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

We present the discovery 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′′yr−1) and confirmed 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] ≤ -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.