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 the
m 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.
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 detectabl
e 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 hoststars 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]
[Context] The first detection of an atmosphere around an extrasolar planet was presented by Charbonneau and collaborators in 2002. In the optical transmission spectrum of the transiting exoplanet HD209458b, an absorption signal from sodium was measur
ed at a level of 0.023+-0.006%, using the STIS spectrograph on the Hubble Space Telescope. Despite several attempts, so far only upper limits to the Na D absorption have been obtained using telescopes from the ground, and the HST result has yet to be confirmed. [Aims] The aims of this paper are to re-analyse data taken with the High Dispersion Spectrograph on the Subaru telescope, to correct for systematic effects dominating the data quality, and to improve on previous results presented in the literature. [Methods] The data reduction process was altered in several places, most importantly allowing for small shifts in the wavelength solution. The relative depth of all lines in the spectra, including the two sodium D lines, are found to correlate strongly with the continuum count level in the spectra. These variations are attributed to non-linearity effects in the CCDs. After removal of this empirical relation the uncertainties in the line depths are only a fraction above that expected from photon statistics. [Results] The sodium absorption due to the planets atmosphere is detected at >5 sigma, at a level of 0.056+-0.007% (2x3.0 Ang band), 0.070+-0.011% (2x1.5 Ang band), and 0.135+-0.017% (2x0.75 Ang band). There is no evidence that the planetary absorption signal is shifted with respect to the stellar absorption, as recently claimed for HD189733b. The measurements in the two most narrow bands indicate that some signal is being resolved.[abridged]
