288 research outputs found
Probing the atmosphere of a solar-like star by galactic microlensing at high magnification
We report a measurement of limb darkening of a solar-like star in the very
high magnification microlensing event MOA 2002-BLG-33. A 15 hour deviation from
the light curve profile expected for a single lens was monitored intensively in
V and I passbands by five telescopes spanning the globe. Our modelling of the
light curve showed the lens to be a close binary system whose centre-of-mass
passed almost directly in front of the source star. The source star was
identified as an F8-G2 main sequence turn-off star. The measured stellar
profiles agree with current stellar atmosphere theory to within ~4% in two
passbands. The effective angular resolution of the measurements is <1
micro-arcsec. These are the first limb darkening measurements obtained by
microlensing for a Solar-like star.Comment: Accepted for publication in A&A Letters. 5 pages, 2 embedded colour
ps figures plus 1 jpg figure. Version with all figures embedded available
from: http://www.roe.ac.uk/~iab/moa33paper
Microlensing optical depth towards the Galactic bulge from MOA observations during 2000 with Difference Image Analysis
We analyze the data of the gravitational microlensing survey carried out by
by the MOA group during 2000 towards the Galactic Bulge (GB). Our observations
are designed to detect efficiently high magnification events with faint source
stars and short timescale events, by increasing the the sampling rate up to 6
times per night and using Difference Image Analysis (DIA). We detect 28
microlensing candidates in 12 GB fields corresponding to 16 deg^2. We use Monte
Carlo simulations to estimate our microlensing event detection efficiency,
where we construct the I-band extinction map of our GB fields in order to find
dereddened magnitudes. We find a systematic bias and large uncertainty in the
measured value of the timescale in our simulations. They are
associated with blending and unresolved sources, and are allowed for in our
measurements. We compute an optical depth tau = 2.59_{-0.64}^{+0.84} \times
10^{-6} towards the GB for events with timescales 0.3<t_E<200 days. We consider
disk-disk lensing, and obtain an optical depth tau_{bulge} =
3.36_{-0.81}^{+1.11} \times 10^{-6}[0.77/(1-f_{disk})] for the bulge component
assuming a 23% stellar contribution from disk stars. These observed optical
depths are consistent with previous measurements by the MACHO and OGLE groups,
and still higher than those predicted by existing Galactic models. We present
the timescale distribution of the observed events, and find there are no
significant short events of a few days, in spite of our high detection
efficiency for short timescale events down to t_E = 0.3 days. We find that half
of all our detected events have high magnification (>10). These events are
useful for studies of extra-solar planets.Comment: 65 pages and 30 figures, accepted for publication in ApJ. A
systematic bias and uncertainty in the optical depth measurement has been
quantified by simulation
Determining the Physical Lens Parameters of the Binary Gravitational Microlensing Event MOA-2009-BLG-016
We report the result of the analysis of the light curve of the microlensing
event MOA-2009-BLG-016. The light curve is characterized by a short-duration
anomaly near the peak and an overall asymmetry. We find that the peak anomaly
is due to a binary companion to the primary lens and the asymmetry of the light
curve is explained by the parallax effect caused by the acceleration of the
observer over the course of the event due to the orbital motion of the Earth
around the Sun. In addition, we detect evidence for the effect of the finite
size of the source near the peak of the event, which allows us to measure the
angular Einstein radius of the lens system. The Einstein radius combined with
the microlens parallax allows us to determine the total mass of the lens and
the distance to the lens. We identify three distinct classes of degenerate
solutions for the binary lens parameters, where two are manifestations of the
previously identified degeneracies of close/wide binaries and positive/negative
impact parameters, while the third class is caused by the symmetric cycloid
shape of the caustic. We find that, for the best-fit solution, the estimated
mass of the lower-mass component of the binary is (0.04 +- 0.01) M_sun,
implying a brown-dwarf companion. However, there exists a solution that is
worse only by \Delta\chi^2 ~ 3 for which the mass of the secondary is above the
hydrogen-burning limit. Unfortunately, resolving these two degenerate solutions
will be difficult as the relative lens-source proper motions for both are
similar and small (~ 1 mas/yr) and thus the lens will remain blended with the
source for the next several decades.Comment: 7 pages, 2 tables, and 5 figure
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