Evaporating Exoplanet Atmospheres

Fig.1 - Athena++ simulation of the evaporating atmosphere of the planet and the stellar wind. We see the co-rotating frame, the planet (left) is actually moving counter-clockwise around the star (right). We can see how the shape of the outflow varies in time.

Close-in exoplanets are exposed to an extreme environment: high temperatures, intense radiation, and strong stellar winds. The high temperatures lead, among other phenomena, to a hydrodynamic escape of the planet's atmosphere. This process has major implications for the evolution of planets in this classification. Research is still in progress to understand the observed lack of short-period Neptune-sized planets, and atmospheric mass loss is assumed to be the underlying process. Therefore, it is substantial to investigate further the nature of this fascinating phenomenon.
For example, the interaction between the planet's outflowing gas and the stellar wind can produce a variety of density patterns whose spectroscopic signatures we can observe during transit. In my current project, we investigate how the interactions of planetary and stellar wind affect the properties of the He-1083 nm line . Past observations and studies have shown that this absorption line is a powerful tracer for evaporating atmospheres. We focus in particular on an anisotropic escape from a tidally-locked planet caused by its day-night-side temperature contrast. We model with 3D hydrodynamic simulations how the stellar wind shapes the planetary outflow and compare isotropic with anisotropic temperature structures of the planet.
Based on the obtained density structures and a radiative transfer analysis, we generate synthetic transmission spectra and light curves of the He-1083nm line. For anisotropic structures, we find a blue shift up to -5 km/s in the helium line compared to the isotropic case. Furthermore, we can find a dependency of temperature contrast and shift velocity . The helium line shape is also affected, as the egress shows a higher absorption for a stronger day-night temperature difference. According to these results, one could reinterpret the observational findings in the He-1083 nm line in the sense of asymmetric atmospheric escape.
The advantage of our approach is that we remain agnostic about the wind-driving mechanisms. Therefore, we only parameterize the wind instead of simulating it in a self-consistent way, which allows us to cover a wide range of parameters. In the future, I would like to apply the same analysis to a planetary outflow restricted to the poles. This type of asymmetry could arise, for example, from a magnetically-driven outflow.

Chemical Composition in the Upper Atmosphere of hot Jupiter HD 189733 b

Fig. 2 - H/C/N/O chemistry of HD189733b. We assumed different temperature profiles in the upper atmosphere (u.a.) for the different regions of the planet (day and night side, terminator regions). The relative abundance of a molecule increases in the positive direction of the x-axis, and the height of the atmosphere increases in the positive direction of the y-axis.

The irradiation of the host star has also an enormous effect on the formation of molecules in the planet's atmosphere; especially in the upper layers where the absorbed photon flux is highest. For our simulations of the atmospheric gas-phase chemistry, we use pressure-temperature profiles that originate from a global circulation model which includes hydro- and thermodynamical modeling. These models are limited by the altitude at which the circulation stops.
However, this is not the physical limit of the atmosphere. In my master's thesis, we parameterized different pressure-temperature profiles for the region above these limits for hot Jupiter HD 189733b, towards lower pressures (log(p [bar]) ~ -10). This covers the region below the exosphere of the planet, which begins per definition where the mean free path is equal to or larger than the scale height. There, the gas is no longer collision-dominated and thus no longer suitable for simulation with the chemical network used here.
Since we do not know the exact temperature profile in the region below the exosphere, we generated three different scenarios: (1) isothermal, (2) increasing, and (3) decreasing temperature for increasing height . These 1D parameterizations were created for the day- and night-side, as well as the terminator regions. The results of the gas-phase chemistry - considering the ionization effects of stellar energetic particles, XUV flux of the star, and cosmic rays - provide interesting insights into the formation of ions and hydrocarbons at high altitudes. For example, hydrocarbon haze precursors such as C2H2, HCN, and CH4 are most abundant on the night side and at the morning terminator. The day side of HD189733b is strongly ionized and H-dominated in the upper atmosphere, while the night side and the terminators are completely H2-dominated.