Physical Properties of Young Brown Dwarfs and Very Low-Mass Stars Inferred from High-Resolution Model Spectra

TL;DR

This study uses high-resolution near-infrared spectra and atmospheric models to determine physical properties of young brown dwarfs and very low-mass stars, revealing successes and limitations of current models.

Contribution

It provides detailed measurements of temperature, gravity, and rotation for a diverse sample, highlighting model strengths and deficiencies, especially regarding FeH opacity.

Findings

Successful reproduction of pressure-broadened KI lines

Identification of missing FeH opacity in models

Estimated parameter precision for future studies

Abstract

By comparing near-infrared spectra with atmosphere models, we infer the effective temperature, surface gravity, projected rotational velocity, and radial velocity for 21 very-low-mass stars and brown dwarfs. The unique sample consists of two sequences in spectral type from M6-M9, one of 5-10 Myr objects and one of >1 Gyr field objects. A third sequence is comprised of only ~M6 objects with ages ranging from <1 Myr to >1 Gyr. Spectra were obtained in the J band at medium (R~2,000) and high (R~20,000) resolutions with NIRSPEC on the Keck II telescope. Synthetic spectra were generated from atmospheric structures calculated with the PHOENIX model atmosphere code. Using multi-dimensional least-squares fitting and Monte Carlo routines we determine the best-fit model parameters for each observed spectrum and note which spectral regions provide consistent results. We identify successes in the reproduction of observed features by atmospheric models, including pressure-broadened KI lines, and investigate deficiencies in the models, particularly missing FeH opacity, that will need to be addressed in order to extend our analysis to cooler objects. The precision that can be obtained for each parameter using medium- and high- resolution near-infrared spectra is estimated and the implications for future studies of very low mass stars and brown dwarfs are discussed.