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تسجيل مستخدم جديدThe Luminosity and Mass Functions of Low-Mass Stars in the Galactic Disk: II. The Field
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We report on new measurements of the luminosity function (LF) and mass function (MF) of field low-mass dwarfs derived from Sloan Digital Sky Survey (SDSS) Data Release 6 (DR6) photometry. The analysis incorporates ~15 million low-mass stars (0.1 Msun < M < 0.8 Msun), spread over 8,400 square degrees. Stellar distances are estimated using new photometric parallax relations, constructed from ugriz photometry of nearby low-mass stars with trigonometric parallaxes. We use a technique that simultaneously measures Galactic structure and the stellar LF from 7 < M_r < 16. We compare the LF to previous studies and convert to a MF using the mass-luminosity relations of Delfosse et al., 2000. The system MF, measured over -1.0 < log M/Msun < -0.1, is well-described by a log-normal distribution with Mo = 0.25 Msun. We stress that our results should not be extrapolated to other mass regimes. Our work generally agrees with prior low-mass stellar MFs and places strong constraints on future star-formation studies of the Milky Way.
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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 2 Micron All Sky Survey (2MASS) detections. This photometric catalog contains more than 25,000 ma
tched SDSS and 2MASS point sources spanning ~30 square degrees on the sky. We have obtained follow-up spectroscopy, complete to J=16, of more than 500 low mass dwarf candidates within a 1 square degree sub-sample, and thousands of additional dwarf candidates in the remaining 29 square degrees. 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). 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 alpha=1.1 (where the Salpeter slope is alpha = 2.35). These results agree well with most previous investigations, though differences in the analytic formalisms adopted to describe those mass functions can give the false impression of disagreement. Given the richness of modern-day astronomical datasets, 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. (abridged)
Modern sky surveys, such as the Sloan Digital Sky Survey and the Two-Micron All Sky Survey, have revolutionized the study of low-mass stars. With millions of photometric and spectroscopic observations, intrinsic stellar properties can be studied with
unprecedented statistical significance. Low-mass stars dominate the local Milky Way and are ideal tracers of the Galactic potential and the thin and thick disks. Recent efforts, driven by SDSS observations, have sought to place the local low-mass stellar population in a broader Galactic context. I highlight a recent measurement of the luminosity and mass functions of M dwarfs, using a new technique optimized for large surveys. Starting with SDSS photometry, the field luminosity function and local Galactic structure are measured simultaneously. The sample size used to estimate the LF is nearly three orders of magnitude larger than any previous study, offering a definitive measurement of this quantity. The observed LF is transformed into a mass function and compared to previous studies. Ongoing investigations employing M dwarfs as tracers of Galactic kinematics are also discussed. SDSS spectroscopy has produced databases containing tens of thousands of low-mass stars, forming a powerful probe of the kinematic structure of the Milky Way. SDSS spectroscopic studies are complemented by large proper motion surveys, which have uncovered thousands of common proper motion binaries containing low-mass stars. Additionally, the SDSS spectroscopic data explore the intrinsic properties of M dwarfs, including metallicity and magnetic activity. The highlighted projects demonstrate the advantages and problems with using large data sets and will pave the way for studies with next-generation surveys, such as PanSTARRS and LSST.
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