Simultaneous follow-up of planetary transits: revised physical properties for the planetary systems HAT-P-16 and WASP-21^⋆

¹ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
e-mail: ciceri@mpia.de
² Astrophysics Group, Keele University, Staffordshire, ST5 5BG, UK
³ Astrophysics Group, University of Exeter, Stocker Road, EX4 4QL, Exeter, UK
⁴ Department of Physics, University of Salerno, via Ponte Don Melillo, 84084 Fisciano (SA), Italy
⁵ Istituto Nazionale di Fisica Nucleare, Sezione di Napoli, Napoli, Italy
⁶ INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁷ Istituto Internazionale per gli Alti Studi Scientifici (IIASS), Vietri Sul Mare (SA), Italy

Received: 8 April 2013
Accepted: 19 July 2013

Abstract

Context. By now, more than 300 planets transiting their host star have been found, and much effort is being put into measuring the properties of each system. Light curves of planetary transits often contain deviations from a simple transit shape, and it is generally difficult to differentiate between anomalies of astrophysical nature (e.g. starspots) and correlated noise due to instrumental or atmospheric effects. Our solution is to observe transit events simultaneously with two telescopes located at different observatories.

Aims. Using this observational strategy, we look for anomalies in the light curves of two transiting planetary systems and accurately estimate their physical parameters.

Methods. We present the first photometric follow-up of the transiting planet HAT-P-16 b, and new photometric observations of WASP-21 b, obtained simultaneously with two medium-class telescopes located in different countries, using the telescope defocusing technique. We modeled these and other published data in order to estimate the physical parameters of the two planetary systems.

Results. The simultaneous observations did not highlight particular features in the light curves, which is consistent with the low activity levels of the two stars. For HAT-P-16, we calculated a new ephemeris and found that the planet is 1.3σ colder and smaller (R_b = 1.190 ± 0.037 R_Jup) than the initial estimates, suggesting the presence of a massive core. Our physical parameters for this system point toward a younger age than previously thought. The results obtained for WASP-21 reveal lower values for the mass and the density of the planet (by 1.0 and 1.4σ respectively) with respect to those found in the discovery paper, in agreement with a subsequent study. We found no evidence of any transit timing variations in either system.