82 research outputs found
OGLE-2005-BLG-018: Characterization of Full Physical and Orbital Parameters of a Gravitational Binary Lens
We present the analysis result of a gravitational binary-lensing event
OGLE-2005-BLG-018. The light curve of the event is characterized by 2 adjacent
strong features and a single weak feature separated from the strong features.
The light curve exhibits noticeable deviations from the best-fit model based on
standard binary parameters. To explain the deviation, we test models including
various higher-order effects of the motions of the observer, source, and lens.
From this, we find that it is necessary to account for the orbital motion of
the lens in describing the light curve. From modeling of the light curve
considering the parallax effect and Keplerian orbital motion, we are able to
measure not only the physical parameters but also a complete orbital solution
of the lens system. It is found that the event was produced by a binary lens
located in the Galactic bulge with a distance kpc from the Earth.
The individual lens components with masses and are separated with a semi-major axis of AU and
orbiting each other with a period yr. The event demonstrates
that it is possible to extract detailed information about binary lens systems
from well-resolved lensing light curves.Comment: 19 pages, 6 figure
A systematic fitting scheme for caustic-crossing microlensing events
We outline a method for fitting binary-lens caustic-crossing microlensing
events based on the alternative model parameterisation proposed and detailed in
Cassan (2008). As an illustration of our methodology, we present an analysis of
OGLE-2007-BLG-472, a double-peaked Galactic microlensing event with a source
crossing the whole caustic structure in less than three days. In order to
identify all possible models we conduct an extensive search of the parameter
space, followed by a refinement of the parameters with a Markov Chain-Monte
Carlo algorithm. We find a number of low-chi2 regions in the parameter space,
which lead to several distinct competitive best models. We examine the
parameters for each of them, and estimate their physical properties. We find
that our fitting strategy locates several minima that are difficult to find
with other modelling strategies and is therefore a more appropriate method to
fit this type of events.Comment: 12 pages, 11 figure
A Low-Mass Planet with a Possible Sub-Stellar-Mass Host in Microlensing Event MOA-2007-BLG-192
We report the detection of an extrasolar planet of mass ratio q ~ 2 x 10^(-4)
in microlensing event MOA-2007-BLG-192. The best fit microlensing model shows
both the microlensing parallax and finite source effects, and these can be
combined to obtain the lens masses of M = 0.060 (+0.028 -0.021) M_sun for the
primary and m = 3.3 (+4.9 -1.6) M_earth for the planet. However, the
observational coverage of the planetary deviation is sparse and incomplete, and
the radius of the source was estimated without the benefit of a source star
color measurement. As a result, the 2-sigma limits on the mass ratio and finite
source measurements are weak. Nevertheless, the microlensing parallax signal
clearly favors a sub-stellar mass planetary host, and the measurement of finite
source effects in the light curve supports this conclusion. Adaptive optics
images taken with the Very Large Telescope (VLT) NACO instrument are consistent
with a lens star that is either a brown dwarf or a star at the bottom of the
main sequence. Follow-up VLT and/or Hubble Space Telescope (HST) observations
will either confirm that the primary is a brown dwarf or detect the low-mass
lens star and enable a precise determination of its mass. In either case, the
lens star, MOA-2007-BLG-192L, is the lowest mass primary known to have a
companion with a planetary mass ratio, and the planet, MOA-2007-BLG-192Lb, is
probably the lowest mass exoplanet found to date, aside from the lowest mass
pulsar planet.Comment: Accepted for publication in the Astrophysical Journal. Scheduled for
the Sept. 1, 2008 issu
OGLE-2005-BLG-153: Microlensing Discovery and Characterization of A Very Low Mass Binary
The mass function and statistics of binaries provide important diagnostics of
the star formation process. Despite this importance, the mass function at low
masses remains poorly known due to observational difficulties caused by the
faintness of the objects. Here we report the microlensing discovery and
characterization of a binary lens composed of very low-mass stars just above
the hydrogen-burning limit. From the combined measurements of the Einstein
radius and microlens parallax, we measure the masses of the binary components
of and . This discovery
demonstrates that microlensing will provide a method to measure the mass
function of all Galactic populations of very low mass binaries that is
