573 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
A New Photometric Model of the Galactic Bar using Red Clump Giants
We present a study of the luminosity density distribution of the Galactic bar
using number counts of red clump giants (RCGs) from the OGLE-III survey. The
data were recently published by Nataf et al. (2013) for 9019 fields towards the
bulge and have RC stars over a viewing area of . The data include the number counts, mean distance modulus
(), dispersion in and full error matrix, from which we fit the data
with several tri-axial parametric models. We use the Markov Chain Monte Carlo
(MCMC) method to explore the parameter space and find that the best-fit model
is the model, with the distance to the GC is 8.13 kpc, the ratio of
semi-major and semi-minor bar axis scale lengths in the Galactic plane
, and vertical bar scale length , is (close to being prolate). The scale length of the stellar
density profile along the bar's major axis is 0.67 kpc and has an angle
of , slightly larger than the value obtained from a similar study
based on OGLE-II data. The number of estimated RC stars within the field of
view is , which is systematically lower than the observed
value. We subtract the smooth parametric model from the observed counts and
find that the residuals are consistent with the presence of an X-shaped
structure in the Galactic centre, the excess to the estimated mass content is
. We estimate the total mass of the bar is . Our results can be used as a key ingredient to construct new density
models of the Milky Way and will have implications on the predictions of the
optical depth to gravitational microlensing and the patterns of hydrodynamical
gas flow in the Milky Way.Comment: 15 pages, 6 figures, 4 tables. MNRAS accepte
Interferometric Visibility and Closure Phase of Microlensing Events with Finite Source Size
Interferometers from the ground and space will be able to resolve the two
images in a microlensing event. This will at least partially lift the inherent
degeneracy between physical parameters in microlensing events. To increase the
signal-to-noise ratio, intrinsically bright events with large magnifications
will be preferentially selected as targets. These events may be influenced by
finite source size effects both photometrically and astrometrically. Using
observed finite source size events as examples, we show that the fringe
visibility can be affected by 5% - 10%, and the closure phase by a few degrees:
readily detectable by ground and space interferometers. Such detections will
offer unique information about the lens-source trajectory relative to the
baseline of the interferometers. Combined with photometric finite source size
effects, interferometry offers a way to measure the angular sizes of the source
and the Einstein radius accurately.
Limb-darkening changes the visibility by a small amount compared with a
source with uniform surface brightness, marginally detectable with ground-based
instruments.
We discuss the implications of our results for the plans to make
interferometric observations of future microlensing events.Comment: 18 pages, 9 figures, submitted to MNRA
Faint-source-star planetary microlensing: the discovery of the cold gas-giant planet OGLE-2014-BLG-0676Lb
We report the discovery of a planet – OGLE-2014-BLG-0676Lb– via gravitational microlensing. Observations for the lensing event were made by the following groups: Microlensing Observations in Astrophysics; Optical Gravitational Lensing Experiment; Wise Observatory; RoboNET/Las Cumbres Observatory Global Telescope; Microlensing Network for the Detection of Small Terrestrial Exoplanets; and μ-FUN. All analyses of the light-curve data favour a lens system comprising a planetary mass orbiting a host star. The most-favoured binary lens model has a mass ratio between the two lens masses of (4.78 ± 0.13) × 10−3. Subject to some important assumptions, a Bayesian probability density analysis suggests the lens system comprises a
3.09^(+1.02)_(−1.12) MJ planet orbiting a 0.62^(+0.20)_(−0.22) M_⊙ host star at a deprojected orbital separation of
4.40^(+2.16)_(−1.46) au. The distance to the lens system is
2.22^(+0.96)_(−0.83) kpc. Planet OGLE-2014-BLG-0676Lb provides additional data to the growing number of cool planets discovered using gravitational microlensing against which planetary formation theories may be tested. Most of the light in the baseline of this event is expected to come from the lens and thus high-resolution imaging observations could confirm our planetary model interpretation
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
Planetary Microlensing at High Magnification
Simulations of planetary microlensing at high magnification that were carried
out on a cluster computer are presented. It was found that the perturbations
due to two-thirds of all planets occur in the time interval [-0.5t_FWHM, 0.5t_
FWHM] with respect to the peak of the microlensing light curve, where t_FWHM is
typically about 14 hours. This implies that only this restricted portion of the
light curve need be intensively monitored for planets, a very significant
practical advantage. Nearly all planetary detections in high magnification
events will not involve caustic crossings. We discuss the issues involved in
determining the planetary parameters in high magnification microlensing events.
Earth mass planets may be detected with 1-m class telescopes if their projected
orbital radii lie within about 1.5 - 2.5 AU. Giant planets are detectable over
a much larger region. For multi-planet systems the perturbations due to
individual planets can be separated under certain conditions. The size of the
source star needs to be determined independently, but the presence of spots on
the source star is likely to be negligible, as is the effect of planetary
motion during an event.Comment: 12 pages, 13 embedded figures, accepted for publication by MNRA
ADAPTr Exhibition
The book is one of the outcomes of the grant (funded by the EU seventh framework Programme grant number 317325. Period of grant 01.01.2013 to 31.12.20160. It describes the exhibition held in Ambika P3. It includes a double page statement from each of the seven partners and from each of the 42 research fellows employed under the scheme. There are four new essays (Prof Richard Blythe, Prof Kester Rattenbury, Prof Leon van Schaik, Dr Fleur Watson) a preface by Prof John Verbeke, and introduction by Prof Katharine Heron. It is included on the ADAPTr website and submitted to the EU as one of the deliverable outputs
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
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