In this paper , 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 initial mass functions ( 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 ( signal-to-noise \sim 1.2 ) in the simultaneous differential images ( SDI ) when one narrow-band image is subtracted from another , but that it does appear in the individual SDI frames as well as the images at J , H , and K _ { s } .
Our broad band photometry for AB Dor C is consistent with that of Close .
However , the photometric uncertainties that we measure are larger than those derived by Close ; our uncertainties are consistent with those measured in other studies using the same adaptive optics system .
Using the age of \tau = 75 -150 Myr recently estimated for AB Dor by Luhman , Stauffer , and Mamajek , 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 \pm 1 from the K -band spectrum of AB Dor C , which is earlier than the value of M8 \pm 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 two 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 .