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 $M_{\mathrm{V}}\sim -1.5$ 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.

arXiv comment10 pages, 6 figures, Table 1 can be found in its entirety online

References in corpus (39)

• The chemical composition of the Sun
• A grid of MARCS model atmospheres for late-type stars I. Methods and general properties
• Towards Better Age Estimates for Stellar Populations: The Y^2 Isochrones for Solar Mixture
• Na-O Anticorrelation and HB. VII. The chemical composition of first and second-generation stars in 15 globular clusters from GIRAFFE spectra
• Na-O Anticorrelation and HB. VIII. Proton-capture elements and metallicities in 17 globular clusters from UVES spectra
• The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters. I. Overview of the Project and Detection of Multiple Stellar Populations
• Catalogue of [Fe/H] determinations for FGK stars : 2001 edition
• Thermohaline instability and rotation-induced mixing. III - Grid of stellar models and asymptotic asteroseismic quantities from the pre-main sequence up to the AGB for low- and intermediate-mass stars at various metallicities
• Deep Mixing of He-3: Reconciling Big Bang and Stellar Nucleosynthesis
• The Chemical Composition Contrast between M3 and M13 Revisited: New Abundances for 28 Giant Stars in M3
• Rotational mixing in low-mass stars II. Self-consistent models of Pop II RGB stars
• CN abundance variations on the main sequence of 47 Tuc
• Comparison between predicted and empirical DV(Bump -HB) in Galactic Globular Clusters
• Thermohaline mixing in evolved low-mass stars
• On the C and N Abundances of 47 Tucanae Main Sequence Stars
• C and N Abundances in Stars At the Base of the Red Giant Branch in M15
• Carbon in Red Giants in Globular Clusters and Dwarf Spheroidal Galaxies
• Oxygen and Sodium Abundances in M13 (NGC 6205) Giants: Linking Globular Cluster Formation Scenarios, Deep Mixing, and Post-RGB Evolution
• Deep Mixing and Metallicity: Carbon Depletion in Globular Cluster Giants
• Inhibition of thermohaline mixing by a magnetic field in Ap star descendants: Implications for the Galactic evolution of 3He
• The Role of Thermohaline Mixing in Intermediate- and Low-Metallicity Globular Clusters
• Thermohaline Mixing and its Role in the Evolution of Carbon and Nitrogen Abundances in Globular Cluster Red Giants: The Test Case of Messier 3
• Outer Versus Inner Halo Globular Clusters: NGC 7492 Abundances
• Metallicity variations in the Type II globular cluster NGC6934
• 3He-Driven Mixing in Low-Mass Red Giants: Convective Instability in Radiative and Adiabatic Limits
• A spectroscopic study of the Globular Cluster NGC 4147
• Mass Outflow and Chromospheric Activity of Red Giant Stars in Globular Clusters I: M15
• Carbon abundances and 12C/13C from globular cluster giants
• Mass Outflow and Chromospheric Activity of Red Giant Stars in Globular Clusters II. M13 and M92
• A phenomenological modification of thermohaline mixing in globular cluster red giants
• Light-Element Abundance Variations at Low Metallicity: the Globular Cluster NGC 5466
• Exploring the Upper Red Giant and Asymptotic Giant Branches: the Globular Cluster M5
• Study of Globular Cluster M53: new variables, distance, metallicity
• 12C/13C isotopic ratios in red-giant stars of the open cluster NGC 6791
• Chemical Abundances in NGC 5053: A Very Metal-Poor and Dynamically Complex Globular Cluster
• Light Element Abundances and Multiple Populations in M10
• Spectral classification and HR diagram of pre-main sequence stars in NGC6530
• Various Modes of Helium Mixing in Globular Cluster Giants and Their Possible Effects on the Horizontal Branch Morphology
• High resolution spectroscopic analysis of seven giants in the bulge globular cluster NGC 6723

Cited by in corpus (2)

• The Pristine survey -- XV. A CFHT ESPaDOnS view on the Milky Way halo and disc populations
• Light Element Abundances and Multiple Populations in M53