Heavy elements in globular clusters: The role of asymptotic giant branch stars

Journal Article · Thu Apr 10 04:00:00 EDT 2014 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/785/1/77· OSTI ID:22357141

Straniero, O.; Cristallo, S.

Recent observations of heavy elements in globular clusters reveal intriguing deviations from the standard paradigm of the early galactic nucleosynthesis. If the r-process contamination is a common feature of halo stars, s-process enhancements are found in a few globular clusters only. We show that the combined pollution of asymptotic giant branch (AGB) stars with a mass ranging between 3 to 6 M {sub ☉} may account for most of the features of the s-process overabundance in M4 and M22. In these stars, the s process is a mixture of two very different neutron-capture nucleosynthesis episodes. The first is due to the {sup 13}C(α, n){sup 16}O reaction and takes place during the interpulse periods. The second is due to the {sup 22}Ne(α, n){sup 25}Mg reaction and takes place in the convective zones generated by thermal pulses. The production of the heaviest s elements (from Ba to Pb) requires the first neutron burst, while the second produces large overabundances of light s (Rb, Sr, Y, Zr). The first mainly operates in the less massive AGB stars, while the second dominates in the more massive. From the heavy-s/light-s ratio, we derive that the pollution phase should last for 150 ± 50 Myr, a period short enough compared to the formation timescale of the globular cluster system, but long enough to explain why the s-process pollution is observed in a few cases only. With few exceptions, our theoretical prediction provides a reasonable reproduction of the observed s-process abundances, from Sr to Hf. However, Ce is probably underproduced by our models, while Rb and Pb are overproduced. Possible solutions are discussed.

• OSTI ID: 22357141
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 1 Vol. 785; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

73 NUCLEAR PHYSICS AND RADIATION PHYSICS
79 ASTRONOMY AND ASTROPHYSICS
ABUNDANCE
ALPHA REACTIONS
ASYMPTOTIC SOLUTIONS
CARBON 13 TARGET
GALAXIES
GIANT STARS
MASS
NEON 22 TARGET
NEUTRON EMISSION
NEUTRON REACTIONS
NUCLEOSYNTHESIS
OXYGEN 16
R PROCESS
S PROCESS
STAR CLUSTERS
STARS
VISIBLE RADIATION