Spectroscopic characterization of 78 DENIS ultracool dwarf candidates in the solar neighborhood and the Upper Sco OB association

TL;DR

This study provides spectroscopic classifications and youth indicators for 78 ultracool dwarf candidates, confirming new members of young stellar associations and identifying nearby young objects, thereby enhancing understanding of low-mass star populations.

Contribution

It introduces a new criterion based on NaI doublet equivalent width to identify young ultracool dwarfs and confirms several new members of nearby young stellar groups.

Findings

Confirmed 7 new members of the Upper Sco OB association.

Identified 2 new members of the R Cr-A star-forming region.

Discovered 3 new L dwarfs within 20 pc.

Abstract

Aims: Low-resolution optical spectroscopic observations for 78 very low-mass star and brown dwarf candidates that have been photometrically selected using the DENIS survey point source catalogue. Methods: Spectral types are derived for them using measurements of the PC3 index. They range from M6 to L4. H_alpha emission and NaI subordinate doublet (818.3 nm and 819.9 nm) equivalent widths are measured in the spectra. Spectroscopic indices of TiO, VO, CrH and FeH molecular features are also reported. Results: A rule-of-thumb criterion to select young very low-mass objects using the NaI doublet equivalent width is given. It is used to confirm seven new members of the Upper Sco OB association and two new members of the R Cr-A star-forming region. Four of our field objects are also classified as very young, but are not members of any known nearby young association. The frequency of lower-gravity young objects in our field ultracool sample is 8.5%. Our results provide the first spectroscopic classification for 38 ultracool dwarfs in the solar vicinity with spectrophotometric distances in the range 17 pc to 65 pc (3 of them are new L dwarfs within 20 pc).