INFRARED PHOTOMETRY AND SPECTROSCOPY OF THE BROWN DWARF CANDIDATE PC0025+0447

We have obtained I, J, H, K, and L' photometry and infrared spectroscopy (lambda/DELTAlambda congruent-to 100) from 1.2-2.4 mum of the unusual emission line M star PC0025+0447. The near infrared photometry shows that this star has infrared colors which are comparable to the latest M dwarf stars such as LHS 2924 and LHS 2065. The infrared colors of PC0025+0447 establish that this star is a dwarf and not a distant giant. The infrared spectrum is dominated by strong H2O absorption bands, and these bands are the strongest measured in any dwarf. Since H2O band strength increases with decreasing temperature it is very likely that PC0025+0447 is substantially cooler than any of the stars calibrated by Berriman & Reid [MNRAS, 227, 315 (1987) ], i.e. T(eff) < 2450 K. The CO first overtone absorption is much weaker in PC0025+0447 than in hotter M dwarfs. Comparison of the spectroscopy of PC0025+0447 with model atmospheres shows correspondence between the observed strength and shape of individual H2O bands, although the H2O band strength is systematically greater in the models. A practical spectral classification sequence and ultimately a temperature scale based on the strength of the infrared H2O bands may be feasible for stars later than M6.5. A preliminary effective temperature for PC0025+0447 based on integrating the observed fluxes yields T(eff) = 1900 +/- 600 K. Spectroscopically the band strength suggests the lowest T(eff) for any dwarf so far observed. We infer from the observed T(eff) and theoretical evolutionary models that PC0025+0447 is either an old (>10(9) yr) H burning star, with M congruent-to 0.08M., or a younger brown dwarf.