Infrared photometry and spectroscopy of the brown dwarf candidate PC0025+0447

Abstract

We have obtained I, J, H, Kf and L' photometry and infrared spectroscopy (λ/Δλ ≃ 100) from 1.2-2.4 µm 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 H_2O absorption bands, and these bands are the strongest measured in any dwarf. Since H_2O band strength increases with decreasing temperature it is very likely that PC0025-f 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-I-0447 with model atmospheres shows correspondence between the observed strength and shape of individual H_2O bands, although the H_2O 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 H_2O bands may be feasible for stars later than M6.5. A preliminary effective temperature for PC0025+0447 based on integrating the observed fluxes yields 1900 ±600 K. Spectroscopically the band strength suggests the lowest T_(eff) for any dwarf so far observed. We infer from the observed and theoretical evolutionary models that PC0025 + 0447 is either an old ( > 10^9 yr) H burning star, with M ≃ 0.08 M_☉, or a younger brown dwarf.