The Luminosity and Mass Functions of Low-Mass Stars in the Galactic Disk. I. The Calibration Region

We present measurements of the luminosity and mass functions of low-mass stars constructed from a catalog of matched Sloan Digital Sky Survey (SDSS) and Two Micron All Sky Survey (2MASS) detections. This photometric catalog contains more than 25,000 matched SDSS and 2MASS point sources spanning ~30 deg² on the sky. We have obtained follow-up spectroscopy, complete to J = 16, of more than 500 low-mass dwarf candidates within a 1 deg² subsample, and thousands of additional dwarf candidates in the remaining 29 deg². This spectroscopic sample verifies that the photometric sample is complete, uncontaminated, and unbiased at the 99% level globally, and at the 95% level in each color range. We use this sample to derive the luminosity and mass functions of low-mass stars over nearly a decade in mass (0.7 M _sun > M _* > 0.1 M _sun). The luminosity function of the Galactic disk is statistically consistent with that measured from volume-complete samples in the solar neighborhood. We find that the logarithmically binned mass function is best fit with an M_c = 0.29 log-normal distribution, with a 90% confidence interval of M_c = 0.20-0.50. These 90% confidence intervals correspond to linearly binned mass functions peaking between 0.27 M _sun and 0.12 M _sun, where the best fit MF turns over at 0.17 M _sun. A power-law fit to the entire mass range sampled here, however, returns a best fit of α = 1.1 (where the Salpeter slope is α = 2.35); a broken power law returns α = 2.04 at masses greater than log M = -0.5 (M = 0.32 M _sun), and α = 0.2 at lower masses. These results agree well with most previous investigations, though differences in the analytic formalisms adopted to describe those mass functions, as well as the range over which the data are fit, can give the false impression of disagreement. Given the richness of modern-day astronomical data sets, we are entering the regime whereby stronger conclusions can be drawn by comparing the actual datapoints measured in different mass functions, rather than the results of analytic analyses that impose structure on the data a priori. Having validated this method to generate a low-mass luminosity function from matched SDSS/2MASS data sets, future studies will extend this technique to the entirety of the SDSS footprint.

Based in part on observations obtained with the Apache Point Observatory 3.5 m telescope, which is owned and operated by the Astrophysical Research Consortium.