INTRODUCTION
1.4 Possible r-process sites
The r-process requires a very neutron-rich environment. Numerous sites have been suggested where the right conditions for the r-process could be achieved. These include shock or jet ejecta in neutron-rich supernovae, inhomogeneous Big Bang cosmologies, ejecta from the coalescence and tidal disruption of binary neutron stars, nova outbursts, shock-induced explosive helium or carbon burning, core helium flashes in low-mass stars, or neutron star accretion disks (see Mathews and Cowan, 1990, and references therein). Based on observations of europium in metal-poor stars and galactic chemical evolution models, Mathews and Cowan (1990) concluded that CCSN were the most likely site for the r-process. Recent studies find that CCSN and neutron star mergers are the only viable r-process site candidates (e.g., Thielemann et al.,2011).
1.4.1 Core-collapse supernovae
After the iron core of a massive star collapses, a proto-neutron star (PNS) forms. This PNS deleptonizes to eventually form a neutron star. This process emits∼1053 erg of binding energy in the form of neutrinos. This large neutrino irradiation drives a
Figure 1.9: Final abundances of r-process nucleosynthesis calculations in the neutrino-driven wind from a PNS during a CCSN. The abundances are shown as a function of the PNS mass. To produce the full r-process up to the third peak (A∼ 195), M & 2M is required, which is unrealistic based on the observed mass distribution of neutron stars. For smaller PNS masses, however, the r-process fairly robustly synthesizes elements up to A ∼ 130. Figure 8 from Wanajo (2013); see that reference for details. © 2013 The American Astronomical Society
hot wind off the surface of the PNS, called a neutrino-driven wind (e.g., Qian and Woosley, 1996). Early simulations and models of this wind indicated that it could have the right conditions for the r-process to take place in some cases (e.g., Woosley et al.,1994; Wanajo,2006).
However, the most recent investigations of r-process nucleosynthesis in neutrino- driven winds have shown that it is unlikely that the full r-process (producing nuclides up to the third peak, c.f. Figure 1.1) operates in those conditions, since the wind does not seem to be neutron-rich enough. It appears that only a weak version of the r-process (producing heavy elements up to A∼130) can take place in the neutrino- driven wind from CCSNe (e.g., Qian and Woosley,1996; Thompson et al., 2001;
Fischer et al., 2010; Roberts et al., 2010; Martínez-Pinedo et al., 2012; Wanajo, 2013). Figure 1.9 (Figure 8 from Wanajo,2013) shows a calculation of r-process nucleosynthesis in the neutrino-driven wind from a CCSN.
There is another process, though, the so-calledνp-process, that can produce nuclides up to A∼ 110 in a proton-rich neutrino-driven wind. In the hot, proton-rich wind,
Figure 1.10: r-Process nucleosynthesis results in a special type of CCSN that is driven magnetorotationally. This type of supernova can produce the full r-process up to the third peak, but because of the large magnetic field and amount of rotation that is required, it is expected to be a very rare (. 0.1%) class of CCSN. Figure 13 from Nishimura et al. (2015); see that reference for details. © 2015 The American Astronomical Society
proton capture produces proton-rich nuclei but stops at 64Ge, which has a long β-decay half-life (∼ 64 s) compared to the expansion time scale (∼ 10 s) and a small proton capture cross section. The νp-process can get past this 64Ge barrier by converting a free proton to a neutron via electron antineutrino capture. The reaction64Ge+n→ 64Ga+p is much faster than proton capture on64Ge and allows the nucleosynthesis to proceed past A = 64 with subsequent proton captures (e.g., Fröhlich et al.,2006; Pruet et al.,2006; Wanajo et al.,2011; Arcones et al.,2012).
Finally, a certain rare type of CCSNe may be able to produce the full r-process. If the progenitor star has a strong magnetic field and its core rotates rapidly, then the supernova explosion could be powered by magnetorotational processes (possibly the magnetorotational instability), which could create a bipolar jet (e.g., Wheeler et al.,2000; Akiyama et al.,2003; Burrows et al.,2007; Mösta et al.,2014, 2015).
It is possible that r-process nucleosynthesis takes place in this jet (e.g., Winteler et al., 2012; Nishimura et al., 2015) and creates all the heavy elements up to the third peak. Figure 1.10 shows an r-process nucleosynthesis calculation in such a magnetorotationally driven CCSN by Nishimura et al. (2015, their Figure 13).
However, because an enormous magnetic field and large amount of rotation is
required in this type of supernova, it is expected that only a small fraction (. 0.1%) of all CCSNe would result in a magnetorotational supernova (Nishimura et al., 2015).
1.4.2 Neutron star mergers
Since an ordinary CCSN most likely cannot produce the full r-process, this leaves neutron star mergers as the only remaining viable site for r-process nucleosynthesis.
