Where, oh where has the r-process gone?

By Y. Qian, G. Wasserburg. 2007

197 citations recorded by the source corpus.

We present a review of the possible sources for r-process nuclei (r-nuclei). It is known that there is as yet no self-consistent mechanism to provide abundant neutrons for a robust r-process in the neutrino-driven winds from nascent neutron stars. We consider that the heavy r-nuclei with mass numbers A>130 (Ba and above) cannot be produced in the neutrino-driven winds. Nonetheless, the r-process and the neutrino-driven winds may be directly or indirectly related by some unknown additional mechanism, which, for example, could provide ejecta with very short dynamic timescales of ≾0.004 s. This undetermined mechanism must supply a neutron source within the same general stellar sites that undergo core collapse to produce the neutron star. Observational data on low-metallicity stars in the Galactic halo show that sites producing the heavy r-nuclei do not produce Fe or any other elements between N and Ge. Insofar as a forming neutron star is key to producing the heavy r-nuclei, then the only possible sources are supernovae resulting from collapse of O–Ne–Mg cores or accretion-induced collapse of white dwarfs, neither of which produce the elements of the Fe group or those of intermediate mass (above C and N). Observational evidence on s and r-nuclei in low-metallicity stars with high C and N abundances shows that the r-process is also active in binary systems. The nuclei with A∼90–110 produced by charged-particle reactions (CPR) in the neutrino-driven winds are in general present in metal-poor stars with high or low abundances of heavy r-nuclei. The CPR nuclei and the heavy r-nuclei are not strongly coupled. Some metal-poor stars show extremely high enrichments of heavy r-nuclei and have established that the abundance patterns of these nuclei are universally close to the solar abundance pattern of heavy r-nuclei. Using a template star with high enrichments of heavy r-nuclei and another with low enrichments we develop a two-component model based on the abundances of Eu (from sources for heavy r-nuclei) and Fe (from Fe core-collapse supernovae). This model gives very good quantitative predictions for the abundances of all the other elements in those metal-poor stars with [Fe/H]≲-1.5 for which the Eu and Fe abundances are known. We attribute the CPR elements such as Sr, Y, and Zr to reactions in the neutrino-driven winds from a nascent neutron star and the heavy r-nuclei to the hypothecated true “r-process”. The CPR nuclei should be produced whenever a neutron star is formed regardless of whether heavy r-nuclei are produced or not. Using the two-component model we estimate the yield of the CPR element Sr to be ∼10^(-6)M_⊙ for a single neutron star formation event. Self-consistent astrophysical models are needed to establish that the CPR nuclei are common to the neutron stars produced in both sources for the heavy r-nuclei and those for Fe. We show that the observational data appear fully consistent with the two-component model. The specific mechanism and site for the production of heavy r-nuclei remains to be found.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/96b78d5bee781ee40e8b6075fdf772ce90fa49fb

