The Link Between Rare-Earth Peak Formation and the Astrophysical Site of the R Process

Abstract

The primary astrophysical source of the rare earth elements is the rapid neutron capture process (r process). The rare earth peak that is seen in the solar r-process residuals has been proposed to originate as a pile-up of nuclei during the end of the r process. We introduce a new method utilizing Monte Carlo studies of nuclear masses in the rare earth region, that includes self-consistently adjusting β-decay rates and neutron capture rates, to find the mass surfaces necessary for the formation of the rare earth peak. We demonstrate our method with two types of astrophysical scenarios, one corresponding conditions typical of core-collapse supernova winds and one corresponding to conditions typical of the ejection of the material from the tidal tails of neutron star mergers. In each type of astrophysical conditions, this method successfully locates a region of enhanced stability in the mass surface that is responsible for the rare earth peak. For each scenario, we find that the change in the mass surface has qualitatively different features, thus future measurements can shed light on the type of environment in which the r process occurred.

The Rare Earth Peak and the Astrophysical Location of the r Process

“…These two calculations show the mass predictions for the neodymium isotopic chain with a systematic increase of β-decay rates by a factor of 2 and Monte Carlo parameter C = 60 held fixed. We find the overall trends in neutron number remain the same as our previous predictions [6,7], which means the dynamical mechanism for peak formation does not change with faster β-decay rates. However, a more extreme change in the mass surface (larger deviations from DZ) is required to produce the REP which counteracts the faster β-decay rates.…”

“…1. We find that a systematic shift of a factor of 2 faster β-decay rates in the rare earth region under produces the REP relative to the A = 195 peak in both a low entropy hot and very neutron-rich cold trajectories, see [6] for details of the trajectories. In the hot r process, the shift in β-decay rates increases the ratio of A = 195 peak to REP to a value of ∼ 14, which is well beyond the solar ratio of ∼ 4.7.…”

“…The formation of the REP has been shown to be sensitive to both astrophysical conditions and nuclear physics inputs [2][3][4][5]. Recently, Monte Carlo studies of nuclear masses have been used in the region to explore the trends required to reproduce the solar REP [6,7]. In these studies, other nuclear properties (e.g.…”

“…The assumptions, approximations and details of the calculations for reverse engineering nuclear properties in the rare earth region responsible for the formation of the peak have been discussed extensively in Refs. [6,7]. Neutron-rich β-decay rates in the rare earth region play an important role in REP formation.…”

Neutron-Star Implosions as Heavy-Element Sources

“…The large neutron excess in this scenario, relatively unmolested by neutrino charged-current capture-induced reprocessing of the neutron-to-proton ratio, could lead to fission cycling [70,71], thereby tying together the nuclear mass number A ¼ 130 and A ¼ 195 r-process abundance peaks. Unlike COM r-process ejecta, which will have a wide range of neutrino exposures, entropy, and electron fraction, and thereby can reproduce the solar system r-process abundance pattern [72], the PBH scenario may be challenged in producing the low-mass, A < 100, r-process material.…”