C_2 in Peculiar DQ White Dwarfs

TL;DR

This paper investigates the origin of peculiar absorption bands in cool DQ white dwarfs, proposing they are pressure-shifted C_2 Swan bands or highly rotationally excited C_2, which can help constrain atmospheric conditions.

Contribution

It introduces the hypothesis that peculiar DQ bands are pressure-shifted or rotationally excited C_2, challenging previous attributions to hydrocarbons like C_2H and suggesting new avenues for atmospheric modeling.

Findings

Peculiar DQ bands are unlikely due to C_2H or other simple molecules.

Pressure-shifted C_2 Swan bands are a plausible explanation for the peculiar bands.

Rotational excitation of C_2 could also explain the observed features.

Abstract

White dwarfs (WDs) with carbon absorption features in their optical spectra are known as DQ WDs. The subclass of peculiar DQ WDs are cool objects (T_eff<6000 K) which show molecular absorption bands that have centroid wavelengths ~100-300 Angstroms shortward of the bandheads of the C_2 Swan bands. These "peculiar DQ bands" have been attributed to a hydrocarbon such as C_2H. We point out that C_2H does not show strong absorption bands with wavelengths matching those of the peculiar DQ bands and neither does any other simple molecule or ion likely to be present in a cool WD atmosphere. The most straightforward explanation for the peculiar DQ bands is that they are pressure-shifted Swan bands of C_2. While current models of WD atmospheres suggest that, in general, peculiar DQ WDs do not have higher photospheric pressures than normal DQ WDs do, that finding requires confirmation by improved models of WD atmospheres and of the behavior of C_2 at high pressures and temperatures. If it is eventually shown that the peculiar DQ bands cannot be explained as pressure-shifted Swan bands, the only explanation remaining would seem to be that they arise from highly rotationally excited C_2 (J_peak>45). In either case, the absorption band profiles can in principle be used to constrain the pressure and the rotational temperature of C_2 in the line-forming regions of normal and peculiar DQ WD atmospheres, which will be useful for comparison with models. Finally, we note that progress in understanding magnetic DQ WDs may require models which simultaneously consider magnetic fields, high pressures and rotational excitation of C_2.