X-shooter spectroscopy of young stellar objects: III. Photospheric and chromospheric properties of Class III objects

Traditionally the chromospheres of late type stars are studied through their strongest emission lines, H alpha and CaHK emission. Our knowledge on the whole emission line spectrum is more elusive as a result of the limited spectral range and sensitivity of most available spectrographs. We intend to reduce this gap with a comprehensive spectroscopic study of the chromospheric emission line spectrum of non-accreting pre-main sequence stars. We analyzed X-shooter/VLT spectra of 24 Class III sources from three nearby star-forming regions. We determined the effective temperature, surface gravity, rotational velocity, and radial velocity by comparing the observed spectra with synthetic BT-Settl model spectra. We investigated in detail the emission lines emerging from the stellar chromospheres and combined these data with archival X-ray data to allow for a comparison between chromospheric and coronal emissions. Flux ratios between individual optical emission lines show a smooth dependence on the effective temperature. Future, more complete chromospheric model grids can be tested against this data set.