Revealing Anisotropic Evaporating Exoplanet Atmospheres

Fabienne Nail , Antonija Oklopčić ¹ , Morgan MacLeod ²

Abstract

Exoplanets that orbit their host stars closely are exposed to an extreme environment: high temperatures, intense radiation, and in some cases strong stellar winds. The high temperatures may lead to a hydrodynamic escape of the planet's atmosphere. This process can have major implications for the evolution of close-in planets and has been proposed as an underlying cause of the observed lack of short-period Neptunes. Therefore, it is important to investigate the nature of this phenomenon and its spectroscopic signatures during planetary transits in more detail. In this work, we investigate how the interaction between the planetary and stellar wind affects the properties of the observed He-1083 nm line, which has been shown to be a powerful tracer of evaporating atmospheres. We focus on anisotropic atmospheric escape caused by a combination of dayside-nightside temperature contrast and a global planetary magnetic field. Using 3D hydrodynamic simulations, we investigate how the stellar wind shapes the planetary outflows with varying degrees of day-night and equator-pole anisotropy and generate synthetic transmission spectra and light curves in the He-1083 nm line. For day-to-night anisotropic structures, we find an overall blue shift of the helium line center of a few km/s compared to the isotropic case. Furthermore, we find a dependency of the blueshift on the degree of assumed anisotropy. The shape of the helium light curve is also affected, as the egress shows a higher absorption for a stronger day-night contrast. These findings may enable identifying avenues for the characterization of asymmetric outflow geometries, including potentially those governed by the presence of large-scale planetary magnetic fields, through helium observations.

Helium Blue-Shift and Day/Night Anisotropy

Based on our simulations, we found a relation between the mid-transit velocity shift of the helium line at 1083 nm and the degree of day-to-night side anisotropy.

**Fig. 1** - Mid-transit velocity shift of helium-line centroid as a function of night-to-day side anisotropy (left). The results are based on 3D hydrodynamic simulations. The points toward the left side correspond to models in which the pressure on the night side is a smaller fraction of the dayside pressure, corresponding to a larger day-night anisotropy. Velocity shifts measured for planets with helium detections are marked in the right panel. (No uncertainties were provided for HD 73583 b and HAT-P-11 b.)

This result suggests that measuring the precise wavelength shifts can be used to constrain the day-to-night side pressure and temperature contrast, and thus probe the efficiency of atmospheric circulation and heat transport in the upper atmosphere.

References:
[1] Kirk et al. 2022, arXiv:2205.11579; [2] Zhang et al. 2022a, AJ, 163, 67; [3] Salz et al. 2018, A&A, 620, A97; [4] Nortmann et al. 2018, Science, 362, 1388; [5] Alonso-Floriano et al. 2019, A&A 629, A110; [6] Allart et al. 2018, Science, 362, 1384; [7] Kirk et al. 2020, AJ, 159, 115; [8] Palle et al. 2020, A&A, 638, A61; [9] Owen 2019, Annu. Rev. Earth Planet. Sci., 47, 67; [10] Szabó & Kiss 2011, ApJL, 727, L44; [11] Owen & Wu 2013, ApJ, 775, 105; [12] Fulton et al. 2017, AJ, 154, 109; [13] Vidal-Madjar et al. 2003, Nature, 422, 143; [14] Lecavelier Des Etangs et al. 2010, A&A, 514, A72; [15] Ehrenreich et al. 2015, Nature, 522, 459; [16] Oklopčić & Hirata 2018, ApJ, 855, L11; [17] MacLeod & Oklopčić 2022, ApJ, 926, 226.

Simulation

For more information scan the QR code (or go to MacLeod & Oklopčić 2022).

3D Hydrodynamic Simulations with Athena++

Instead of simulating the wind launching and atmospheric heating self-consistently, we parameterize the planetary wind hydrodynamically, to remain agnostic about the wind-driving mechanisms. This means that we do not generate the wind based on the physical conditions, but embed it and its shape "by hand" in the simulation.

We assume a tidally-locked planet (super-Neptune) orbiting a K-type star. The simulation parameters are based on the system WASP-107.

M_star= 0.68 M_sun, R_star= 0.67 R_sun,
M_p= 0.096 M_J, R_p= 0.94 R_J.

The same setup is used in the work of MacLeod & Oklopčić (2022) [17], where they investigate the effects of the stellar wind strength on the shape of the planetary outflow. There, they assume an isotropic planetary wind.

Anisotropic Outflow

Close-in and tidally locked planets should have an enormous temperature difference between day and night side.

Therefore, we parameterized an anisotropic planetary wind that takes this temperature difference into account. For this purpose, we have created different scenarios where the night side pressure of the wind is reduced compared to the maximum pressure on the day side (substellar point).

The highest degree of anisotropy we investigate is when the pressure on the night side reaches 1% of the day side pressure. To investigate the dependence of the degree of anisotropy and the observational helium signatures, we further parametrize cases in steps of 10% (e.g. 10%, 20%, ..., 90% ratio of night-to-day side pressure, see Fig. 1).

Fig. 3 shows the density distribution of such an anisotropic simulation. It can be seen that the wind is stronger on the day side due to the pressure difference.

In our future work, we will also parameterize an equator-pole anisotropy to model magnetically-controlled outflows.

Blue Shift

**Fig. 4** - Line-of-sight optical depth and velocity of He-1083 nm (observer left, star right).

**Fig. 5** - Time series of synthetic transmission spectra of the helium triplet (stellar rest frame).

Blue Shift of He-1083nm

The helium line forms in the vicinity of the planet, where the planetary wind is still unshocked by the stellar wind, as shown in Fig. 4 (see contour lines for optical depth). The star is located in the positive x-direction, the observer in the negative x-direction.

The gas on the day side flows to the night side to compensate for the lower pressure given by the parameterized anisotropy.

This causes the gas to move towards the observer and the helium line appears blue-shifted (see Fig. 5). This effect is stronger for a higher degree of anisotropy between day and night side (see Fig. 1).

Motivation

**Fig. 2** - Planetary occurrence rate corrected for observational bias[9].

Hot-Neptune Desert and Radius Valley

Exoplanets that orbit their host stars closely are exposed to extreme environments of intense stellar radiation and stellar winds. The resulting high atmospheric temperatures may lead to hydrodynamic escape of the planet’s atmosphere.

The intense mass loss can have major implications for the evolution of close-in planets and has been proposed as an underlying cause of the observed lack of hot Neptunes [10] and the bimodal radius distribution [11, 12] (see Fig. 2).

To fully assess the importance of atmospheric escape for the evolution of close-in exoplanets, it is critical to improve our understanding of the nature of this phenomenon.

Helium triplet at 1083 nm

Extended and evaporating atmospheres have been observed through high-resolution transmission spectroscopy at wavelengths of strong atomic lines, such as Lyman-alpha (e.g. [13], [14], [15]).

The helium triplet at 1083 nm also performs well as a diagnostic for low-density regions ([4], [6], [3], [16]).

The feature lies in the infrared and is not as strongly absorbed by the interstellar medium as the Lyman-alpha line.

Our work aims to find observational signatures in the He-1083 nm line that will allow us to characterize evaporating exoplanet atmospheres and thus understand the nature of atmospheric escape more precisely.

Helium Blue-Shift and Day/Night Anisotropy

Simulation

Contact

Blue Shift

Motivation