Pressure Distortion of the H$_2$-He Collision-Induced Absorption at the Photosphere of Cool White Dwarf Stars

TL;DR

This study uses advanced quantum chemistry methods to show that pressure significantly distorts the collision-induced absorption of molecular hydrogen in cool white dwarf atmospheres, affecting their infrared spectra.

Contribution

It provides the first ab initio simulation of CIA opacity under extreme high-density conditions relevant to white dwarf atmospheres, revealing significant distortions not captured by previous models.

Findings

CIA profiles are significantly distorted above 0.1 g/cm³ density.

The roto-translational band is enhanced and shifted to higher frequencies.

The vibrational band is blueward shifted, split, and less intense.

Abstract

Collision-induced absorption (CIA) from molecular hydrogen is a dominant opacity source in the atmosphere of cool white dwarfs. It results in a significant flux depletion in the near-IR and IR parts of their spectra. Because of the extreme conditions of helium-rich atmospheres (where the density can be as high as a few g/cm$^3$), this opacity source is expected to undergo strong pressure distortion and the currently used opacities have not been validated at such extreme conditions. To check the distortion of the CIA opacity we applied state-of-the-art ab initio methods of computational quantum chemistry to simulate the CIA opacity at high densities. The results show that the CIA profiles are significantly distorted above densities of $0.1\,{\rm g/cm}^3$ in a way that is not captured by the existing models. The roto-translational band is enhanced and shifted to higher frequencies as an effect of the decrease of the interatomic separation of the H$_2$ molecule. The vibrational band is blueward shifted and split into $Q_R$ and $Q_P$ branches, separated by a pronounced interference dip. Its intensity is also substantially reduced. The distortions result in a shift of the maximum of the absorption from $2.3\,\mu{\rm m}$ to $3-7 \mu{\rm m}$, which could potentially explain the spectra of some very cool, helium-rich white dwarfs.