Spectroscopy of Young Brown Dwarfs and Isolated Planetary Mass Objects

Abstract

This paper presents an overview of the spectroscopic properties of substellar objects found in young (age between 1 and 20 Myr) open clusters and stellar associations. An updated status of spectral classification and temperature calibrations is given. A reddening-independent spectral index is provided for L-type dwarfs. The spectral types that separate stars from brown dwarfs, and brown dwarfs from planetary mass bodies, are discussed. The behaviour of gravity sensitive features, which potentially can play a key role in an age scale for star forming regions and young open clusters, is described. Current one-dimensional schemes for spectral classification of ultracool dwarfs should be taken with caution. The observed emission lines in young brown dwarfs and isolated superplanets are discussed. A possible selection bias against finding accreting brown dwarfs and isolated superplanets (substellar equivalents of the classical T Tauri stars) is identified. Finally, we comment on the possibility of using deuterium burning to provide an age scale for young associations and clusters.