Star-Planet Interactions: Emission Spectroscopy of H$_{3}^{+}$ in Extrasolar Giant Planet Atmospheres

Abstract

Extrasolar giant planets (EGP) in a close-in orbit undergo extreme irradiation by their host star. Emission of H$_{3}^{+}$ is expected to support the stabilization of the planetary atmosphere, since the ion radiates in the near-infrared (NIR) and thereby cools the upper layers of the atmosphere. The simulations of H$_{3}^{+}$ emission from hot Jupiter planets differ in their predicted strengths by several orders of magnitude. H$_{3}^{+}$ emissions from EGP atmospheres have not been detected yet. Previous to this study, only a few other attempts for the observation of H$_{3}^{+}$ from hot Jupiter atmospheres have been made and did not reach the emission limits of the theoretical predictions. We obtained ground based spectroscopy with CRIRES at the Very Large Telescope (VLT) for the three hot Jupiter systems HD 209458, HD 217107 and GJ 86, and search for signatures of H$_{3}^{+}$ emission in the $\mathrm{L}'$ band. CRIRES is a NIR high resolution spectrograph and provides adequate resolution to observe atmospheric emission lines from exoplanet atmospheres. For the transiting hot Jupiter HD 209458 b, we obtained spectra during and after secondary eclipse. We use the information on the ephemeris of the planet to search for H$_{3}^{+}$ emission lines directly in the spectra after correcting for the planetary radial velocity. We apply a cross-correlation approach, subtracting the stellar signal with the secondary eclipse spectra. We also apply a `shift-and-add' technique, combining all non-eclipse spectra in order to detect possibly hidden H$_{3}^{+}$ emission. We do not find any signs of H$_{3}^{+}$ emission from the atmosphere of the hot Jupiter HD 209458 b in the obtained spectra. We derive the emission limits for the H$_{3}^{+}$ line at 3953.0 nm $[Q(1,0)]$ to be $8.32\cdot 10^{18}\,\mathrm{W}$ and $5.34\cdot 10^{18}\,\mathrm{W}$ for the line at 3985.5 nm $[Q(3,0)]$. These emission limits are in good agreement to other ground based measurements of H$_{3}^{+}$ emission limits in hot Jupiter atmospheres. However, comparing our emission limits to the theoretical predictions, there is a deficiency of up to three magnitudes of sensitivity in order to measure H$_{3}^{+}$ emission in our data. From our detection limits we derived the constraints for pushing the H$_{3}^{+}$ emission limit down to $5 \cdot 10^{16}\,\mathrm{W}$ with new CRIRES observations and successfully obtained spectra of the two non-transiting systems HD 217107 and GJ 86. We searched for H$_{3}^{+}$ directly and also tested the cross-correlation approach, similar to the first attempt. The analysis of the spectra from both hot Jupiter systems did not reveal signatures of atmospheric H$_{3}^{+}$ emission. We derive our best result for HD 217107 b with an emission limit of $2.88\cdot 10^{16}\,\mathrm{W}$ for the emission line at $3985.5\,\mathrm{nm}$. This limit is below the estimations by Miller et. al. (2000) and in range of the limit derived by Yelle (2004). Future observational attempts of atmospheric H$_{3}^{+}$ of hot Jupiters with ground based instruments, aiming at even lower emission limits preferably would be carried out with high resolution spectroscopy at future giant telescopes, such as the E-ELT.