101 research outputs found
Modelling the Galactic bar using OGLE-II Red Clump Giant Stars
Red clump giant stars can be used as distance indicators to trace the mass
distribution of the Galactic bar. We use RCG stars from 44 bulge fields from
the OGLE-II microlensing collaboration database to constrain analytic tri-axial
models for the Galactic bar. We find the bar major axis is oriented at an angle
of 24 - 27 degrees to the Sun-Galactic centre line-of-sight. The ratio of
semi-major and semi-minor bar axis scale lengths in the Galactic plane x_0,
y_0, and vertical bar scale length z_0, is x_0 : y_0 : z_0 = 10 : 3.5 : 2.6,
suggesting a slightly more prolate bar structure than the working model of
Gerhard (2002) which gives the scale length ratios as x_0 : y_0 : z_0 = 10 : 4
: 3 .Comment: 15 pages, 10 figures, accepted for publication in MNRAS.
Supplementary material available online: 10 pages, 10 figure
Predictions for the Detection and Characterization of a Population ofFree-floating Planets with K2 Campaign 9
K2 Campaign 9 (K2C9) offers the first chance to measure parallaxes and masses of members of the large population of free-floating planets (FFPs) that has previously been inferred from measurements of the rate of short-timescale microlensing events. Using detailed simulations of the nominal campaign (ignoring the loss of events due to Kepler\u27s emergency mode) and ground-based microlensing surveys, we predict the number of events that can be detected if there is a population of 1 MjupiterFFPs matching current observational constraints. Using a Fisher matrix analysis, we also estimate the number of detections for which it will be possible to measure the microlensing parallax, angular Einstein radius, and FFP mass. We predict that between 1.4 and 7.9 events will be detected in the K2 data, depending on the noise floor that can be reached, but with the optimistic scenario being more likely. For nearly all of these, it will be possible to either measure the parallax or constrain it to be probabilistically consistent with only planetary-mass lenses. We expect that for between 0.42 and 0.98 events it will be possible to gain a complete solution and measure the FFP mass. For the emergency-mode truncated campaign, these numbers are reduced by 20 percent. We argue that when combined with prompt high-resolution imaging of a larger sample of short-timescale events, K2C9 will conclusively determine if the putative FFP population is indeed both planetary and free-floating
Microlensing of close binary stars
The gravity due to a multiple-mass system has a remarkable gravitational
effect: the extreme magnification of background light sources along extended
so-called caustic lines. This property has been the channel for some remarkable
astrophysical discoveries over the past decade, including the detection and
characterisation of extra-solar planets, the routine analysis of
limb-darkening, and, in one case, limits set on the apparent shape of a star
several kiloparsec distant. In this paper we investigate the properties of the
microlensing of close binary star systems. We show that in some cases it is
possible to detect flux from the Roche lobes of close binary stars. Such
observations could constrain models of close binary stellar systems.Comment: 10 pages, accepted to MNRA
Prediction of Planet Yields by the PRime-focus Infrared Microlensing Experiment Microlensing Survey
The PRime-focus Infrared Microlensing Experiment (PRIME) will be the first to
conduct a dedicated near infrared (NIR) microlensing survey by using a 1.8m
telescope with a wide field of view of 1.45 at the South
African Astronomical Observatory (SAAO). The major goals of the PRIME
microlensing survey are to measure the microlensing event rate in the inner
Galactic bulge to help design the observing strategy for the exoplanet
microlensing survey by the {\it Nancy Grace Roman Space Telescope} and to make
a first statistical measurement of exoplanet demographics in the central bulge
fields where optical observations are very difficult owing to the high
extinction in these fields. Here we conduct a simulation of the PRIME
microlensing survey to estimate its planet yields and determine the optimal
survey strategy, using a Galactic model optimized for the inner Galactic bulge.
