Letter to the Editor

Evolution and nucleosynthesis of extremely metal-poor and metal-free low- and intermediate-mass stars

II. s-process nucleosynthesis during the core He flash

¹ Departament de Física i Enginyeria NuclearUniversitat Politècnica de Catalunya, EUETIB, Carrer Comte d’Urgell 187, 08036 Barcelona, Spain
e-mail: simon.w.campbell@upc.edu
² Institut de Ciències de l’Espai (ICE-CSIC), Campus UAB, Fac. Ciències, Torre C5 parell 2, 08193 Bellaterra, Spain
³ Centre for Stellar and Planetary Astrophysics, Monash University, Clayton, VIC 3800, Australia
e-mail: maria.lugaro@monash.edu
⁴ Research School of Astronomy and Astrophysics, Australian National University, Canberra, Australia
e-mail: akarakas@mso.anu.edu.au

Received: 19 July 2010
Accepted: 6 October 2010

Abstract

Context. Models of primordial and hyper-metal-poor stars that have masses similar to the Sun are known to experience an ingestion of protons into the hot core during the core helium flash phase at the end of their red giant branch evolution. This produces a concurrent secondary flash powered by hydrogen burning that gives rise to further nucleosynthesis in the core.

Aims. We aim to model the nucleosynthesis occurring during the proton ingestion event to ascertain if any significant neutron-capture nucleosynthesis occurs.

Methods. We perform post-process nucleosynthesis calculations on a one-dimensional stellar evolution calculation of a star with mass 1 M_⊙ and a metallicity of  [ Fe/H ]  =  − 6.5 that suffers a proton ingestion episode. Our network includes 320 nuclear species and 2366 reactions and treats mixing and burning simultaneously.

Results. We find that the mixing and burning of protons into the hot convective core leads to the production of ¹³C, which then burns via the ¹³C(α, n)¹⁶O reaction, releasing a large number of free neutrons. During the first two years of neutron production the neutron poison ¹⁴N abundance is low, allowing the prodigious production of heavy elements such as strontium, barium, and lead via slow neutron captures (the s process). These nucleosynthetic products are later carried to the stellar surface and ejected via stellar winds. We compare our results with observations of the hyper-metal-poor halo star HE 1327-2326, which shows a strong Sr overabundance.

Conclusions. Our model provides the possibility of self-consistently explaining the Sr overabundance in HE 1327-2326 together with its C, N, and O overabundances (all within a factor of  ~4) if the material were heavily diluted, for example, via mass transfer in a wide binary system. The model produces at least 18 times too much Ba than observed, but this may be within the large modelling uncertainties. In this scenario, binary systems of low mass must have formed in the early Universe. If this is true, it puts constraints on the primordial initial mass function.