1. Brown dwarfs do not make a major contribution to the dark halo of the Milky Way galaxy;

  2. In most environments (globular and open clusters, the solar neighborhood, and the galactic bulge) the MF for low-mass stars (#DXGT#0.1 solar masses) is approximately “flat”; i.e., the number per log-mass interval is constant; and

  3. To the extent that it can be directly measured, the MF for brown dwarfs continues the flat behavior seen for low-mass stars. Brown dwarfs should therefore constitute a large fraction of all stellar objects, but only a small fraction of the total mass in such objects.

However, the data, particularly the microlensing data, contain several major puzzles whose resolution may ultimately undermine this simple picture.1


The Hubble Space Telescope's (HST's) Wide-Field/Planetary Camera has been used to measure the LF of the solar neighborhood to V = 18 and the LF of the bulge to V = 12. Since both populations are approximately solar in metallicity, 2 one can convert from LF to MF using the local solar-metallicity, mass-luminosity relation derived empirically from nearby binary stars. Theoretical stellar models are in almost perfect agreement with these empirical mass measurements, within the accuracy of the scatter of the observations on a Hertzsprung-Russell (H-R) diagram. In both cases, the MFs (after correction for unseen binary companions) are consistent with flat down to the limit of the observations (0.1 solar masses for the solar-neighborhood MF and 0.3 solar masses for the bulge MF). The 2-Micron All Sky Survey (2MASS) and Deep Near-Infrared Survey of the Southern Sky (DENIS) are both sensitive to nearby field brown dwarfs provided that they are sufficiently young (and so are luminous). Only about 1% of the data have been analyzed, but already three brown dwarfs have been confirmed by detection of lithium in their spectra. Because of small-number statistics, no attempt has yet been made to estimate the local brown dwarf MF, although this should be possible in the relatively near future.

About 10 globular cluster LFs have now been measured down to (or in some cases nearly to) the bottom of the hydrogen-burning main sequence using HST. In particular, it is possible to use proper motions to separate the cluster main sequence from contaminating field stars and so demonstrate that the cluster sequence ends before the limit of detectability. Stellar models predict the shape of the cluster color-magnitude diagram very well and (together with their empirically tested predictions for solar-metallicity stars) give confidence that the theoretical mass-luminosity relations are correct. The resulting MFs reveal a range of behavior from flat to falling moderately (number per log mass interval proportional to mass). At the present time it is not known whether the underlying (initial) MF is universally flat and the observed differences among clusters reflect different histories, with some clusters losing their low-mass stars due to dynamical evolution; or whether there is some intrinsic variety among initial MFs.3 In either


For a recent review, see, for example, A. Gould, “Microlensing and the Stellar Mass Fanction,”Publications of the Astronomical Society of the Pacific, 108: 465, 1996.


A. McWilliam and R.M. Rich, “The First Detailed Abundance Analysis of Galactic Bulge K Giants in Baade's Window,”Astrophysical Journal Supplement, 91: 749, 1994.


K.L. Luhman and G.H. Rieke, “The Low-Mass Initial Mass Function in Young Clusters: L1495E,” Astrophysical Journal, 497: 354, 1998.

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