14 research outputs found
The Optical Gravitational Lensing Experiment. Short Distance Scale to the LMC
We present {\it UBVI} photometry of the eclipsing binary HV2274 - the system
which has been recently used for distance determination to the LMC by Guinan et
al. (1998). We determine the interstellar reddening to the star,
E(B-V)=0.149+/-0.015 mag, based on observed colors of the star. This value is
in excellent agreement with the mean reddening towards HV2274 obtained from
photometry of the red clump stars in the surrounding field. The reddening is
almost twice as large as determined by Guinan et al. (1998).
We discuss the consequences of reddening underestimate. Most likely HV2274 is
located much closer with the distance modulus to the star and the LMC: m-M =
18.22+/-0.13 mag supporting the short distance scale to the LMC. Such a
distance modulus is in excellent agreement with the recent distance
determinations with RR Lyr and red clump stars.Comment: 11 pages, Latex, 2 Figures. Accepted for publication in Astrophysical
Journal Letters. New version - trimmed to fit ApJL. Additional determination
of the reddening towards HV2274 with OB star
Planetary Detection Efficiency of the Magnification 3000 Microlensing Event OGLE-2004-BLG-343
OGLE-2004-BLG-343 was a microlensing event with peak magnification
A_{max}=3000+/-1100, by far the highest-magnification event ever analyzed and
hence potentially extremely sensitive to planets orbiting the lens star. Due to
human error, intensive monitoring did not begin until 43 minutes after peak, at
which point the magnification had fallen to A~1200, still by far the highest
ever observed. As the light curve does not show significant deviations due to a
planet, we place upper limits on the presence of such planets by extending the
method of Yoo et al. (2004b), which combines light-curve analysis with priors
from a Galactic model of the source and lens populations, to take account of
finite-source effects. This is the first event so analyzed for which
finite-source effects are important, and hence we develop two new techniques
for evaluating these effects. Somewhat surprisingly, we find that
OGLE-2004-BLG-343 is no more sensitive to planets than two previously analyzed
events with A_{max}~100, despite the fact that it was observed at ~12 times
higher magnification. However, we show that had the event been observed over
its peak, it would have been sensitive to almost all Neptune-mass planets over
a factor of 5 of projected separation and even would have had some sensitivity
to Earth-mass planets. This shows that some microlensing events being detected
in current experiments are sensitive to very low-mass planets. We also give
suggestions on how extremely high-magnification events can be more promptly
monitored in the future.Comment: 50 pages, 13 figures, published in The Astrophysical Journa
Constraints on Planetary Companions in the Magnification A=256 Microlensing Event: OGLE-2003-BLG-423
We develop a new method of modeling microlensing events based on a Monte
Carlo simulation that incorporates both a Galactic model and the constraints
imposed by the observed characteristics of the event. The method provides an
unbiased way to analyze the event especially when parameters are poorly
constrained by the observed lightcurve. We apply this method to search for
planetary companions of the lens in OGLE-2003-BLG-423, whose maximum
magnification A_max=256+-43 (or A_max=400+-115 from the lightcurve data alone)
is the highest among single-lens events ever recorded. The method permits us,
for the first time, to place constraints directly in the
planet-mass/projected-physical-separation plane rather than in the
mass-ratio/Einstein-radius plane as was done previously. For example,
Jupiter-mass companions of main-sequence stars at 2.5 AU are excluded with 80%
efficiency.Comment: 10 pages, 7 figures, accepted for publication in The Astrophysical
Journa
Microlensing optical depth toward the Galactic Bulge using bright sources from OGLE-II
We present a measurement of the microlensing optical depth toward the
Galactic Bulge based on 4 years of the OGLE-II survey using Red Clump Giant
(RCG). Using 32 events we find tau=2.55_{-0.46}^{+0.57}* 10^{-6} at
(l,b)=(1.16, -2.75). Taking into account the measured gradient along the
Galactic latitude b, tau = [ (4.48+/- 2.37) + (0.78+/- 0.84)* b]* 10^{-6}, this
value is consistent with previous measurements using RCG sources and recent
theoretical predictions. We determine the microlensing parameters and select
events using a model light curve with the flux blending. We find that ~38% of
the OGLE-II events which appear to have RCG sources are actually due to much
fainter stars blended with a bright companion. We show explicitly that model
fits without blending result in similar tau estimates through partial
cancellation of contributions from higher detection efficiency, underestimated
time-scales and larger number of selected events. This approach, however, leads
to biased time-scale distributions and event rates. Consequently, microlensing
studies should carefully consider source confusion effects even for bright
stars.Comment: 49 pages and 18 figures, ApJ in press, the value changed due to the
systematic correctio
Microlensing of Relativistic Knots in the Quasar HE1104-1805
We present 3 years of photometry of the ``Double Hamburger'' lensed quasar,
HE1104-1805, obtained on 102 separate nights using the OGLE 1.3-m telescope.
Both the A and B images show variations, but with substantial differences in
the lighcurves at all time delays. At the 310 day delay reported by Wisotzki
and collaborators the difference lightcurve has an rms amplitude of 0.060 mag.
The structure functions for the A and B images are quite different, with image
A more than twice as variable as image B (a factor of 4 in structure function)
on timescales of less than a month. Adopting microlensing as a working
hypothesis for the uncorrelated variability, the short timescale argues for the
relativistic motion of one or more components of the source. We argue that the
small amplitude of the fluctuations is due to the finite size of the source
with respect to the microlenses.Comment: As accepted for publication in ApJ. 22 pages. The discussion of
microlensing at high optical depth has been shortened and a few minor points
have been clarifie
WR 20a is an Eclipsing Binary: Accurate Determination of Parameters for an Extremely Massive Wolf-Rayet System
We present a high-precision I-band light curve for the Wolf-Rayet binary WR
20a, obtained as a sub-project of the Optical Gravitational Lensing Experiment.
