High-precision K-band photometry of the secondary eclipse of HD 209458

Abstract

Recently, mid-infrared Spitzer observations have been presented that show the light decrement due to the passage of a planet behind its host star. These measurements of HD209458b and TrES-1 are the first detections of direct light from an extra-solar planet. Interpretation of these results in terms of planet equipartition temperature and bond albedo is however strongly model dependent and require additional observations at shorter wavelengths. Here we report on two attempts to detect the secondary eclipse of HD209458b from the ground in K-band, using the UK InfraRed Telescope (UKIRT). A photometry precision of 0.12% relative to two nearby reference stars was reached during both occasions, but no firm detection of the eclipses were obtained. The first observation shows a flux decrement of -0.13+-0.18%, and the second of -0.10+-0.10%. A detailed description of the observing strategy, data reduction and analysis is given, and a discussion on how the precision in ground-based K-band photometry could be further improved. In addition we show that the relative photometry between the target and the reference stars between the two epochs is consistent down to the <0.1% level, which is interesting in the light of possible near-infrared surveys to search for transiting planets around M and L dwarfs.