130 research outputs found
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&
RR Lyrae Stars In The GCVS Observed By The Qatar Exoplanet Survey
We used the light curve archive of the Qatar Exoplanet Survey (QES) to
investigate the RR Lyrae variable stars listed in the General Catalogue of
Variable Stars (GCVS). Of 588 variables studied, we reclassify 14 as eclipsing
binaries, one as an RS Canum Venaticorum-type variable, one as an irregular
variable, four as classical Cepheids, and one as a type II Cepheid, while also
improving their periods. We also report new RR Lyrae sub-type classifications
for 65 variables and improve on the GCVS period estimates for 135 RR Lyrae
variables. There are seven double-mode RR Lyrae stars in the sample for which
we measured their fundamental and first overtone periods. Finally, we detect
the Blazhko effect in 38 of the RR Lyrae stars for the first time and we
successfully measured the Blazhko period for 26 of them.Comment: Accepted IBV
Difference image analysis : automatic kernel design using information criteria
This publication was made possible by NPRP grant # X-019-1-006 from the Qatar National Research Fund (a member of Qatar Foundation).We present a selection of methods for automatically constructing an optimal kernel model for difference image analysis which require very few external parameters to control the kernel design. Each method consists of two components; namely, a kernel design algorithm to generate a set of candidate kernel models, and a model selection criterion to select the simplest kernel model from the candidate models that provides a sufficiently good fit to the target image. We restricted our attention to the case of solving for a spatially invariant convolution kernel composed of delta basis functions, and we considered 19 different kernel solution methods including six employing kernel regularization. We tested these kernel solution methods by performing a comprehensive set of image simulations and investigating how their performance in terms of model error, fit quality, and photometric accuracy depends on the properties of the reference and target images. We find that the irregular kernel design algorithm employing unregularized delta basis functions, combined with either the Akaike or Takeuchi information criterion, is the best kernel solution method in terms of photometric accuracy. Our results are validated by tests performed on two independent sets of real data. Finally, we provide some important recommendations for software implementations of difference image analysis.Publisher PDFPeer reviewe
An Improved Method for Estimating the Masses of Stars with Transiting Planets
To determine the physical parameters of a transiting planet and its host star
from photometric and spectroscopic analysis, it is essential to independently
measure the stellar mass. This is often achieved by the use of evolutionary
tracks and isochrones, but the mass result is only as reliable as the models
used. The recent paper by Torres et al (2009) showed that accurate values for
stellar masses and radii could be obtained from a calibration using T_eff, log
g and [Fe/H]. We investigate whether a similarly good calibration can be
obtained by substituting log rho - the fundamental parameter measured for the
host star of a transiting planet - for log g, and apply this to star-exoplanet
systems. We perform a polynomial fit to stellar binary data provided in Torres
et al (2009) to obtain the stellar mass and radius as functions of T_eff, log
rho and [Fe/H], with uncertainties on the fit produced from a Monte Carlo
analysis. We apply the resulting equations to measurements for seventeen
SuperWASP host stars, and also demonstrate the application of the calibration
in a Markov Chain Monte Carlo analysis to obtain accurate system parameters
where spectroscopic estimates of effective stellar temperature and metallicity
are available. We show that the calibration using log rho produces accurate
values for the stellar masses and radii; we obtain masses and radii of the
SuperWASP stars in good agreement with isochrone analysis results. We ascertain
that the mass calibration is robust against uncertainties resulting from poor
photometry, although a good estimate of stellar radius requires good-quality
transit light curve to determine the duration of ingress and egress.Comment: 5 pages, 2 figures, accepted for publication in A&
The first WASP public data release
The WASP (wide angle search for planets) project is an exoplanet transit survey that has been automatically taking wide field images since 2004. Two instruments, one in La Palma and the other in South Africa, continually monitor the night sky, building up light curves of millions of unique objects. These light curves are used to search for the characteristics of exoplanetary transits. This first public data release (DR1) of the WASP archive makes available all the light curve data and images from 2004 up to 2008 in both the Northern and Southern hemispheres. A web interface () to the data allows easy access over the Internet. The data set contains 3 631 972 raw images and 17 970 937 light curves. In total the light curves have 119 930 299 362 data points available between them
Qatar-1b: a hot Jupiter orbiting a metal-rich K dwarf star
We report the discovery and initial characterisation of Qatar-1b, a hot
Jupiter orbiting a metal-rich K dwarf star, the first planet discovered by the
Alsubai Project exoplanet transit survey. We describe the strategy used to
select candidate transiting planets from photometry generated by the Alsubai
Project instrument. We examine the rate of astrophysical and other false
positives found during the spectroscopic reconnaissance of the initial batch of
candidates. A simultaneous fit to the follow-up radial velocities and
photometry of Qatar-1b yield a planetary mass of 1.09+/-0.08 Mjup and a radius
of 1.16+/-0.05 Rjup. The orbital period and separation are 1.420033 days and
0.0234 AU for an orbit assumed to be circular. The stellar density, effective
temperature and rotation rate indicate an age greater than 4 Gyr for the
system.Comment: 8 pages, 5 figures, submitted to Monthly Notices of the Royal
Astronomical Societ
The 0.5MJ transiting exoplanet WASP-13b
We report the discovery of WASP-13b, a low-mass M_{\rm p} = 0.46 ^_~M_J transiting exoplanet with an orbital period of 4.35298 0.00004 days. The transit has a depth of 9 mmag, and although our follow-up photometry does not allow us to constrain the impact parameter well (0 < b < 0.46), with radius in the range ~ 1.06-1.21 RJ the location of WASP-13b in the mass-radius plane is nevertheless consistent with H/He-dominated, irradiated, low core mass and core-free theoretical models. The G1V host star is similar to the Sun in mass (M__ ~M_{\odot}) and metallicity ([M/H] = 0.00.2), but is possibly older ( 8.5^_{\rm -4.9} Gyr)
Qatar-2: A K dwarf orbited by a transiting hot Jupiter and a more massive companion in an outer orbit
We report the discovery and initial characterization of Qatar-2b, a hot
Jupiter transiting a V = 13.3 mag K dwarf in a circular orbit with a short
period, P_ b = 1.34 days. The mass and radius of Qatar-2b are M_p = 2.49 M_j
and R_p = 1.14 R_j, respectively. Radial-velocity monitoring of Qatar-2 over a
span of 153 days revealed the presence of a second companion in an outer orbit.
The Systemic Console yielded plausible orbits for the outer companion, with
periods on the order of a year and a companion mass of at least several M_j.
Thus Qatar-2 joins the short but growing list of systems with a transiting hot
Jupiter and an outer companion with a much longer period. This system
architecture is in sharp contrast to that found by Kepler for multi-transiting
systems, which are dominated by objects smaller than Neptune, usually with
tightly spaced orbits that must be nearly coplanar
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