Consistent Gas-phase C/O Abundances from UV and Optical Emission Lines: A Robust Scale for Chemical Evolution across Cosmic Time

The carbon-to-oxygen (C/O) abundance ratio is a valuable tracer of star formation history, as C and O enrichment occurs on different timescales. However, measurements based on ultraviolet (UV) collisionally excited lines and those based on optical recombination lines may be subject to biases from the abundance discrepancy factor (ADF), which is well established for oxygen but uncertain for carbon. We present precise UV-based measurements of gas-phase C²⁺/O²⁺ ionic abundance in four H ii regions, which have prior optical-based measurements, combined with archival UV data for two additional H II regions, in order to establish a reliable abundance scale and to investigate biases between the two methods. We find a clear ADF for the C²⁺ ion, which is consistent with that of O²⁺, assuming a similar temperature structure in the zones of the nebula that these ions occupy. The C/O abundance derived from UV collisional lines and optical recombination lines is therefore also consistent to within <0.1 dex, with an offset of 0.05 ± 0.03 dex in C²⁺/O²⁺ for the standard T_e method. While the absolute C/H and O/H abundances are subject to large uncertainty from the ADF, our results establish that C/O abundances measured from these different methods can be reliably compared. Thus, we confirm the robustness of gas-phase C/O measurements for studying galaxy evolution and star formation timescales, including from rest-UV observations of high-redshift galaxies with JWST.