2006PASP..118..740S

The globular cluster Messier 13 (NGC 6205) has been shown by a number of different studies to be chemically inhomogeneous with regard to elements ranging from C through Al. A database of λ3883 CN-band indices, carbon, oxygen, and sodium abundances for red giants in M13 has been compiled and homogenized from a variety of literature sources. The data are used to document the distribution of CN band strength among both red giant branch (RGB) and asymptotic giant branch (AGB) stars brighter than M_V=+0.8, as well as the relationships between the CN inhomogeneities and the dispersions in other elements. The CN distribution among the M13 RGB stars is bimodal, although a few intermediate-CN red giants are present in the cluster. The AGB stars show uniformly weak CN bands. The spread in CN band strength, once empirically corrected for effective temperature and gravity differences among the red giants, is very well correlated with the sodium abundance and is anticorrelated with oxygen. Above M_V=+0.8, the carbon abundance of red giants decreases with increasing luminosity, as first found by N. Suntzeff. There is a rather modest anticorrelation between CN band strength and [C/Fe] among RGB stars of comparable magnitude. The abundance patterns within M13 can be interpreted as a primordial (or pre-RGB phase) abundance spread, coupled with the later effects of deep mixing within the red giants. In the CN-weak giants and those CN-strong stars with [O/Fe]~-0.2±0.2 dex on the upper RGB, deep mixing seems to have been limited largely to the dredge-up of C⟶N-processed material. By contrast, there are some M13 stars, most notably a group of very oxygen-deficient giants near the RGB tip, but perhaps also a fraction of CN-strong giants with lesser oxygen depletions, in which some measure of O⟶N-processed material also appears to have been brought to the surface.