Carbon Abundance Inhomogeneities and Deep Mixing Rates in Galactic Globular Clusters

Abstract

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 V ∼ −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 λ4300 CH and λ3883 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.

The Pristine survey – XV. A CFHT ESPaDOnS view on the Milky Way halo and disc populations

“…In a more advanced stage of evolution along the RGB, these stars experience an additional mixing episode just after the RGB bump, when the molecular weight barrier (the μ-barrier) left by the conv ectiv e env elope at the point of deepest inward progress is canceled out by the outward expansion of the H-burning shell. This extra-mixing episode (Sweigart & Mengel 1979 ;Charbonnel 1995 ;Angelou et al 2012 ) produces a decline in the surface abundance of carbon ([C/Fe]) and 12 C/ 13 C (Briley et al 1990 ;Gratton et al 2000 ;Martell, Smith & Briley 2008 ;Gerber, Briley & Smith 2019 ) and lithium (Lind et al 2009 ), and an increase in the nitrogen abundance (Gratton et al 2000 ).…”

Light Element Abundances and Multiple Populations in M10

“…RGB stars in GCs brighter than the LFB have been shown to have decreasing surface C abundances and increasing surface N abundances as a function of magnitude. Previous studies have tried to constrain the rate of C depletion by studying the brightest RGB stars in GCs and determining a rate based on an assumed initial C abundance (Martell et al 2008b;Gerber et al 2019). However, because these stars can be observed to have a large range of initial C abundances, these studies have struggled to calculate precise C-depletion rates.…”

Blanco DECam Bulge Survey (BDBS). VII. Multiple Populations in Globular Clusters of the Galactic Bulge

“…These color spreads are well correlated with the light element spreads found in spectroscopic studies (Marino et al 2008;Carretta et al 2009). The MPs found in MS stars suggest that the abundance variations are primordial, and not the result of self-enrichment via mixing processes in evolved stars (Gratton et al 2001;Gerber et al 2019). The commonly adopted picture is that the first generation (1G) of stars formed with chemical abundances similar to field stars at the same metallicity and subsequently polluted the intracluster environment, giving rise to a second generation (2G) at the same metallicity but enriched in proton-capture reaction products of the CNO, NeNa, and MgAl chains (Carretta et al 2010a).…”