METHANE IN THE ATMOSPHERE OF THE TRANSITING HOT NEPTUNE GJ436B?

Abstract

We present an analysis of seven primary transit observations of the hot\nNeptune GJ436b at 3.6, 4.5 and $8~\\mu$m obtained with the Infrared Array Camera\n(IRAC) on the Spitzer Space Telescope. After correcting for systematic effects,\nwe fitted the light curves using the Markov Chain Monte Carlo technique.\nCombining these new data with the EPOXI, HST and ground-based $V, I, H$ and\n$K_s$ published observations, the range $0.5-10~\\mu$m can be covered. Due to\nthe low level of activity of GJ436, the effect of starspots on the combination\nof transits at different epochs is negligible at the accuracy of the dataset.\nRepresentative climate models were calculated by using a three-dimensional,\npseudo-spectral general circulation model with idealised thermal forcing.\nSimulated transit spectra of GJ436b were generated using line-by-line radiative\ntransfer models including the opacities of the molecular species expected to be\npresent in such a planetary atmosphere. A new, ab-initio calculated, linelist\nfor hot ammonia has been used for the first time. The photometric data observed\nat multiple wavelengths can be interpreted with methane being the dominant\nabsorption after molecular hydrogen, possibly with minor contributions from\nammonia, water and other molecules. No clear evidence of carbon monoxide and\ndioxide is found from transit photometry. We discuss this result in the light\nof a recent paper where photochemical disequilibrium is hypothesised to\ninterpret secondary transit photometric data. We show that the emission\nphotometric data are not incompatible with the presence of abundant methane,\nbut further spectroscopic data are desirable to confirm this scenario.\n