Alkali Line Profiles in Degenerate Dwarfs

TL;DR

This paper investigates the broadening of alkali lines in ultracool dwarf atmospheres, emphasizing the importance of multiple-perturber interactions for accurate spectral modeling of the coolest, densest objects.

Contribution

It introduces a unified theory approach to accurately model alkali line profiles affected by multiple perturbers in ultracool dwarf atmospheres, surpassing previous density expansion methods.

Findings

Line profiles are dominated by multiple-perturber interactions.

Current models show missing opacity in the far red wings of K I lines.

Unified theory calculations improve spectral models for ultracool dwarfs.

Abstract

Ultracool stellar atmospheres show absorption by alkali resonance lines severely broadened by collisions with neutral perturbers. In the coolest and densest atmospheres, such as those of T dwarfs, Na I and K I broadened by molecular hydrogen and helium can come to dominate the entire optical spectrum. Their profiles have been successfully modelled with accurate interaction potentials in the adiabatic theory, computing line profiles from the first few orders of a density expansion of the autocorrelation function. The line shapes in the emergent spectrum also depend on the distribution of absorbers as a function of depth, which can be modelled with improved accuracy by new models of dust condensation and settling. The far red K I wings of the latest T dwarfs still show missing opacity in these models, a phenomenon similar to what has been found for the Na I line profiles observed in extremely cool, metal-rich white dwarfs. We show that the line profile in both cases is strongly determined by multiple-perturber interactions at short distances and can no longer be reproduced by a density expansion, but requires calculation of the full profile in a unified theory. Including such line profiles in stellar atmosphere codes will further improve models for the coolest and densest dwarfs as well as for the deeper atmosphere layers of substellar objects in general.