Detection of Methane in the Closest Extreme Metal-poor T Dwarf WISEA J181006.18−101000.5*

WISEA J181006.18−101000.5 (WISE1810) is the nearest metal-poor ultracool dwarf to the Sun. It has a low effective temperature and has been classified as an extreme early-T subdwarf. However, methane, the characteristic molecule of the spectral class T, was not seen in the previous low-resolution spectrum. Using the 10.4 m Gran Telescopio Canarias, we collected a high-quality JHK-band intermediate-resolution R ≈ 5000 spectrum of WISE1810, in which 17 ± 6 ppm of methane is clearly detected, while carbon monoxide is absent. Based on a custom computed ATMO2020++ model, we estimated an effective temperature of 1000 ± 100 K, a high surface gravity of dex, and a carbon abundance of [C/H] = −1.5 ± 0.2 dex, inferring [Fe/H] = −1.7 ± 0.2 dex. Potassium is not seen in our data, and the upper limits of the pseudoequivalent width of the J-band atomic lines are at least 25–60 times weaker than those measured from solar-metallicity early-T counterparts. We measured a heliocentric radial velocity of −83 ± 13 km s−1, inferring that WISE1810 is more likely a thick-disk member.