Transmission spectroscopy of exoplanet XO-2b observed with HST NICMOS

TL;DR

This study demonstrates the challenges of extracting exoplanet atmospheric spectra with HST NICMOS due to instrument systematics, providing a water vapor absorption upper limit for XO-2b and analyzing XO-1b.

Contribution

The paper introduces a method to extract exoplanet spectra using a companion star as a reference, highlighting NICMOS's limitations and providing atmospheric constraints for XO-2b.

Findings

Spectral noise correlates with instrumental parameters.

Extracted spectra are limited by NICMOS systematics.

Water vapor absorption in XO-2b's atmosphere is below 1570 ppm.

Abstract

Spectroscopy during planetary transits is a powerful tool to probe exoplanet atmospheres. We present the near-infrared transit spectroscopy of XO-2b obtained with HST NICMOS. Uniquely for NICMOS transit spectroscopy, a companion star of similar properties to XO-2 is present in the field of view. We derive improved star and planet parameters through a photometric white-light analysis. We show a clear correlation of the spectrum noise with instrumental parameters, in particular the angle of the spectral trace on the detector. An MCMC method using a decorrelation from instrumental parameters is used to extract the planetary spectrum. Spectra derived independently from each of the 3 visits have a RMS of 430, 510, and 1000 ppm respectively. The same analysis is performed on the companion star after numerical injection of a transit with a depth constant at all wavelengths. The extracted spectra exhibit residuals of similar amplitude as for XO-2, which represent the level of remaining NICMOS systematics. This shows that extracting planetary spectra is at the limit of NICMOS' capability. We derive a spectrum for the planet XO-2b using the companion star as a reference. The derived spectrum can be represented by a theoretical model including atmospheric water vapor or by a flat spectrum model. We derive a 3-sigma upper limit of 1570 ppm on the presence of water vapor absorption in the atmosphere of XO-2b. In an appendix, we perform a similar analysis for the gas giant planet XO-1b.