Optical Spectra of Ultracool Dwarfs with the Southern African Large Telescope

TL;DR

This paper presents new optical spectra for 81 ultracool dwarfs, evaluates automated spectral classification methods, and reports significant H-alpha flares in a known binary, advancing spectral analysis techniques for these objects.

Contribution

It introduces an automated classification scheme for ultracool dwarfs using template fitting and spectral indices, with uncertainty quantification and new spectral classifications.

Findings

Automated classification generally accurate within two subclasses.

Large H-alpha flares detected in a known ultracool dwarf binary.

Alternative classifications provided for several objects.

Abstract

New spectra of 81 ultracool dwarfs (spectral types M7 and later) are discussed. Spectral classifications of 49 objects are available in the literature, while 32 objects are newly classified. The known spectral types were used to test an automated classification scheme, which relies primarily on template fitting, supplemented by matching of spectral indices calibrated against the template spectra. An attempt was made to quantify the uncertainty in the spectral types, which is generally better than two subclasses. Objects for which spectral types differ by more than one subclass from the literature classifications are discussed individually. Discrepancies between automated classifications based on respectively template fitting and spectral index matching, may be useful for flagging objects with unusual spectra. Aside from the 32 first-time classifications, alternative classifications are presented for 32 previously classified dwarfs. Very large (equivalent width greater than 130 \AA) H$\alpha$ flares are reported for the known ultracool dwarf binary 2MASS J15200224$-$4422419; curiously, the object does not appear to have quiescent emission lines. Non-zero equivalent width measurements are listed for a further 29 objects.