Infrared Colors at the Stellar / Substellar Boundary

We present new infrared JHK photometry for 61 halo and disk stars around the stellar/substellar boundary. We also present new L ′ photometry for 21 of these stars and for 40 low–mass stars taken from the Leggett 1992 photometry compilation. These data are combined with available optical photometry and astrometric data to produce color–color and absolute magnitude–color diagrams — the current sample extends the similar work presented in the 1992 paper into more metal–poor and lower mass regimes. The disk and halo sequences are compared to the predictions of the latest model atmospheres and structural models. We find good agreement between observation and theory except for known problems in the V and H passbands probably due to incomplete molecular data for TiO, metal hydrides and H 2 O. The metal–poor M subdwarfs are well matched by the models as oxide opacity sources are less important in this case. The known extreme M subdwarfs have metallicities about one–hundredth solar, and the coolest subdwarfs have T ef f ∼ 3000 K with masses ∼0.09M/M ⊙. The grainless models are not able – 2 – to reproduce the flux distributions of disk objects with T ef f < 2500 K, however a preliminary version of the NextGen–Dusty models which includes homogeneous formation and extinction by dust grains is able to match the colors of these very cool objects. The least luminous objects in this sample are GD165B, three DENIS objects — DBD0205, DBD1058 and DBD1228 — and Kelu-1. These have T ef f ∼ 2000 K and are at or below the stellar limit with masses ≤0.075M/M ⊙. Photometry alone cannot constrain these parameters further as the age is unknown, but published lithium detections for two of these objects (Kelu-1 and DBD1228) imply that they are young (aged about 1 Gyr) and substellar (mass ≤0.06M/M ⊙).