Determining the physical properties of very-low-mass stars and brown dwarfs in the near-infrared

Accurate measurements of the fundamental physical properties of very-low-mass stars and brown dwarfs are crucial for calibrating evolutionary models. Photometry and low-resolution spectroscopy effectively average over absorption features that sample different layers in complex cool atmospheres. By studying a large sample of objects bright enough for high-resolution spectroscopy, we can develop methods for determining physical properties as accurately and efficiently as possible. As part of the Brown Dwarf Spectroscopic Survey (BDSS; [1, 2]), we are conducting a detailed comparison of observed and synthetic spectra for a sample of young M and L dwarfs and field M, L, and T dwarfs (∼50 objects in total). High-resolution near-infrared spectra from NIR-SPEC on Keck II provide an unequaled combination of resolving power and wavelength coverage. Synthetic spectra were created from PHOENIX atmosphere models calculated exclusively for this project with updated line lists and solar abundances. Combined with spectral types from photometric studies and low-resolution spectra and surface gravity estimates from age determination, the high-resolution spectra enable precise measurements of effective temperature and surface gravity, as well as accurate determination of radial velocity and projected rotational velocity. Our preliminary observation-model comparisons distinguish between wavelength regimes for which the models reproduce observed high-resolution spectra and regimes in which model data (line lists, oscillator strengths, etc.) are lacking.