Charged-Particle and Neutron-Capture Processes in the High-Entropy Wind of Core-Collapse Supernovae

Abstract

The astrophysical site of the r-process is still uncertain, and a full exploration of the systematics of this process in terms of its dependence on nuclear properties from stability to the neutron drip-line within realistic stellar environments has still to be undertaken. Sufficiently high neutron-to-seed ratios can only be obtained either in very neutron-rich low-entropy environments or moderately neutron-rich high-entropy environments, related to neutron star mergers (or jets of neutron star matter) and the high-entropy wind of core-collapse supernova explosions. As chemical evolution models seem to disfavor neutron star mergers, we focus here on high-entropy environments characterized by entropy S, electron abundance Y e , and expansion velocity V exp . We investigate the termination point of charged-particle reactions, and we define a maximum entropy S final for a given V exp and Y e , beyond which the seed production of heavy elements fails due to the very small matter density. We then investigate whether an r-process subsequent to the charged-particle freeze-out can in principle be understood on the basis of the classical approach, which assumes a chemical equilibrium between neutron captures and photodisintegrations, possibly followed by a β-flow equilibrium. In particular, we illustrate how long such a chemical equilibrium approximation holds, how the freeze-out from such conditions affects the abundance pattern, and which role the late capture of neutrons originating from β-delayed neutron emission can play. Furthermore, we analyze the impact of nuclear properties from different theoretical mass models on the final abundances after these late freeze-out phases and β-decays back to stability. As only a superposition of astrophysical conditions can provide a good fit to the solar r-abundances, the question remains how such superpositions are attained, resulting in the apparently robust r-process pattern observed in low metallicity stars.

SYSTEMATIC NON-LTE STUDY OF THE −2.6 ≤ [Fe/H] ≤ 0.2 F AND G DWARFS IN THE SOLAR NEIGHBORHOOD. II. ABUNDANCE PATTERNS FROM Li TO Eu*

“…Europium is referred to as an r-process element, because only 6% of solar Eu originates from the s-process (Travaglio et al 1999). Theoretical predictions of a pure r-process production of Eu and Ba give [Eu/Ba] r ;0.67 in the classical waiting-point (WP) approximation (Kratz et al 2007) and [Eu/Ba] r ;=0.87 in the large-scale parameterized dynamical network calculations of Farouqi et al (2010) in the context of an adiabatically expanding high-entropy wind, as is expected to occur in core-collapse SNe. The solar r-residual, i.e., the difference between solar total and s-abundance, where the s-abundance is deduced from the Galactic chemical evolution models, ranges between [Eu/Ba] r = 0.71 (Travaglio et al 1999) and 0.80 (Bisterzo et al 2014).…”

Impact of the β-Delayed Neutron Emission Probabilities Around A = 100–125 on the r-Process and the BRIKEN Project

“…[4], which includes reaction rates from the compilation Reaclib v2.0 [5] and masses from the FRDM mass model [6]. As an input for the code, the parameterized trajectory follows a simple model of adiabatically expanding homogeneous mass zone described in [7], which is the main feature of the hydrodynamical trajectory for most of r-process simulation of different r-process scenarios. Then, the sensitivity study starts with choosing a baseline for the simulation which fits the solar abundance pattern within the mass range 95-120.…”

“…The HEW is parameterized as in [7] with electron fraction Y e =0.45, expansion timescale τ =30 ms and entropy per baryon s/k varied from 135 to 195. To simulate the condition of the dynamical ejecta from the merging neutron stars, we used a set of parameters described in [9] with expansion timescale τ =20 ms, entropy per baryon s/k=10 and electron fraction Y e varied from 0.25 to 0.4.…”

The r-Java 2.0 code: nuclear physics

“…The differences between the final abundances from r-Java 2.0 and the Clemson code seen in both cases can be credited to the fact that the two codes use different mass models (the Clemson code used the finite range droplet model and r-Java 2.0 HFB21), which has been shown to affect the r-process abundance yield (e.g. Farouqi et al 2010). …”

“…Explosive, neutron-rich environments provide the ideal conditions for r-processes to occur. The predominant astrophysical sites being studied as possible locations for r-process: the high-entropy winds (HEW) from protoneutron stars (see Qian & Woosley 1996;Farouqi et al 2010) and ejected matter from neutron star mergers (see Freiburghaus et al 1999a;Goriely et al 2011). However, both of these scenarios face significant hurdles that must be overcome.…”