101 research outputs found
Microlensing Events of the LMC are Better Explained by Stars within the LMC Than by Machos
The recently reported microlensing events of the LMC have caused much
excitement, and have been interpreted as due to `dark objects' (MACHOs) in the
halo of our Galaxy. It is shown here that the stars within the LMC play a
dominant role as gravitational lenses and can indeed account for the observed
events. The MACHO event and one of the EROS events lie within the bar for which
the probability of microlensing is consistent with being caused by an object
within the LMC. If the stars within the LMC play a dominant role as lenses, the
events should be concentrated towards the center of the LMC. If MACHOs play a
dominant role as lenses then, for a given number of monitored stars, the events
should be uniformly distributed over the whole of the LMC. Thus the galactic
and the LMC lenses can be statistically distinguished in most cases. It is
further shown that, under certain conditions, the light curve of an event
caused by a star within the LMC would be different from the one caused by a
MACHO. This can also be a distinguishing signature, and seems to have been
observed in case of the MACHO event. 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, which further indicates that the
lensing is due to a low mass star within the LMC itself.Comment: 14 pages, To appear in Sept., 1994 issue of Pub. Astron. Soc. Pac.,
Figures available on request from [email protected]
A Re-examination of the "Planetary" Lensing Events in M22
We have carried out further analysis of the tentative, short-term
brightenings reported by Sahu et al. (2001), which were suggested to be
possible lensings of Galactic-bulge stars by free-floating planets in the
globular cluster M22. Closer examination shows that--unlikely as it may
seem--small, point-like cosmic rays had hit very close to the same star in both
of a pair of cosmic-ray-split images, which cause the apparent brightenings of
stars at the times and locations reported. We show that the observed number of
double hits is consistent with the frequency of cosmic rays in WFPC2 images,
given the number of stars and epochs observed. Finally, we point to ways in
which cosmic rays can be more directly distinguished.Comment: Accepted Dec 11, 2001 by ApJ Letter
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