AESOPUS 2.1: Low-Temperature Opacities Extended to High Pressure

TL;DR

This paper extends the AESOPUS opacity database to high-pressure, low-temperature environments, incorporating molecular line broadening, electron degeneracy, and non-ideal effects, enabling more accurate modeling of low-mass stars, brown dwarfs, and exoplanet atmospheres.

Contribution

The study significantly broadens AESOPUS opacity calculations to cover higher pressures and densities, including effects like pressure broadening and electron degeneracy, with new pre-computed tables and an expanded web interface.

Findings

Expanded opacity tables for $ ext{log}(R)$ from -8 to +6.

Inclusion of pressure broadening and non-ideal effects.

Application to low-mass star evolution models.

Abstract

We address the critical need for accurate Rosseland mean gas opacities in high-pressure environments, spanning temperatures from 100 K to 32000 K. Current opacity tables from Wichita State University and AESOPUS 2.0 are limited to $\log(R) \le 1$, where $R=\rho\, T_6^{-3}$ in units of $\mathrm{g}\,\mathrm{cm}^{-3}(10^6\mathrm{K})^{-3}$. This is insufficient for modeling very low-mass stars, brown dwarfs, and planets with atmospheres exhibiting higher densities and pressures ($\log(R) > 1$). Leveraging extensive databases such as ExoMol, ExoMolOP, MoLLIST, and HITEMP, we focus on expanding the AESOPUS opacity calculations to cover a broad range of pressure and density conditions ($-8 \leq \log(R) \leq +6$). We incorporate the thermal Doppler mechanism and micro-turbulence velocity. Pressure broadening effects on molecular transitions, leading to Lorentzian or Voigt profiles, are explored in the context of atmospheric profiles for exoplanets, brown dwarfs, and low-mass stars. We also delve into the impact of electron degeneracy and non-ideal effects such as ionization potential depression under high-density conditions, emphasizing its notable influence on Rosseland mean opacities at temperatures exceeding $10,000$ K. As a result, this study expands AESOPUS public web interface for customized gas chemical mixtures, promoting flexibility in opacity calculations based on specific research needs. Additionally, pre-computed opacity tables, inclusive of condensates, are provided. We present a preliminary application to evolutionary models for very low-mass stars.