1,917 research outputs found
Early Estimation of Microlensing Event Magnifications
Gravitational microlensing events with high peak magnifications provide a
much enhanced sensitivity to the detection of planets around the lens star.
However, estimates of peak magnification during the early stages of an event by
means of chi^2 minimization frequently involve an overprediction, making
observing campaigns with strategies that rely on these predictions inefficient.
I show that a rudimentary Bayesian formulation, incorporating the known
statistical characteristics of a detection system, produces much more accurate
predictions of peak magnification than chi^2 minimisation. Implementation of
this system will allow efficient follow-up observing programs that focus solely
on events that contribute to planetary abundance statistics.Comment: Accepted by ApJ. 19 pages, incl 7 figures and 2 table
The Optical Gravitational Lensing Experiment. Red Clump Stars as a Distance Indicator
We present relation of the mean I-band brightness of red clump stars on
metallicity. Red clump stars were proposed to be a very attractive standard
candle for distance determination. The calibration is based on 284 nearby red
giant stars whose high quality spectra allowed to determine accurate individual
metal abundances. High quality parallaxes (\sigma_\pi / \pi < 10%) and
photometry of these very bright stars come from Hipparcos measurements.
Metallicity of the sample covers a large range: -0.6<[Fe/H]<+0.2 dex. We find a
weak dependence of the mean I-band brightness on metallicity (about 0.13
mag/dex).
What is more important, the range of metallicity of the Hipparcos sample
partially overlaps with metallicity of field giants in the LMC, thus making it
possible to determine the distance to the LMC by almost direct comparison of
brightness of the local Hipparcos red clump giants with that of LMC stars.
Photometry of field red clump giants in nine low extinction fields of the LMC
halo collected during the OGLE-II microlensing survey compared with the
Hipparcos red clump stars data yields the distance modulus to the LMC:
(m-M)_LMC=18.24+/-0.08 mag.Comment: 11 pages. Latex+psfig. Accepted for publictation in ApJ Letters.
Major revision: 30% larger sample of Hipparcos red giants with spectroscopic
metallicities and extended photometry of the LMC field red clump stars (9
lines-of-sight). Due to journal space limitation, sections on comparison of
the LMC red clump distance modulus with previous determinations and relations
between the color indices and metallicity removed from this versio
Eclipsing Binaries in the OGLE Variable Star Catalog.III. Long-Period Contact Systems
A sample of contact binaries discovered by the OGLE project in Baade's
Window, with orbital periods longer than one day and with available color and
light-curve data, has been analyzed. It consists of only 32 systems, in
contrast to 388 WUMa-type systems with shorter periods which were analyzed
before. Most systems are very distant and are probably located close to or in
the galactic Bulge. Two groups of contact binaries are seen in the sample: (1)
a continuation of the WUMa-type sequence, extending up to the orbital periods
of 1.3 - 1.5 day, but rather sharply ending in this period range; (2) an
inhomogeneous group of rare systems with long periods up to 26 days, all with
red colors and relatively shallow eclipses. While the systems of the first
group share most of the characteristics of the typical WUMa-type systems
(except that they are on the average brighter and more distant, hence more
reddened), the long-period systems do not seem to form an early-type extension
of contact binaries, but may consist of a mixture of late-type objects,
including tidally distorted red giants with invisible companions.Comment: 24 pages including 10 figures (inserted with psfig) and one table;
submitted to A
Search for Planetary Candidates within the OGLE Stars
We propose a method to distinguish between planetary and stellar companions
to stars which present a periodic decrease in brightness, interpreted as a
transit. Light curves from a total of 177 stars from the OGLE project were
fitted by the model which simulates planetary transits using an opaque disk in
front of an image of the Sun. The simulation results yield the orbital radius
in units of stellar radii, the orbital inclination angle, and the ratio of the
planet to the star radii. Combining Kepler's third law with a mass-radius
relation for main sequence stars, it was possible to estimate values for the
masses and radii of both the primary and secondary objects. This model was
successfully tested with the confirmed planets orbiting the stars HD 209458,
TrES-1, OGLE-TR-10, 56, 111, 113, and 132. The method consists of selecting as