We know that binary neutron star systems exist in our galaxy and that their orbit is shrinking due to gravitational wave emission (e.g., Hulse and Taylor, 1975;
Lattimer and Prakash, 2005), which will eventually cause the two neutron stars to merge (e.g., Price and Rosswog, 2006). Numerous groups have performed hydrodynamical simulations of the merger of two neutron stars or the merger of a neutron star and a black hole. Such mergers can eject neutron-rich matter through a variety of processes. There are two types of dynamical ejecta, which are launched shortly before or during the merger. As the two neutron stars get close to each other, or a single neutron star gets close to its black hole companion, the neutron star(s) get tidally deformed and disrupted, which produces a stream of neutron star material that is flung out into space and unbound from the system (e.g., Price and Rosswog,2006; Foucart et al.,2014; Sekiguchi et al., 2015; Kyutoku et al.,2015;
Radice et al.,2016). This type of ejecta is referred to tidal tails and an example is shown in Figure 1.11. The second type of dynamical ejecta is material squeezed out from the collision interface of the two neutron stars. This type of dynamical ejecta only occurs in neutron star–neutron star (NSNS) mergers (e.g., Bauswein et al.,2013; Hotokezaka et al.,2013b). The dynamical ejecta mass ranges between 10−4to a few×10−2solar masses and the electron fraction distribution ranges from Ye∼ 0.05−0.45 in the binary neutron star case. Black hole-neutron star binaries can eject up to∼ 0.1M, but only if the black hole is of a similar mass as the neutron star and has a fairly high spin, otherwise there is typically no ejecta at all, because the neutron star gets disrupted inside the event horizon of the black hole. The electron fraction of the ejecta from a BHNS merger is typically below 0.2 (Foucart et al.,2014). I discuss r-process nucleosynthesis in the dynamical ejecta of a BHNS merger in detail in ChapterIV.
Neutron star mergers can produce additional outflows after the merger. In most cases, an accretion disk or torus forms around the central compact object, which is either a black hole or a hot hypermassive neutron star (HMNS). The lifetime of the HMNS before it collapses to a black hole ranges from a few milliseconds to
Figure 1.11: Density rendering of a binary neutron star merger simulation. The dark central blobs are the two neutron stars just before merging. The dynamical ejecta of neutron-rich matter in the form of two tidal tails can be seen clearly. This material is unbound and r-process nucleosynthesis takes place in the ejecta. Figure fromhttp://users.monash.edu.au/~dprice/research/nsmag, see Price and Rosswog (2006) for details. © 2006 Daniel Price and Stephan Rosswog much longer than 30 ms (e.g., Sekiguchi et al.,2011; Hotokezaka et al.,2013a). If there is a HMNS, it will emit neutrinos, and the hot accretion disk also cools via neutrino emission. This can drive a neutrino-driven wind from the disk surface, see Figure1.12(Figure 1 from Perego et al.,2014). An outflow from the disk can also be triggered by viscous heating and alpha recombination in the disk. Since this outflow happens at later times, the neutrino irradiation has enough time to significantly raise the electron fraction of the outflow, so that most simulations find Ye ∼ 0.2−0.45 with a few×10−3M ejected in these disk outflows (e.g., Surman et al.,2008; Wanajo and Janka,2012; Fernández and Metzger,2013; Perego et al., 2014; Just et al.,2015; Foucart et al.,2015). r-Process nucleosynthesis in the disk outflow after a NSNS merger is the subject of ChapterV.
Because the ejecta from neutron star mergers is so neutron-rich, the r-process can easily create all elements up to A ∼ 250, which is beyond the third peak. In fact, during r-process nucleosynthesis even heavier nuclides (A > 300) are produced, however, those nuclides are unstable to fission (either spontaneous or neutron- induced). Their fission products quickly capture more neutrons, grow to A > 300, and then fission themselves. This so-called fission cycle continues until the free neutrons are exhausted. The remarkable result is that the final r-process abundance pattern is very robust to variations in the detailed properties of the ejecta. If fission
Figure 1.12: Cartoon of an accretion disk around a HMNS and the resulting neutrino- driven wind. Figure 1 from Perego et al. (2014); see that reference for details. © 2014 Albino Perego and coauthors
cycling is reached, the final abundances are independent of the exact number of cycles (e.g., Korobkin et al., 2012; Bauswein et al., 2013; Mendoza-Temis et al., 2015). Figure 1.13 (Figure 4 from Korobkin et al., 2012) shows the outcome of r-process nucleosynthesis in a variety of NSNS and BHNS mergers. All merger scenarios produce essentially identical final abundances, thus demonstrating the robustness of the r-process in neutron star mergers.