Measuring Fundamental Parameters of Substellar Objects. I: Surface Gravities

We present an analysis of high resolution optical spectra for a sample of very young, midto late M, low-mass stellar and substellar objects: 11 in the Upper Scorpius association, and 2 (GG Tau Ba and Bb) in the Taurus star-forming region. Effective temperatures and surface gravities are derived from a multifeature spectral analysis using TiO, Na I and K I , through comparison with the latest synthetic spectra. We show that these spectral diagnostics complement each other, removing degeneracies with temperature and gravity in the behavior of each. In combination, they allow us to determine temperature to within 50K and gravity to within 0.25 dex, in very cool young objects. Our highresolution spectral analysis does not require extinction estimates. Moreover, it yields temperatures and gravities independent of theoretical evolutionary models (though our estimates do depend on the synthetic spectral modeling). We find that our gravities for most of the sample agree remarkably well with the isochrone predictions for the likely cluster ages. However, discrepancies appear in our coolest targets: these appear to have significantly lower gravity (by upto 0.75 dex) than our hotter objects, even though our entire sample covers a relatively narrow range in effective temperature (∼ 300K). This drop in gravity is also implied by inter-comparisons of the data alone, without recourse to synthetic spectra. We consider, and argue against, dust opacity, cool stellar spots or Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138. [email protected] Astronomy Department, University of California, Berkeley, CA 94720. [email protected] Astronomy Department, University of Michigan, Ann Arbor, MI 48109. [email protected] CRAL, École Normale Supérieure, 46 Allee d’Italie, Lyon, 69364, France. [email protected] Hamburger Sternwarte, Universitaet Hamburg, 21029 Hamburg, Germany. [email protected] Bloomberg Center for Physics & Astronomy, Johns Hopkins University, Baltimore, MD 21218. [email protected]