# Self-luminous and irradiated exoplanetary atmospheres explored with HELIOS

## TL;DR

This paper enhances the HELIOS radiative transfer code for exoplanet atmospheres, validating it against established models and exploring the effects of various physical ingredients on temperature profiles and spectra.

## Contribution

The authors introduce new features in HELIOS, including improved formalism and physical ingredients, and provide a comprehensive comparison with other models, along with applications to exoplanet atmospheric predictions.

## Key findings

- Temperature-pressure profiles are highly sensitive to included opacities.
- Stellar path length adjustment significantly affects upper atmospheric temperatures.
- Scattering correction improves reflected light calculations by ~10%.

## Abstract

We present new methodological features and physical ingredients included in the 1D radiative transfer code HELIOS, improving the hemispheric two-stream formalism. We conduct a thorough intercomparison survey with several established forward models, including COOLTLUSTY, PHOENIX, and find satisfactory consistency with their results. Then, we explore the impact of (i) different groups of opacity sources, (ii) a stellar path length adjustment, and (iii) a scattering correction on self-consistently calculated atmospheric temperatures and planetary emission spectra. First, we observe that temperature-pressure (T-P) profiles are very sensitive to the opacities included, with metal oxides, hydrides, the alkali atoms (and ionized hydrogen) playing an important role for the absorption of shortwave radiation (in very hot surroundings). Moreover, if these species are sufficiently abundant, they are likely to induce non-monotonic T-P profiles. Second, without the stellar path length adjustment, the incoming stellar flux is significantly underestimated for zenith angles above 80{\deg}, which somewhat affects the upper atmospheric temperatures and the planetary emission. Third, the scattering correction improves the accuracy of the computation of the reflected stellar light by ~10%. We use HELIOS to calculate a grid of cloud-free atmospheres in radiative-convective equilibrium for self-luminous planets for a range of effective temperatures, surface gravities, metallicities, and C/O ratios, to be used by planetary evolution studies. Furthermore, we calculate dayside temperatures and secondary eclipse spectra for a sample of exoplanets for varying chemistry and heat redistribution. These results may be used to make predictions on the feasibility of atmospheric characterizations with future observations.