Effects of the stellar wind on the Ly alpha transit of close-in planets

STEPHEN CAROLAN; ALINE VIDOTTO; CAROLINA VILLARREAL D'ANGELO; GOPAL HAZRA

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY

WILEY-BLACKWELL PUBLISHING, INC

Lugar: Londres; Año: 2021

0035-8711

We use 3D hydrodynamics simulations followed by synthetic line profile calculations to examine the effect increasing the strength of the stellar wind has on observed Ly alpha transits of a hot Jupiter (HJ) and a warm Neptune (WN). We find that increasing the stellar wind mass-loss rate from 0 (no wind) to 100 times the solar mass-loss rate value causes reduced atmospheric escape in both planets (a reduction of 65 per cent and 40 per cent for the HJ and WN, respectively, compared to the ´no wind´ case). For weaker stellar winds (lower ram pressure), the reduction in planetary escape rate is very small. However, as the stellar wind becomes stronger, the interaction happens deeper in the planetary atmosphere, and, once this interaction occurs below the sonic surface of the planetary outflow, further reduction in evaporation rates is seen. We classify these regimes in terms of the geometry of the planetary sonic surface. ´Closed´ refers to scenarios where the sonic surface is undisturbed, while ´open´ refers to those where the surface is disrupted. We find that the change in stellar wind strength affects the Ly alpha transit in a non-linear way (note that here we do not include charge-exchange processes). Although little change is seen in planetary escape rates (~ 5.5 × 1011 g s-1) in the closed to partially open regimes, the Ly alpha absorption (sum of the blue [-300, -40 km s-1] and red [40, 300 km s-1] wings) changes from 21 to 6 per cent as the stellar wind mass-loss rate is increased in the HJ set of simulations. For the WN simulations, escape rates of ~ 6.5 × 1010 g s-1 can cause transit absorptions that vary from 8.8 to 3.7 per cent, depending on the stellar wind strength. We conclude that the same atmospheric escape rate can produce a range of absorptions depending on the stellar wind and that neglecting this in the interpretation of Ly alpha transits can lead to underestimation of planetary escape rates.