The puzzling dredge-up pattern in NGC 1978

Abstract

Low-mass stars are element factories that efficiently release their products in the final stages of their evolution by means of stellar winds. Since they are large in number, they contribute significantly to the cosmic matter cycle. To assess this contribution quantitatively, it is crucial to obtain a detailed picture of the stellar interior, particularly with regard to nucleosynthesis and mixing mechanisms. We seek to benchmark stellar evolutionary models of low-mass stars. In particular, we measure the surface abundance of ^{12}C in thermally pulsing AGB stars with well-known mass and metallicity, which can be used to infer information about the onset and efficiency of the third dredge-up. We recorded high-resolution near-infrared spectra of AGB stars in the LMC cluster NGC 1978. The sample comprised both oxygen-rich and carbon-rich stars, and is well-constrained in terms of the stellar mass, metallicity, and age. We derived the C/O and ^{12}C/^{13}C ratio from the target spectra by a comparison to synthetic spectra. Then, we compared the outcomes of stellar evolutionary models with our measurements. The M stars in NGC 1978 show values of C/O and ^{12}C/^{13}C that can best be explained with moderate extra-mixing on the RGB coupled to a moderate oxygen enhancement in the chemical composition. These oxygen-rich stars do not seem to have undergone third dredge-up episodes (yet). The C stars show carbon-to-oxygen and carbon isotopic ratios consistent with the occurrence of the third dredge-up. We did not find S stars in this cluster. None of the theoretical schemes that we considered was able to reproduce the observations appropriately. Instead, we discuss some non-standard scenarios to explain the puzzling abundance pattern in NGC 1978.

16 pages, 9 figures, 4 tables, accepted for publication in A&A, language revised

References in corpus (19)

• A grid of MARCS model atmospheres for late-type stars I. Methods and general properties
• Thermohaline mixing: A physical mechanism governing the photospheric composition of low-mass giants
• Stellar Models and Yields of Asymptotic Giant Branch Stars
• A double main sequence turn-off in the rich star cluster NGC 1846 in the Large Magellanic Cloud
• Origin of multiple stellar populations in globular clusters and their helium enrichment
• The chemical composition of red giant stars in four intermediate-age clusters of the Large Magellanic Cloud
• The Chemical Enrichment History of the Large Magellanic Cloud
• Population Parameters of Intermediate-Age Star Clusters in the Large Magellanic Cloud. I. NGC 1846 and its Wide Main Sequence Turnoff
• Low temperature Rosseland opacities with varied abundances of carbon and nitrogen
• Abundances in intermediate-mass AGB stars undergoing third dredge-up and hot-bottom burning
• Chemical abundances in giants stars of the tidally disrupted globular cluster NGC 6712 from high-resolution infrared spectroscopy
• The globular cluster NGC 1978 in the Large Magellanic Cloud
• Distances to Populous Clusters in the LMC via the K-Band Luminosity of the Red Clump
• 3He-Driven Mixing in Low-Mass Red Giants: Convective Instability in Radiative and Adiabatic Limits
• On the Iron content of NGC 1978 in the LMC: a metal rich, chemically homogeneous cluster
• The Flattening of Globular Clusters
• Three clusters of the SMC from ACS/WFC HST archive data: NGC 265, K~29 and NGC 290 and their fields
• The intermediate-age globular cluster NGC 1783 in the Large Magellanic Cloud
• AGB nucleosynthesis in the Large Magellanic Cloud. Detailed abundance analysis of the RV Tauri star MACHO47.2496.8

Cited by in corpus (17)

• Evolution, nucleosynthesis and yields of low mass AGB stars at different metallicities (II): the FRUITY database
• On the Formation of Multiple Stellar Populations in Globular Clusters
• Helium enrichment and Carbon-star Production in Metal-rich Populations
• Heavy-element yields and abundances of Asymptotic Giant Branch models with a Small Magellanic Cloud metallicity
• Is Extra Mixing Really Needed in Asymptotic Giant Branch Stars?
• On the Need for Deep Mixing in AGB Stars of Low Mass
• Carbon and oxygen isotopic ratios for nearby miras
• Evolution and Nucleosynthesis of AGB stars in Three Magellanic Cloud Clusters
• Comparative Modelling of the Spectra of Cool Giants
• The Pulsation of AGB Stars in the Magellanic Cloud Clusters NGC 1978 and NGC 419
• The evolutionary state of Miras with changing pulsation periods
• CRIRES-POP: A library of high resolution spectra in the near-infrared
• Oxygen isotopic ratios in intermediate-mass red giants
• The impact of the revised ${}^{17}$O$(p,\alpha {)}^{14}$N reaction rate on ${}^{17}$O stellar abundances and yields
• Abundance analysis for long-period variables II. RGB and AGB stars in the globular cluster 47\,Tuc
• Seeing Red in NGC 1978, NGC 55, and NGC 3109
• Search for a metallicity spread in the multiple population Large Magellanic Cloud cluster NGC 1978