Rauw et al. have recently presented spectroscopy for this system, strongly
suggesting extremely large minimum masses of 70.7 +/- 4.0 Mo and 68.8 +/- 3.8
Mo for the component stars of the system, with the exact values depending
strongly on the period of the system. We detect deep eclipses of about 0.4 mag
in the light curve of WR 20a, confirming and refining the suspected period of
P=3.686 days and deriving an inclination angle of i=74.5 +/- 2 deg. Using these
photometric data and the radial velocity data of Rauw et al., we derive the
masses for the two components of WR 20a to be 83.0 +/- 5.0 Mo and 82.0 +/- 5.0
Mo. Therefore, WR 20a is confirmed to consist of two extremely massive stars
and to be the most massive binary known with an accurate mass determination.Comment: 13 pages, 2 figures, 3 tables, accepted for publication in ApJ
The Optical Gravitational Lensing Experiment. Catalog of stellar proper motions in the OGLE-II Galactic bulge fields
We present a proper motion (\mu) catalogue of 5,080,236 stars in 49 Optical
Gravitational Lensing Experiment II (OGLE-II) Galactic bulge (GB) fields,
covering a range of -11 deg. <l< 11 deg. and -6 deg. <b<3 deg., the total area
close to 11 square degrees. The proper motion measurements are based on 138 -
555 I-band images taken during four observing seasons: 1997-2000. The catalogue
stars are in the magnitude range 11 < I < 18 mag. In particular, the catalogue
includes Red Clump Giants (RCGs) and Red Giants in the GB, and main sequence
stars in the Galactic disc. The proper motions up to \mu = 500 mas/yr were
measured with the mean accuracy of 0.8-3.5 mas/yr, depending on the brightness
of a star. This catalogue may be useful for studying the kinematic of stars in
the GB and the Galactic disk.Comment: 13 pages, 16 figures, MNRAS in pres
OGLE-2003-BLG-262: Finite-Source Effects from a Point-Mass Lens
We analyze OGLE-2003-BLG-262, a relatively short, t_E=12.5+-0.1day,
microlensing event generated by a point-mass lens transiting the face of a K
giant source in the Galactic bulge. We use the resulting finite-source effects
to measure the angular Einstein radius, theta_E=195+-17muas, and so constrain
the lens mass to the full-width half-maximum interval 0.08 < M/M_sun < 0.54.
The lens-source relative proper motion is mu_rel = 27+-2 km/s/kpc. Both values
are typical of what is expected for lenses detected toward the bulge. Despite
the short duration of the event, we detect marginal evidence for a "parallax
asymmetry", but argue that this is more likely to be induced by acceleration of
the source, a binary lens, or possibly by statistical fluctuations. Although
OGLE-2003-BLG-262 is only the second published event to date in which the lens
transits the source, such events will become more common with the new OGLE-III
survey in place. We therefore give a detailed account of the analysis of this
event to facilitate the study of future events of this type.Comment: 9 pages, 6 figures, accepted for publication in the Astrophysical
Journa
Variability of Luminous Stars in the Large Magellanic Cloud Using 10 Years of ASAS Data
Motivated by the detection of a recent outburst of the massive luminous blue
variable LMC-R71, which reached an absolute magnitude M_V = -9.3 mag, we
undertook a systematic study of the optical variability of 1268 massive stars
in the Large Magellanic Cloud, using a recent catalog by Bonanos et al. (2009)
as the input. The ASAS All Star Catalog (Pojmanski 2002) provided well-sampled
light curves of these bright stars spanning 10 years. Combining the two
catalogs resulted in 599 matches, on which we performed a variability search.
We identified 117 variable stars, 38 of which were not known before, despite
their brightness and large amplitude of variation. We found 13 periodic stars
that we classify as eclipsing binary (EB) stars, eight of which are newly
discovered bright, massive eclipsing binaries composed of OB type stars. The
remaining 104 variables are either semi- or non-periodic, the majority (85)
being red supergiants. Most (26) of the newly discovered variables in this
category are also red supergiants with only three B and four O stars.Comment: 23 pages, 10 figures and 3 tables; published in A
Optical Gravitational Lensing Experiment. OGLE-1999-BUL-19: The First Multi-Peak Parallax Event
We describe a highly unusual microlensing event, OGLE-1999-BUL-19, which
exhibits multiple peaks in its light curve. The Einstein radius crossing time
for this event is approximately one year, which is unusually long. We show that
the motion of the Earth induces these multiple peaks in the light curve, since
the relative transverse velocity of the lens projected into the observer plane
is very small (v = 12.5 km/s). This is the lowest velocity so far published and
we believe that this is the first multiple-peak parallax event ever observed.
We also believe that this event may be exhibiting slight binary-source
signatures in addition to these parallax-induced multiple peaks. With
spectroscopic observations it is possible to test this `parallax plus
binary-source' hypothesis and (if this hypothesis turns out to be correct) to
simultaneously fit both models and obtain a measurement of the lens mass.
Furthermore, spectroscopic observations could also supply information regarding
the lens properties, possibly providing another avenue for determining the lens
mass. We found that most of the I-band blending is probably caused by light
from the lens or a binary companion to the source. However, in the V-band,
there appears to be a second blended source 0.35" away from the lensed source.
HST observations will be very useful for understanding the nature of the
blends. We also suggest that a radial velocity survey of all parallax events
will be very useful for further constraining the lensing kinematics and
understanding the origins of these events and the excess of long events toward
the bulge.Comment: 36 pages, 7 figures. Accepted for publication in MNRA