Testing Theoretical Evolutionary Models with AB Doradus C and the Initial Mass Function

Abstract

We assess the constraints on the evolutionary models of young low-mass objects that are provided by the measurements of the companion AB Dor C by Close and coworkers and by a new comparison of model-derived IMFs of star-forming regions to the well-calibrated IMF of the solar neighborhood. After performing an independent analysis of all of the imaging and spectroscopic data for AB Dor C that were obtained by Close, we find that AB Dor C (which has no methane) is not detected at a significant level (S/N ~ 1.2) in the SDI data when one narrowband image is subtracted from another but that it does appear in the individual SDI frames, as well as the images at J, H, and Ks. Although our broadband photometry for AB Dor C is consistent with that of Close, the uncertainties that we measure are larger. Using the age of τ = 75-150 Myr recently estimated for AB Dor by Luhman and coworkers, the luminosity predicted by the models of Chabrier and Baraffe is consistent with the value that we estimate from the photometry for AB Dor C. We measure a spectral type of M6 ± 1 from the K-band spectrum of AB Dor C, which is earlier than the value of M8 ± 1 reported by Close and is consistent with the model predictions when a dwarf temperature scale is adopted. In a test of these evolutionary models at much younger ages, we show that the low-mass IMFs that they produce for star-forming regions are similar to the IMF of the solar neighborhood. If the masses of the low-mass stars and brown dwarfs in these IMFs of star-forming regions were underestimated by a factor of 2 as suggested by Close, then the IMF characterizing the current generation of Galactic star formation would have to be radically different from the IMF of the solar neighborhood.