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

Abstract

We have investigated a highly energetic H-ingestion event during shell He burning leading to H-burning luminosities of log(L H /L ) ∼ 13 in a 45M Pop III massive stellar model. In order to track the nucleosynthesis which may occur in such an event, we run a series of single-zone nucleosynthesis models for typical conditions found in the stellar evolution model. Such nucleosynthesis conditions may lead to i-process neutron densities of up to ∼ 10 13 cm −3 . The resulting simulation abundance pattern, where Mg comes from He burning and Ca from the i process, agrees with the general observed pattern of the most iron-poor star currently known, SMSS J031300.36-670839.3. However, Na is also efficiently produced in these i process conditions, and the prediction exceeds observations by ∼ 2.5dex. While this probably rules out this model for SMSS J031300.36-670839.3, the typical i-process signature of combined He burning and i process of higher than solar [Na/Mg], [Mg/Al] and low [Ca/Mg] is reproducing abundance features of the two next most iron-poor stars HE 1017-5240 and HE 1327-2326 very well. The i process does not reach Fe which would have to come from a low level of additional enrichment. i process in hyper-metal poor or Pop III massive stars may be able to explain certain abundance patterns observed in some of the most-metal poor CEMP-no stars.

Neutron Star Mergers Might Not Be the Only Source of r-process Elements in the Milky Way

“…The r-process solar residual pattern does not take into account the potential contribution of the i-process (Cowan & Rose 1977). The i-process can be activated at all metallicities in massive stars (even down to metal-free stars, e.g., Banerjee et al 2018;Clarkson et al 2018), in rapidly accreting white dwarfs (Denissenkov et al 2017), and in AGB and post-AGB stars of different types (e.g., Herwig et al 2011;Jones et al 2016). Observations of possible i-process abundance signatures have been detected in metal-poor stars (e.g., Dardelet et al 2014;Lugaro et al 2015;Abate et al 2016;Hampel et al 2016;Roederer et al 2016b,a) and in young open stellar clusters (Mishenina et al 2015), for different elements including Eu all the way up to Pb.…”

The periodic table and the physics that drives it

“…The i-process is thought to occur in carbon enhanced metal poor (CEMP) stars and the n-process in supernova shock front traversing the He burning shell [175][176][177] . Concerning the r-capture process, there are a number of possible astrophysical environments.…”

Neutron-capture elements in dwarf galaxies

“…The nucleosynthetic sites of the i-process remain unconfirmed (e.g., Frebel 2018;Koch et al 2019), but among the proposed scenarios are: low-mass, lowmetallicity ([Fe/H] −3) stars (Campbell & Lattanzio 2008;Campbell et al 2010;Cruz et al 2013;Cristallo et al 2016), massive (5−10 M ) super-AGB stars (Doherty et al 2015;Jones et al 2016), evolved low-mass stars (Herwig et al 2011;Hampel et al 2019), and rapidly accreting white dwarfs (Herwig et al 2014;Denissenkov et al 2017). Finally, massive (m > 20 M ), metalpoor stars could also play a role in the production of i-process elements (Clarkson et al 2018;Banerjee et al 2018).…”