A Test For An Intervening Stellar Population As The Origin of Microlensing Events Toward the Large Magellanic Cloud

Zaritsky and Lin (1997) have recently presented observations which can be interpreted as evidence for an intervening stellar population along the line-of-sight to the LMC, at a distance of about 34 kpc. The evidence is based on detected populations of red-clump and lower main-sequence stars which appear brighter than the principal distributions by about 0.9 magnitude, and which have similar fractional surface densities of ∼6%. ZL97 suggest that the intervening population may belong to a low surface-brightness dwarf galaxy, or to the tidal debris tail of a disrupted galaxy. They derive an optical depth for gravitational lensing through the intervening system which is consistent with the observed value, thus suggesting that MACHOs comprise at most a small fraction of the Galactic dark halo. Such interpretation of the microlensing events toward the LMC has major implications for the nature of the dark matter, and thus should be tested in every conceivable way. We suggest a test which is based on the microlensing data themselves: if the observed events arise in the ZL97 intervening population, then all the lenses are at nearly the same distance, and have nearly the same transverse velocity. Thus, the predicted distribution of event durations should directly reflect the stellar mass function in the intervening population, which is probably not significantly different from that in the Galactic disk. We find that the first eight MACHO events toward the LMC are as consistent with a standard halo distribution of 0.5M⊙ MACHOs (best fit), as they are with an intervening population having a transverse speed of ∼ 240 km s and a Miller-Scalo stellar IMF. Monte Carlo simulations reveal that in order to completely rule out either the standard halo distribution of MACHOs or the ZL97 intervening population, about 40 events will be required if the IMF is rising below the H-burning mass limit, and ≈ 140 if it is falling. If the intervening population hypothesis is correct then: (a) the currently observed range of event durations directly implies a lens mass range of ≈ 0.06–0.8M⊙, which brackets the mass of ≈0.2M⊙ where many investigations claim that the stellar mass function peaks, and (b) the event distribution provides valuable information on the low-mass end of the stellar IMF. Subject headings: gravitational lenses – dark matter Galaxy: halo – Magellanic Clouds – stars: low-mass, brown dwarfs, mass function