Letter to the Editor

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

¹ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
e-mail: mlampon@iaa.es
² Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
³ Leiden Observatory, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands
⁴ Centro de Astrobiología (CSIC-INTA), ESAC, Camino bajo del castillo s/n, 28692, Villanueva de la Cañada, Madrid, Spain
⁵ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁶ Landessternwarte, Zentrum für Astronomie der Universität Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
⁷ Instituto de Astrofísica de Canarias (IAC), Calle Vía Láctea s/n, 38200, La Laguna Tenerife, Spain
⁸ Departamento de Astrofísica, Universidad de La Laguna, 38026, La Laguna Tenerife, Spain
⁹ Institut für Astrophysik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
¹⁰ Departamento de Física de la Tierra y Astrofísica & IPARCOS-UCM (Instituto de Física de Partículas y del Cosmos de la UCM), Facultad de Ciencias Físicas, Universidad Complutense de Madrid, 28040 Madrid, Spain
¹¹ Institut de Ciències de l’Espai (CSIC-IEEC), Campus UAB, c/ de Can Magrans s/n, 08193 Bellaterra, Barcelona, Spain
¹² Institut d’Estudis Espacials de Catalunya (IEEC), 08034 Barcelona, Spain

Received: 26 January 2021
Accepted: 19 March 2021

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 photon-limited. 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 10 830 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.