Abstract This work presents atmospheric modeling of multi-wavelength spectra for eight cool white dwarfs with carbon-enriched atmospheres, with four each of spectral type DQ and DQp. The Deslanders-d’Azambuja bands of C2 are detected in all six DQ stars that have data covering wavelengths shorter than 4000 Å. These bands are blue shifted in the DQp types, consistent with that observed for the Swan bands, and reproduced with the same pressure distortion model. The coolest stars in the sample show significant near-infrared flux suppression, identified here as collision-induced absorption from dense helium, with trace hydrogen H/He −6, estimated from the absence of CH features. Notably, the near-ultraviolet through near-infrared spectral energy distribution of WD 0038−226 is correctly reproduced using an atmospheric model with H/He =−6.8; however, beyond 2 μm the predicted flux is too high, and a carbon-free atmosphere is able to reproduce the mid-infrared observations well. The multi-wavelength coverage permits accurate modeling characterization of these stars, including constraining the ionization equilibrium in fluid helium and the resulting density profiles. The densities found in this work are 2–3 times higher than determined in previous studies, approaching 2 g cm−3 for the coolest stars. Excluding the uncertain high masses obtained for the two coolest DQp stars, the sample stars have masses that are broadly consistent with those found for the wider white dwarf population, with no difference across the DQ-DQp temperature boundary.
Farihi et al. (Thu,) studied this question.