The stellar obliquity, planet mass, and very low albedo of Qatar-2

The stellar obliquity, planet mass, and very low albedo of Qatar-2 from K2 Photometry

Authors:

Dai et al

Abstract:

The Qatar-2 transiting exoplanet system was recently observed by the {\it Kepler} telescope as part of {\it K2} Campaign 6. The photometric time series has one-minute time sampling and a precision of about 690~ppm, after filtering out artifacts and spurious trends. We identify dozens of starspot-crossing events, when the planet eclipsed a relatively dark region of the stellar photosphere. The observed patterns in the sequence of these events demonstrate that the planet always transits over the same range of stellar latitudes, and therefore that the stellar obliquity is less than about 10∘. We support this conclusion with two different modeling approaches: one based on explicit identification and timing of the events, and the other based on fitting the light curves with a spotted-star model. We are also able to refine the usual transit parameters and measure the stellar rotation period (18.5±1.9~days), corresponding to a 'gyrochronological' age of 1.4±0.3 Gyr. Coherent flux variations with the same period as the transits are seen throughout the entire light curve. These variations are well modeled as the combined effects of ellipsoidal light variations (17.4±2.8~ppm) and Doppler boosting (11.9±2.5~ppm). The magnitudes of these effects are both consistent with a planetary mass of 2.6±0.5 MJup, which is in turn consistent with the mass determined by the Doppler technique. No occultations are detected, giving a 2σ upper limit of 0.013 on the planet's visual geometric albedo. The measured transit times are consistent with a constant orbital period. In particular we find no evidence for orbital decay, although we are only able to place a weak lower bound on the relevant tidal quality factor: Q′⋆>1.5×104~(95\% confidence).

K2 Campaigns 5 & 6 Have IDed 152 Exoplanet Candidates

Transiting exoplanet candidates from K2 Campaigns 5 and 6

Authors:

Pope et al

Abstract:

We introduce a new transit search and vetting pipeline for observations from the K2 mission, and present the candidate transiting planets identified by this pipeline out of the targets in Campaigns 5 and 6. Our pipeline uses the Gaussian Process-based K2SC code to correct for the K2 pointing systematics and simultaneously model stellar variability. The systematics-corrected, variability-detrended light curves are searched for transits with the Box Least Squares method, and a period-dependent detection threshold is used to generate a preliminary candidate list. Two or three individuals vet each candidate manually to produce the final candidate list, using a set of automatically-generated transit fits and assorted diagnostic tests to inform the vetting. We detect 147 single-planet system candidates and 5 multi-planet systems, independently recovering the previously-published hot~Jupiters EPIC 212110888b, WASP-55b (EPIC 212300977b) and Qatar-2b (EPIC 212756297b). We also report the outcome of reconnaissance spectroscopy carried out for all candidates with Kepler magnitude Kp less than 13, identifying 12 targets as likely false positives. We compare our results to those of other K2 transit search pipelines, noting that ours performs particularly well for variable and/or active stars, but that the results are very similar overall. All the light curves and code used in the transit search and vetting process are publicly available, as are the follow-up spectra.

Exoplanet Qatar-2b Atmosphere Detected to have Hydrogen & Helium

Physical properties, starspot activity, orbital obliquity, and transmission spectrum of the Qatar-2 planetary system from multi-colour photometr

Authors:

Mancini et al

Abstract:

We present seventeen high-precision light curves of five transits of the planet Qatar-2b, obtained from four defocussed 2m-class telescopes. Three of the transits were observed simultaneously in the SDSS griz passbands using the seven-beam GROND imager on the MPG/ESO 2.2-m telescope. A fourth was observed simultaneously in Gunn grz using the CAHA 2.2-m telescope with BUSCA, and in r using the Cassini 1.52-m telescope. Every light curve shows small anomalies due to the passage of the planetary shadow over a cool spot on the surface of the host star. We fit the light curves with the prism+gemc model to obtain the photometric parameters of the system and the position, size and contrast of each spot. We use these photometric parameters and published spectroscopic measurements to obtain the physical properties of the system to high precision, finding a larger radius and lower density for both star and planet than previously thought. By tracking the change in position of one starspot between two transit observations we measure the orbital obliquity of Qatar-2 b to be 4.3 \pm 4.5 degree, strongly indicating an alignment of the stellar spin with the orbit of the planet. We calculate the rotation period and velocity of the cool host star to be 11.4 \pm 0.5 d and 3.28 \pm 0.13 km/s at a colatitude of 74 degree. We assemble the planet's transmission spectrum over the 386-976 nm wavelength range and search for variations of the measured radius of Qatar-2 b as a function of wavelength. Our analysis highlights a possible H2/He Rayleigh scattering in the blue.