Spectrophotometric properties of pre-main-sequence stars: The ε Chamaeleontis cluster

We present a study of flux-calibrated low-resolution optical spectroscopy of 10 stars belonging to eight systems in the ∼5-Myr-old ε Chamaeleontis (ε Cha) 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 ε Cha stars to a slightly older PMS cluster, the ≈8-Myr-old η Cha 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 ε Cha cluster members is indistinguishable from that of Gyr-old dwarfs. This result is identical to that found earlier in η Cha. Gravity-sensitive line indices place the cluster between the giant and dwarf sequences, and there is clear evidence that ε Cha stars have lower surface gravity than η Cha stars. This result is consistent with ε Cha 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 ε Cha cluster members show a B-band flux excess of ≈0.2 mag 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.