The origin of peculiar molecular bands in cool DQ white dwarfs

TL;DR

This study investigates the cause of blueshifted molecular bands in cool DQ white dwarfs, proposing that pressure-induced shifts of C2 molecules in dense helium atmospheres explain the observed spectral features.

Contribution

The paper demonstrates that increased helium density causes electronic transition energy shifts in C2, leading to the observed spectral blueshift in cool DQ white dwarfs.

Findings

Electronic transition energy T_e increases with helium density.

Pressure shifts of C2 bands explain observed spectral blueshifts.

Vibrational frequency changes are insufficient to account for shifts.

Abstract

The DQ white dwarfs are stars whose atmosphere is enriched with carbon, which for cool stars ($T_{\rm eff}<8000\rm \, K$) is indicated by the Swan bands of $\rm C_2$ in the optical part of their spectra. With decreasing effective temperature these molecular bands undergo a significant blueshift ($\sim 100-300 \AA$). The origin of this phenomenon has been disputed over the last two decades and has remained unknown. We attempt to address this problem by investigating the impact of dense helium on the spectroscopic properties of molecular carbon under the physical conditions encountered inside helium-rich, fluid-like atmospheres of cool DQ white dwarfs. We found that the electronic transition energy $T_e$ increases monotonically with the helium density ($\Delta T_{\rm e}\rm\, (eV)\sim1.6 \, \it \rho \rm \, (g/cm^3)$). This causes the Swan absorption to occur at shorter wavelengths compared with unperturbed $\rm C_2$. On the other hand the pressure-induced increase in the vibrational frequency is insufficient to account for the observed Swan bands shifts. This is consistent with the observations and indicates that the observed Swan-like molecular bands are most likely the pressure-shifted bands of $\rm C_2$.