CN and CH band strengths of 283 47 tucanae giants.
Citations Over Time
Abstract
To characterize further the star-to-star CN and CH band strength variations observed among 47 Tucanae giants, CCD B, V and DDO photometry has been obtained for a 14 x 12 arcminute field centered 8.5 arcminutes NE of the cluster center. From this sample, C(4142) and C(4245) colors have been used to characterize the CN and CH bands of 283 cluster giants down to V = 16 (M_v ~ +2.5). The analysis examines four aspects of the present data: 1) Observed C(4142) and C(4245) colors are compared against model DDO colors as a function of luminosity (evolutionary state). As opposed to the more metal-poor clusters, little change in the range and distribution of C and N is seen with evolutionary state on the 47 Tuc red giant branch (RGB) over the 4 magnitudes observed. 2) The ratio of CN-weak to CN-strong stars appears to remain the same both before and after the luminosity function ''bump'' believed to signal the onset of RGB mixing. 3) The spatial distribution of CN-strong and CN-weak stars is determined for 294 red giants exterior to 4 arc minutes from the cluster center. The distribution of CN among stars exterior to 13 - 20 arc minutes is clearly biased toward CN-weak stars. The DDO colors are also consistent with the suggestion that CN-strong stars dominate the central 0 - 5 arc minutes. This result confirms earlier studies which suggest a radial trend in the CN distribution of 47 Tuc. 4) An examination of the positions of horizontal branch (HB) stars on the color-magnitude diagram reveals that the CN-strong HB stars are on average, about 0.05 mags more luminous in V and tend to be bluer than their CN-weak counterparts. This result appears consistent with the idea that the CN-strong stars of 47 Tuc have mixed small amounts of additional He into their envelopes during RGB ascent, as outlined by \cite{alan97}. It is therefore suggested that the bulk C and N abundance differences observed among 47 Tuc giants are not likely the result of a progressive deep mixing of CN-cycle material to the stellar surface during the later phases of RGB ascent, but are rather the signature of an earlier process. That some deep dredge-up may have occurred among the evolved giants cannot be ruled out, and is consistent with the HB morphology described below as well as with previously observed (12}C/({13)) C ratios, but the extent of such mixing is clearly less than that believed to occur in stars of more metal-poor clusters.