Microlensing events of the LMC are better explained by stars in the LMC than by MACHOs
Session 63 -- Large Scale Structure and Cosmology
Oral presentation, Thursday, 2, 1994, 10:00-11:30

## [63.04] Microlensing events of the LMC are better explained by stars in the LMC than by MACHOs

Kailash C. Sahu (IAC)

It is shown here that, for observations within the bar of the LMC, the probability of microlensing being caused by a star within the LMC is about 5 $\times$ 10$^{-8}$. Outside of the bar, the probability of a microlensing being caused by a star in the LMC is about 2 orders of magnitude lower. The event detected by Alcock et al. (Nature, 1993, 365 , 621) is well within the bar and its probability is consistent with being caused by an object within the LMC.

It is further shown that, under certain conditions, the light curve of an event caused by a star within the LMC would be different near the peak of the light curve from the one caused by a MACHO. This is exactly what is observed in the event detected by Alcock et al. The fit discrepancy near the peak which the authors say is not yet understood" would be a natural consequence if the event is caused by an object within the LMC, providing clear evidence that the microlensing is caused by a low mass star within the LMC itself.

Out of the two events detected by Auberg et al. (Nature, 1993, 365 , 623), one is at the edge of the bar, and hence the probability may be close to the observed one. Further work, with the surface photometry carried out in this region, is warranted. The other one is far from the bar, and if this is genuine microlensing, it cannot be due to an object within the LMC. However, the fact that this event shows some indication of colour change, albeit with low $\sigma$, is worrying and if true, would suggest that this may not be genuine microlensing. Further observations, particularly spectroscopic observations to determine the nature of this star, would be of great interest.

The possible cause as to why the microlensing time scales are of the order of a month in all the observed events are discussed.