24 research outputs found
Minimizing follow-up for space-based transit surveys using full lightcurve analysis
One of the biggest challenges facing large transit surveys is the elimination
of false-positives from the vast number of transit candidates. We investigate
to what extent information from the lightcurves can identify blend scenarios
and eliminate them as planet candidates, to significantly decrease the amount
of follow-up observing time required to identify the true exoplanet systems. If
a lightcurve has a sufficiently high signal-to-noise ratio, a distinction can
be made between the lightcurve of a stellar binary blended with a third star
and the lightcurve of a transiting exoplanet system. We perform simulations to
determine what signal-to-noise level is required to make the distinction
between blended and non-blended systems as function of transit depth and impact
parameter. Subsequently we test our method on real data from the first IRa01
field observed by the CoRoT satellite, concentrating on the 51 candidates
already identified by the CoRoT team. About 70% of the planet candidates in the
CoRoT IRa01 field are best fit with an impact parameter of b>0.85, while less
than 15% are expected in this range considering random orbital inclinations. By
applying a cut at b<0.85, meaning that ~15% of the potential planet population
would be missed, the candidate sample decreases from 41 to 11. The lightcurves
of 6 of those are best fit with such low host star densities that the
planet-to-star size ratii imply unrealistic planet radii of R>2RJup. Two of the
five remaining systems, CoRoT1b and CoRoT4b, have been identified as planets by
the CoRoT team, for which the lightcurves alone rule out blended light at 14%
(2sigma) and 31% (2sigma). We propose to use this method on the Kepler database
to study the fraction of real planets and to potentially increase the
efficiency of follow-up.Comment: 13 pages, 11 figures, 2 tables. Accepted for publication in A&
Searching for transits in the Wide Field Camera Transit Survey with difference-imaging light curves
The Wide Field Camera Transit Survey is a pioneer program aiming at for searching extra-solar planets in the near-infrared. The images from the survey are processed by a data reduction pipeline, which uses aperture photometry to construct the light curves. We produce an alternative set of light curves using the difference-imaging method for the most complete field in the survey and carry out a quantitative comparison between the photometric precision achieved with both methods. The results show that differencephotometry light curves present an important improvement for stars with J > 16. We report an implementation on the box-fitting transit detection algorithm, which performs a trapezoid-fit to the folded light curve, providing more accurate results than the boxfitting model. We describe and optimize a set of selection criteria to search for transit candidates, including the V-shape parameter calculated by our detection algorithm. The optimized selection criteria are applied to the aperture photometry and difference-imaging light curves, resulting in the automatic detection of the best 200 transit candidates from a sample of ~475 000 sources. We carry out a detailed analysis in the 18 best detections and classify them as transiting planet and eclipsing binary candidates. We present one planet candidate orbiting a late G-type star. No planet candidate around M-stars has been found, confirming the null detection hypothesis and upper limits on the occurrence rate of short-period giant planets around M-dwarfs presented in a prior study. We extend the search for transiting planets to stars with J ≤ 18, which enables us to set a stricter upper limit of 1.1%. Furthermore, we present the detection of five faint extremely-short period eclipsing binaries and three M-dwarf/M-dwarf binary candidates. The detections demonstrate the benefits of using the difference-imaging light curves, especially when going to fainter magnitudes.Peer reviewe
The GROUSE project II: Detection of the Ks-band secondary eclipse of exoplanet HAT-P-1b
Context: Only recently it has become possible to measure the thermal emission
from hot-Jupiters at near-Infrared wavelengths using ground-based telescopes,
by secondary eclipse observations. This allows the planet flux to be probed
around the peak of its spectral energy distribution, which is vital for the
understanding of its energy budget. Aims: The aim of the reported work is to
measure the eclipse depth of the planet HAT-P-1b at 2.2micron. This planet is
an interesting case, since the amount of stellar irradiation it receives falls
in between that of the two best studied systems (HD209458 and HD189733), and it
has been suggested to have a weak thermal inversion layer. Methods: We have
used the LIRIS instrument on the William Herschel Telescope (WHT) to observe
the secondary eclipse of HATP-1b in the Ks-band, as part of our Ground-based
secondary eclipse (GROUSE) project. The observations were done in staring mode,
while significantly defocusing the telescope to avoid saturation on the K=8.4
star. With an average cadence of 2.5 seconds, we collected 6520 frames during
one night. Results: The eclipse is detected at the 4sigma level, the measured
depth being 0.109+/-0.025%. The uncertainties are dominated by residual
systematic effects, as estimated from different reduction/analysis procedures.
The measured depth corresponds to a brightness temperature of 2136+150-170K.
This brightness temperature is significantly higher than those derived from
longer wavelengths, making it difficult to fit all available data points with a
plausible atmospheric model. However, it may be that we underestimate the true
uncertainties of our measurements, since it is notoriously difficult to assign
precise statistical significance to a result when systematic effects are
important.Comment: 7 pages, 10 figures, Accepted for publication in A&
Optical to near-infrared transit observations of super-Earth GJ1214b: water-world or mini-Neptune?
