200 research outputs found
An algorithm to detect blends with eclipsing binaries in planet transit searches
We present an algorithm that can detect blends of bright stars with fainter,
un-associated eclipsing binaries. Such systems contaminate searches for
transiting planets, in particular in crowded fields where blends are common.
Spectroscopic follow-up observations on large aperture telescopes have been
used to reject these blends, but the results are not always conclusive. Our
approach exploits the fact that a blend with a eclipsing binary changes its
shape during eclipse. We analyze original imaging data from the Optical
Gravitational Lensing Experiment (OGLE), which were used to discover planet
transit candidates. Adopting a technique developed in weak gravitational
lensing to carefully correct for the point spread function which varies both
with time and across the field, we demonstrate that ellipticities can be
measured with great accuracy using an ensemble of images. Applied to OGLE-TR-3
and OGLE-TR-56, two of the planetary transit candidates, we show that both
systems are blended with fainter stars, as are most other stars in the OGLE
fields. Moreover, while we do not detect shape change when TR-56 undergoes
transits, TR-3 exhibits a significant shape change during eclipses. We
therefore conclude that TR-3 is indeed a blend with an eclipsing binary, as has
been suggested from other lines of evidence. The probability that its shape
change is caused by residual systematics is found to be less than 0.6%. Our
technique incurs no follow-up cost and requires little human interaction. As
such it could become part of the data pipeline for any planetary transit search
to minimize contamination by blends. We briefly discuss its relevance for the
Kepler mission and for binary star detection.Comment: Submitted to ApJ, 10 pages, 10 figure
The Araucaria Project. The Distance to the Local Group Galaxy NGC 6822 from Cepheid Variables discovered in a Wide-Field Imaging Survey
We have obtained mosaic images of NGC 6822 in V and I bands on 77 nights.
From these data, we have conducted an extensive search for Cepheid variables
over the entire field of the galaxy, and we have found 116 such variables with
periods ranging from 1.7 to 124 days. We used the long-period ( 5.6 days)
Cepheids to establish the period-luminosity relations in V, I and in the
reddening-independent Wesenheit index, which are all very tightly defined.
Fitting the OGLE LMC slopes in the various bands to our data, we have derived
distance values for NGC 6822 in V, I and which agree very
well among themselves. Our adopted best distance value from the reddening-free
Wesenheit index is 23.34 0.04 (statistical) 0.05 (systematic) mag.
This value agrees within the combined 1 sigma uncertainties with a previous
distance value derived for NGC 6822 by McAlary et al. from near-IR photometry
of 9 Cepheids, but our new value is significantly more accurate. We compare the
slopes of the Cepheid PL relation in V and I as determined in the five
best-observed nearby galaxies, which span a metallicity range from -1.0 to -0.3
dex, and find the data consistent with no metallicity dependence of the PL
relation slope in this range. Comparing the magnitudes of 10-day Cepheids with
the I-band magnitudes of the TRGB in the same set of galaxies, there is no
evidence either for a significant variation of the period-luminosity zero
points in V and I. The available data limit such a zero point variation to less
than 0.03 mag, in the considered low-metallicity regime.Comment: Latex, Astronomical Journal accepte
Systematic Analysis of 22 Microlensing Parallax Candidates
We attempt to identify all microlensing parallax events for which the
parallax fit improves \Delta\chi^2 > 100 relative to a standard microlensing
model. We outline a procedure to identify three types of discrete degeneracies
(including a new one that we dub the ``ecliptic degeneracy'') and find many new
degenerate solutions in 16 previously published and 6 unpublished events. Only
four events have one unique solution and the other 18 events have a total of 44
solutions. Our sample includes three previously identified black-hole (BH)
candidates. We consider the newly discovered degenerate solutions and determine
the relative likelihood that each of these is a BH. We find the lens of event
MACHO-99-BLG-22 is a strong BH candidate (78%), event MACHO-96-BLG-5 is a
marginal BH candidate (37%), and MACHO-98-BLG-6 is a weak BH candidate (2.2%).
The lens of event OGLE-2003-BLG-84 may be a Jupiter-mass free-floating planet
candidate based on a weak 3 sigma detection of finite-source effects. We find
that event MACHO-179-A is a brown dwarf candidate within ~100 pc of the Sun,
mostly due to its very small projected Einstein radius, \tilde r_E = 0.23+-0.05
AU. As expected, these microlensing parallax events are biased toward lenses
that are heavier and closer than average. These events were examined for
xallarap (or binary-source motion), which can mimic parallax. We find that 23%
of these events are strongly affected by xallarap.Comment: 69 Pages, 10 Figures, 24 Tables, Submitted to Ap
The Araucaria Project. Population effects on the V and I band magnitudes of red clump stars
We present measurements of the V and I band magnitudes of red clump stars in
15 nearby galaxies obtained from recently published homogenous HST photometry.
Supplementing these results with similar data for another 8 galaxies available
in the literature the populational effects on the V and I band magnitudes of
red clump stars were investigated. Comparing red clump magnitudes with the
I-band magnitude of the TRGB in a total sample of 23 galaxies possessing very
different environments we demonstrate that population effects strongly affect
both the V and I band magnitude of red clump stars in a complex way. Our
empirical results basically confirm the theoretical results of Girardi and
Salaris, and show that optical (VI) photometry of red clump stars is not an
accurate method for the determination of distances to nearby galaxies at the
present moment, as long as the population effects are not better calibrated,
both empirically and theoretically. Near infrared photometry is a much better
way to measure galaxy distances with red clump stars given its smaller
sensitivity to population effects.Comment: AJ in pres
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