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Register a new userEuropium production: neutron star mergers versus core-collapse supernovae
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We have explored the Eu production in the Milky Way by means of a very detailed chemical evolution model. In particular, we have assumed that Eu is formed in merging neutron star (or neutron star black hole) binaries as well as in type II supernovae. We have tested the effects of several important parameters influencing the production of Eu during the merging of two neutron stars, such as: i) the time scale of coalescence, ii) the Eu yields and iii) the range of initial masses for the progenitors of the neutron stars. The yields of Eu from type II supernovae are very uncertain, more than those from coalescing neutron stars, so we have explored several possibilities. We have compared our model results with the observed rate of coalescence of neutron stars, the solar Eu abundance, the [Eu/Fe] versus [Fe/H] relation in the solar vicinity and the [Eu/H] gradient along the Galactic disc. Our main results can be summarized as follows: i) neutron star mergers can be entirely responsible for the production of Eu in the Galaxy if the coalescence time scale is no longer than 1 Myr for the bulk of binary systems, the Eu yield is around $3 times 10^{-7}$ M$_odot$, and the mass range of progenitors of neutron stars is 9-50 M$_odot$; ii) both type II supernovae and merging neutron stars can produce the right amount of Eu if the neutron star mergers produce $2 times 10^{-7}$ M$_odot$ per system and type II supernovae, with progenitors in the range 20-50 M$_odot$, produce yields of Eu of the order of $10^{-8}-10^{-9}$ M$_odot$; iii) either models with only neutron stars producing Eu or mixed ones can reproduce the observed Eu abundance gradient along the Galactic disc.
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We use cosmological, magnetohydrodynamical simulations of Milky Way-mass galaxies from the Auriga project to study their enrichment with rapid neutron capture (r-process) elements. We implement a variety of enrichment models from both binary neutron star mergers and rare core-collapse supernovae. We focus on the abundances of (extremely) metal-poor stars, most of which were formed during the first ~Gyr of the Universe in external galaxies and later accreted onto the main galaxy. We find that the majority of metal-poor stars are r-process enriched in all our enrichment models. Neutron star merger models result in a median r-process abundance ratio which increases with metallicity, whereas the median trend in rare core-collapse supernova models is approximately flat. The scatter in r-process abundance increases for models with longer delay times or lower rates of r-process producing events. Our results are nearly perfectly converged, in part due to the mixing of gas between mesh cells in the simulations. Additionally, different Milky Way-mass galaxies show only small variation in their respective r-process abundance ratios. Current (sparse and potentially biased) observations of metal-poor stars in the Milky Way seem to prefer rare core-collapse supernovae over neutron star mergers as the dominant source of r-process elements at low metallicity, but we discuss possible caveats to our models. Dwarf galaxies which experience a single r-process event early in their history show highly enhanced r-process abundances at low metallicity, which is seen both in observations and in our simulations. We also find that the elements produced in a single event are mixed with ~10^8 Msun of gas relatively quickly, distributing the r-process elements over a large region.
Neutron star mergers (NSM) are likely to be the main production sites for the rapid (r-) neutron capture process elements. We study the r-process enrichment of the stellar halo of the Milky Way through NSM, by tracing the typical r-process element Eu in the Munich-Groningen semi-analytic galaxy formation model, applied to three high resolution Aquarius dark matter simulations. In particular, we investigate the effect of the kick velocities that neutron star binaries receive upon their formation, in the building block galaxies (BBs) that partly formed the stellar halo by merging with our Galaxy. When this kick is large enough to overcome the escape velocity of the BB, the NSM takes place outside the BB with the consequence that there is no r-process enrichment. We find that a standard distribution of NS kick velocities decreases [Eu/Mg] abundances of halo stars by $sim 0.5$~dex compared to models where NS do not receive a kick. With low NS kick velocities, our simulations match observed [Eu/Mg] abundances of halo stars reasonably well, for stars with metallicities [Mg/H]$geq -1.5$. Only in Aquarius halo B-2 also the lower metallicity stars have [Eu/Mg] values similar to observations. We conclude that our assumption of instantaneous mixing is most likely inaccurate for modelling the r-process enrichment of the Galactic halo, or an additional production site for r-process elements is necessary to explain the presence of low-metallicity halo stars with high Eu abundances.