GJ1214b is thought to be either a mini-Neptune with a thick, hydrogen-rich
atmosphere, or a planet with a composition dominated by water. In the case of a
hydrogen-rich atmosphere, molecular absorption and scattering processes may
result in detectable radius variations as a function of wavelength. The aim of
this paper is to measure these variations. We have obtained observations of the
transit of GJ1214b in the r- and I-band with the INT, in the g, r, i and z
bands with the 2.2 meter MPI/ESO telescope, in the Ks-band with the NOT, and in
the Kc-band with the WHT. By comparing the transit depth between the the
different bands, which is a measure for the planet-to-star size ratio, the
atmosphere is investigated. We do not detect clearly significant variations in
the planet-to-star size ratio as function of wavelength. Although the ratio at
the shortest measured wavelength, in g-band, is 2sigma larger than in the other
bands. The uncertainties in the Ks and Kc bands are large, due to systematic
features in the light curves. The tentative increase in the planet-to-star size
ratio at the shortest wavelength could be a sign of an increase in the
effective planet-size due to Rayleigh scattering, which would require GJ1214b
to have a hydrogen-rich atmosphere. If true, then the atmosphere has to have
both clouds, to suppress planet-size variations at red optical wavelengths, as
well as a sub-solar metallicity, to suppress strong molecular features in the
near- and mid-infrared. However, star spots, which are known to be present on
the hoststar's surface, can (partly) cancel out the expected variations in
planet-to-star size ratio, due to the lower surface temperature of the spots .
A hypothetical spot-fraction of 10% would be able to raise the infrared points
sufficiently with respect to the optical measurements to be inconsistent with a
water-dominated atmosphere. [abridged]Comment: 13 pages, 8 figures. Accepted for publication in A&
Four ultra-short period eclipsing M-dwarf binaries in the WFCAM Transit Survey
We report on the discovery of four ultra-short period (P<0.18 days) eclipsing
M-dwarf binaries in the WFCAM Transit Survey. Their orbital periods are
significantly shorter than of any other known main-sequence binary system, and
are all significantly below the sharp period cut-off at P~0.22 days as seen in
binaries of earlier type stars. The shortest-period binary consists of two M4
type stars in a P=0.112 day orbit. The binaries are discovered as part of an
extensive search for short-period eclipsing systems in over 260,000 stellar
lightcurves, including over 10,000 M-dwarfs down to J=18 mag, yielding 25
binaries with P<0.23 days. In a popular paradigm, the evolution of short period
binaries of cool main-sequence stars is driven by loss of angular momentum
through magnetised winds. In this scheme, the observed P~0.22 day period
cut-off is explained as being due to timescales that are too long for
lower-mass binaries to decay into tighter orbits. Our discovery of low-mass
binaries with significantly shorter orbits implies that either these timescales
have been overestimated for M-dwarfs, e.g. due to a higher effective magnetic
activity, or that the mechanism for forming these tight M-dwarf binaries is
different from that of earlier type main-sequence stars.Comment: 22 pages, 17 figures, 3 tables Accepted for publication in MNRA
The first planet detected in the WTS: an inflated hot-Jupiter in a 3.35 d orbit around a late F star [Erratum]
We report the discovery of WTS-1b, the first extrasolar planet found by the
WFCAM Transit Survey, which began observations at the 3.8-m United Kingdom
Infrared Telescope (UKIRT) in August 2007. Light curves comprising almost 1200
epochs with a photometric precision of better than 1 per cent to J ~ 16 were
constructed for ~60000 stars and searched for periodic transit signals. For one
of the most promising transiting candidates, high-resolution spectra taken at
the Hobby-Eberly Telescope (HET) allowed us to estimate the spectroscopic
parameters of the host star, a late-F main sequence dwarf (V=16.13) with
possibly slightly subsolar metallicity, and to measure its radial velocity
variations. The combined analysis of the light curves and spectroscopic data
resulted in an orbital period of the substellar companion of 3.35 days, a
planetary mass of 4.01 +- 0.35 Mj and a planetary radius of 1.49+0.16-0.18 Rj.
WTS-1b has one of the largest radius anomalies among the known hot Jupiters in
the mass range 3-5 Mj. The high irradiation from the host star ranks the planet
in the pM class.Comment: 16 pages, 10 figure
The first planet detected in the WTS: an inflated hot Jupiter in a 3.35 d orbit around a late F star
We report the discovery of WTS-1b, the first extrasolar planet found by the WFCAM Transit Survey, which began observations at the 3.8-m United Kingdom Infrared Telescope (UKIRT) in 2007 August. Light curves comprising almost 1200 epochs with a photometric precision of better than 1 per cent to J ˜ 16 were constructed for ˜60 000 stars and searched for periodic transit signals. For one of the most promising transiting candidates, high-resolution spectra taken at the Hobby-Eberly Telescope (HET) allowed us to estimate the spectroscopic parameters of the host star, a late-F main-sequence dwarf (V = 16.13) with possibly slightly subsolar metallicity, and to measure its radial velocity variations. The combined analysis of the light curves and spectroscopic data resulted in an orbital period of the substellar companion of 3.35 d, a planetary mass of 4.01 ± 0.35 MJ and a planetary radius of 1.49-0.18+0.16 RJ. WTS-1b has one of the largest radius anomalies among the known hot Jupiters in the mass range 3-5 MJ. The high irradiation from the host star ranks the planet in the pM class