The origin of peculiar molecular bands in cool DQ white dwarfs

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 $\rm C_2$.