Limits on the12C to13C isotopic ratio in two white dwarfs

Abstract

A search for the molecular12C13C isotopic bands in two white dwarfs is described. Spectroscopic observations of the two carbon band white dwarfs, BPM 27606 (=2153-51) andL 879-14 (=0435-08) have been obtained. These data have a resolution slightly better than 2 A, higher than usually employed for white dwarfs and cover the Δv=1 vibrational swan bands of the C2 molecule where the isotopic shift is of order 8 A. The isotopic bands have not been detected. However, upper limits to the12C:13C abundance ratio derived from the data yield12C:13C>40 for BPM 27606 and12C:13C>8 for L879-14. If these ratios are representative of the material in the interiors of the carbon band white dwarfs and taking the low upper limits on the N and O abundances relative to C, this is consistent with the carbon having been formed by the 3α process and not having undergone any subsequent mixing with H-rich material. In addition, the C2 vibrational bandheads are blueshifted, most probably by pressure shifts. This shows that the employment of higher resolutions reveals additional physical effects in the spectra of the carbon band white dwarfs that may become important for interpreting the cool end of the helium rich white dwarf sequence.