The schizophrenic spectrum of LSR 1610-0040: A peculiar M dwarf/subdwarf

We present a moderate-resolution (R equivalent to lambda/Delta lambda approximate to 2000), 0.8-4.1 mu m spectrum of LSR 1610-0040, a high proper motion star classified as an early-type L subdwarf by Lepine and collaborators based on its red-optical spectrum. The near-infrared spectrum of LSR 1610-0040 does not fit into the (tentative) M/L subdwarf sequence but rather exhibits a mix of characteristics found in the spectra of both M dwarfs and M subdwarfs. In particular, the near-infrared spectrum exhibits an Na I doublet and CO overtone band heads in the K band and Al I and K I lines and an FeH band head in the H band, all of which have strengths more typical of field M dwarfs. Furthermore, the spectrum of Gl 406 (M6 V) provides a reasonably good match to the 0.6-4.1 m spectral energy distribution of LSR 1610-0040. Nevertheless, the near-infrared spectrum of LSR 1610-0040 also exhibits features common to the spectra of M subdwarfs, including a strong Ti I multiplet centered at similar to 0.97 mu m, a weak VO band at similar to 1.06 mu m, and possible collision-induced H(2) absorption in the H and K bands. We discuss a number of possible explanations for the appearance of the red-optical and near-infrared spectra of LSR 1610-0040. Although we are unable to definitively classify LSR 1610-0040, the preponderance of evidence suggests that it is a mildly metal-poor M dwarf. Finally, we tentatively identify a new band of TiO at similar to 0.93 mu m in the spectra of M dwarfs.