Evolution of atmospheric escape in close-in giant planets and their associated Ly α and H α transit predictions

Abstract

Strong atmospheric escape has been detected in several close-in exoplanets. As these planets consist mostly of hydrogen, observations in hydrogen lines, such as Lyα and Hα, are powerful diagnostics of escape. Here, we simulate the evolution of atmospheric escape of close-in giant planets and calculate their associated Lyα and Hα transits. We use a one-dimensional hydrodynamic escape model to compute physical properties of the atmosphere and a ray-tracing technique to simulate spectroscopic transits. We consider giant (0.3 and 1M jup ) planets orbiting a solar-like star at 0.045au, evolving from 10 to 5000 Myr. We find that younger giants show higher rates of escape, owing to a favourable combination of higher irradiation fluxes and weaker gravities. Less massive planets show higher escape rates (10 10 -10 13 g/s) than those more massive (10 9 -10 12 g/s) over their evolution. We estimate that the 1-M jup planet would lose at most 1% of its initial mass due to escape, while the 0.3-M jup planet, could lose up to 20%. This supports the idea that the Neptunian desert has been formed due to significant mass loss in low-gravity planets. At younger ages, we find that the mid-transit Lyα line is saturated at line centre, while Hα exhibits transit depths of at most 3 -4% in excess of their geometric transit. While at older ages, Lyα absorption is still significant (and possibly saturated for the lower mass planet), the Hα absorption nearly disappears. This is because the extended atmosphere of neutral hydrogen becomes predominantly in the ground state after ∼ 1.2 Gyr.

Stellar wind effects on the atmospheres of close-in giants: a possible reduction in escape instead of increased erosion

“…Here, we set the base at r = R pl and assume the temperature there is 1000 K and the density is 4 × 10 −13 g cm −3 . These are similar to values used in Murray-Clay et al (2009); Allan & Vidotto (2019). We remind however that Murray-Clay et al (2009) demonstrated that changing these values had negligible effect on the physical properties of the atmospheric escape.…”

“…It is important at high incident fluxes -it takes place at close-in planets and/or planets orbiting active stars, when the incident extreme UV (EUV) fluxes are several orders of magnitude larger than the EUV solar flux at Earth. Here, we use the model from Allan & Vidotto (2019) to compute the hydrodynamic properties of escaping atmospheres of close-in giants. This model is based on Murray-Clay et al (2009), and it considers planetary gravity, stellar tidal forces, photoionisation heating from stellar EUV radiation, cooling from Ly-α radiation and ionisation balance.…”

Mass-loss rate and local thermodynamic state of the KELT-9 b thermosphere from the hydrogen Balmer series

“…This confirms that such planets undergo a process of evaporation (Sing et al 2019). However, it is important to get a more precise idea of the mass loss rate of such objects to understand the role of the stellar irradiation and its processes of interaction with the planet atmosphere (Fossati et al 2018;Allan & Vidotto 2019;García Muñoz & Schneider 2019). This paper reports on the results of the Sensing Planetary Atmospheres with Differential Echelle Spectroscopy (SPADES) program (Bourrier et al 2017b(Bourrier et al , 2018Hoeijmakers et al 2018Hoeijmakers et al , 2019.…”

“…The Boltzmann equilibrium is assumed by default in our code. However, because we are probing high in the thermosphere, the collisions between particles become less numerous and the electronic state distribution can depart from the Boltzmann distribution, the atmosphere is in non-LTE or NLTE (Barman et al 2002;Fortney et al 2003;Christie et al 2013;Huang et al 2017;Oklopčić & Hirata 2018;Fisher & Heng 2019;Allan & Vidotto 2019;García Muñoz & Schneider 2019;Lampón et al 2020). For the NLTE case, we define the departure coefficients (e.g., Mihalas 1970;Salem & Brocklehurst 1979;Barman et al 2002;Draine 2011;Hubeny & Mihalas 2014) as:…”

Different types of star-planet interactions

“…than older, Jupiter-mass planets orbiting at close distances to their host stars (Allan & Vidotto 2019).…”