21 research outputs found
On Planetary Companions to the MACHO-98-BLG-35 Microlens Star
We present observations of microlensing event MACHO-98-BLG-35 which reached a
peak magnification factor of almost 80. These observations by the Microlensing
Planet Search (MPS) and the MOA Collaborations place strong constraints on the
possible planetary system of the lens star and show intriguing evidence for a
low mass planet with a mass fraction . A giant planet with is excluded from 95%
of the region between 0.4 and 2.5 from the lens star, where is the
Einstein ring radius of the lens. This exclusion region is more extensive than
the generic "lensing zone" which is . For smaller mass planets,
we can exclude 57% of the "lensing zone" for and 14% of
the lensing zone for . The mass fraction corresponds to an Earth mass planet for a lensing star of mass \sim
0.3 \msun. A number of similar events will provide statistically significant
constraints on the prevalence of Earth mass planets. In order to put our limits
in more familiar terms, we have compared our results to those expected for a
Solar System clone averaging over possible lens system distances and
orientations. We find that such a system is ruled out at the 90% confidence
level. A copy of the Solar System with Jupiter replaced by a second Saturn mass
planet can be ruled out at 70% confidence. Our low mass planetary signal (few
Earth masses to Neptune mass) is significant at the confidence
level. If this planetary interpretation is correct, the MACHO-98-BLG-35 lens
system constitutes the first detection of a low mass planet orbiting an
ordinary star without gas giant planets.Comment: ApJ, April 1, 2000; 27 pages including 8 color postscript figure
Study of variable stars in the MOA data base: long-period red variables in the Large Magellanic Cloud
One hundred and forty six long-period red variable stars in the Large
Magellanic Cloud (LMC) from the three year MOA project database were analysed.
A careful periodic analysis was performed on these stars and a catalogue of
their magnitudes, colours, periods and amplitudes is presented. We convert our
blue and red magnitudes to band values using 19 oxygen-rich stars. A group
of red short-period stars separated from the Mira sequence has been found on a
(log P, K) diagram. They are located at the short period side of the Mira
sequence consistent with the work of Wood and Sebo (1996). There are two
interpretations for such stars; a difference in pulsation mode or a difference
in chemical composition. We investigated the properties of these stars together
with their colour, amplitude and periodicity. We conclude that they have small
amplitudes and less regular variability. They are likely to be higher mode
pulsators. A large scatter has been also found on the long period side of the
(log P, K) diagram. This is possibly a systematic spread given that the blue
band of our photometric system covers both standard B and V bands and affects
carbon-rich stars.Comment: 19 pages, 19 figures, accepted for publication in MNRA
Study by MOA of extra-solar planets in gravitational microlensing events of high magnification
A search for extra-solar planets was carried out in three gravitational
microlensing events of high magnification, MACHO 98-BLG-35, MACHO 99-LMC-2, and
OGLE 00-BUL-12. Photometry was derived from observational images by the MOA and
OGLE groups using an image subtraction technique. For MACHO 98-BLG-35,
additional photometry derived from the MPS and PLANET groups was included.
Planetary modeling of the three events was carried out in a super-cluster
computing environment. The estimated probability for explaining the data on
MACHO 98-BLG-35 without a planet is <1%. The best planetary model has a planet
of mass ~(0.4-1.5) X 10^-5 M_Earth at a projected radius of either ~1.5 or ~2.3
AU. We show how multi-planet models can be applied to the data. We calculated
exclusion regions for the three events and found that Jupiter-mass planets can
be excluded with projected radii from as wide as about 30 AU to as close as
around 0.5 AU for MACHO 98-BLG-35 and OGLE 00-BUL-12. For MACHO 99-LMC-2, the
exclusion region extends out to around 10 AU and constitutes the first limit
placed on a planetary companion to an extragalactic star. We derive a
particularly high peak magnification of ~160 for OGLE 00-BUL-12. We discuss the
detectability of planets with masses as low as Mercury in this and similar
events.Comment: 14 pages, 16 embedded postscript figures, 3 PNG figures, revised
version accepted by MNRA
MACHO Alert 95-30 : First Real-Time Observation of Extended Source Effects in Gravitational Microlensing
We present analysis of MACHO Alert 95-30, a dramatic gravitational
microlensing event towards the Galactic bulge whose peak magnification departs
significantly from the standard point-source microlensing model. Alert 95-30
was observed in real-time by the Global Microlensing Alert Network (GMAN),
which obtained densely sampled photometric and spectroscopic data throughout
the event. We interpret the light-curve ``fine structure'' as indicating
transit of the lens across the extended face of the source star. This signifies
resolution of a star several kpc distant. We find a lens angular impact
parameter theta_{min}/theta_{source} = 0.715 +/- 0.003. This information, along
with the radius and distance of the source, provides an additional constraint
on the lensing system. Spectroscopic and photometric data indicate the source
is an M4 III star of radius 61 +/- 12 Rsun, located on the far side of the
bulge at 9 kpc. We derive a lens angular velocity, relative to the source, of
21.5 +/- 4.9 km/s/kpc, where the error is dominated by uncertainty in the
source radius. Likelihood analysis yields a median lens mass of
0.67{+2.53}{-0.46} Msun, located with 80% probability in the Galactic bulge at
a distance of 6.93{+1.56}{-2.25} kpc. If the lens is a main-sequence star, we
can include constraints on the lens luminosity. This modifies our estimates to
M_lens = 0.53{+0.52}{-0.35} Msun and D_lens = 6.57{+0.99}{-2.25} kpc. Spectra
taken during the event show that the absorption line equivalent widths of H
alpha and the TiO bands near 6700 A vary, as predicted for microlensing of an
extended source. This is most likely due to center-to-limb variation in the
stellar spectral lines. These data demonstrate the feasibility of using
microlensing limb crossings as a tool to probe stellar atmospheres directly.Comment: 32 pages including 6 tables, and 15 figures; Uses AASTeX 4.0;
submitted to The Astrophysical Journa
On the optical identification of SCO X-1
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