Erratum: Physical properties, star-spot activity, orbital obliquity and transmission spectrum of the Qatar-2 planetary system from multicolour photometry

Abstract

We present 17 high-precision light curves of five transits of the planet Qatar-2 b, obtained from four defocused 2 m-class telescopes. Three of the transits were observed simultaneously in the Sloan g r i z passbands using the seven-beam Gamma Ray Burst Optical and Near-Infrared Detector imager on the MPG/ESO 2.2-m telescope. A fourth was observed simultaneously in Gunn grz using the Centro Astronómico Hispano Alemán 2.2-m telescope with Bonn University Simultaneous Camera, 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 star-spot between two transit observations, we measure the orbital obliquity of Qatar-2 b to be λ = 4.• 3 ± 4.• 5, 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 11.5 ± 0.2 d and 3.28 ± 0.04 km s −1 at a colatitude of 74 • . 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 H 2 /He Rayleigh scattering in the blue.

Recurring sets of recurring starspot occultations on exoplanet host Qatar-2

“…The recurring occultation pair indicated a sky-projected obliquity of 4.3 ± 4.5 • . The rotational period was reported as 11.4 ± 0.5 d, which has since been corrected to 14.8 ± 0.3 d due to a calculation error (Mancini et al 2016). However, the analysis presented in this paper proves that the two starspot occultations analysed by Mancini et al (2014Mancini et al ( , 2016 were in fact caused by two different starspots and that even their corrected rotational period is incorrect.…”

The Stellar Obliquity, Planet Mass, and Very Low Albedo of Qatar-2 From K2 Photometry

“…For example, Mancini et al (2014) did not have the stellar rotation period as an independent check for their model of Qatar-2b. By assuming that two particular spot-crossing anomalies they observed were associated with a single spot, they derived a stellar rotation period of 14.8±0.3 days (as later revised by Mancini et al 2016). This is now known to be incorrect; most likely, the two observed anomalies were produced by crossings over two different spots.…”

“…Without the ability to calculate the rotational phase of each transit, Mancini et al (2014) had to make the assumption that two particular spot-crossing anomalies they observed were caused by eclipses of the same spot. With this assumption, they found the stellar rotation period to be 14.8±0.3 days (after the correction described by Mancini et al 2016) and the skyprojected obliquity (the angle between the sky projections of the stellar rotation axis and the orbit normal) to be l =    4 . 3 4 .…”

MuSCAT: a multicolor simultaneous camera for studying atmospheres of transiting exoplanets

“…This kind of study is known as transmission spectroscopy, and numbers of multi-color transit observations for this purpose have been reported so far. [10][11][12][13][14][15][16][17] Multi-color simultaneous cameras are very fruitful for the studies described above for two reasons. First, multi-color simultaneous cameras provide not only higher efficiency but also more feasibility to accomplish aimed studies than single-color cameras, since observable transits from a specific ground site are very limited.…”

“…Second, simultaneity of multi-color transit photometry is important to avoid systematic differences of transit depths due to luminosity change in host stars possibly caused by existence of starspots, plages, stellar activity, and so on. For the reasons, multicolor simultaneous cameras such as GROND, 10,11 BUSCA, 12 ULTRACAM, 13 SIRIUS, 14,15 and MITSuME 16,17 have been actively used for transit observations. Considering the fact that more interesting transiting planets will be discovered in the near future by advanced ground-based surveys, and also space-based surveys, like K2, TESS, and PLATO, developments of new multi-color simultaneous cameras are highly desired.…”