A Comparison between Carbon and Nitrogen Abundances of Bright Giants in the Globular Clusters M13, M10, and NGC 7006

Abstract

Carbon and nitrogen abundances have been derived for red giants in the globular clusters M13, M10, and NGC 7006. These constitute a trio of clusters having very similar metallicities ([Fe/H] ≈ -1.55) but different horizontal-branch (HB) morphologies, with NGC 7006 having a much higher fraction of red HB stars than either M13 or M10. The red giants observed have absolute magnitudes in the range -2.5 < MV < 0.7 in the case of M13, -1.8 < MV < 0.4 for M10, and -2.1 < MV < -1.2 for the more distant NGC 7006. Most of the spectra were obtained with the Lick Observatory 3 m Shane reflector, supplemented by observations of M10 with the McDonald Observatory 2.7 m telescope. These data were reduced in a homogeneous manner and compared with synthetic spectra computed from model atmospheres in order to derive [C/Fe] abundances from the λ4300 G band and [N/Fe] abundances from the λ3883 CN band. The giants in all three clusters appear to exhibit a common relationship between [C/Fe] and MV in which the carbon abundance becomes increasingly diminished with increasing luminosity on the giant branch. Such behavior was previously discovered for M13 by Suntzeff and is known to occur in other low-metallicity globular clusters, such as M3, M92, and M15, as well as among halo field red giants. The phenomenon is widely recognized as indicating a progressive dredge-up of CNO-processed material from regions close to the H-burning shell within Population II giants. The similarities between the [C/Fe]-MV trends in the three second-parameter clusters indicates that the deep-mixing mechanism is relatively uncorrelated with cluster HB type. This result may be inconsistent with scenarios that rely on rotation to both drive deep mixing within metal-poor red giants and to set the eventual zero-age HB position of these stars, unless there is a threshold for rotation-induced mixing that is well below that needed to delay the He-flash. The nitrogen abundance is found to differ by as much as 1.0 dex between giants in the program clusters and to be anticorrelated with the carbon abundance. It is speculated that the behavior of [N/Fe] with respect to MV can be interpreted as a combination of deep mixing and primordial contributions.