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 thatthis source is of later spectral type than the esdM7 APMPM 0559-2903, with thepresence of strong alkali lines (including Rb I), VO absorption at 7400 A andH2O absorption at 1.4 microns. Current optical classification schemes yield aspectral type of esdM8, making LEHPM 2-59 one of only two ultracool esdMsknown. The substantial space velocity of this object (V_galactic ~ -180 km/s)identifies it as a halo star. Spectral model fits to the optical and nearinfrared spectral data for this and four other late-type esdMs indicate thatLEHPM 2-59 is the coolest esdM currently known, with Teff = 2800-3000 K and-1.5 <~ [M/H] <~ -2.0. Comparison of Teff determinations for M dwarfs and esdMsbased on spectral model fits from this study and the literature demonstrate adivergence in Teff scales beyond spectral types M5/esdM5, as large as 600-800 Kby types M8/esdM8. While this divergence is likely an artifact of theunderlying classification scheme, it may lead to systematic errors in thederived properties of intermediate metallicity subdwarfs. We comment on thefuture of ultracool subdwarf classification, and suggest several ideas foraddressing shortcomings in current (largely extrapolated) schemes.