Keck HIRES Spectroscopy of Extragalactic HII Regions: C and O Abundances
  from Recombination Lines
  We present very deep spectrophotometry of 14 bright extragalactic HII regions
belonging to spiral, irregular, and blue compact galaxies. The data for 13
objects were taken with the HIRES echelle spectrograph on the Keck I telescope.
We have measured CII recombination lines in 10 of the objects and OII
recombination lines in 8 of them. We have determined electron temperatures from
line ratios of several ions, specially of low ionization potential ones. We
have found a rather tight linear empirical relation between Te([NII]) and
Te([OIII]). We have found that OII lines give always larger abundances than
[OIII] lines. Moreover, the difference of both O++ abundance determinations
--the so-called abundance discrepancy factor-- is very similar in all the
objects, with a mean value of 0.26+/-0.09 dex, independently of the properties
of the HII region and of the parent galaxy. Using the observed recombination
lines, we have determined the O, C, and C/O radial abundance gradients for 3
spiral galaxies: M33, M101, and NGC2403, finding that C abundance gradients are
always steeper than those of O, producing negative C/O gradients accross the
galactic disks. This result is similar to that found in the Milky Way and has
important implications for chemical evolution models and the nucleosynthesis of
C.
