2008MNRAS.389.1461L

We present a study of flux-calibrated low-resolution optical spectroscopy of 10 stars belonging to eight systems in the ∼5-Myr-old εChamaeleontis (εCha) pre-main-sequence (PMS) star cluster. Using synthetic broad-band colours, narrow-band continuum, atomic and molecular lines derived from the spectra, we compare the εCha stars to a slightly older PMS cluster, the ~8-Myr-old ηCha cluster and to main-sequence dwarfs. Using synthetic VRI colours and other indices, we find that the relationship between broad-band colours and spectroscopic temperature indicators for εCha cluster members is indistinguishable from that of Gyr-old dwarfs. This result is identical to that found earlier in ηCha. Gravity-sensitive line indices place the cluster between the giant and dwarf sequences, and there is clear evidence that εCha stars have lower surface gravity than ηCha stars. This result is consistent with εCha being the slightly younger PMS association, a few Myr younger according to the Hertzsprung-Russell (HR) diagram placement of these two clusters and comparison with PMS evolutionary grids. Late M-type εCha cluster members show a B-band flux excess of ~0.2mag compared to observations of standard dwarfs, which might be related to enhanced magnetic activity. A similar level of excess B-band emission appears to be a ubiquitous feature of low-mass members of young stellar populations with ages less than a few hundred Myr, a very similar time-scale to the PMS phase of elevated relative X-ray luminosity.