Three-dimensional hydrodynamic simulations of the upper atmosphere of π\pi Men c: comparison with Lyα\alpha transit observations

Aims: We aim at constraining the conditions of the wind and high-energy emission of the host star reproducing the non-detection of Ly$\alpha$ planetary absorption. Methods: We model the escaping planetary atmosphere, the stellar wind, and their interaction employing a multi-fluid, three-dimensional hydrodynamic code. We assume a planetary atmosphere composed of hydrogen and helium. We run models varying the stellar high-energy emission and stellar mass-loss rate, further computing for each case the Ly$\alpha$ synthetic planetary atmospheric absorption and comparing it with the observations. Results: We find that a non-detection of Ly$\alpha$ in absorption employing the stellar high-energy emission estimated from far-ultraviolet and X-ray data requires a stellar wind with a stellar mass-loss rate about six times lower than solar. This result is a consequence of the fact that, for $\pi$ Men c, detectable Ly$\alpha$ absorption can be caused exclusively by energetic neutral atoms, which become more abundant with increasing the velocity and/or the density of the stellar wind. By considering, instead, that the star has a solar-like wind, the non-detection requires a stellar ionising radiation about four times higher than estimated. This is because, despite the fact that a stronger stellar high-energy emission ionises hydrogen more rapidly, it also increases the upper atmosphere heating and expansion, pushing the interaction region with the stellar wind farther away from the planet, where the planet atmospheric density that remains neutral becomes smaller and the production of energetic neutral atoms less efficient. Conclusions: Comparing the results of our grid of models with what is expected and estimated for the stellar wind and high-energy emission, respectively, we support the idea that the atmosphere of $\pi$ Men c is likely not hydrogen-dominated.Comment: Accepted for publication in A&A. The abstract has been shortened to fit the arXiv for

On the origin of the non-detection of metastable HeI in the upper atmosphere of the hot Jupiter WASP-80b

We aim to narrow down the origin of the non-detection of the metastable HeI triplet at about 10830 A obtained for the hot Jupiter WASP-80b. We measure the X-ray flux of WASP-80 from archival observations and use it as input to scaling relations accounting for the coronal [Fe/O] abundance ratio to infer the extreme-ultraviolet (EUV) flux in the 200-504 A range, which controls the formation of metastable HeI. We run three dimensional (magneto) hydrodynamic simulations of the expanding planetary upper atmosphere interacting with the stellar wind to study the impact on the HeI absorption of the stellar high-energy emission, the He/H abundance ratio, the stellar wind, and the possible presence of a planetary magnetic field up to 1 G. For a low stellar EUV emission, which is favoured by the measured logR'HK value, the HeI non-detection can be explained by a solar He/H abundance ratio in combination with a strong stellar wind, or by a sub-solar He/H abundance ratio, or by a combination of the two. For a high stellar EUV emission, the non-detection implies a sub-solar He/H abundance ratio. A planetary magnetic field is unlikely to be the cause of the non-detection. The low EUV stellar flux, driven by the low [Fe/O] coronal abundance, is the likely primary cause of the HeI non-detection. High-quality EUV spectra of nearby stars are urgently needed to improve the accuracy of high-energy emission estimates, which would then enable one to employ the observations to constrain the planetary He/H abundance ratio and the stellar wind strength. This would greatly enhance the information that can be extracted from HeI atmospheric characterisation observations.Comment: Accepted for publication on A&A, 20 page

The GAPS Programme at TNG : XXXII. The revealing non-detection of metastable He I in the atmosphere of the hot Jupiter WASP-80b

