A ground-based K_S-band detection of the thermal emission from the transiting exoplanet WASP-4b^⋆,⋆⋆

¹ Department of Astronomy and AstrophysicsP. Universidad Católica de Chile. Av. Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile
e-mail: cccacere@astro.puc.cl
² European Southern Observatory, Av. Alonso de Córdova 3107, Casilla 19, Santiago 19001, Chile
³ Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA
⁴ Observatoire de Genève, Université de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland
⁵ ESA-STScI, 3700 San Martin Drive, Batlimore, MD 21218, USA
⁶ Department of Astronomy, Universidad de Concepción, Casilla 160-C, Concepción, Chile
⁷ Obserwatorium Astronomiczne Uniwersytetu Warszawskiego, Aleje Ujazdowskie 4, 00-478 Warszawa, Poland

Received: 29 November 2010
Accepted: 30 March 2011

Abstract

Context. Secondary eclipses are a powerful tool to measure directly the thermal emission from extrasolar planets, and to constrain their type and physical parameters.

Aims. We started a project to obtain reliable broad-band measurements of the thermal emission of transiting exoplanets.

Methods. Ground-based high-cadence near-infrared relative photometry was used to obtain a sub-millimagnitude precision light curve of a secondary eclipse of WASP-4b – a 1.12 M_J hot Jupiter on a 1.34 day orbit around G7V star.

Results. The data show a clear ≥10σ detection of the planet’s thermal emission at 2.2 μm. The calculated thermal emission corresponds to a fractional eclipse depth of $\mathrm{0.18}{\mathrm{5}}_{-0.013}^{\mathrm{+}\mathrm{0.014}}$%, with a related brightness temperature in K_S of T_B = 1995 ± 40 K, centered at ${\mathit{T}}_{\mathrm{C}}\mathrm{=}\mathrm{2455102.6116}{\mathrm{2}}_{-0.00077}^{\mathrm{+}\mathrm{0.00071}}$ HJD. We could set a limit on the eccentricity of ecosω = 0.0027 ± 0.0018, compatible with a near-circular orbit.

Conclusions. The calculated brightness temperature and the specific models suggest a highly inefficient redistribution of heat from the day-side to the night-side of the planet, and a consequent emission mainly from the day-side. The high-cadence ground-based technique is capable of detecting the faint signal of the secondary eclipse of extrasolar planets, which makes it a valuable complement to space-based mid-IR observations.