Characterizing the atmosphere of the warm Neptune HAT-P-11 b with high resolution spectroscopy

High resolution spectra of transit events enable the search for constituents in exoplanet atmospheres with a variety of methods.
Using cross correlation, the observations can be compared to models across a wide wavelength range, taking advantage of the large number of absorption lines of some species. For other species, the excess absorption in strong individual lines is measurable directly by comparing the spectra taken during the transit with those before and after the transit. We analyze transmission spectra of HAT-P-11 b obtained with CARMENES using these two methods to search for a multitude of species in the planetary atmosphere. We remove stellar and telluric lines with SYSREM and find a tentative detection of CH4 using cross correlation. In addition, the influence of clouds on the detection strength is investigated. We confirm the previously detected presence of He by examining the near-infrared helium triplet. Helium is known to be a tracer for extended atmospheres and to be related to stellar activity. The large eccentricity of HAT-P-11 b's orbit helps to disentangle the planetary from the stellar absorption lines. Furthermore, we search for other species that are expected to be present in the atmosphere of warm Neptunes.