On the sensitivity of extrasolar mass-loss rate ranges: HD 209458b a case study

Abstract

We present a 3D hydrodynamic study of the effects that different stellar wind conditions and planetary wind structures have on the calculated Lyα absorptions produced by a cometary tail during transit. We concentrate, as a case study, on the known HD 209458b case. Initially, we assume a broad range of possible planetary mass-loss rate values: M˙ p = [1−7] × 1010 g s−1. Then, by comparing the observational Lyα absorption with the numerically derived ones, we could constrain the M˙ p values within the given range. We find that the planetary massloss rate does not change dramatically for large changes in stellar wind speeds ∼[250– 800] km s−1 and temperature ∼[3–7] × 106 K while keeping fixed the stellar mass-loss rate (M˙ = 9.0 × 10−14 M yr−1). The M˙ p range found is ∼ [3−5] × 1010 g s−1, depending upon the efficiency of the stellar wind to transport heat (polytropic index ∼ [1.01–1.13]), leading to different stellar wind speeds. Several models with anisotropic evaporation profiles for the planetary escaping atmosphere were carried out, showing that both, the escape through polar regions, resembling the emission associated with reconnection processes, and through the night side, produced by a strong stellar wind that compresses the planetary atmosphere and inhibits its escape from the day hemisphere yields larger absorptions than an isotropic planetary wind.