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Issue A&A
Volume 493, Number 2, January II 2009
Page(s) L35 - L38
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:200811239
Published online 04 December 2008



A&A 493, L35-L38 (2009)
DOI: 10.1051/0004-6361:200811239

Letter

Ground-based $\sf { \textit K}$-band detection of thermal emission from the exoplanet TrES-3b

E. J. W. de Mooij and I. A. G. Snellen

Leiden University, Postbus 9513, 2300 RA, Leiden, The Netherlands
    e-mail: demooij@strw.leidenuniv.nl

Received 28 October 2008 / Accepted 28 November 2008

Abstract
Context. Secondary eclipse measurements of transiting extrasolar planets with the Spitzer Space Telescope have yielded several direct detections of thermal exoplanet light. Since Spitzer operates at wavelengths longward of 3.6 $\mu$m, arguably one of the most interesting parts of the planet spectrum (from 1 to 3 $\mu$m) is inaccessible with this satellite. This region is at the peak of the planet's spectral energy distribution and is also the regime where molecular absorption bands can significantly influence the measured emission.
Aims. So far, 2.2 $\mu$m K-band secondary eclipse measurements, which are possible from the ground, have not yet lead to secure detections. The aim of this paper is to measure the secondary eclipse of the very hot Jupiter TrES-3b in K-band, and in addition to observe its transit, to obtain an accurate planet radius in the near infrared.
Methods. We have used the william herschell telescope (WHT) to observe the secondary eclipse, and the united kingdom infrared telescope (UKIRT) to observe the transit of TrES-3b. Both observations involved significant defocusing of the telescope, aimed to produce high-cadence time series of several thousand frames at high efficiency, with the starlight spread out over many pixels.
Results. We detect the secondary eclipse of TrES-3b with a depth of $-0.241 \pm 0.043$% (~6$\sigma$). This corresponds to a day-side brightness temperature of T$_{\rm B}$(2.2 $\mu$m) =  $2040 \pm 185$ K, which is consistent with current models of the physical properties of this planet's upper atmosphere. The centre of the eclipse seems slightly offset from phase $\phi=0.5$ by $\Delta\phi = -0.0042 \pm 0.0027$, which could indicate that the orbit of TrES-3b is non-circular. Analysis of the transit data shows that TrES-3b has a near-infrared radius of $1.338 \pm 0.016$  $R_{\rm Jup}$, showing no significant deviation from optical measurements.


Key words: techniques: photometric -- stars: individual: TrES-3 -- planetary systems



© ESO 2009

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