Carbon and Oxygen Abundances in Low Metallicity Dwarf Galaxies
  The study of carbon and oxygen abundances yields information on the time
evolution and nucleosynthetic origins of these elements, yet remains relatively
unexplored. At low metallicities (12+log(O/H) < 8.0), nebular carbon
measurements are limited to rest-frame UV collisionally excited emission lines.
Therefore, we present UV spectrophotometry of 12 nearby, low-metallicity,
high-ionization HII regions in dwarf galaxies obtained with the Cosmic Origins
Spectrograph on the Hubble Space Telescope. We present the first analysis of
the C/O ratio in local galaxies based solely on simultaneous significant
detections of the UV O^+2 and C^+2 collisionally excited lines in seven of our
targets and five objects from the literature, to create a final sample of 12
significant detections. Our sample is complemented by optical SDSS spectra,
from which we measured the nebular physical conditions and oxygen abundances
using the direct method.
  At low metallicity (12+log(O/H) < 8.0), no clear trend is evident in C/O vs.
O/H for the present sample given the large dispersion observed. When combined
with recombination line observations at higher values of O/H, a general trend
of increasing C/O with increasing O/H is also viable, but with some significant
outliers. Additionally, we find the C/N ratio appears to be constant (but with
significant scatter) over a large range in oxygen abundance, indicating carbon
is predominantly produced by similar nucleosynthetic mechanisms as nitrogen. If
true, and our current understanding of nitrogen production is correct, this
would indicate that primary production of carbon (a flat trend) dominates at
low metallicity, but quasi-secondary production (an increasing trend) becomes
prominent at higher metallicities. A larger sample will be needed to determine
the true nature and dispersion of the relation.
