Searching for transits in the WTS with the difference imaging light curves

The search for exo-planets is currently one of the most exiting and
active topics in astronomy. Small and rocky planets are
particularly the subject of intense research, since if they are
suitably located from their host star, they may be warm and
potentially habitable worlds. On the other hand, the discovery of
giant planets in short-period orbits provides important constraints
on models that describe planet formation and orbital migration
theories. Several projects are dedicated to discover and
characterize planets outside of our solar system. Among them, the
Wide-Field Camera Transit Survey (WTS) is a pioneer program aimed
to search for extra-solar planets, that stands out for its
particular aims and methodology. The WTS has been in operation
since August 2007 with observations from the United Kingdom
Infrared Telescope, and represents the first survey that searches
for transiting planets in the near-infrared wavelengths; hence the
WTS is designed to discover planets around M-dwarfs. The survey was
originally assigned about 200 nights, observing four fields that
were selected seasonally (RA = 03, 07, 17 and 19h) during a year.
The images from the survey are processed by a data reduction
pipeline, which uses aperture photometry to construct the light
curves. For the most complete field (19h-1145 epochs) in the
survey, we produce an alternative set of light curves by using the
method of difference imaging, which is a photometric technique that
has shown important advantages when used in crowded fields. A
quantitative comparison between the photometric precision achieved
with both methods is carried out in this work. We remove systematic
effects using the sysrem algorithm, scale the error bars on the
light curves, and perform a comparison of the corrected light
curves. The results show that the aperture photometry light curves
provide slightly better precision for objects with J < 16.
However, difference photometry light curves present a significant
improvement for fainter stars. In order to detect transits in the
WTS light curves, we use a modified version of the box-fitting
algorithm. The implementation on the detection algorithm performs a
trapezoid-fit to the folded light curve. We show that the new fit
is able to produce more accurate results than the box-fit model. We
describe a set of selection criteria to search for transit
candidates that include a parameter calculated by our detection
algorithm: the V-shape parameter, which has proven to be useful to
automatically identify and remove eclipsing binaries from the
survey. The criteria are optimized using Monte-Carlo simulations of
artificial transit signals that are injected into the real WTS
light curves and subsequently analyzed by our detection algorithm.
We separately optimize the selection criteria for two different
sets of light curves, one for F-G-K stars, and another for
M-dwarfs. In order to search for transiting planet candidates, the
optimized selection criteria are applied to the aperture photometry
and difference imaging light curves. In this way, the best 200
transit candidates from a sample of ~ 475 000 sources are
automatically selected. A visual inspection of the folded light
curves of these detections is carried out to eliminate clear
false-positives or false-detections. Subsequently, several analysis
steps are performed on the 18 best detections, which allow us to
classify these objects as transiting planet and eclipsing binary
candidates. We report one planet candidate orbiting a late G-type
star, which is proposed for photometric follow-up. The independent
analysis on the M-dwarf sample provides no planet candidates around
these stars. Therefore, the null detection hypothesis and upper
limits on the occurrence rate of giant planets around M-dwarfs with
J < 17 mag presented in a prior study are confirmed. In this
work, we extended the search for transiting planets to stars with J
< 18 mag, which enables us to impose a more strict upper limit
of 1.1 % on the occurrence rate of short-period giant planets
around M-dwarfs, which is significantly lower than other limit
published so far. The lack of Hot Jupiters around M-dwarfs play an
important role in the existing theories of planet formation and
orbital migration of exo-planets around low-mass stars. The dearth
of gas-giant planets in short-period orbit detections around M
stars indicates that it is not necessary to invoke the disk
instability formation mechanism, coupled with an orbital migration
process to explain the presence of such planets around low-mass
stars. The much reduced efficiency of the core-accretion model to
form Jupiters around cool stars seems to be in agreement with the
current null result. However, our upper limit value, the lowest
reported sofar, is still higher than the detection rates of
short-period gas-giant planets around hotter stars. Therefore, we
cannot yet reach any firm conclusion about Jovian planet formation
models around low-mass and cool main-sequence stars, since there
are currently not sufficient observational evidences to support the
argument that Hot Jupiters are less common around M-dwarfs than
around Sun-like stars. The way to improve this situation is to
monitor larger samples of M-stars. For example, an extended
analysis of the remaining three WTS fields and currently running
M-dwarf transit surveys (like Pan-Planets and PTF/M-dwarfs
projects, which are monitoring up to 100 000 objects) may reduce
this upper limit. Current and future space missions like Kepler and
GAIA could also help to either set stricter upper limits or finally
detect Hot Jupiters around low-mass stars. In the last part of this
thesis, we present other applications of the difference imaging
light curves. We report the detection of five faint
extremely-short-period eclipsing binary systems with periods
shorter than 0.23 d, as well as two candidates and one confirmed
M-dwarf/M-dwarf eclipsing binaries. The etections and results
presented in this work demonstrate the benefits of using the
difference imaging light curves, especially when going to fainter
magnitudes.