Search for the presence of helium in the atmospheres of ten exoplanets with SPIRou

The near-infrared helium triplet is now an established tracer of the upper atmosphere of exoplanets. However, the presence of metastable helium in exoplanet atmospheres lies on a delicate balance between the extreme ultra-violet flux emitted by the star and the distance of the planet to its star. Exoplanets in mild-condition of irradiation around K-type stars are, therefore, expected to be the most amenable for its detection, which is in agreement with the first studies.

The use of the new generation of high-resolution near-infrared spectrographs (CARMENES, NIRSPec, GIANO, SPIRou, …) allows to spectrally and temporally resolve the helium triplet. The high spectral resolution enables the line shape determination providing constraints on the altitude of metastable helium particles and the dynamic taking place. The high temporal resolution tracks its distribution during the transit and can put in evidence cometary-like tails through post-transit absorption. Therefore, the helium triplet is a unique probe of the transition from the thermosphere, physically bound to the exoplanet, to the exosphere, where particles escape into space. Constraints on the mass loss rate of the atmospheres inform us of the evolution of exoplanets and can explain the presence of the so-called evaporation desert and the general lack of Neptune-mass planets.

While previous studies focused their analysis on a couple of targets at a time, we will present the first SPIRou survey composed of ten exoplanets analyzed homogeneously. Those planets have mini-Neptune to Jupiter masses, sample two orders of magnitude in stellar irradiation, and orbit M, K, and G-type stars. We report the detection of helium for only three planets and the non-detections for the others including some discrepancies with the literature. We applied the open-source 1D radiative transfer model, p-winds to constrain the mass loss rate, temperature, and bulk velocity of the upper atmosphere of these 10 exoplanets. We then explore correlations that could explain the presence or lack thereof of metastable helium. Finally, a special focus will be made on HD189733b for which four transits and two half-transits were obtained leading to high data quality. We will report on the lack of post transit absorption and temporal variation.