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

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 RP, 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).

p-winds: An open-source Python code to model planetary outflows and upper atmospheres

“…More complex, three-dimensional models are necessary to completely describe these features (e.g., Bourrier et al 2016;Villarreal D'Angelo et al 2021;Wang & Dai 2021a,b;Allart et al 2019;MacLeod & Oklopčić 2021). Simple one-dimensional models are nevertheless capable of retrieving atmospheric escape parameters with the assumption that the mass loss process takes place spherically and homogeneously throughout the surface of the planet (e.g., Lampón et al 2020Lampón et al , 2021. Models that are faster to calculate are also useful when there is a need to explore a large parameter space, which is what we discuss in Sect.…”

“…The disadvantage is that the formation of metastable He in the upper atmospheres of exoplanets depends on a specific level of irradiation arriving at the planet (e.g., Nortmann et al 2018;Oklopčić 2019;Dos Santos et al 2020a). Nevertheless, He spectroscopy has the potential to become the main technique of atmospheric escape observations (Allart et al 2019;Alonso-Floriano et al 2019;Kirk et al 2020;Vissapragada et al 2020;Lampón et al 2021;Paragas et al 2021).…”

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

“…As electrons are predominantly produced by the photo-ionisation of H and lost by advection and H + recombination, the He(2 3 S) concentration profile is thus mainly controlled by the H production and loss rates. Therefore, by constraining the He(2 3 S) concentration profile, as was done by Lampón et al (2020Lampón et al ( , 2021, we simultaneously constrain the production and loss rates of H. Hence, in this work we analysed the production and loss rates of H corresponding to the T -Ṁ ranges and H/He ratios of 99/1 for HD 189733 b and 98/2 for HD 209458 b and GJ 3470 b, which were previously obtained by Lampón et al (2020Lampón et al ( , 2021 and are summarised in Fig. 1.…”

“…Spake et al 2018;Nortmann et al 2018;Allart et al 2018). Some examples of how these measurements help to better characterise the escaping atmospheres of gas giants were provided by Allart et al (2018), Mansfield et al (2018), Lampón et al (2020Lampón et al ( , 2021, and Shaikhislamov et al (2020).…”

“…Fig.1. Ranges of temperature and mass-loss rates for HD 209458 b, HD 189733 b, and GJ 3470 b for H/He ratios of 98/2, 99/1, and 98/2, respectively, derived byLampón et al (2020Lampón et al ( , 2021. Symbols show the derived ranges and dotted lines the extended ranges (see Sect.…”