# Primeval very low-mass stars and brown dwarfs - VI. Population properties of metal-poor degenerate brown dwarfs

## TL;DR

This paper reports the discovery and analysis of metal-poor degenerate brown dwarfs, exploring their spectral properties, population characteristics, and the transition zones between stars and brown dwarfs, using new and existing survey data.

## Contribution

It presents 15 new T dwarfs, analyzes their spectral and population properties, and discusses the transition zones and discovery potential of deep sky surveys for metal-poor brown dwarfs.

## Key findings

- Identification of a mildly metal-poor T dwarf with suppressed K-band flux.
- Analysis of spectral-type and colour correlations for T5+ subdwarfs.
- Discussion of the population and transition zones of low-mass and degenerate brown dwarfs.

## Abstract

We presented 15 new T dwarfs that were selected from UKIRT Infrared Deep Sky Survey, Visible and Infrared Survey Telescope for Astronomy, and Wide-field Infrared Survey Explorer surveys, and confirmed with optical to near infrared spectra obtained with the Very Large Telescope and the Gran Telescopio Canarias. One of these new T dwarfs is mildly metal-poor with slightly suppressed $K$-band flux. We presented a new X-shooter spectrum of a known benchmark sdT5.5 subdwarf, HIP 73786B. To better understand observational properties of brown dwarfs, we discussed transition zones (mass ranges) with low-rate hydrogen, lithium, and deuterium burning in brown dwarf population. The hydrogen burning transition zone is also the substellar transition zone that separates very low-mass stars, transitional, and degenerate brown dwarfs. Transitional brown dwarfs have been discussed in previous works of the Primeval series. Degenerate brown dwarfs without hydrogen fusion are the majority of brown dwarfs. Metal-poor degenerate brown dwarfs of the Galactic thick disc and halo have become T5+ subdwarfs. We selected 41 T5+ subdwarfs from the literature by their suppressed $K$-band flux. We studied the spectral-type - colour correlations, spectral-type - absolute magnitude correlations, colour-colour plots, and HR diagrams of T5+ subdwarfs, in comparison to these of L-T dwarfs and L subdwarfs. We discussed the T5+ subdwarf discovery capability of deep sky surveys in the 2020s.