The Intermediate Neutron-Capture Process and Carbon-Enhanced Metal-Poor Stars

Abstract

Carbon-enhanced metal-poor (CEMP) stars in the Galactic Halo display enrichments in heavy elements associated with either the s (slow) or the r (rapid) neutron-capture process (e.g., barium and europium respectively), and in some cases they display evidence of both. The abundance patterns of these CEMP-s/r stars, which show both Ba and Eu enrichment, are particularly puzzling since the s and the r processes require neutron densities that are more than ten orders of magnitude apart, and hence are thought to occur in very different stellar sites with very different physical conditions. We investigate whether the abundance patterns of CEMP-s/r stars can arise from the nucleosynthesis of the intermediate neutron-capture process (the i process), which is characterised by neutron densities between those of the s and the r processes. Using nuclear network calculations, we study neutron capture nucleosynthesis at different constant neutron densities n ranging from 10 7 to 10 15 cm −3 . With respect to the classical s process resulting from neutron densities on the lowest side of this range, neutron densities on the highest side result in abundance patterns that show an increased production of heavy s-process and r-process elements but similar abundances of the light s-process elements.Such high values of n may occur in the thermal pulses of asymptotic giant branch (AGB) stars due to proton ingestion episodes. Comparison to the surface abundances of 20 CEMP-s/r stars show that our modelled i-process abundances successfully reproduce observed abundance patterns that could not be previously explained by s-process nucleosynthesis. Because the i-process models fit the abundances of CEMP-s/r stars so well, we propose that this class should be renamed as CEMP-i.

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

“…Such events would amount to a light-element version of the i process, a neutron capture process with neutron densities in the range 10 13 -10 15 cm −3 , that is activated in convective-reactive, combined H and He-burning events (Cowan & Rose 1977;Dardelet et al 2014;Herwig et al 2011;Hampel et al 2016). We propose that this event may produce sufficient energy to expel a portion of the H/He convective-reactive layer of the star as discussed by Jones et al (2016).…”

“…The [Na/Mg] ratio could be as small as ≈ −0.3 dex in these conditions. But the presence Fegroup n-capture seeds, or even a too high neutron exposure with Pop III initial abundance, would lead to an efficient production of trans-iron elements with the i-process abundance signature, leading not to CEMP-no but to CEMP-i (or CEMP-r/s) star abundance signatures (Dardelet et al 2014;Herwig et al 2011;Hampel et al 2016). …”

Are some CEMP-s stars the daughters of spinstars?

“…CEMP stars are divided into several subclasses depending on their enrichment in s-and r-elements: CEMP-s, CEMP-r, CEMP-r/s (or CEMP-i, Hampel et al 2016) and CEMP-no (little enriched in s-or r-elements). Most CEMP-s stars have −3 < [Fe/H] < −2 (Norris et al 2013).…”

Carbon-Enhanced Metal-Poor Stars and the Need for an Intermediate Neutron Capture Process

“…The success of our model is reproduced for almost all of the 20 stars in our sample (we refer the reader to [12] for full details of these fits). We are able to reproduce the majority of the elemental abundances within the uncertainty of the observational measurements, as shown in Fig.…”