Measuring the orbit shrinkage rate of hot Jupiters due to tides

Abstract

Context. A tidal interaction between a star and a close-in exoplanet leads to shrinkage of the planetary orbit and eventual tidal dis- ruption of the planet. Measuring the shrinkage of the orbits will allow for the tidal quality parameter of the star (Q★′) to be measured, which is an important parameter to obtain information about stellar interiors. Aims. We analyse data from the Transiting Exoplanet Survey Satellite (TESS) for two targets known to host close-in hot Jupiters, which have significant data available and are expected to have a fast decay: WASP-18 and WASP-19. We aim to measure the current limits on orbital period variation and provide new constrains on Q★′ for our targets. Methods. We modelled the transit shape using all the available TESS observations and fitted the individual transit times of each tran- sit. We used previously published transit times together with our results to fit two models, a constant period model, and a quadratic orbital decay model, using Markov chain Monte Carlo (MCMC) algorithms. Results. We obtain new constrains on Q★′ for both targets and improve the precision of the known planet parameters with the newest observations from TESS. We find period change rates of (−0.11 ± 0.21) × 10−10 for WASP-18b and (−0.35 ± 0.22) × 10−10 for WASP-19b and we do not find significant evidence of orbital decay in these targets. We obtain new lower limits for Q★′ of (1.42 ± 0.34) × 107 in WASP-18 and (1.26 ± 0.10) × 106 in WASP-19, corresponding to upper limits of the orbital decay rate of −0.45 × 10−10 and −0.71 × 10−10, respectively, with a 95% confidence level. We compare our results with other relevant targets for tidal decay studies. Conclusions. We find that the orbital decay rate in both WASP-18b and WASP-19b appears to be smaller than the measured orbital decay of WASP-12b. We show that the minimum value of Q★′ in WASP-18 is two orders of magnitude higher than that of WASP-12, while WASP-19 has a minimum value one order of magnitude higher, which is consistent with other similar targets. Further observations are required to constrain the orbital decay of WASP-18 and WASP-19.