Context. Because of its proximity to an active K-type star, the hot Jupiter WASP-80b has been identified as a possible excellent target for detecting and measuring He I absorption in the upper atmosphere. Aims. Our aim was to look for, and eventually measure and model, metastable He I atmospheric absorption. Methods. We observed four primary transits of WASP-80b in the optical and near-infrared using the HARPS-N and GIANO-B high-resolution spectrographs attached to the Telescopio Nazionale Galileo telescope, focusing the analysis on the He I triplet. We further employed a three-dimensional hydrodynamic aeronomy model to understand the observational results. Results. We did not find any signature of planetary absorption at the position of the He I triplet with an upper limit of 0.7% (i.e. 1.11 planetary radii; 95% confidence level). We re-estimated the high-energy stellar emission, which we combined with a stellar photospheric model, to generate the input for the hydrodynamic modelling. We determined that, assuming a solar He to H abundance ratio, He I absorption should have been detected. Considering a stellar wind 25 times weaker than solar, we could reproduce the non-detection only by assuming a He to H abundance ratio about 16 times smaller than solar. Instead, considering a stellar wind ten times stronger than solar, we could reproduce the non-detection only with a He to H abundance ratio about ten times smaller than solar. We attempted to understand this result by collecting all past He I measurements and looking for correlations with high-energy stellar emission and planetary gravity, but without success. Conclusions. WASP-80b is not the only planet with an estimated sub-solar He to H abundance ratio, which suggests the presence of efficient physical mechanisms (e.g. phase separation, magnetic fields) capable of significantly modifying the He to H content in the upper atmosphere of hot Jupiters. The planetary macroscopic properties and the shape of the stellar spectral energy distribution are not sufficient for predicting the presence or absence of detectable metastable He in a planetary atmosphere, since the He abundance also appears to play a major role

Global 3D simulation of the upper atmosphere of HD189733b and absorption in metastable HeI and Ly{\alpha} lines

A 3D fully self-consistent multi-fluid hydrodynamic aeronomy model is applied to simulate the hydrogen-helium expanding upper atmosphere of the hot Jupiter HD189733b, and related absorption in the Lya line and the 10830 A line of metastable helium. We studied the influence of a high-energy stellar flux, stellar wind, and Lya cooling to reproduce the available observations. We found that to fit the width of the absorption profile in 10830 A line the escaping upper atmosphere of planet should be close to the energy limited escape achieved with a significantly reduced Lya cooling at the altitudes with HI density higher than 3*10^6 cm^-3. Based on the preformed simulations, we constrain the helium abundance in the upper atmosphere of HD189733b by a rather low value of He/H~0.005. We show that under conditions of a moderate stellar wind similar to that of the Sun the absorption of Lya line takes place mostly within the Roche lobe due to thermal broadening at a level of about 7%. At an order of magnitude stronger wind, a significant absorption of about 15% at high blue shifted velocities of up to 100 km/s is generated in the bowshock region, due to Doppler broadening. These blue shifted velocities are still lower than those (~200 km/s) detected in one of the observations. We explain the differences between performed observations, though not in all the details, by the stellar activity and the related fluctuations of the ionizing radiation (in case of 10830 A line), and stellar wind (in case of Lya line)

The GAPS programme at TNG: XXXII. The revealing non-detection of metastable He I in the atmosphere of the hot Jupiter WASP-80b

Context. Because of its proximity to an active K-type star, the hot Jupiter WASP-80b has been identified as a possible excellent target for detecting and measuring HeI absorption in the upper atmosphere.Aims. Our aim was to look for, and eventually measure and model, metastable HeI atmospheric absorption.Methods. We observed four primary transits of WASP-80b in the optical and near-infrared using the HARPS-N and GIANO-B high-resolution spectrographs attached to the Telescopio Nazionale Galileo telescope, focusing the analysis on the HeI triplet. We further employed a three-dimensional hydrodynamic aeronomy model to understand the observational results.Results. We did not find any signature of planetary absorption at the position of the HeI triplet with an upper limit of 0.7% (i.e. 1.11 planetary radii; 95% confidence level). We re-estimated the high-energy stellar emission, which we combined with a stellar photospheric model, to generate the input for the hydrodynamic modelling. We determined that, assuming a solar He to H abundance ratio, HeI absorption should have been detected. Considering a stellar wind 25 times weaker than solar, we could reproduce the non-detection only by assuming a He to H abundance ratio about 16 times smaller than solar. Instead, considering a stellar wind ten times stronger than solar, we could reproduce the non-detection only with a He to H abundance ratio about ten times smaller than solar. We attempted to understand this result by collecting all past HeI measurements and looking for correlations with high-energy stellar emission and planetary gravity, but without success.Conclusions. WASP-80b is not the only planet with an estimated sub-solar He to H abundance ratio, which suggests the presence of efficient physical mechanisms (e.g. phase separation, magnetic fields) capable of significantly modifying the He to H content in the upper atmosphere of hot Jupiters. The planetary macroscopic properties and the shape of the stellar spectral energy distribution are not sufficient for predicting the presence or absence of detectable metastable He in a planetary atmosphere, since the He abundance also appears to play a major role