Halo Stars near the Hydrogen-burning Limit: The M/L Subdwarf Transition

We present the first far-red spectra of the L subdwarf 2MASS J16262034+3925190 and the late-M/early-L subdwarf 2MASS J16403197+1231068. We confirm the ultracool subdwarf nature of these objects. Like an L4 dwarf, 2M 1626+3925 shows strong K I absorption and is very similar to, but hotter than, the late-L subdwarf 2MASS J05325346+8246465. It is unambiguously an L subdwarf. 2M 1640+1231 is very similar to SSSPM J1444-2019, which has been classified as sdM9 or early sdL. In contrast to the hotter M subdwarfs, L subdwarfs are characterized by not only enhanced hydrides but also strong TiO. Progress in a classification system requires identification of more subdwarfs to map out their variations.