The He I 10830 line in exoplanets, triggered by coronal emission Jorge Sanz-Forcada1, Manuel López-Puertas2 , Lisa Nortmann3, Manuel Lampón2 Introduction Despite of early efforts back in 2000 to find the He I 10830 line (actually a triplet) in transiting exoplanets, it was not until 2018 that the line was finally observed in an exoplanet. An increasing number of detections in transiting exoplanets is allowing us to trace an unambiguous relation between this line and the incoming XUV irradiation from the corona of the star. The stellar radiation below 504 Å ionizes He atoms in the planet atmosphere, producing the He I 10830 Å triplet after recombination takes place. This line is becoming a useful tool to measure the effects of photoevaporation in planets, and even to detect the presence of winds. In contrast with the H Lyman-α line, the He triplet is not absorbed by the interstellar medium, posing it as the best tracer of photoevaporation for the future. The analysis of the stellar coronal emission is essential to predict the line detection in exoplanets. The careful work carried out in the X-exoplanets database provides an easy, and reliable, way to calculate the expected He-ionizing XUV stellar flux. 1Centro de Astrobiología (CAB, CSIC-INTA), Spain; 2Instituto de Astrofísica de Andalucía (IAA-CSIC), Spain; 3Institut für Astrophysik, George-August-Universität, Gottingen jsanz@cab.inta-csic.es Figure 3. Coronal model (Emission Measure Distribution, EMD) of AU Mic. The upper panel shows the coronal model, with a solid black thick line (dashed if uncertain determination of EMD). Thin colored lines are indicative of the emissivity functions of the spectral lines, with the ions numbered below, weighted by the EMD. In the lower panel it is shown the observed-to-predicted line fluxes ratio for the measured spectral lines. Color code of the elements is the same in both panels. Figure 5. He I 10830 equivalent width (EW) against XUV (5-504 A) coronal irradiation at the planet orbit. The latter is expressed as its ratio over the gravitational potential of the planet, in Jovian units, for a tighter relation. Figure 4. X-ray luminosity as measured in a sample of X- exoplanets, against modeled EUV luminosity, in two spectral ranges relevant for ionization processes in the planet atmosphere Figure 2. He I 10830 triplet as observed in WASP-69 b (Nortmann et al. 2018) during transits. Vertical lines mark the positions of the three lines. Note that this triplet is produced in the planet, and not in the star. A 3% absorption was detected in the line during the transit. The X-exoplanets project (Sanz-Forcada+ 2011) observes planet- hosting stars in X-rays to carefully model its corona, in order to predict the EUV spectral response. Figure 4 includes stars present in the original X-exoplanets sample, with the addition of other planet-hosting stars, some of them analyzed in the CARMENES and the PanCET projects. Best fit to the current sample provides useful X-ray to EUV relations: log LEUV, H = (0.793 ± 0.058) log LX + (6.53 ± 1.61) [100-920 Å] log LEUV, He = (0.856 ± 0.042) log LX + (4.38 ± 1.16) [100-504 Å] CONCLUSIONS • He I 10830 is a line that is formed in the exoplanets atmosphere triggered by coronal radiation • A tight relation between X-ray and EUV luminosities allows for a quick calculation of the ionizing irradiation reaching a planet atmosphere. If no X-ray flux is available it is still possible to use a relation between rotation period and X-ray flux to have an estimate. • A careful analysis of high-resolution X-ray and UV spectra provides the best coronal models. Tighter relations are expected to be constrained in the future as the X-exoplanet sample improves its quality. Other relations that use X-ray surface flux (Chadney+ 2015, King+ 2018) instead of X-ray luminosity (using a very small sample of stars) seem to fail when applied to the X-exoplanets sample. Figure 1. He I levels diagram, from Zirin (1988) X The He line triplet needs