planetary candidates only those objects with primary densities between 0.7 and
2.3 solar densities (F, G, and K stars) and secondaries with radius less than
1.5 Jupiter radius. The method is not able to distinguish between a planet and
a dwarf star with mass less than 0.1 , such as OGLE-TR-122. We propose
a selection of 28 planetary candidates (OGLE-TR-49, 51, 55, 63, 71, 76, 90, 97,
100, 109, 114, 127, 130, 131, 134, 138, 140, 146, 151, 155, 159, 164, 165, 169,
170, 171, 172, and 174) for high resolution spectroscopy follow up.Comment: 4 figures, 2 table
Photometric standard stars in the BVI system in a wide field centered on the spiral galaxy NGC 300
Based on 13 nights of observations of four fields in NGC 300, we have set up
an extensive sequence of stars with accurate BVI photometry covering a
relatively large (25 x 25 arcmin) region centered on this galaxy. This sequence
of standard stars is very useful for calibrating the photometry of variable
stars and other objects in NGC 300 and other galaxies obtained from wide field
mosaic images. Our standard star list contains B, V and I measurements for 390
stars. The accuracy of the zero points in the V filter and B-V color is better
than 0.02 mag, and about 0.03 mag for the V-I color. We found very good
agreement between our measurements and those previously obtained by Walker for
26 stars near NGC 300.Comment: 12 pages, Latex, 1 Figure in gif format, 1 Table as postscript file.
P.A.S.P. in pres
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
On the Period Distribution of Close-In Extrasolar Giant Planets
Transit (TR) surveys for extrasolar planets have recently uncovered a
population of ``very hot Jupiters,'' planets with orbital periods of P< 3 d. At
first sight this may seem surprising, given that radial velocity (RV) surveys
have found a dearth of such planets, despite the fact that their sensitivity
increases with decreasing P. We examine the confrontation between RV and TR
survey results, paying particular attention to selection biases that favor
short-period planets in transit surveys. We demonstrate that, when such biases
and small-number statistics are properly taken into account, the period
distribution of planets found by RV and TR surveys are consistent at better
than the 1-sigma level. This consistency holds for a large range of reasonable
assumptions. In other words, there are not enough planets detected to robustly
conclude that the RV and TR short-period planet results are inconsistent.
Assuming a logarithmic distribution of periods, we find that the relative
frequency of very hot Jupiters (VHJ: P=1-3 d) to hot Jupiters (HJ: P=3-9 d) is
10-20%. Given an absolute frequency of HJ of ~1%, this implies that
approximately one star in ~500-1000 has a VHJ. We also note that VHJ and HJ
appear to be distinct in terms of their upper mass limit. We discuss the
implications of our results for planetary migration theories, as well as
present and future TR and RV surveys.Comment: 11 pages, 4 figures, 2 tables. Minor changes. Accepted to ApJ, to
appear in the April 20, 2005 issue (v623
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
Microlensing of Circumstellar Disks
We investigate the microlensing effects on a source star surrounded by a
circumstellar disk, as a function of wavelength. The microlensing light curve
of the system encodes the geometry and surface brightness profile of the disk.
In the mid- and far-infrared, the emission of the system is dominated by the
thermal emission from the cold dusty disk. For a system located at the Galactic
center, we find typical magnifications to be of order 10-20% or higher,
depending on the disk surface brightness profile, and the event lasts over one
year. At around 20 microns, where the emission for the star and the disk are
comparable, the difference in the emission areas results in a chromatic
microlensing event. Finally, in the near-infrared and visible, where the
emission of the star dominates, the fraction of star light directly reflected
by the disk slightly modifies the light curve of the system which is no longer
that of a point source. In each case, the corresponding light curve can be used
to probe some of the disk properties. A fraction of 0.1% to 1% optical
microlensing events are expected to be associated with circumstellar disk
systems. We show that the lensing signal of the disk can be detected with
sparse follow-up observations of the next generation space telescopes. While
direct imaging studies of circumstellar disks are limited to the solar
neighborhood, this microlensing technique can probe very distant disk systems
living in various environments and has the potential to reveal a larger
diversity of circumstellar disks.Comment: 9 pages, 7 figures. Accepted for publication in Ap
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