A Near-Infrared Spectroscopic Study of Young Field Ultracool Dwarfs

TL;DR

This study develops a near-infrared spectroscopic method to classify the spectral type and gravity of young ultracool dwarfs, providing a new tool for age estimation and understanding their properties.

Contribution

It introduces a gravity-sensitive index-based classification system for ultracool dwarfs in the near-IR, independent of optical spectra, and proposes low-gravity spectral standards.

Findings

Spectral types from near-IR match optical types with small offsets.

Gravity-sensitive indices effectively classify youth in ultracool dwarfs.

The method distinguishes young, dusty objects and estimates their ages.

Abstract

We present a near-infrared (0.9-2.4 microns) spectroscopic study of 73 field ultracool dwarfs having spectroscopic and/or kinematic evidence of youth (~10-300 Myr). Our sample is composed of 48 low-resolution (R~100) spectra and 41 moderate-resolution spectra (R>~750-2000). First, we establish a method for spectral typing M5-L7 dwarfs at near-IR wavelengths that is independent of gravity. We find that both visual and index-based classification in the near-IR provide consistent spectral types with optical spectral types, though with a small systematic offset in the case of visual classification at J and K band. Second, we examine features in the spectra of ~10 Myr ultracool dwarfs to define a set of gravity-sensitive indices based on FeH, VO, K, Na and H-band continuum shape. We then create an index-based method for classifying the gravities of M6-L5 dwarfs that provides consistent results with gravity classifications from optical spectroscopy. Our index-based classification can distinguish between young and dusty objects. Guided by the resulting classifications, we propose a set of low-gravity spectral standards for the near-IR. Finally, we estimate the ages corresponding to our gravity classifications.