Anisotropic winds from close-in extra-solar planets

TL;DR

This study models anisotropic, thermally driven winds from close-in exoplanets, revealing significant multidimensional effects on wind structure, mass loss rates, and observational signatures compared to spherical models.

Contribution

It provides the first detailed multidimensional hydrodynamic simulations of planetary winds, highlighting differences from spherical symmetry and implications for observations.

Findings

Polar flows form above the planet surface in hot, ionized winds.

Mass loss rate is reduced by nearly a factor of four in multidimensional models.

Enhanced column density near the planet affects absorption line observations.

Abstract

We present two-dimensional hydrodynamic models of thermally driven winds from highly irradiated, close-in extra-solar planets. We adopt a very simple treatment of the radiative heating processes at the base of the wind, and instead focus on the differences between the properties of outflows in multidimensions in comparison to spherically symmetric models computed with the same methods. For hot (T > 2 x 10^{4} K) or highly ionized gas, we find strong (supersonic) polar flows are formed above the planet surface which produce weak shocks and outflow on the night-side. In comparison to a spherically symmetric wind with the same parameters, the sonic surface on the day-side is much closer to the planet surface in multidimensions, and the total mass loss rate is reduced by almost a factor of four. We also compute the steady-state structure of interacting planetary and stellar winds. Both winds end in a termination shock, with a parabolic contact discontinuity which is draped over the planet separating the two shocked winds. The planetary wind termination shock and the sonic surface in the wind are well separated, so that the mass loss rate from the planet is essentially unaffected. However, the confinement of the planetary wind to the small volume bounded by the contact discontinuity greatly enhances the column density close to the planet, which might be important for the interpretation of observations of absorption lines formed by gas surrounding transiting planets.