Name/Affiliation: Tobias Schmidt (Hamburger Sternwarte, Germany)
Mass determination of young directly imaged brown dwarf and planetary companions of cool G to M stars
About 25 sub-stellar companions with large separations (>~ 50 AU) to their young cool primary stars and brown dwarfs are confirmed by both/either common proper motion and late-M to late-L type spectra. The origin and early evolution of these objects is still under debate. While often these sub-stellar companions are regarded as brown dwarfs, at least few of them are likely massive planets, the mass estimates are very uncertain so far. They are companions to late-type primary stars or brown dwarfs in young associations and star forming regions like Taurus, Upper Scorpius, the TW Hya association, Beta Pic moving group, TucHor association, Lupus, Ophiuchus, and Chamaeleon, hence their ages and distances are well known, in contrast to free-floating brown dwarfs. Here we present how mass estimates of such young directly imaged companions can be derived, using not only evolutionary models, which are currently almost uncalibrated by direct mass measurements of young objects. An empirical classification by medium-resolution spectroscopy is hard, as there is an apparent mismatch between spectra of old field type objects and young low-mass companions at the same effective temperature, hampering a determination of temperature and surface gravity independent from models. We show that from spectra of the objects, using the advantages of light concentration by an AO-assisted integral field spectrograph, temperature, extinction, metallicity and surface gravity can be derived using non-equilibrium radiative transfer atmosphere models as comparison and that this procedure as well allows a mass determination in combination with the luminosities found by the direct observations. We show that uncertainties of log g of 0.2 dex can be reached at the deuterium burning limit. Further to spectra of known objects we plan to present a new sub-stellar companion, whose data analysis for confirmation is currently under way.