Status Report

A Brown Dwarf Microlens Candidate in the OGLE-II Database

By SpaceRef Editor
January 17, 2003
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Astrophysics, abstract

From: Martin C. Smith <[email protected]>
Date: Thu, 16 Jan 2003 15:03:23 GMT (71kb)

A Brown Dwarf Microlens Candidate in the OGLE-II Database

M. C. Smith,
S. Mao,
P. R. Wozniak

Comments: 12 pages, 2 figures. Accepted for publication in ApJ Letters

We describe a unique mass determination for a microlensing event from the
second phase of the Optical Gravitational Microlensing Experiment (OGLE-II).
The event, sc26_2218, which is very bright (baseline magnitude I=15.10),
appears to exhibit both parallax and finite source effects. The parallax effect
allows us to determine the projected Einstein radius on the observer plane
(approx. 3.8 AU), while the finite source effect allows us to determine the
ratio of the angular source size and the angular Einstein radius. As the
angular size of the star can be estimated using its color and magnitude, we can
hence determine the angular Einstein radius (approx. 0.1 mas). By combining
both the projected and angular Einstein radius we can determine the lens mass M
= 0.050^{+0.016}_{-0.011} solar masses, independent of the source distance. The
lens is therefore a brown dwarf candidate. However, the `parallax’ signature is
weak and so we cannot completely discount the possibility that these signatures
originate from binary rotation of the source (which would prevent any estimate
of the lens mass), rather than parallax. However, this can be tested by future
spectroscopic observations. This event highlights the scientific returns for
intense monitoring of bright microlensing events, since the parallax and finite
source effects can be more easily identified due to their high signal-to-noise

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