The young, active binary star EK Draconis

Abstract

EK Dra (HD 129333) is a young, active, nearby star that is orbited by a low mass companion. By combining new speckle observations with old and new radial velocity measurements we find that the orbit is highly eccentric with e=0.82\pm0.03, and we derive the true masses of both components. The masses are $0.9\pm0.1 {\rm M}_\odot$ and $0.5\pm0.1 {\rm M}_\odot$, for the primary and secondary, respectively. From high resolution spectra we derive a new $T_{\rm eff}$ of $5700\pm70$ K, and a $\log g$ of $4.37\pm0.10$, which is different to previous estimates. However, the new spectroscopic distance differs by only 5.8% to the distance derived by parallax measurement of the Hipparcos satellite and thus the stellar parameters are presumably more realistic than older determinations. We derive a somewhat higher value for the metallicity of $[Fe/H]=-0.16\pm0.07$. EK Dra turns out to be one of the few nearby young stars which will evolve similar to the sun. The precise radial velocity measurements taken in the course of this program also allows us to shed more light on to the activity of this star. In 2001 and 2002 we find a periodic signal of the radial velocity variations with a period of $2.767\pm0.005$ days which we interpret as the rotation period. This signal vanishes in 2003. However the signal can be recovered if only the spectra in which the photospheric lines are asymmetric are used. On the other hand, we do not find a close correlation between the asymmetry of photospheric lines and the radial velocity.