence is insufficient to decide between the two possibilities for KM Hya. It is interesting that rotation appears to be synchronized in KM Hya which is much younger than the synchronization times computed with the formulae by Press et al. (1975).