EXOA8

The key role that hydrodynamic escape has on the mass loss of the planets and their evolution is well known. This is particularly important for close-in planets because the extreme stellar XUV irradiation they receive causes them to undergo hydrodynamic atmospheric escape. Although the mass loss caused by this mechanism may not be high enough to significantly alter the state of hot Jupiters, it strongly affects the evolution of lower-mass planets. For the latter, atmospheric escape drives and controls their evolution, shaping our currently observed exoplanet population. The whole process, from stellar irradiation to the planet, is not currently well understood, mainly because of the scarcity of appropriate observations. 

Recently though, high-resolution absorption measurements of the metastable He I triplet state at 10830 Å have become available. These have opened a new window to study the upper atmospheres of exoplanets. In particular, the CARMENES high-resolution spectrograph at CAHA has provided in the last few years He I triplet absorption measurements of about ten diverse exoplanets. 

This talk will present those observations and a thorough analysis of them. In particular, we will show results about key parameters of those planets' upper atmospheres like their mass-loss rates, H/He abundances, and their diverse hydrodynamic escape regimes. The number of planets observed is already large enough that we can draw some general conclusions.

How to cite: López-Puertas, M., Lampón, M., Sanz-Forcada, J., Czesla, S., Sánchez-López, A., Molaverdikhani, K., Nortmann, L., Orell-Miquel, J., and CARMENES Consortium, T.: On the study of atmospheric escape of exoplanets using the new window of the He 10830 line, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-1011, https://doi.org/10.5194/epsc2022-1011, 2022.

With over 5000 confirmed exoplanetary detections so far, the characterisation of these new worlds is arguably more important than ever. The imminent arrival of the first JWST observations, followed by the launch of the exoplanet-dedicated Ariel mission towards the end of the decade, means that the key goal of the exoplanet scientific community is ensuring that we can extract as much information from these observations as possible. In recent years, attention has been brought to the importance of accurately characterising exoplanet host stars. Excluding the small number of planets that are favourable to direct imaging, the star-planet system is observed as a blended source. These intertwined planetary and stellar signals can only be successfully disentangled with sufficiently complex stellar models and external, supplementary observations of the stellar properties. One of the largest ways in which the host star can contaminate exoplanet transmission spectra is tied to its activity.  Active F-M dwarfs are expected to be characterised by heterogeneous photospheres due to the presence of cool spots and hot faculae generated by their magnetic fields. These active regions result in contamination regardless of if they are occulted or unocculted. Their presence causes the transit chord to deviate from the pre-transit disk integrated stellar spectrum, the assumed light source for transmission spectroscopy. This contamination also varies strongly as a function of wavelength with the strongest contamination observed in the optical regime (Fig. 1). Inaccurate or incomplete treatments of the star can therefore lead to significant biases in the retrieved planetary and atmospheric parameters.

In contrast to unocculted features, occulted features are much easier to identify and correct for at the light curve fitting stage. This work focuses on quantifying the effects of unocculted spots and faculae by using the exoplanet retrieval code TauREx 3 to fit for parameters related to stellar activity, the covering fractions and temperatures of spots/faculae, for both existing observations and for JWST and Ariel simulations. Subsequently, the probability of the host star being active can be constrained through analysis of the Bayes factor. Overall, it is still rare that stellar parameters are considered in retrievals and this work firmly advocates for a greater exploration of combined stellar and planetary retrievals. The efficacy of the activity plugin is demonstrated through simulations, for which the input stellar parameters are already known. The plugin has also been used extensively to analyse HST STIS spectra for WASP-17b (Saba et al. 2022).

After demonstrating a simplified, proof-of-concept correction method for stellar activity in TauREx 3, subsequent focus has been on implementing a more realistic stellar model in order to better capture the degeneracies that spots may introduce. This is being conducted in collaboration with the INAF Palermo Royal Observatory with the model taking into account the increased complexity of limb darkening and the use of fixed spot positions on the stellar disk (Fig. 2). A comparison between the two models will be presented to quantify the benefits of using a more realistic stellar model and validate the increased complexity with a particular focus on JWST and Ariel simulations.

Fig. 1 - Forward model transmission spectra for a hot-Jupiter (R_P = 1 R_Jup , M_P = 1 M_Jup , T_P = 1000K, log(H₂O)= -3) orbiting a K5V host star with a photospheric temperature of 4450K. The models show how the stellar contamination varies as a function of increasing covering fractions of spots (3480K) or faculae (4550K). Temperatures of the three stellar components (spots, faculae and quiet photosphere) are taken from Rackham et al. (2018).

Fig. 2 – Left: The position of two large spots on the photosphere with co-ordinates of 10°N, 10°E and 30°N, 30°E and radii of 0.1R_* and 0.2 R_* respectively. The temperature contrast between the spots and the photosphere is 1000K. Right: The effect of these two spots on the intensity profiles at 1.7μm (orange) and 5.2μm (blue). The impact of the spots is greater at shorter wavelengths due to the heightened effects of limb darkening.

