ÆSOPUS 2.1: Low-temperature Opacities Extended to High Pressure

We address the critical need for accurate Rosseland mean gas opacities in high-pressure environments, spanning temperatures from 100 K to 32,000 K. Current opacity tables from Wichita State University and Æ SOPUS 2.0 are limited to log ( R ) ≤ 1 , where R = ρ T 6 − 3 in units of g cm − 3 ( 10 6 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 Æ SOPUS opacity calculations to cover a broad range of pressure and density conditions ( − 8 ≤ log ( R ) ≤ + 6 ). We incorporate the thermal Doppler mechanism and microturbulence 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 nonideal 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 the Æ SOPUS public web interface for customized gas chemical mixtures, promoting flexibility in opacity calculations based on specific research needs. Additionally, precomputed opacity tables, inclusive of condensates, are provided. We present a preliminary application to evolutionary models for very low-mass stars.