independent of the biases caused by the luminosity of the population.Comment: 6 pages, 3 figures, 1 tabl
The First Neptune Analog or Super-Earth with Neptune-like Orbit: MOA-2013-BLG-605Lb
We present the discovery of the first Neptune analog exoplanet or super-Earth
with Neptune-like orbit, MOA-2013-BLG-605Lb. This planet has a mass similar to
that of Neptune or a super-Earth and it orbits at times the expected
position of the snow-line, , which is similar to Neptune's
separation of from the Sun. The planet/host-star mass ratio
is and the projected separation normalized by the
Einstein radius is . There are three degenerate physical
solutions and two of these are due to a new type of degeneracy in the
microlensing parallax parameters, which we designate "the wide degeneracy". The
three models have (i) a Neptune-mass planet with a mass of orbiting a low-mass M-dwarf with a mass of , (ii) a mini-Neptune with orbiting a brown dwarf host with and (iii) a super-Earth with orbiting a low-mass brown dwarf host with which is slightly favored. The 3-D
planet-host separations are 4.6 AU, 2.1 AU and
0.94 AU, which are , or
times larger than for these models,
respectively. The Keck AO observation confirm that the lens is faint. This
discovery suggests that low-mass planets with Neptune-like orbit are common. So
processes similar to the one that formed Neptune in our own Solar System or
cold super-Earth may be common in other solar systems.Comment: 54 pages, 10 figures, 13 tables, Accepted for publication in the Ap
Probing the atmosphere of the bulge G5III star OGLE-2002-BUL-069 by analysis of microlense H alpha line
We discuss high-resolution, time-resolved spectra of the caustic exit of the
binary microlensing event OGLE 2002-BUL-69 obtained with UVES on the VLT. The
source star is a G5III giant in the Galactic Bulge. During such events, the
source star is highly magnified, and a strong differential magnification around
the caustic resolves its surface. Using an appropriate model stellar atmosphere
generated by the NextGEN code we obtained a model light curve for the caustic
exit and compared it with a dense set of photometric observations obtained by
the PLANET microlensing follow up network. We further compared predicted
variations in the H alpha equivalent width with those measured from our
spectra. While the model and observations agree in the gross features, there
are discrepancies suggesting shortcomings in the model, particularly for the H
alpha line core, where we have detected amplified emission from the stellar
chromosphere as the source star's trailing limb exited the caustic. This
achievement became possible by the provision of the OGLE-III Early Warning
System, a network of small telescopes capable of nearly-continuous
round-the-clock photometric monitoring, on-line data reduction, daily
near-real-time modelling in order to predict caustic crossing parameters, and a
fast and efficient response of a 8m-class telescope to a
``Target-Of-Opportunity'' observation request.Comment: 4 pages Latex, 3 figures, accepted for publication to astronomy and
astrophysics letter
Characterizing Low-Mass Binaries From Observation of Long Time-scale Caustic-crossing Gravitational Microlensing Events
Despite astrophysical importance of binary star systems, detections are
limited to those located in small ranges of separations, distances, and masses
and thus it is necessary to use a variety of observational techniques for a
complete view of stellar multiplicity across a broad range of physical
parameters. In this paper, we report the detections and measurements of 2
binaries discovered from observations of microlensing events MOA-2011-BLG-090
and OGLE-2011-BLG-0417. Determinations of the binary masses are possible by
simultaneously measuring the Einstein radius and the lens parallax. The
measured masses of the binary components are 0.43 and 0.39
for MOA-2011-BLG-090 and 0.57 and 0.17 for
OGLE-2011-BLG-0417 and thus both lens components of MOA-2011-BLG-090 and one
component of OGLE-2011-BLG-0417 are M dwarfs, demonstrating the usefulness of
microlensing in detecting binaries composed of low-mass components. From
modeling of the light curves considering full Keplerian motion of the lens, we
also measure the orbital parameters of the binaries. The blended light of
OGLE-2011-BLG-0417 comes very likely from the lens itself, making it possible
to check the microlensing orbital solution by follow-up radial-velocity
observation. For both events, the caustic-crossing parts of the light curves,
which are critical for determining the physical lens parameters, were resolved
by high-cadence survey observations and thus it is expected that the number of
microlensing binaries with measured physical parameters will increase in the
future.Comment: 8 pages, 5 figures, 4 table
OGLE-2005-BLG-071Lb, the Most Massive M-Dwarf Planetary Companion?
