373 research outputs found
Predicted microlensing events from analysis of Gaia Data Release 2
Astrometric microlensing can be used to make precise measurements of the
masses of lens stars that are independent of their assumed internal physics.
Such direct mass measurements, obtained purely by observing the gravitational
effects of the stars on external objects, are crucial for validating
theoretical stellar models. Specifically, astrometric microlensing provides a
channel to direct mass measurements of single stars for which so few
measurements exist. To use the astrometric solutions and photometric
measurements of ~1.7 billion stars from Gaia Data Release 2 to predict
microlensing events during the nominal Gaia mission and beyond. This will
enable astronomers to observe the entirety of each event with appropriate
observing resources. The data will allow precise lens mass measurements for
white dwarfs and low-mass main sequence stars helping to constrain stellar
evolutionary models. I search for source-lens pairs in GDR2 that could lead to
events between 25/07/2014 and 25/07/2026. I estimate lens masses using GDR2
photometry and parallaxes, and appropriate model isochrones. Combined with
source and lens parallax measurements from GDR2, this allows the Einstein
radius to be computed for each pair. By considering the paths on the sky, I
calculate the microlensing signals that are to be expected. I present a list of
76 predicted microlensing events. 9 and 5 astrometric events will be caused by
LAWD37 and Stein2051B. 9 events will exhibit detectable photometric and
astrometric signatures. Of the remaining events, ten will exhibit astrometric
signals with amplitudes above 0.5 mas, while the rest are low-amplitude
astrometric events with amplitudes between 0.131 and 0.5 mas. 5 and 2 events
will reach their peaks during 2018 and 2019. 5 of the photometric events have
the potential to evolve into high-magnification events, which may also probe
for planetary companions to the lenses.Comment: Accepted A&
Difference image analysis: The interplay between the photometric scale factor and systematic photometric errors
Context: Understanding the source of systematic errors in photometry is
essential for their calibration. Aims: We investigate how photometry performed
on difference images can be influenced by errors in the photometric scale
factor. Methods: We explore the equations for difference image analysis (DIA)
and we derive an expression describing how errors in the difference flux, the
photometric scale factor and the reference flux are propagated to the object
photometry. Results: We find that the error in the photometric scale factor is
important, and while a few studies have shown that it can be at a significant
level, it is currently neglected by the vast majority of photometric surveys
employing DIA. Conclusions: Minimising the error in the photometric scale
factor, or compensating for it in a post-calibration model, is crucial for
reducing the systematic errors in DIA photometry.Comment: Accepted A&
A New Algorithm For Difference Image Analysis
In the context of difference image analysis (DIA), we present a new method
for determining the convolution kernel matching a pair of images of the same
field. Unlike the standard DIA technique which involves modelling the kernel as
a linear combination of basis functions, we consider the kernel as a discrete
pixel array and solve for the kernel pixel values directly using linear
least-squares. The removal of basis functions from the kernel model is
advantageous for a number of compelling reasons. Firstly, it removes the need
for the user to specify such functions, which makes for a much simpler user
application and avoids the risk of an inappropriate choice. Secondly, basis
functions are constructed around the origin of the kernel coordinate system,
which requires that the two images are perfectly aligned for an optimal result.
The pixel kernel model is sufficiently flexible to correct for image
misalignments, and in the case of a simple translation between images, image
resampling becomes unnecessary. Our new algorithm can be extended to spatially
varying kernels by solving for individual pixel kernels in a grid of image
sub-regions and interpolating the solutions to obtain the kernel at any one
pixel.Comment: MNRAS Letters Accepte
Upper limits on the hot Jupiter fraction in the field of NGC 7789
We describe a method of estimating the abundance of short-period extrasolar
planets based on the results of a photometric survey for planetary transits. We
apply the method to a 21-night survey with the 2.5m Isaac Newton Telescope of
\~32000 stars in a ~0.5 deg by 0.5 deg square field including the open cluster
NGC 7789. From the colour-magnitude diagram we estimate the mass and radius of
each star by comparison with the cluster main sequence. We search for injected
synthetic transits throughout the lightcurve of each star in order to determine
their recovery rate, and thus calculate the expected number of transit
detections and false alarms in the survey. We take proper account of the
photometric accuracy, time sampling of the observations and criteria
(signal-to-noise and number of transits) adopted for transit detection.
Assuming that none of the transit candidates found in the survey will be
confirmed as real planets, we place conservative upper limits on the abundance
of planets as a function of planet radius, orbital period and spectral type.Comment: Submitted to MNRAS (04/11/2005
The unusually large population of Blazhko variables in the globular cluster NGC 5024 (M53)
We report the discovery of amplitude and phase modulations typical of the
Blazhko effect in 22 RRc and 9 RRab type RR Lyrae stars in NGC 5024 (M53). This
brings the confirmed Blazhko variables in this cluster to 23 RRc and 11 RRab,
that represent 66% and 37% of the total population of RRc and RRab stars in the
cluster respectively, making NGC 5024 the globular cluster with the largest
presently known population of Blazhko RRc stars. We place a lower limit on the
overall incidence rate of the Blazhko effect among the RR Lyrae population in
this cluster of 52%. New data have allowed us to refine the pulsation periods.
The limitations imposed by the time span and sampling of our data prevents
reliable estimations of the modulation periods. The amplitudes of the
modulations range between 0.02 and 0.39 mag. The RRab and RRc are neatly
separated in the CMD, and the RRc Blazhko variables are on averge redder than
their stable couterparts; these two facts may support the hypothesis that the
HB evolution in this cluster is towards the red and that the Blazhko
modulations in the RRc stars are connected with the pulsation mode switch.Comment: ACCEPTED IN MNRAS 14 pages, 9 figures and 6 table
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