Erratum: Pop III i-process nucleosynthesis and the elemental abundances of SMSS J0313-6708 the most iron-poor stars

This is an erratum to the paper ‘Pop III i-process nucleosynthesis and the elemental abundances of SMSS J0313-6708 the most iron-poor stars’ that was published in Mon. Not. R. Astron. Soc. Letters, 474, L37–L41 (2018). An error in decaying the abundances for the purpose of plotting was corrected and this changes only the abundances of N and Sc noticeably (Fig. 1, cf. fig. 4 in Clarkson, Herwig & Pignatari 2018). Due to the β− decay of 45Ca, which has a terrestrial half-life of 343 days, the Sc prediction is |${\sim } 1\, \mathrm{dex}$| greater than the observed upper limits in all three stars. However, it depends on the (n, γ) cross-section for the unstable 45Ca isotope, for which, to our knowledge, no measurement has been made. The theoretical 45Ca(n, γ)46Ca reaction rate has an uncertainty variation factor of 5.62 (Georgios Perdikakis, private communication). Error bars on the Sc abundance shown in Fig. 1 reflect this uncertainty. Taking these uncertainties into account, the observations of Sc do not impose strong constraints on the model. Further investigation of the impact neutron-capture uncertainties in this scenario is planned. Abundances of single-zone calculations with unstable isotopes decayed. Plot corresponds to fig. 4 in (Clarkson et al. 2018). The N abundances are higher than the observations due to the decay of 14C. In the simulation, the 14C abundance is a result of the 13C(n, γ)14C and 14N(n, p)14C reactions. The final N abundance is artificially high relative to more realistic 3D hydro-based, multi-zone simulations as a consequence of the single-zone modelling approach. In realistic conditions, the high neutron densities of the i process originates from fast 13C(α, n)16O reactions at high temperatures found at the bottom of the He-shell. However, at such high temperatures in a single-zone simulation, 13N(p, γ)14O exceeds the beta decay of 13N and thereby the production of the 13C neutron source is bypassed. In our model, we add 1|${{\ \rm per\ cent}}$| protons, by mass, to the He-burning material which aims to mimic the nucleosynthesis that would be seen given the mixing behaviour observed in similar scenarios where multi-zone and 3D simulations have been used (Herwig et al. 2011; Stancliffe et al. 2011). In reality, the 12C(p, γ)13N reaction occurs at the top of the He-shell and the convective motion would then entrain this material downward to higher temperature conditions where then 13N would decay and the 13C(α, n)16O reaction could take place. By using a value of 1|${{\ \rm per\ cent}}$| protons, |$2\, \mathrm{dex}$| higher than the typical value in multi-zone simulations, we artificially increase the 12C(p, γ)13N reaction rate by |${\sim } 2 \, \mathrm{dex}$| (cf. Fig. 2 of Herwig et al. 2011) in order to account for our conducting the one-zone simulation at lower temperatures. In our stellar evolution simulation, convective velocities reach over |$100\, \mathrm{km \, s^{-1}}$| during the event and the remaining He-shell has a radius of |$9\times 10^{4}\, \mathrm{km}$|⁠. Using the velocity estimate from MLT, 13N would be advected ∼3/4 of the way to the base of the shell before decaying into 13C, at a temperature of |$2.5\times 10^{8}\, \mathrm{K}$|⁠, where the neutron-capture rate is about |$2\, \mathrm{dex}$| higher than it is at |$2\times 10^{8}\, \mathrm{K}$| (Cyburt et al. 2010). As a result of our assumptions, 13C, and therefore N abundances are enhanced to levels higher than we expect from a full 3D hydro modelling approach. Our original conclusion that HE 0107-5240 and HE 1327-2326 are reasonable matches to i-process neutron-capture simulations remains unaffected. The details of the burning and mixing of CNO isotopes requires the use of 3D hydrodynamics simulations and a range of CNO elemental rations are feasible.

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