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Citations in the corpus

The s-Process in Low Metallicity Stars. II. Interpretation of High-Resolution Spectroscopic Observations with AGB models (2011, background): “From the theoretical point of view, several models have been advanced (Farouqi et al. 2010; Qian & Wasserburg 2008, 2007; Kratz et al. 2007; Wanajo & Ishimaru 2006), but an exhaustive interpretation is still lacking.”
THE CHEMICAL EVOLUTION OF THE DRACO DWARF SPHEROIDAL GALAXY (2009, background): “Suggested sites include SNII, hypernovae (Qian & Wasserburg 2008), accretion-induced collapse of a white dwarf into a neutron star in a binary system (Qian & Wasserburg 2003), and neutrino-driven winds from a nascent neutron star as reviewed by Qian & Wasserburg (2007).”
The impact of individual nuclear properties on $r$-process nucleosynthesis (2015, methodology): “echanisms of and astrophysical sites for the creation of the s-process [6] and heavy p-process [7, 8] nuclei are on a rm footing. The site or sites of the rprocess still evade denitive determination [9, 10, 11]. The r-process pattern is extracted from the solar system abundances by subtracting the s”
NEUTRINO-DRIVEN WINDS IN THE AFTERMATH OF A NEUTRON STAR MERGER: NUCLEOSYNTHESIS AND ELECTROMAGNETIC TRANSIENTS (2015, background): “Observations indicate that there are at least two types of abundance patterns among the oldest r-process stars (see, e.g., Sneden et al. (2008); Qian & Wasserburg (2007)): 1) stars with high enhancement of heavy elements (Z > 50) present a robust pattern for those and some variations for the…”
Colloquium : Superheavy elements: Oganesson and beyond (2019, background): “This nucleosynthesis process occurs in astrophysical scenario(s) where the timescale for neutron capture is much shorter than the timescale for beta decay (Arnould et al., 2007; Qian and Wasserburg, 2007).”
Delayed outflows from black hole accretion tori following neutron star binary coalescence (2013, background): “This issue is particularly important because the astrophysical origin of r-process nuclei remains a mystery (Qian & Wasserburg 2007), and binary NS mergers could be an important−and possibly dominant−source (Lattimer & Schramm 1974; Freiburghaus et al. 1999; Korobkin et al. 2012).”
Nucleosynthesis in the Early Galaxy (2007, methodology): “After submission of this paper, Qian & Wasserburg (2007) presented a refinement of their phenomenological model that is based on similar observational constraints as presented here.”
First stars VIII. Enrichment of the neutron-capture elements in the early Galaxy (2007, unknown): “…elements, called the “weak” r-process (Busso et al. 1999; Qian & Wasserburg 2000; Wanajo et al. 2001), LEPP process (Travaglio et al. 2004), or CPR process (Qian & Wasserburg 2007), or even an entirely new nucleosynthesis mechanism in massive, metal-poor stars (νp-process, Froehlich et al. (2006).”
Short-duration gamma-ray bursts with extended emission from protomagnetar spin-down (2007, background): “Isolated NS birth via AIC is also one of the only Galactic r-process sites consistent with current observations of elemental abundances in metal-poor halo stars (Qian & Wasserburg 2007).”
CHARGED-PARTICLE AND NEUTRON-CAPTURE PROCESSES IN THE HIGH-ENTROPY WIND OF CORE-COLLAPSE SUPERNOVAE (2010, background): “…the full superposition of conditions required, is a matter of debate (see, e.g., Kratz et al. 1993; Wasserburg et al. 1996; Meyer & Brown 1997; Pfeiffer et al. 2001b; Sneden & Cowan 2003; Honda et al. 2006; Qian & Wasserburg 2007; Ott et al. 2008; Farouqi et al. 2009a, and references therein).”
The s process: Nuclear physics, stellar models, and observations (2010, background): “There is general consensus for the need of an additional LEPP source of yet unknown origin for the light isotopes, including Sr, Y, and Zr, an issue that is still highly debated (Farouqi et al., 2010; Pignatari et al., 2010; Qian and Wasserburg, 2007).”
r-Process Nucleosynthesis in Hot Accretion Disk Flows from Black Hole-Neutron Star Mergers (2008, background): “…process that contributed to the entire range of r-process nuclei; recent data from meteorites and metal-poor halo stars has changed that view (see Qian & Wasserburg 2007 and ref- 1 Permanent address: Department of Physics and Astronomy, Union College, Schenectady, NY 12308. erences therein).”
Galactic r-process enrichment by neutron star mergers in cosmological simulations of a Milky Way-mass galaxy (2014, background): “First, observations of stars with a range of metallicities find that heavy r-process nuclei are also accompanied by elements with atomic masses of 90–120 (Qian & Wasserburg 2007; Sneden et al. 2008).”
Production of the entire range of r-process nuclides by black hole accretion disc outflows from neutron star mergers (2016, methodology): “The astrophysical site of this process has been under debate for more than 50 yrs (see, e.g., Qian & Wasserburg 2007, Arnould et al. 2007, Sneden et al. 2008, Thielemann et al. 2011 for reviews).”