Three-dimensional hydrodynamic simulations of the upper atmosphere of π Men c: Comparison with Lyα transit observations

Abstract

Context. π Men c is the first planet to have been discovered by the Transiting Exoplanet Survey Satellite. It orbits a bright, nearby star and has a relatively low average density, making it an excellent target for atmospheric characterisation. The existing planetary upper atmosphere models of π Men c predict significant atmospheric escape, but Lyα transit observations indicate the non-detection of hydrogen escaping from the planet. Aims. Our study is aimed at constraining the conditions of the wind and high-energy emission of the host star and reproducing the non-detection of Lyα planetary absorption. Methods. We modelled the escaping planetary atmosphere, the stellar wind, and their interaction employing a multi-fluid, three-dimensional hydrodynamic code. We assumed a planetary atmosphere composed of hydrogen and helium. We ran models varying the stellar high-energy emission and stellar mass-loss rate, and, for each case, we further computed the Lyα synthetic planetary atmospheric absorption and compared it with the observations. Results. We find that a non-detection of Lyα 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 π Men c, detectable Lyα absorption can be caused exclusively by energetic neutral atoms, which become more abundant with increasing velocity or 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. The reason for this is that 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 it is likely that the atmosphere of π Men c is not hydrogen-dominated. Therefore, future observations should focus on the search for planetary atmospheric absorption at the position of lines of heavier elements, such as He, C, and O.

A Heavy Molecular Weight Atmosphere for the Super-Earth π Men c

“…Unlike the case for π Men c, there is strong evidence that these other exoplanets' atmospheres are H 2 /He-dominated. The transit depth and Doppler velocities reported here are consistent with the C II ions being swept by the stellar wind into a ∼15 R p wide (about the extent of the Roche lobe in the substellar direction, and closer to the planet than the interface between the planetary and stellar winds) tail and accelerated to high velocities, a scenario suggested by 3D models of π Men c and other exoplanets (Shaikhislamov et al 2020a(Shaikhislamov et al , 2020b. We found by means of a simplified phenomenological model of π Men cʼs tail (Appendix B) that the C II measurements can be explained if the planet loses carbon at a rate 10 8 g s −1 , requiring that the atmosphere contains this atom in at least solar abundance.…”

“…The spectroscopic velocity of +10 km s −1 for the C II ions is consistent with our photochemical-hydrodynamic predictions, and likely traces absorption in the vicinity of the planet's dayside as the gas accelerates toward the star. Comparable velocities are predicted by 3D models on the planet's dayside (Shaikhislamov et al 2020a) before the escaping gas interacts with the impinging stellar wind. Negative velocities of −70 km s −1 (and probably faster, as the stellar line becomes weak and the S/N poor at the corresponding wavelengths) suggest that the C II ions are accelerated away from the star by other mechanisms such as tidal forces and magnetohydrodynamic interactions with the stellar wind.…”

“…Our models indicate that the neutral hydrogen flux (the slow component for ENA generation, see above) at a reference location defined by the sonic point is  m H I (at Mach = 1) ∼ 5 × 10 9 g s −1 for H 2 /He-dominated atmospheres. From published 3D models for H 2 /He-dominated atmospheres incorporating ENAs (Holmström et al 2008;Shaikhislamov et al 2020a), we estimate that a neutral hydrogen flux about 1/4 times that value will bring the ENA generation in line with the nondetection of Lyα absorption. Therefore, we set  m H I (at Mach = 1)  1.25 × 10 9 g s −1 as the approximate threshold for bulk compositions consistent with insufficient ENA generation and with the nondetection of Lyα absorption.…”

“…A prior observation of π Men c with the HST Space Telescope Imaging Spectrograph (STIS) revealed no evidence for in-transit absorption of the stellar Lyα line (García Muñoz et al 2020). When present, absorption in the Lyα wings is primarily caused by energetic neutral atoms (ENAs; Holmström et al 2008;Tremblin & Chiang 2013), which are fast neutral hydrogen atoms generated when the low-velocity neutral hydrogen escaping the planet and the high-velocity protons in the stellar wind exchange charge: Three-dimensional modeling shows that if π Men cʼs atmosphere is H 2 /He-dominated, large amounts of ENAs are generated that produce measurable Lyα transit depths (Shaikhislamov et al 2020a). Conversely, reduced ENA generation occurs if either the flux of stellar wind protons or the flux of neutral hydrogen from the planet are weak.…”

Hydrogen Dominated Atmospheres on Terrestrial Mass Planets: Evidence, Origin and Evolution

“…The most important characteristic -the mass-loss rate -is equal to about 3 × 10 9 g/s in Loyd et al (2017) and 3.3 × 10 9 g/s in the 3D calculation. Thus, for the first time, 3D simulations have indicated that the main quantitative parameters of atmospheric escape from highly irradiated exoplanets can be accurately simulated by 1D models (see also Shaikhislamov et al 2020, for the case of the super-Earth π Men c). This is because the outflow is driven mostly in the upper atmosphere and because the day-side heating by XUV is efficiently redistributed over the whole atmosphere by zonal flows.…”