Optical spectroscopy of a brown dwarf candidate ⋆

We have used the Low-Resolution Imaging Spectrograph on the Keck II telescope to observe the brown dwarf candidate D04 (Hawkins et al, 1998). The spectrum matches that of a spectral-type M7 dwarf, implying a photospheric temperature of ≈ 2600K. This is consistent with the available (R-I)C and (I-K) colours. If the parallax measured by Hawkins et al is correct, then the implication is that D04 has a radius of ∼ 0.035R⊙, or one-third that of Jupiter. This contradicts the predictions made by current stellar models that electron degeneracy leads to nearly constant radii for stars and brown dwarfs at masses below 0.1 M⊙. We suggest that an equally valid interpretation of the data is that D04 is a VB8 analogue at a distance of ≈ 150 parsecs.