Modelling the He I triplet absorption at 10 830 Å in the atmospheres of HD 189733 b and GJ 3470 b

¹ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
e-mail: mlampon@iaa.es
² Centro de Astrobiología (CSIC-INTA), ESAC, Camino bajo del castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
³ Leiden Observatory, Leiden University, Postbus 9513, 2300 RA, Leiden, The Netherlands
⁴ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁵ Landessternwarte, Zentrum für Astronomie der Universität Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
⁶ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, 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 Observatorio de Calar Alto, Sierra de los Filabres, 04550 Gérgal, Almería, Spain
¹¹ 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
¹² Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
¹³ Institut de Ciències de l’Espai (CSIC-IEEC), Campus UAB, c/ de Can Magrans s/n, 08193 Bellaterra, Barcelona, Spain
¹⁴ Institut d’Estudis Espacials de Catalunya (IEEC), 08034 Barcelona, Spain

Received: 13 September 2020
Accepted: 20 January 2021

Abstract

Characterising the atmospheres of exoplanets is key to understanding their nature and provides hints about their formation and evolution. High resolution measurements of the helium triplet absorption of highly irradiated planets have been recently reported, which provide a new means of studying their atmospheric escape. In this work we study the escape of the upper atmospheres of HD 189733 b and GJ 3470 b by analysing high resolution He I triplet absorption measurements and using a 1D hydrodynamic spherically symmetric model coupled with a non-local thermodynamic model for the He I triplet state. We also use the H density derived from Lyα observations to further constrain their temperatures, mass-loss rates, and H/He ratios. We have significantly improved our knowledge of the upper atmospheres of these planets. While HD 189733 b has a rather compressed atmosphere and small gas radial velocities, GJ 3470 b, on the other hand with a gravitational potential ten times smaller, exhibits a very extended atmosphere and large radial outflow velocities. Hence, although GJ 3470 b is much less irradiated in the X-ray and extreme ultraviolet radiation, and its upper atmosphere is much cooler, it evaporates at a comparable rate. In particular, we find that the upper atmosphere of HD 189733 b is compact and hot, with a maximum temperature of 12 400₋₃₀₀⁺⁴⁰⁰ K, with a very low mean molecular mass (H/He = (99.2/0.8) ± 0.1), which is almost fully ionised above 1.1 R_P, and with a mass-loss rate of (1.1 ± 0.1) × 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)_−1.5^+1.0), which is not strongly ionised, and with a mass-loss rate of (1.9 ± 1.1) × 10¹¹ g s⁻¹. Furthermore, our results suggest that upper atmospheres of giant planets undergoing hydrodynamic escape tend to have a very low mean molecular mass (H/He ≳ 97/3).