WISEA J041451.67-585456.7 and WISEA J181006.18-101000.5: The First Extreme T-type Subdwarfs?

TL;DR

This paper reports the discovery of the first known extreme T-type subdwarfs, low-temperature brown dwarfs with very low metallicity, identified through high proper motion surveys and confirmed by spectroscopy.

Contribution

It presents the identification and spectroscopic confirmation of the first extreme T-type subdwarfs, expanding the understanding of low-metallicity substellar objects.

Findings

Both objects are low-temperature, high proper motion brown dwarfs.

Their spectra suggest very low metallicities, possibly the first of their kind.

They occupy the substellar transition zone with the lowest masses and temperatures.

Abstract

We present the discoveries of WISEA J041451.67-585456.7 and WISEA J181006.18-101000.5, two low-temperature (1200$-$1400 K), high proper motion T-type subdwarfs. Both objects were discovered via their high proper motion ($>$0.5 arcsec yr$^{-1}$); WISEA J181006.18-101000.5 as part of the NEOWISE proper motion survey and WISEA J041451.67-585456.7 as part of the citizen science project Backyard Worlds; Planet 9. We have confirmed both as brown dwarfs with follow-up near-infrared spectroscopy. Their spectra and near-infrared colors are unique amongst known brown dwarfs, with some colors consistent with L-type brown dwarfs and other colors resembling those of the latest-type T dwarfs. While no forward model consistently reproduces the features seen in their near-infrared spectra, the closest matches suggest very low metallicities ([Fe/H] $\leq$ -1), making these objects likely the first examples of extreme subdwarfs of the T spectral class (esdT). WISEA J041451.67-585456.7 and WISEA J181006.18-101000.5 are found to be part of a small population of objects that occupy the "substellar transition zone," and have the lowest masses and effective temperatures of all objects in this group.