Transits and starspots in the WASP-19 planetary system

Abstract

We have developed a new model for analysing light curves of planetary transits when there are starspots on the stellar disc. Because the parameter space contains a profusion of local minima we developed a new optimisation algorithm which combines the global minimisation power of a genetic algorithm and the Bayesian statistical analysis of the Markov chain. With these tools we modelled three transit light curves of WASP-19. Two light curves were obtained on consecutive nights and contain anomalies which we confirm as being due to the same spot. Using these data we measure the star's rotation period and velocity to be 11.76 ± 0.09 d and 3.88 ± 0.15 km s −1 , respectively, at a latitude of 65 • . We find that the sky-projected angle between the stellar spin axis and the planetary orbital axis is λ = 1.0 • ± 1.2 • , indicating axial alignment. Our results are consistent with and more precise than published spectroscopic measurements of the Rossiter-McLaughlin effect.

Refined System Parameters and TTV Study of Transiting Exoplanetary System Hat-P-20

“…This effects can: (1) modify the total flux of the visible stellar surface and hence change the entire shape of the transit light curve when normalized to the out-of-transit flux (Oshagh et al 2013b;Daassou et al 2014;Herrero et al 2015); in the case of short transit duration (namely, transit duration T 14 = stellar rotational period P rot ) and no dramatic transient events, like the eruption of an optical flare, the shape of a transit light curve which is not distorted by other systematic noises is still symmetric like the transit light curve of a quiet star; and (2) modify the local shape of the transit light curve while the planetary disc overlays the spot and/or faculae on the stellar disc during the planet transit, namely spot crossing events. The distribution and size of spots occulted by the planet can be obtained precisely through fitting the anomalies of high-precision transit light curves induced by the spot crossing events (Silva 2003;Pont et al 2007;SilvaValio 2008;Désert et al 2011;Nutzman et al 2011;SanchisOjeda et al 2011SanchisOjeda et al , 2013Tregloan-Reed et al 2013, 2015. Hence, even when a symmetric light curve modified by several un-occulted spots on the stellar surface is fitted with the theoretical model of the transit event, most transiting parameters directly characterizing the properties of the system cannot be obtained accurately and then the physical parameters of the system deduced from these transit parameters are biased, such as the orbital inclination of the planet, the orbital semimajor axis of the planet, and even the radii and mass of both the host star and planet.…”

“…Examples include the model by Silva (2003), the model by Wolter et al (2009), SOAP-T by Oshagh et al (2013a), PRISM by Tregloan-Reed et al (2013), SPOTROD by Béky et al (2014), KSint by Montalto et al (2014), and ELLC by Maxted (2016). These models, except those of Montalto et al (2014) and Maxted (2016), numerically integrate over the stellar surface or projected disc to calculate the transit light curve.…”

“…Nonetheless, the entire modification of the transit light curve is only partially attributed to the non-occulted spots in the photosphere because the occulted spots also play the same role in the modification, although the properties of occulted spots by the planet can be diagnosed by modeling transiting data with sufficiently high precision and cadence (Tregloan-Reed et al 2013). Meanwhile, the transit light curve imposes no more constraints on the properties of non-occulted spots than total fractional deficit of stellar flux when only a single, shortobserving-window transit light curve is available for each transit event, which is ubiquitous in ground-based observation of transit events of exoplanetary systems.…”

“…Meanwhile, Mandel & Agol (2002)'s analytic Occultquad model is modified by adding the parameter x non to model highly symmetric transit light curves which are influenced by only non-occulted spots. In addition, we define the location of spots on the stellar surface in the reference system in which the spin of the host star is to both perpendicular to the line of sight and the transiting trajectory on the stellar disc, as in Tregloan-Reed et al (2013), in order to elucidate the degeneracies between the parameters characterizing the spot properties and the orientation of stellar spin axis. We implement both adapted models in STMT to simulate the transit light curves of the spotted star.…”

“…Therefore, accounting for the effects of stellar activity on transit light curves becomes essential for acquiring accurate physical parameters and appropriately characterizing transiting exoplanetary systems. Based on previous work of Tregloan-Reed et al (2013, 2015 and Montalto et al (2014), we develop a highly efficient tool by employing the DREAM algorithm  (Vrugt et al 2009) to simultaneously model multiple transiting light curves deformed by spots in the stellar photosphere. Utilizing our new tool, we comprehensively analyze the published and new transit light curves of HAT-P-20.…”