CHAOS II: Gas-Phase Abundances in NGC 5194
  We have observed NGC5194 (M51a) as part of the CHemical Abundances of Spirals
(CHAOS) project. Using the Multi Object Double Spectrographs (MODS) on the
Large Binocular Telescope (LBT) we are able to measure one or more of the
temperature-sensitive auroral lines ([O III] 4363, [N II] 5755, [S III] 6312)
and thus measure "direct" gas-phase abundances in 29 individual HII regions. [O
III] 4363 is only detected in two HII regions both of which show indications of
excitation by shocks. We compare our data to previous direct abundances
measured in NGC5194 and find excellent agreement for all but one region
(Delta[log(O/H)] ~ 0.04). We find no evidence of trends in Ar/O, Ne/O, or S/O
within NGC5194 or compared to other galaxies. We find modest negative gradients
in both O/H and N/O with very little scatter (sigma < 0.08 dex), most of which
can be attributed to random error and not to intrinsic dispersion. The
gas-phase abundance gradient is consistent with the gradients observed in other
interacting galaxies, which tend to be shallower than gradients measured in
isolated galaxies. The N/O ratio (<log(N/O)> = -0.62) suggests secondary
nitrogen production is responsible for a significantly larger fraction of
nitrogen (e.g., factor of 8-10) relative to primary production mechanisms than
predicted by theoretical models.
