THE EMERGENT 1.1-1.7 μm SPECTRUM OF THE EXOPLANET COROT-2B AS MEASURED USING THE<i>HUBBLE SPACE TELESCOPE</i>

Abstract

We have used Hubble/WFC3 and the G141 grism to measure the secondary eclipse\nof the transiting very hot Jupiter CoRoT-2b in the 1.1-1.7$\\mu$m spectral\nregion. We find an eclipse depth averaged over this band equal to\n$395^{+69}_{-45}$ parts per million, equivalent to a blackbody temperature of\n$1788\\pm18$K. We study and characterize several WFC3 instrumental effects,\nespecially the "hook" phenomenon described by Deming et al. (2013). We use data\nfrom several transiting exoplanet systems to find a quantitative relation\nbetween the amplitude of the hook and the exposure level of a given pixel.\nAlthough the uncertainties in this relation are too large to allow us to\ndevelop an empirical correction for our data, our study provides a useful guide\nfor optimizing exposure levels in future WFC3 observations. We derive the\nplanet's spectrum using a differential method. The planet-to-star contrast\nincreases to longer wavelength within the WFC3 bandpass, but without water\nabsorption or emission to a $3\\sigma$ limit of 85 ppm. The slope of the WFC3\nspectrum is significantly less than the slope of the best-fit blackbody. We\ncompare all existing eclipse data for this planet to a blackbody spectrum, and\nto spectra from both solar abundance and carbon-rich (C/O=1) models. A\nblackbody spectrum is an acceptable fit to the full dataset. Extra continuous\nopacity due to clouds or haze, and flattened temperature profiles, are strong\ncandidates to produce quasi-blackbody spectra, and to account for the amplitude\nof the optical eclipses. Our results show ambiguous evidence for a temperature\ninversion in this planet.\n