We combine all available information to constrain the nature of
OGLE-2005-BLG-071Lb, the second planet discovered by microlensing and the first
in a high-magnification event. These include photometric and astrometric
measurements from Hubble Space Telescope, as well as constraints from higher
order effects extracted from the ground-based light curve, such as microlens
parallax, planetary orbital motion and finite-source effects. Our primary
analysis leads to the conclusion that the host of Jovian planet
OGLE-2005-BLG-071Lb is an M dwarf in the foreground disk with mass M= 0.46 +/-
0.04 Msun, distance D_l = 3.3 +/- 0.4 kpc, and thick-disk kinematics v_LSR ~
103 km/s. From the best-fit model, the planet has mass M_p = 3.8 +/- 0.4 M_Jup,
lies at a projected separation r_perp = 3.6 +/- 0.2 AU from its host and so has
an equilibrium temperature of T ~ 55 K, i.e., similar to Neptune. A degenerate
model less favored by \Delta\chi^2 = 2.1 (or 2.2, depending on the sign of the
impact parameter) gives similar planetary mass M_p = 3.4 +/- 0.4 M_Jup with a
smaller projected separation, r_\perp = 2.1 +/- 0.1 AU, and higher equilibrium
temperature T ~ 71 K. These results from the primary analysis suggest that
OGLE-2005-BLG-071Lb is likely to be the most massive planet yet discovered that
is hosted by an M dwarf. However, the formation of such high-mass planetary
companions in the outer regions of M-dwarf planetary systems is predicted to be
unlikely within the core-accretion scenario. There are a number of caveats to
this primary analysis, which assumes (based on real but limited evidence) that
the unlensed light coincident with the source is actually due to the lens, that
is, the planetary host. However, these caveats could mostly be resolved by a
single astrometric measurement a few years after the event.Comment: 51 pages, 12 figures, 3 tables, Published in Ap
Limb-darkening measurements for a cool red giant in microlensing event OGLE 2004-BLG-482
Aims: We present a detailed analysis of OGLE 2004-BLG-482, a relatively
high-magnification single-lens microlensing event which exhibits clear
extended-source effects. These events are relatively rare, but they potentially
contain unique information on the stellar atmosphere properties of their source
star, as shown in this study. Methods: Our dense photometric coverage of the
overall light curve and a proper microlensing modelling allow us to derive
measurements of the OGLE 2004-BLG-482 source star's linear limb-darkening
coefficients in three bands, including standard Johnson-Cousins I and R, as
well as in a broad clear filter. In particular, we discuss in detail the
problems of multi-band and multi-site modelling on the expected precision of
our results. We also obtained high-resolution UVES spectra as part of a ToO
programme at ESO VLT from which we derive the source star's precise fundamental
parameters. Results: From the high-resolution UVES spectra, we find that OGLE
2004-BLG-482's source star is a red giant of MK type a bit later than M3, with
Teff = 3667 +/- 150 K, log g = 2.1 +/- 1.0 and an assumed solar metallicity.
This is confirmed by an OGLE calibrated colour-magnitude diagram. We then
obtain from a detailed microlensing modelling of the light curve linear
limb-darkening coefficients that we compare to model-atmosphere predictions
available in the literature, and find a very good agreement for the I and R
bands. In addition, we perform a similar analysis using an alternative
description of limb darkening based on a principal component analysis of ATLAS
limb-darkening profiles, and also find a very good agreement between
measurements and model predictions.Comment: Accepted in A&
OGLE-2009-BLG-092/MOA-2009-BLG-137: A Dramatic Repeating Event With the Second Perturbation Predicted by Real-Time Analysis
We report the result of the analysis of a dramatic repeating gravitational
microlensing event OGLE-2009-BLG-092/MOA-2009-BLG-137, for which the light
curve is characterized by two distinct peaks with perturbations near both
peaks. We find that the event is produced by the passage of the source
trajectory over the central perturbation regions associated with the individual
components of a wide-separation binary. The event is special in the sense that
the second perturbation, occurring days after the first, was
predicted by the real-time analysis conducted after the first peak,
demonstrating that real-time modeling can be routinely done for binary and
planetary events. With the data obtained from follow-up observations covering
the second peak, we are able to uniquely determine the physical parameters of
the lens system. We find that the event occurred on a bulge clump giant and it
was produced by a binary lens composed of a K and M-type main-sequence stars.
The estimated masses of the binary components are
and , respectively, and they are separated in
projection by . The measured distance to the
lens is . We also detect the orbital motion
of the lens system.Comment: 18 pages, 5 figures, 1 tabl
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