In the last decade there has been a remarkable increase in our knowledge about core-collapse supernovae (CC-SNe), and the birthplace of neutron stars, from both the observational and the theoretical point of view. Since the 1930s, with the first systematic supernova search, the techniques for discovering and studying extragalactic SNe have improved. Many SNe have been observed, and some of them, have been followed through efficiently and with detail. Furthermore, there has been a significant progress in the theoretical modelling of the scenario, boosted by the arrival of new generations of supercomputers that have allowed to perform multidimensional numerical simulations with unprecedented detail and realism. The joint work of observational and theoretical studies of individual SNe over the whole range of the electromagnetic spectrum has allowed to derive physical parameters, which constrain the nature of the progenitor, and the composition and structure of the stars envelope at the time of the explosion. The observed properties of a CC-SN are an imprint of the physical parameters of the explosion such as mass of the ejecta, kinetic energy of the explosion, the mass loss rate, or the structure of the star before the explosion. In this chapter, we review the current status of SNe observations and theoretical modelling, the connection with their progenitor stars, and the properties of the neutron stars left behind.
The level of star formation in elliptical galaxies is poorly constrained, due to difficulties in quantifying the contamination of flux-based estimates of star formation from unrelated phenomena, such as AGN and old stellar populations. We here utilise core-collapse supernovae (CCSNe) as unambiguous tracers of recent star formation in ellipticals within a cosmic volume. We firstly isolate a sample of 421 z < 0.2, r < 21.8 mag CCSNe from the SDSS-II Supernova Survey. We then introduce a Bayesian method of identifying ellipticals via their colours and morphologies in a manner unbiased by redshift and yet consistent with manual classification from Galaxy Zoo 1. We find ~ 25 % of z < 0.2 r < 20 mag galaxies in the Stripe 82 region are ellipticals (~ 28000 galaxies). In total, 36 CCSNe are found to reside in ellipticals. We demonstrate that such early-types contribute a non-negligible fraction of star formation to the present-day cosmic budget, at 11.2 $pm$ 3.1 (stat) $^{+3.0}_{-4.2}$ (sys) %. Coupling this result with the galaxy stellar mass function of ellipticals, the mean specific star formation rate (SSFR; $overline{S}$) of these systems is derived. The best-fit slope is given by log ($overline{S}(M)$/yr) = - (0.80 $pm$ 0.59) log ($M/10^{10.5}rm{M}_{odot}$) - 10.83 $pm$ 0.18. The mean SSFR for all log ($M/rm{M}_{odot}$) > 10.0 ellipticals is found to be $overline{S} = 9.2 pm 2.4$ (stat) $^{+2.7}_{-2.3}$ (sys) $times 10^{-12}$ yr$^{-1}$, which is consistent with recent estimates via SED-fitting, and is 11.8 $pm$ 3.7 (stat) $^{+3.5}_{-2.9}$ (sys) % of the mean SSFR level on the main sequence as also derived from CCSNe. We find the median optical spectrum of elliptical CCSN hosts is statistically consistent with that of a control sample of ellipticals that do not host CCSNe, implying that these SN-derived results are well-representative of the total low-z elliptical population.
Knowledge of the progenitors of core-collapse supernovae is a fundamental component in understanding the explosions. The recent progress in finding such stars is reviewed. The minimum initial mass that can produce a supernova has converged to 8 +/- 1 solar masses, from direct detections of red supergiant progenitors of II-P SNe and the most massive white dwarf progenitors, although this value is model dependent. It appears that most type Ibc supernovae arise from moderate mass interacting binaries. The highly energetic, broad-lined Ic supernovae are likely produced by massive, Wolf-Rayet progenitors. There is some evidence to suggest that the majority of massive stars above ~20 solar masses may collapse quietly to black-holes and that the explosions remain undetected. The recent discovery of a class of ultra-bright type II supernovae and the direct detection of some progenitor stars bearing luminous blue variable characteristics suggests some very massive stars do produce highly energetic explosions. The physical mechanism is open to debate and these SNe pose a challenge to stellar evolutionary theory.
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