Among stars in Galactic globular clusters the carbon abundance tends to decrease with increasing luminosity on the upper red giant branch , particularly within the lowest metallicity clusters .
While such a phenomena is not predicted by canonical models of stellar interiors and evolution , it is widely held to be the result of some extra mixing operating during red giant branch ascent which transports material exposed to the CN ( O ) -cycle across the radiative zone in the stellar interior and into the base of the convective envelope , whereupon it is brought rapidly to the stellar surface .

Here we present measurements of [ C/Fe ] abundances among 67 red giants in 19 globular clusters within the Milky Way .
Building on the work of Martell et al. , we have concentrated on giants with absolute magnitudes of M _ { \mathrm { V } } \sim - 1.5 within clusters encompassing a range of metallicity ( -2.4 < [ Fe/H ] < -0.3 ) .
The Kitt Peak National Observatory ( KPNO ) 4 m and Southern Astrophysical Research ( SOAR ) 4.1 m telescopes were used to obtain spectra covering the \lambda 4300 CH and \lambda 3883 CN bands .
The CH absorption features in these spectra have been analyzed via synthetic spectra in order to obtain [ C/Fe ] abundances .
These abundances and the luminosities of the observed stars were used to infer the rate at which C abundances change with time during upper red giant branch evolution ( i.e. , the mixing efficiency ) .
By establishing rates over a range of metallicity , the dependence of deep mixing on metallicity is explored .
We find that the inferred carbon depletion rate decreases as a function of metallicity , although our results are dependent on the initial [ C/Fe ] composition assumed for each star .