Discovery of the Coolest Extreme Subdwarf

We report the discovery of LEHPM 2-59 as the coolest extreme M subdwarf (esdM) found to date. Optical and near-infrared spectroscopy demonstrate that this source is of later spectral type than the esdM7 APMPM J0559-2903, with the presence of strong alkali lines (including Rb I), VO absorption at 7400 Å, and H₂O absorption at 1.4 μm. Current optical classification schemes yield a spectral type of esdM8, making LEHPM 2-59 one of only two ultracool esdMs known. The substantial space velocity of this object (V_galactic~-180 km s^-1) identifies it as a halo star. Spectral model fits to the optical and near-infrared spectral data for this and four other late-type esdMs indicate that LEHPM 2-59 is the coolest esdM currently known, with T_eff=2800-3000 K and -1.5<~[M/H]<~-2.0. Comparison of T_eff determinations for M dwarfs and esdMs based on spectral model fits from this study and the literature demonstrate a divergence in T_eff scales beyond spectral types ~M5/esdM5, as large as 600-800 K by types M8/esdM8. While this divergence is likely an artifact of the underlying classification scheme, it may lead to systematic errors in the derived properties of intermediate metallicity subdwarfs. We comment on the future of ultracool subdwarf classification and suggest several ideas for addressing shortcomings in current (largely extrapolated) schemes.