Limits on the12C to13C isotopic ratio in two white dwarfs

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 Å, higher than usually employed for white dwarfs and cover the v=1 vibrational swan bands of the C₂ molecule where the isotopic shift is of order 8 Å. 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 3 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.