Evidence of energy-, recombination-, and photon-limited escape regimes in giant planet H/He atmospheres

Abstract

Hydrodynamic escape is the most efficient atmospheric mechanism of planetary mass loss and has a large impact on planetary evolution. Three hydrodynamic escape regimes have been identified theoretically: energy-limited, recombination-limited, and photonlimited. However, no evidence of these regimes had been reported until now. Here, we report evidence of these three regimes via an analysis of a helium i triplet at 10830 Å and Lyα absorption involving a 1D hydrodynamic model that allows us to estimate hydrogen recombination and advection rates. In particular, we show that HD 209458 b is in the energy-limited regime, HD 189733 b is in the recombination-limited regime, and GJ 3470 b is in the photon-limited regime. These exoplanets can be considered as benchmark cases for their respective regimes.

A tentative detection of He I in the atmosphere of GJ 1214 b

“…Furthermore, by performing a similar analysis as that in Lampón et al (2021a), we found that GJ 1214 b is in the photonlimited hydrodynamic escape regime. Accordingly, the upper atmosphere of this planet is relatively cold, the neutral H density is dominated by advection over recombination, and the ionisation front encompasses all the escaping flow.…”

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

“…Constraining mass-loss rates in a young, well-characterized multiplanet system remains an important goal. These measurements can help differentiate between the recombinationlimited, energy-limited, and photon-limited regimes for mass loss at early times, which would have crucial implications for the outflow efficiencies (Lampón et al 2021). In turn, this would allow us to benchmark population-level mass-loss models, like those used to infer core masses and compositions of the Kepler planets (Rogers & Owen 2021).…”

Estimating photoevaporative mass loss of exoplanets with PLATYPOS

“…We, therefore, calculated the mass-loss of the planets for a low, intermediate, and high stellar activity scenario (red, gray, and blue, respectively). For more details, see section 4.2.4 in Poppenhaeger et al (2021) escape, have been identified in theoretical studies (e.g., Lammer et al 2003;Murray-Clay et al 2009;Owen & Alvarez 2016;Owen & Jackson 2012), and there is recent observational evidence of giant planets supporting these regimes (Lampón et al 2021). The underlying physics of the escape differs in terms of the production and losses of neutral hydrogen, as well as the processes converting the absorbed stellar radiation into work, which ultimately drives the evaporative outflow.…”