Improved parameters for the transiting hot Jupiters WASP-4b and WASP-5b

The gaseous giant planets WASP-4b and WASP-5b are transiting 12-magnitude solar-type stars in the Southern hemisphere. The aim of the present work is to refine the parameters of these systems using high cadence VLT/FORS2 z-band transit photometry and high-resolution VLT/UVES spectroscopy. For WASP-4, the new estimates for the planet radius and mass from a combined analysis of our VLT data with previously published transit photometry and radial velocities are R_p = 1.30^+0.05_-0.04 R_J and M_p = 1.21^+0.13_-0.08 M_J, resulting in a density ρ_p = 0.55^+0.04_-0.02 ρ_J. The radius and mass for the host star are R_* = 0.87^+0.04_-0.03 R_⊙ and M_* = 0.85^+0.11_-0.07 M_⊙. Our ground-based photometry reaches 550 ppm at time sampling of ~50 s. Nevertheless, we also report the presence of an instrumental effect on the VLT that degraded our photometry for the WASP-5 observations. This effect could be a major problem for similar programs. Our new estimates for the parameters of the WASP-5 system are R_p = 1.09 ± 0.07 R_J, M_p = 1.58^+0.13_-0.10 M_J, ρ_p = 1.23 ^+0.26_-0.16 ρ_J, R_* = 1.03^+0.06_-0.07 R_⊙, and M_* = 0.96^+0.13_-0.09 M_⊙. The measured size of WASP-5b agrees well with the basic models of irradiated planets, while WASP-4b is clearly an “anomalously” large planet.

Based on data collected with the FORS2 imager at the VLT-UT4 telescope and with the UVES spectrograph at the VLT-UT2 telescope (Paranal Observatory, ESO, Chile) in the programme 280.C-5003.