Newly Discovered Brown Dwarfs Not Seen in Microlensing Timescale Frequency Distribution?

The 2-Micron All Sky Survey (2MASS) (Skrutskie et al. 1997) and the DEep NearInfrared Survey of the southern sky (DENIS) (Epchtein et al. 1997) haverevealed a heretofore unknown population of free brown dwarfs that has extendedthe local mass function down to as small as 0.01M_sun (Reid et al. 1999). Ifthis local proportion of brown dwarfs extends throughout the Galaxy---inparticular in the Galactic bulge---one expects an increase in the predictedfraction of short time scale microlensing events in directions toward theGalactic bulge. Zhao et al.(1996) have indicated that a mass function with30-60% of the lens mass in brown dwarfs is not consistent with empiricalmicrolensing data. Here we show that even the much lower mass fraction (~ 10%)of brown dwarfs inferred from the new discoveries appears inconsistent with thedata. The added brown dwarfs do indeed increase the expected number of shorttime scale events, but they appear to drive the peak in the time scalefrequency distribution to time scales smaller than that observed, and do nototherwise match the observed distribution. A reasonably good match to theempirical data (Alcock et al. 1996) is obtained by increasing the fraction ofstars in the range 0.08

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