Metastable Helium Absorptions with 3D Hydrodynamics and Self-consistent Photochemistry. II. WASP-107b, Stellar Wind, Radiation Pressure, and Shear Instability

Abstract

This paper presents simulations of the metastable helium (He * ) observations of WASP-107b, so far the highest signal-to-noise ratio detection that is confirmed by three different instruments. We employ full 3D hydrodynamics coupled with coevolving nonequilibrium thermochemistry and ray-tracing radiation, predicting mass-loss rates, temperature profiles, and synthetic He * line profiles and light curves from first principles. We find that a stellar wind stronger than solar is demanded by the observed heavily blueshifted line profile and asymmetric transit light curve. Radiation pressure can be important for Lyα observations, but not He * . Our model finds that WASP-107b is losing mass at a rate of  ´-Å -M M 1.0 10 yr 9 1 . Although  M varies by <1% given constant wind and irradiation from the host, shear instabilities still emerge from wind impacts, producing ∼10% fluctuations of He * transit depths over hour-long timescales. The common assumption that He * transit depth indicates the fluctuation of  M is problematic. The trailing tail is more susceptible than planet adjacency to the shear instabilities; thus, the line profile is more variable in the blueshifted wing, while the transit light curve is more variable after midtransit. We stress that the synergy between Lyα (higher altitudes, lower density) and He * (lower altitudes, higher density) transit observations, particularly simultaneous ones, yields better understanding of planetary outflows and stellar wind properties.

A Search for Planetary Metastable Helium Absorption in the V1298 Tau System

“…On the other hand, our data only cover half of the transit, which limits the sensitivity of our measurement. If planet b has an extended egress comparable to that of WASP-69 b (Nortmann et al 2018) and WASP-107 b (Wang & Dai 2021b, Spake et al 2021, our data might still be consistent with a strong excess absorption signal during the transit. In this case the the transit-timing offset would need to be even larger, but we cannot exclude this possibility with the current partial phase coverage.…”

“…Additionally, there is a potential degeneracy between the model limb darkening and the outflow geometry. Even for modest outflows, deviations from spherical symmetry in the extended metastable helium distribution can change the morphology of the light curve (Wang & Dai 2021a, 2021b. Both of these factors may contribute to the "V-shaped" appearance of the light curves, especially for planet d.…”

“…The geocentric velocity of V1298 Tau on our WIRC nights ranges between−8 km s −1 (for the 2020 October observations) and 33 km s −1 (for the 2021 January observations). However, for strong planetary outflows the line may be broadened (Wang & Dai 2021b), resulting in overlap with the telluric water features. Even at a modest 10 km s −1 geocentric velocity, the 2020 November HPF observations revealed that the strongly broadened stellar helium line was beginning to overlap with the telluric water lines.…”

The Posttransit Tail of WASP-107b Observed at 10830 Å

“…Hydrodynamic simulations of escaping planetary atmospheres generally predict that the escaped material should form a comet-like tail, further shaped by the interaction with the stellar wind. For WASP-107b specifically, the modeling work of Wang & Dai (2021b) shows that the evidence of a planetary tail should be observable in the He 10830 Å spectra.…”

“…Several studies, including Dai (2021b) andMcCann et al (2019), have found that shear instabilities due to interaction with a stellar wind can change the level of absorption observed in transit light curves of escaping atmospheres. The greatest variability is seen in the pre and posttransit absorption; however, they also report low levels of variability between second and third contact.…”