The spectroscopic characteristics of intermediate-aged pre-main-sequence stars: The η Chamaeleontis cluster

We present a study of calibrated low-resolution spectra of the 18 known primaries of the ≈9-Myr-old η Chamaeleontis (η Cha) pre-main-sequence (PMS) star cluster. Using synthetic broad-band colours and narrow-band continuum-sensitive, temperature-sensitive and gravity-sensitive indices derived from the spectra, we compare the η Cha stars to standard dwarfs. We find that the VRI colours of the PMS stars are indistinguishable from those of main-sequence stars, but that a B-band excess attaining ≈0.2 mag for late-M cluster stars is present, which might be an indicator of gravity, metallicity and/or activity differences between the two samples of stars. The narrow-band spectral indices for the η Cha stars possibly indicate higher metallicity and strongly indicate lower surface gravity than the dwarf indices, consistent with the elevated location of the cluster in the Hertzsprung-Russell evolutionary diagram. Using the derived synthetic colours and indices, we adopt spectral types for the late-type η Cha stars. We then produce a table of absolute optical magnitudes and colours representing the cluster isochrone for comparison with PMS evolutionary models. From our results we also conclude that the η Cha stars are unreddened, consistent with the group being a sample of older PMS stars distant from obscuring molecular clouds, except for the A1 member HD 75505 for which we confirm A_V = 0.4 mag likely due to the presence of circumstellar material.