How to cite: Thompson, A., Saba, A., Changeat, Q., Tinetti, G., Cracchiolo, G., and Micela, G.: Characterising the Effects of Active Host Stars on Exoplanet Transmission Spectra in Retrieval, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-266, https://doi.org/10.5194/epsc2022-266, 2022.

On our search for habitable worlds, we have to account for explosive stellar flaring and coronal mass ejections (CMEs) impacting exoplanets. These stellar outbursts are a double-edged sword. On the one hand, flares and CMEs are capable of stripping off atmospheres and extinguishing existing biology. On the other hand, flares might be the (only) means to deliver the trigger energy for prebiotic chemistry and initiate life. This talk will highlight our study of all stellar flares from the TESS primary mission, driven by a convolutional neural network. I will discuss our new insights on flaring as a function of stellar type, age, rotation, spot coverage, and other factors. Most importantly, I will link our findings to prebiotic chemistry and ozone sterilisation, identifying which worlds might lie in the sweet spot between too much and too little flaring. With future extended missions and increased coverage, flare studies and new exoplanet discoveries will ultimately aid in defining criteria for habitability.

How to cite: Günther, M. N.: Stellar Flares and Habitable(?) Worlds from the TESS Primary Mission, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-1186, https://doi.org/10.5194/epsc2022-1186, 2022.

Close-in exoplanets can interact with their host stars magnetically, producing a variety of observable signatures at different wavelengths. For these interactions to occur, the planet must orbit inside the Alfvén surface of the stellar wind plasma, the region where magnetic forces dominate. As it is not generally possible to measure the plasma properties of the stellar winds of low-mass stars, the location of the Alfvén surface cannot be determined from observations. However, by coupling both observationally-derived magnetic field maps of the stellar surface and constraints on the stellar wind mass-loss rate with sophisticated magnetohydrodynamic models, we can obtain a 3D picture of the stellar wind plasma. This allows us to determine both the size and shape of the Alfvén surface, which in turn can be used to assess the feasibility of magnetic star-planet interactions occurring. In this talk, I will discuss how this approach allows us to predict and interpret hints of star-planet interactions from radio to X-ray wavelengths. I will illustrate how obtaining near-simultaneous observations at these wavelengths is our best bet for benchmarking these magnetohydrodynamic models.

How to cite: Kavanagh, R., Vidotto, A., Vedantham, H., Jardine, M., Klein, B., Callingham, J., and Morin, J.: Signatures of star-planet interactions across the electromagnetic spectrum, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-1173, https://doi.org/10.5194/epsc2022-1173, 2022.

The warm- and hot-Neptune-type planets are known to experience significant atmospheric mass loss and are easier to observe and characterize in comparison to terrestrial-like planets. They represent, therefore, interesting targets in the context of planetary atmospheric evolution and star-planet interaction. The majority of studies, however, employ hydrodynamic simulations relying on simplified approaches in describing photoionization processes, or, on the other hand, use high-level photoionization solvers not accounting for dynamical effects.
In this study, we aim to outline for which types of planets in the Neptune-like mass range the non-LTE effects become important (in particular - in terms of atmospheric mass loss) and to which extent the hydrodynamic effects (as adiabatic expansion) influence the theoretically predicted transmission spectra.
To achieve this goal, we combine our 1D hydrodynamic upper atmosphere model (Kubyshkina et al., 2018) with the latest version of the non-LTE photoionization and radiative transfer code Cloudy (Ferland et al., 2017), which accounts for ionization and dissociation, atomic level transitions and chemical reactions for the lightest chemical elements up to zink. Thus, the former is responsible for resolving the hydrodynamic outflow and the latter solves the realistic photoionization and heating of the planetary atmosphere. We verify this scheme by comparing our results for a dozen of the high profile planets with the predictions of Salz et al., 2016, which use a similar approach.
We apply this hybrid framework to model the upper atmospheres of a range of Neptune-like and terrestrial-like planets with masses between 1 and 50 Mearth, changing systematically the orbital parameters (and thus, equilibrium temperature), and the irradiation level from the host star. We find, that for the majority of warm and hot Neptunes, accounting for realistic non-LTE ionization/heating affects significantly the basic parameters of planetary atmospheres, predicting cooler, slower, and denser outflow compared to the predictions of the pure hydrodynamic model. The predictions of the hydrodynamic and hybrid models become closer at the highest levels of stellar radiation.
In turn, accounting for hydrodynamic effects, in particular for expansion cooling, considerably affects the predictions of Cloudy models, and is therefore important for the interpretation of observations.

How to cite: Kubyshkina, D. and Fossati, L.: Non-LTE effects in atmospheres of warm and hot Neptunes: implications for the atmospheric mass loss, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-1241, https://doi.org/10.5194/epsc2022-1241, 2022.