The origin of peculiar molecular bands in cool DQ white dwarfs

Aims. The DQ white dwarfs are stars whose atmosphere is enriched with carbon, which for cool stars (Teff < 8000 K) is indicated by the Swan bands of C2 in the optical part of their spectra. With decreasing effective temperature these molecular bands undergo a significant blueshift (∼100–300 Å). 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, the electronic Swan band transition energy Te and the vibrational frequency ωe, under the physical conditions encountered inside helium-rich, fluid-like atmospheres of cool DQ white dwarfs. Methods. In our investigation we use a density functional theory based quantum mechanical approach. Results. The electronic transition energy Te increases monotonically with the helium density (ΔTe (eV) ∼ 1.6 ρ (g/cm3)). This causes the Swan absorption to occur at shorter wavelengths compared with unperturbed C2. On the other hand the pressure-induced increase in the vibrational frequency is insufficient to account for the observed Swan bands shifts. Our findings are in line with the shape of the distorted molecular bands observed in DQp stars, but the predicted photospheric density required to reproduce these spectral features is one order of magnitude lower than the one predicted by the current models. This indicates pollution by hydrogen or reflects incomplete knowledge of the properties of fluid-like atmospheres of these stars. Conclusions. Our work shows that at the physical conditions encountered in the fluid-like atmospheres of cool DQ white dwarfs the strong interactions between C2 and helium atoms cause an increase in Te, which should produce a blueward shift of the Swan bands. This is consistent with the observations and indicates that the observed Swan-like molecular bands are most likely the pressure-shifted bands of C2.