EDP Sciences
Free access
Volume 496, Number 1, March II 2009
Page(s) 259 - 267
Section Planets and planetary systems
DOI http://dx.doi.org/10.1051/0004-6361:200810929
Published online 14 January 2009
A&A 496, 259-267 (2009)
DOI: 10.1051/0004-6361:200810929

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

M. Gillon1, B. Smalley2, L. Hebb3, D. R. Anderson2, A. H. M. J. Triaud1, C. Hellier2, P. F. L. Maxted2, D. Queloz1, and D. M. Wilson2

1  Observatoire de Genève, Université de Genève, 51 Chemin des Maillettes, 1290 Sauverny, Switzerland
    e-mail: michael.gillon@obs.unige.ch
2  Astrophysics Group, Keele University, Staffordshire, ST5 5BG, UK
3  School of Physics and Astronomy, University of St. Andrews, North Haugh, Fife, KY16 9SS, UK

Received 7 September 2008 / Accepted 9 December 2008

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 $R_{\rm p} = 1.30^{+0.05}_{-0.04}$ RJ and $M_{\rm p} = 1.21^{+0.13}_{-0.08}$ MJ, resulting in a density $\rho_{\rm p} = 0.55^{+0.04}_{-0.02}$ $\rho_J$. The radius and mass for the host star are $R_\ast = 0.87^{+0.04}_{-0.03}$ $R_\odot$ and $M_\ast = 0.85^{+0.11}_{-0.07}$ $M_\odot$. 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 $R_{\rm p} = 1.09$ $\pm$ 0.07 RJ, $M_{\rm p} = 1.58^{+0.13}_{-0.10}$ MJ, $\rho_{\rm p} = 1.23 ^{+0.26}_{-0.16}$ $\rho_J$, $R_\ast = 1.03^{+0.06}_{-0.07}$ $R_\odot$, and $M_\ast = 0.96^{+0.13}_{-0.09}$ $M_\odot$. The measured size of WASP-5b agrees well with the basic models of irradiated planets, while WASP-4b is clearly an “anomalously” large planet.

Key words: binaries: eclipsing -- planetary systems -- stars: individual: WASP-4 -- stars: individual: WASP-5 -- techniques: photometric -- techniques: spectroscopic

© ESO 2009