A recent analysis comparing the [ Na/Fe ] distributions of red giant branch ( RGB ) and asymptotic giant branch ( AGB ) stars in the Galactic globular cluster NGC 6752 found that the ratio of Na–poor to Na–rich stars changes from 30:70 on the RGB to 100:0 on the AGB .
The surprising paucity of Na–rich stars on the AGB in NGC 6752 warrants additional investigations to determine if the failure of a significant fraction of stars to ascend the AGB is an attribute common to all globular clusters .
Therefore , we present radial velocities , [ Fe/H ] , and [ Na/Fe ] abundances for 35 AGB stars in the Galactic globular cluster 47 Tucanae ( 47 Tuc ; NGC 104 ) , and compare the AGB [ Na/Fe ] distribution with a similar RGB sample published previously .
The abundances and velocities were derived from high resolution spectra obtained with the Michigan/Magellan Fiber System ( M2FS ) and MSpec spectrograph on the Magellan–Clay 6.5m telescope .
We find the average heliocentric radial velocity and [ Fe/H ] values to be \langle RV _ { helio . } \rangle =–18.56 km s ^ { -1 } ( \sigma =10.21 km s ^ { -1 } ) and \langle [ Fe/H ] \rangle =–0.68 ( \sigma =0.08 ) , respectively , in agreement with previous literature estimates .
The average [ Na/Fe ] abundance is 0.12 dex lower in the 47 Tuc AGB sample compared to the RGB sample , and the ratio of Na–poor to Na–rich stars is 63:37 on the AGB and 45:55 on the RGB .
However , in contrast to NGC 6752 , the two 47 Tuc populations have nearly identical [ Na/Fe ] dispersion and interquartile range values .
The data presented here suggest that only a small fraction ( \la 20 \% ) of Na–rich stars in 47 Tuc may fail to ascend the AGB , which is a similar result to that observed in M13 .
Regardless of the cause for the lower average [ Na/Fe ] abundance in AGB stars , we find that Na–poor stars and at least some Na–rich stars in 47 Tuc evolve through the early AGB phase .
The contrasting behavior of Na–rich stars in 47 Tuc and NGC 6752 suggests that the RGB [ Na/Fe ] abundance alone is insufficient for predicting if a star will ascend the AGB .