In order to maximize the number of planet detections and the range of planet
mass, we compare the planet yields among four observation strategies. Assuming
{the \citet{2012Natur.481..167C} mass function as modified by
\citet{2019ApJS..241....3P}}, we predict that PRIME will detect planetary
signals for planets ( planets with ,
planets with mass , planets ), per year depending on the chosen
observation strategy.Comment: 25 pages, 17 figures, and 3 tables. Accept for publication in The
Astronomical Journa
Proper Motion Dispersions of Red Clump Giants in the Galactic Bulge: Observations and Model Comparisons
Red clump giants in the Galactic bulge are approximate standard candles and
hence they can be used as distance indicators. We compute the proper motion
dispersions of RCG stars in the Galactic bulge using the proper motion
catalogue from the second phase of the Optical Gravitational Microlensing
Experiment (OGLE-II, Sumi et al. 2004) for 45 fields. The proper motion
dispersions are measured to a few per cent accuracy due to the large number of
stars in the fields. The observational sample is comprised of 577736 stars.
These observed data are compared to a state-of-the-art particle simulation of
the Galactic bulge region. The predictions are in rough agreement with
observations, but appear to be too anisotropic in the velocity ellipsoid. We
note that there is significant field-to-field variation in the observed proper
motion dispersions. This could either be a real feature, or due to some unknown
systematic effect.Comment: 12 pages, 13 figures, accepted for publication in MNRA
Microlensing Results Challenge the Core Accretion Runaway Growth Scenario for Gas Giants
We compare the planet-to-star mass-ratio distribution measured by
gravitational microlensing to core accretion theory predictions from population
synthesis models. The core accretion theory's runaway gas accretion process
predicts a dearth of intermediate-mass giant planets that is not seen in the
microlensing results. In particular, the models predict fewer
planets at mass ratios of than inferred
from microlensing observations. This tension implies that gas giant formation
may involve processes that have hitherto been overlooked by existing core
accretion models or that the planet-forming environment varies considerably as
a function of host-star mass. Variation from the usual assumptions for the
protoplanetary disk viscosity and thickness could reduce this discrepancy, but
such changes might conflict with microlensing results at larger or smaller mass
ratios, or with other observations. The resolution of this discrepancy may have
important implications for planetary habitability because it has been suggested
that the runaway gas accretion process may have triggered the delivery of water
to our inner solar system. So, an understanding of giant planet formation may
help us to determine the occurrence rate of habitable planets.Comment: 12 pages, 2 figures, 1 table, accepted for publication in ApJ
OGLE-II High Proper Motion Stars towards the Galactic centre
The photometry data base of the second phase of the OGLE microlensing
experiment, OGLE-II, is a rich source of information about the kinematics and
structure of the Galaxy. In this work we use the OGLE-II proper motion
catalogue to identify candidate stars which have high proper motions. 521 stars
with proper motion mu > 50 mas/yr in the OGLE-II proper motion catalogue (Sumi
2004) were cross-identified with stars in the MACHO high proper motion
catalogue, and the DENIS and 2MASS infra-red photometry catalogues. Photometric
distances were computed for stars with colours consistent with G/K and M type
stars. 6 stars were newly identified as possible nearby (< 50 pc) M dwarfs.Comment: 7 figures and 4 tables, MNRAS, accepte
A Planetary Microlensing Event with an Unusually Red Source Star: MOA-2011-BLG-291
We present the analysis of planetary microlensing event MOA-2011-BLG-291,
which has a mass ratio of and a source star that
is redder (or brighter) than the bulge main sequence. This event is located at
a low Galactic latitude in the survey area that is currently planned for NASA's
WFIRST exoplanet microlensing survey. This unusual color for a microlensed
source star implies that we cannot assume that the source star is in the
Galactic bulge. The favored interpretation is that the source star is a lower
main sequence star at a distance of kpc in the Galactic disk.
However, the source could also be a turn-off star on the far side of the bulge
or a sub-giant in the far side of the Galactic disk if it experiences
significantly more reddening than the bulge red clump stars. However, these
possibilities have only a small effect on our mass estimates for the host star
and planet. We find host star and planet masses of and from a Bayesian
analysis with a standard Galactic model under the assumption that the planet
hosting probability does not depend on the host mass or distance. However, if
we attempt to measure the host and planet masses with host star brightness
measurements from high angular resolution follow-up imaging, the implied masses
will be sensitive to the host star distance. The WFIRST exoplanet microlensing
survey is expected to use this method to determine the masses for many of the
planetary systems that it discovers, so this issue has important design
implications for the WFIRST exoplanet microlensing survey
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