INTRODUCTION
1.5 Galactic chemical evolution
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.
Figure 1.13: r-Process nucleosynthesis calculations in a wide range of NSNS and BHNS mergers. All mergers produce virtually identical final abundances, demon- strating the robustness of the r-process in neutron star mergers. Figure 4 from Korobkin et al. (2012); see that reference for details. © 2012 Oleg Korobkin and coauthors
The metallicity (a metal being anything other than H and He) of stars thus serves as a chronometer (e.g., Matteucci, 2012). An important caveat is, however, that the galaxy did not form from a single dark matter halo. Rather, the galaxy today is the product of merging sub-halos and within each sub-halo, the age–metallicity relationship depends on the mass of the sub-halo. Therefore, there may not be a universal age–metallicity relationship for the Milky Way. See Ishimaru et al. (2015) for details. Usually, the iron-to-hydrogen ratio [Fe/H] is used as a proxy for the metallicity. The notation [X/H] denotes the logarithm (base 10) of the abundance ratio of X to hydrogen, normalized to the solar ratio. I.e., the sun has [X/H] = 0 and a star with [Fe/H]= −2 has 100 times less iron compared to hydrogen than the sun. Large-scale spectroscopic surveys of stars that measure various abundances can thus provide valuable insights into the chemical evolution of the galaxy (e.g., Edvardsson et al.,1993; Suda et al.,2008).
Since compact object binaries take a long time, on average, to inspiral under grav- itational wave emission (0.1−1 Gyr, e.g., Dominik et al., 2012), one might not expect to see r-process material in stars that were formed in the first 100 Myr or so.
However, the observed metal-poor stars that contain r-process elements (e.g., Sneden
et al., 2009) were formed within the first 100 Myr of star formation in our galaxy.
Furthermore, neutron star mergers are rare events that release r-process material and that material has to mix with the interstellar medium in the galaxy before it can be incorporated into new stars. Thus one might expect a significant scatter in the r-process abundances in different parts of the galaxy, depending on whether there was a neutron star merger nearby. The observed scatter in the r-process abundances might be lower than what one would expect from mergers (e.g., Argast et al.,2004).
However, even though the average delay time for compact object mergers is 0.1− 1 Gyr, population synthesis models (e.g., De Donder and Vanbeveren, 2004; Do- minik et al.,2012) predict that there are a few percent of binary neutron star systems that have delay times as short as a few Myr. These tight binaries can be created by unstable mass transfer due to Roche lobe overflow. The exact distribution of delay times and especially the minimum delay time depend strongly on the treatment of the common envelope phase of binary stellar evolution (e.g., Dominik et al.,2012). Us- ing advanced population synthesis models and inhomogeneous mixing into account, several authors have found that neutron star mergers could be the dominant source of the r-process in the Milky Way, possibly with some early magnetorotationally- driven CCSNe, and can also account for the observed scatter of heavy elements (e.g., Ishimaru et al., 2015; Cescutti et al., 2015; Wehmeyer et al., 2015; van de Voort et al.,2015). Figure1.14shows a computation by van de Voort et al. (2015, their Figure 1) of galactic r-process enrichment in a cosmological simulation. They find that neutron star mergers alone can account for the observed r-process-to-iron ratios as a function of [Fe/H].
A less theoretical argument for neutron star mergers to be the dominant site of the r-process comes from recent intriguing work by Wallner et al. (2015). They find that the current abundance of 244Pu (half-life of 81 Myr) in the interstellar medium, inferred from measuring 244Pu in the deep-sea crust, is much lower than its abundance in the early solar system. This points to a low-rate/high-yield process, like a neutron star merger, being responsible for the production of244Pu. Hotokezaka et al. (2015) use the same data, but go a step farther. Given the total amount of r-process material in the galaxy, there is a degeneracy between a production site with a high rate and low yield (e.g., CCSN), or a site with a low rate and high yield (e.g., neutron star mergers). With a simple galactic mixing model, Hotokezaka et al.
(2015) compute the number density of244Pu a typical observer would measure given a certain244Pu production rate. With this model, a relation between the production
Figure 1.14: Results of a cosmological simulation with r-process enrichment by neutron star mergers. The colored pixels show the stellar mass of the galaxy that has a particular r-process-to-iron ratio and [Fe/H] value. The black line is the median and dashed lines are the 16th and 84th percentile. The black plusses and downward arrows are observed europium-to-iron ratios in galactic stars (europium is used as a proxy for r-process material since europium is almost exclusively produced by the r-process and readily measurable). The galactic chemical evolution simulation can account for the observations quite well. Figure 1 from van de Voort et al. (2015);
see that reference for details. © 2014 Freeke van de Voort and coauthors
rate and production amount of244Pu can be computed by requiring that the current
244Pu measurement is equal to what a typical observer would expect to see. This relation is shown in Figure 1.15 (Figure 1 from Hotokezaka et al., 2015) and it breaks the degeneracy of the total amount of r-process material. Hotokezaka et al.
(2015) find that the244Pu measurement in the deep sea crust agrees very well with the expected rate and yield of neutron star mergers. Tsujimoto et al. (2017) draw similar conclusions by studying the244Pu abundance in meteorites.
Finally, a recent discovery by Ji et al. (2016) also points to neutron star mergers as the dominant r-process site. They discovered that the ultrafaint dwarf galaxy Reticulum II is highly enhanced in r-process elements compared to all other known ultrafaint dwarf galaxies. Ultrafaint dwarf galaxies are small galaxies that orbit the Milky Way and formed around the same time when the first stars in the Milky Way formed.
Figure1.16(Figure 2 from Ji et al.,2016) shows that most of the stars in Reticulum II have an r-process enrichment that is two to three order of magnitude higher than
Figure 1.15: The green region is the allowed relationship between the r-process event rate R0 and yield Mej per event to produce the total amount of r-process material in the galaxy. The blue region is the constraint obtained from a galactic mixing model and a measurement of244Pu in the deep sea crust. The two constraints intersect at an event rate that is much lower than the CCSN rate but compatible with the expected neutron star merger rate. Furthermore, the predicted ejecta mass is also compatible with what we expect from neutron star mergers and what has been inferred from possible kilonova/macronova observations (see Section 1.6). Figure 1 from Hotokezaka et al. (2015); see that reference for details. © 2015 Macmillan Publishers Limited
the stars in other ultrafaint dwarf galaxies. And the measured abundances in these stars match the universal r-process abundance pattern. This implies that Reticulum II was enriched in r-process elements by a single, rare event, such as a neutron star merger, that has not happened in other ultrafaint dwarf galaxies. Furthermore, Ji et al. (2016) compute the total europium yield in Reticulum II and find that it is three orders of magnitude higher than what would be expected from a CCSN but the yield is compatible with a neutron star merger.