C and N abundances of MS and SGB stars in NGC 1851
  We present the first chemical analysis of stars on the double subgiant branch
(SGB) of the globular cluster NGC 1851. We obtained 48 Magellan IMACS spectra
of subgiants and fainter stars covering the spectral region between
3650-6750\AA, to derive C and N abundances from the spectral features at
4300\AA (G-band) and at ~ 3883\AA (CN). We added to our sample ~ 45 unvevolved
stars previously observed with FORS2 at the VLT. These two datasets were
homogeneously reduced and analyzed. We derived abundances of C and N for a
total of 64 stars and found considerable star-to-star variations in both [C/H]
and [N/H] at all luminosities extending to the red giant branch (RGB) base
(V~18.9). These abundances appear to be strongly anticorrelated, as would be
expected from the CN-cycle enrichment, but we did not detect any bimodality in
the C or N content. We used HST and ground-based photometry to select two
groups of faint- and bright-SGB stars from the visual and Str\"omgren
color-magnitude diagrams. Significant variations in the carbon and nitrogen
abundances are present among stars of each group, which indicates that each SGB
hosts multiple subgenerations of stars. Bright- and faint-SGB stars differ in
the total C+N content, where the fainter SGB have about 2.5 times the C+N
content of the brighter ones. Coupling our results with literature photometric
data and abundance determinations from high-resolution studies, we identify the
fainter SGB with the red-RGB population, which also should be richer on average
in Ba and other s-process elements, as well as in Na and N, when compared to
brighter SGB and the blue-RGB population.
