Newly discovered brown dwarfs not seen in microlensing time scale frequency distribution?

Details Newly discovered brown dwarfs not seen in microlensing time scale frequency distribution? Newly discovered brown dwarfs not seen in microlensing time scale frequency distribution?

1999-08-14

arxiv.org/abs/astro-ph/9908154v2

Astronomy and Astrophysics

Astrophysics

5 pages, 1 figure. PS file using aas2pp4.sty. To appear in ApJ Letters

Scientific paper

10.1086/312295

The 2-Micron All Sky Survey (2MASS) (Skrutskie et al. 1997) and the DEep Near Infrared Survey of the southern sky (DENIS) (Epchtein et al. 1997) have revealed a heretofore unknown population of free brown dwarfs that has extended the local mass function down to as small as 0.01M_sun (Reid et al. 1999). If this local proportion of brown dwarfs extends throughout the Galaxy---in particular in the Galactic bulge---one expects an increase in the predicted fraction of short time scale microlensing events in directions toward the Galactic bulge. Zhao et al.(1996) have indicated that a mass function with 30-60% of the lens mass in brown dwarfs is not consistent with empirical microlensing data. Here we show that even the much lower mass fraction (~ 10%) of brown dwarfs inferred from the new discoveries appears inconsistent with the data. The added brown dwarfs do indeed increase the expected number of short time scale events, but they appear to drive the peak in the time scale frequency distribution to time scales smaller than that observed, and do not otherwise match the observed distribution. A reasonably good match to the empirical data (Alcock et al. 1996) is obtained by increasing the fraction of stars in the range 0.08

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