Ultraviolet nebular emission lines are important for understanding the time evolution and nucleosynthetic origins of their associated elements , but the underlying trends of their relative abundances are unclear .
We present UV spectroscopy of 20 nearby low-metallicity , high-ionization dwarf galaxies obtained using the Hubble Space Telescope .
Building upon previous studies , we analyze the C/O relationship for a combined sample of 40 galaxies with significant detections of the UV O ^ { +2 } /C ^ { +2 } collisionally-excited lines and direct-method oxygen abundance measurements .
Using new analytic carbon ionization correction factor relationships , we confirm the flat trend in C/O versus O/H observed for local metal-poor galaxies .
We find an average { log ( C / O ) } = -0.71 with an intrinsic dispersion of \sigma = 0.17 dex .
The C/N ratio also appears to be constant at { log ( C / N ) } = 0.75 , plus significant scatter ( \sigma = 0.20 dex ) , with the result that carbon and nitrogen show similar evolutionary trends .
This large and real scatter in C/O over a large range in O/H implies that measuring the UV C and O emission lines alone does not provide a reliable indicator of the O/H abundance .
By modeling the chemical evolution of C , N , and O of individual targets , we find that the C/O ratio is very sensitive to both the detailed star formation history and to supernova feedback .
Longer burst durations and lower star formation efficiencies correspond to low C/O ratios , while the escape of oxygen atoms in supernovae winds produces decreased effective oxygen yields and larger C/O ratios .
Further , a declining C/O relationship is seen with increasing baryonic mass due to increasing effective oxygen yields .