247 research outputs found
OGLE-2012-BLG-0455/MOA-2012-BLG-206: Microlensing event with ambiguity in planetary interpretations caused by incomplete coverage of planetary signal
Characterizing a microlensing planet is done from modeling an observed
lensing light curve. In this process, it is often confronted that solutions of
different lensing parameters result in similar light curves, causing
difficulties in uniquely interpreting the lens system, and thus understanding
the causes of different types of degeneracy is important. In this work, we show
that incomplete coverage of a planetary perturbation can result in degenerate
solutions even for events where the planetary signal is detected with a high
level of statistical significance. We demonstrate the degeneracy for an
actually observed event OGLE-2012-BLG-0455/MOA-2012-BLG-206. The peak of this
high-magnification event exhibits very strong deviation
from a point-lens model with for data sets with a
total number of measurement 6963. From detailed modeling of the light curve, we
find that the deviation can be explained by four distinct solutions, i.e., two
very different sets of solutions, each with a two-fold degeneracy. While the
two-fold (so-called "close/wide") degeneracy is well-understood, the degeneracy
between the radically different solutions is not previously known. The model
light curves of this degeneracy differ substantially in the parts that were not
covered by observation, indicating that the degeneracy is caused by the
incomplete coverage of the perturbation. It is expected that the frequency of
the degeneracy introduced in this work will be greatly reduced with the
improvement of the current lensing survey and follow-up experiments and the
advent of new surveys.Comment: 5 pages, 3 figures, ApJ accepte
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
Microlensing Event MOA-2007-BLG-400: Exhuming the Buried Signature of a Cool, Jovian-Mass Planet
We report the detection of the cool, Jovian-mass planet MOA-2007-BLG-400Lb.
The planet was detected in a high-magnification microlensing event (with peak
magnification A_max = 628) in which the primary lens transited the source,
resulting in a dramatic smoothing of the peak of the event. The angular extent
of the region of perturbation due to the planet is significantly smaller than
the angular size of the source, and as a result the planetary signature is also
smoothed out by the finite source size. Thus the deviation from a single-lens
fit is broad and relatively weak (~ few percent). Nevertheless, we demonstrate
that the planetary nature of the deviation can be unambiguously ascertained
from the gross features of the residuals, and detailed analysis yields a fairly
precise planet/star mass ratio of q = 0.0026+/-0.0004, in accord with the large
significance (\Delta\chi^2=1070) of the detection. The planet/star projected
separation is subject to a strong close/wide degeneracy, leading to two
indistinguishable solutions that differ in separation by a factor of ~8.5.
Upper limits on flux from the lens constrain its mass to be M < 0.75 M_Sun
(assuming it is a main-sequence star). A Bayesian analysis that includes all
available observational constraints indicates a primary in the Galactic bulge
with a mass of ~0.2-0.5 M_Sun and thus a planet mass of ~ 0.5-1.3 M_Jupiter.
The separation and equilibrium temperature are ~0.6-1.1AU (~5.3-9.7AU) and
~103K (~34K) for the close (wide) solution. If the primary is a main-sequence
star, follow-up observations would enable the detection of its light and so a
measurement of its mass and distance.Comment: 30 pages, 6 figures, Submitted to Ap
Density functional formalism in the canonical ensemble
Density functional theory, when applied to systems with , is based
on the grand canonical extension of the Hohenberg-Kohn-Sham theorem due to
Mermin (HKSM theorem). While a straightforward canonical ensemble
generalization fails, work in nanopore systems could certainly benefit from
such extension. We show that, if the asymptotic behaviour of the canonical
distribution functions is taken into account, the HKSM theorem can be extended
to the canonical ensemble. We generate -modified correlation and
distribution functions hierarchies and prove that, if they are employed, either
a modified external field or the density profiles can be indistinctly used as
independent variables. We also write down the % -modified free energy
functional and prove that its minimum is reached when the equilibrium values of
the new hierarchy are used. This completes the extension of the HKSM theorem.Comment: revtex, to be submitted to Phys. Rev. Let
MOA-2011-BLG-293Lb: A test of pure survey microlensing planet detections
Because of the development of large-format, wide-field cameras, microlensing
surveys are now able to monitor millions of stars with sufficient cadence to
detect planets. These new discoveries will span the full range of significance
levels including planetary signals too small to be distinguished from the
noise. At present, we do not understand where the threshold is for detecting
planets. MOA-2011-BLG-293Lb is the first planet to be published from the new
surveys, and it also has substantial followup observations. This planet is
robustly detected in survey+followup data (Delta chi^2 ~ 5400). The planet/host
mass ratio is q=5.3+/- 0.2*10^{-3}. The best fit projected separation is
s=0.548+/- 0.005 Einstein radii. However, due to the s-->s^{-1} degeneracy,
projected separations of s^{-1} are only marginally disfavored at Delta
chi^2=3. A Bayesian estimate of the host mass gives M_L = 0.43^{+0.27}_{-0.17}
M_Sun, with a sharp upper limit of M_L < 1.2 M_Sun from upper limits on the
lens flux. Hence, the planet mass is m_p=2.4^{+1.5}_{-0.9} M_Jup, and the
physical projected separation is either r_perp = ~1.0 AU or r_perp = ~3.4 AU.
We show that survey data alone predict this solution and are able to
characterize the planet, but the Delta chi^2 is much smaller (Delta chi^2~500)
than with the followup data. The Delta chi^2 for the survey data alone is
smaller than for any other securely detected planet. This event suggests a
means to probe the detection threshold, by analyzing a large sample of events
like MOA-2011-BLG-293, which have both followup data and high cadence survey
data, to provide a guide for the interpretation of pure survey microlensing
data.Comment: 29 pages, 6 figures, Replaced 7/3/12 with the version accepted to Ap
OGLE-2009-BLG-023/MOA-2009-BLG-028: Characterization of a Binary Microlensing Event Based on Survey Data
We report the result of the analysis of the light curve of a caustic-crossing
binary-lens microlensing event OGLE-2009-BLG-023/MOA-2009-BLG-028. Even though
the event was observed solely by survey experiments, we could uniquely
determine the mass of the lens and distance to it by simultaneously measuring
the Einstein radius and lens parallax. From this, we find that the lens system
is composed of M-type dwarfs with masses and
located in the Galactic disk with a distance of kpc toward the Galactic bulge direction. The event demonstrates that
physical lens parameters of binary-lens events can be routinely determined from
future high-cadence lensing surveys and thus microlensing can provide a new way
to study Galactic binaries.Comment: 8 pages, 5 figure
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