Pressure Distortion of the H₂–He Collision-induced Absorption at the Photosphere of Cool White Dwarf Stars

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 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 µm to 3 − 7µm, which could potentially explain the spectra of some very cool, helium-rich white dwarfs.

A New Generation of Cool White Dwarf Atmosphere Models. III. WD J2356–209: Accretion of a Planetesimal with an Unusual Composition

“…In fact, as already noted in Paper II, the profiles of Jørgensen et al (2000) predict a too strong absorption in the ≈ 1.2 − 2 µm region, possibly because the potential energy and induced dipole surfaces used to derive those profiles were computed with a smaller atomic orbital basis set. Furthermore, Figure 3 shows that significant differences between the profiles of Abel et al (2012) and Blouin et al (2017) appear above 2 µm (particularly near the maximum of the fundamental band at ≈ 2.5 µm). Those differences are due to many-body collisions (which are only included in the profiles of Blouin et al) that lead to an enhancement and a distortion of the absorption profile.…”

The Gaia 20 pc white dwarf sample

“…We currently do not know the reason for this behaviour that was also observed by Gaia Collaboration et al (2018b). In this regime, the effects of collision-induced-absorption and the red-wing of the Lyα line become important for the optical colours (Borysow et al 2001;Kowalski & Saumon 2006;Blouin et al 2017).…”

The Enigmatic Brown Dwarf WISEA J153429.75-104303.3 (a.k.a. “The Accident”)

“…However, a 1950K blackbody would have its peak flux near 1.5 μm, which does not match the photometry of WISE 1534−1043. Although white dwarfs below ∼4000K have strong collision-induced absorption, reshaping their spectral energy distributions markedly away from a blackbody (Lenzuni et al 1991), this cannot explain the colors seen in WISE 1534−1043 (e.g., Blouin et al 2017. ) Besides, theoretical models predict that it is not possible for a white dwarf to have cooled to this T eff considering the age of the Milky Way (e.g., Calcaferro et al 2018).…”