Characterisation of exoplanetary upper atmospheres undergoing hydrodynamic atmospheric escape

Resumen

Among the most important results we find that the upper atmosphere of HD189733 b is compact and hot, with a maximum temperature of 12 400+400 −300 K, with a very low mean molecular mass (H/He=(99.2/0.8)±0.1), with small gas radial velocities, which is almost fully ionised above 1.1RP, and with a mass-loss rate of 1.07+0.08 −0.09 ×1011 g s−1. In contrast, the upper atmosphere of GJ 3470 b is highly extended and relatively cold, with a maximum temperature of 5100±900 K, also with a very low mean molecular mass (H/He=(98.5/1.5)+1.0 −1.5), with large radial outflow velocities, which is not strongly ionised and with a mass-loss rate of (1.87±1.13)×1011 g s−1. HD209458 b seems an intermediate case between HD189733 b and GJ 3470 b, as its upper atmosphere is extended, although not as GJ 3470 b, with an intermediate temperature of 7625±500 K, a mean molecular weight of 98/2, with intermediate gas radial velocities, and a mass-loss rate of (0.71±0.29)×1011 g s−1. Attending to the derived H density profiles we find that while the recombination is the process governing the whole upper atmosphere of HD189733 b, advection dominates in GJ 3470 b, and both processes are not negligible in HD209458 b. In addition to constrain the main atmospheric parameters of these exoplanets we confirm that GJ 3470 b undergoes hydrodynamic escape, and provide the first observational derivation of the H/He ratio of an exoplanet. Moreover, our results suggest that the upper atmospheres of giant planets undergoing hydrodynamic escape tend to have very low mean molecular mass (H/He&97/3). Furthermore, we report observational evidence of the hydrodynamic escape regimes in H/He atmospheres. In particular, we demonstrate that HD209458 b is in the energy-limited regime, HD189733 b is in the recombination-limited regime and GJ 3470 b is in the photonlimited regime. Accordingly, we propose these exoplanets as benchmark cases for their respective regimes. We conclude that this work significantly improves the atmospheric characterisation of this sample of exoplanets and enhances our knowledge of the hydrodynamic escape mechanism.