GTC OSIRIS transiting exoplanet atmospheric survey: detection of sodium in XO-2b from differential long-slit spectroscopy

TL;DR

This study reports the detection of sodium in the atmosphere of exoplanet XO-2b using differential long-slit spectroscopy, demonstrating the potential and limitations of current observational techniques for exoplanet atmospheric characterization.

Contribution

First detection of sodium in XO-2b's atmosphere using GTC long-slit spectroscopy, highlighting challenges with slit losses and suggesting improvements for future observations.

Findings

Detected sodium absorption at 50{ A} bandpass with 5.2-sigma significance

Limited by slit losses, full optical transmission spectra were not obtained

XO-2b is the first hot Jupiter with evidence for both sodium and potassium in its atmosphere.

Abstract

We present two transits of the hot-Jupiter exoplanet XO-2b using the Gran Telescopio Canarias (GTC). The time series observations were performed using long-slit spectroscopy of XO-2 and a nearby reference star with the OSIRIS instrument, enabling differential specrophotometric transit lightcurves capable of measuring the exoplanet's transmission spectrum. Two optical low-resolution grisms were used to cover the optical wavelength range from 3800 to 9300{\AA}. We find that sub-mmag level slit losses between the target and reference star prevent full optical transmission spectra from being constructed, limiting our analysis to differential absorption depths over ~1000{\AA} regions. Wider long slits or multi-object grism spectroscopy with wide masks will likely prove effective in minimising the observed slit-loss trends. During both transits, we detect significant absorption in the planetary atmosphere of XO-2b using a 50{\AA} bandpass centred on the Na I doublet, with absorption depths of Delta(R_pl/R_star)^2=0.049+/-0.017 % using the R500R grism and 0.047+/-0.011 % using the R500B grism (combined 5.2-sigma significance from both transits). The sodium feature is unresolved in our low-resolution spectra, with detailed modelling also likely ruling out significant line-wing absorption over an ~800{\AA} region surrounding the doublet. Combined with narrowband photometric measurements, XO-2b is the first hot Jupiter with evidence for both sodium and potassium present in the planet's atmosphere.