Orbital alignment and star-spot properties in the WASP-52 planetary system

Abstract

We report 13 high-precision light curves of eight transits of the exoplanet WASP-52 b, obtained by using four medium-class telescopes, through different filters, and adopting the defocussing technique. One transit was recorded simultaneously from two different observatories and another one from the same site but with two different instruments, including a multi-band camera. Anomalies were clearly detected in five light curves and modelled as star-spots occulted by the planet during the transit events. We fitted the clean light curves with the jktebop code, and those with the anomalies with the prism+gemc codes in order to simultaneously model the photometric parameters of the transits and the position, size and contrast of each star-spot. We used these new light curves and some from the literature to revise the physical properties of the WASP-52 system. Star-spots with similar characteristics were detected in four transits over a period of 43 d. In the hypothesis that we are dealing with the same starspot, periodically occulted by the transiting planet, we estimated the projected orbital obliquity of WASP-52 b to be λ = 3 • .8 ± 8 • .4. We also determined the true orbital obliquity, ψ = 20 • ± 50 • , which is, although very uncertain, the first measurement of ψ purely from star-spot crossings. We finally assembled an optical transmission spectrum of the planet and searched for variations of its radius as a function of wavelength. Our analysis suggests a flat transmission spectrum within the experimental uncertainties.

The GTC exoplanet transit spectroscopy survey

“…The resulting transit parameters agree well with previous studies (Hébrard et al 2013;Kirk et al 2016;Mancini et al 2017), except for R p /R , which appears shallower than previously reported. This deviation is probably caused by stellar activity, given that occulted star spots (Mancini et al 2017) and faculae (Kirk et al 2016) have been identified several times.…”

“…2, the derived GTC/OSIRIS transmission spectrum for WASP-52b does not exhibit any broad spectral signatures, except for the narrow excess absorption at the Na line. The standard deviation of the 16.5 nm transit depths σ(R p /R ) = 0.00110 is smaller than one atmospheric scale height H/R = kT/(µgR ) = 0.00126, where the stellar radius R = 0.786 R , the planetary surface gravity g = 6.85 m s −2 and the planetary equilibrium temperature T eq = 1315 K were taken from Mancini et al (2017); k is the Boltzmann constant; µ is the mean molecular weight (MMW) and assumed as 2.3× proton mass.…”

“…The host star is active, and exhibits a rotational modulation of 16.4 ± 0.04 days with an amplitude of 9.6 mmag. Activity signatures such as chromospheric emission in the Ca ii H+K lines (Hébrard et al 2013), occulted star spots (Mancini et al 2017) and faculae (Kirk et al 2016) have been identified. Kirk et al (2016) obtained a broadband transmission spectrum in the u , g and a 31.2 nm filter centered on Na using the 4.2 m William Herschel Telescope, and found an increasing planetary radius towards the red optical.…”

“…WASP-52b was discovered to transit a K2 dwarf every 1.75 days (Hébrard et al 2013). This low-density planet (0.43 M Jup , 1.25 R Jup ; Mancini et al 2017) is highly inflated, and thus well-suited for transmission spectroscopy. The host star is active, and exhibits a rotational modulation of 16.4 ± 0.04 days with an amplitude of 9.6 mmag.…”

Physical properties and transmission spectrum of the WASP-74 planetary system from multiband photometry

“…More information about these telescopes and instruments is available in previous papers (e.g. Mancini et al 2018). The same transit was also observed with the Danish telescope using a Bessell-U filter, in order to have a wavelength coverage in the near-ultraviolet as well.…”