arXiv subjects

Takuji Tsujimoto

Publications and source records attributed to Takuji Tsujimoto.

At least 37 records · Page 2Linked to original sources

arXiv2021

Two Sites of r-Process Production Assessed on the Basis of the Age-tagged Abundances of Solar Twins

Solar twins, i.e., stars that are nearly identical to the Sun, including their metallicities, in the solar vicinity show ages widely distributed from 0-10 Gyr. This fact matches the orbital history of solar twins in the new paradigm of galactic dynamics, in which stars radially move on the disk when they encounter transient spiral arms. This finding suggests that older twins were born closer to the Galactic center and traveled a longer distance to reach their present location, according to the hypothesis that chemical enrichment occurs more quickly and that solar metallicity is attained on a shorter timescale with a decreasing Galactocentric distance (R_GC). We show that abundance patterns covering a wide range of heavy elements for solar twins sharing similar ages are identical and that their variation among different age groups can be understood on the basis of the age-R_GC connection within the framework of Galactic chemical evolution. This study identifies the Galactic bulge as the birthplace of the oldest solar twins. Based on this scheme, we find that the relation between [r-process/Fe] and R_GC for the inner Galactic region is incompatible with the hypothesis of a sole site for r-process production, that is, neutron star mergers, whose delay time distribution could be approximated by the power-law form (proportional to t^n). Alternatively, this relation suggests the presence of two distinct sites for r-process production: short-lived massive stars, ending with specific core-collapse supernovae, and neutron star mergers that are heavily inclined to emerge with longer delay times, as represented by n=0-0.5.

astro-ph.GA↗

arXiv2020

Remarkable migration of the solar system from the innermost Galactic disk; a wander, a wobble, and a climate catastrophe on the Earth

Recent knowledge of Galactic dynamics suggests that stars radially move on the disk when they encounter transient spiral arms that are naturally generated during the process of disk formation. We argue that a large movement of the solar system from the innermost disk over its lifetime is inferred from a comparison of the solar composition with those of solar twins within the Galactic chemical evolution framework. The implied metal-rich environment at the Sun's birthplace and formation time is supported by measured silicon isotopic ratios in presolar silicon carbide grains. We perform numerical simulations of the dynamical evolution of disk stars in a Milky Way-like galaxy to identify the lifetime trajectory of the solar system. We find that a solar system born in the proximity of the Galactic bulge could travel to the current locus by the effect of radial migration induced by several major encounters with spiral arms. The frequent feature we identify is the repeated passages of stars inside the same spiral arm owing to the wobble of stars traveling in and out of the spiral arms. We predict that such episodes are evidenced in the Earth's geological history as snowball Earth and that their occurrence times are within our prediction. In particular, the stellar motion that vertically oscillates during passages through spiral arms occasionally leads to a split into two discrete passage episodes with an interval of several tens of Myr, implying two relevant snowball Earth events that occurred in rapid succession (~7.2 and 6.5 hundred Myr ago).

astro-ph.GA↗

arXiv2019

r-Process enrichment in the Galactic halo characterized by nucleosynthesis variation in the ejecta of coalescing neutron star binaries

A large star-to-star variation in the abundances of r-process elements, as seen in the [Eu/Fe] ratio for Galactic halo stars, is a prominent feature that is distinguishable from other heavy elements. It is, in part, caused by the presence of highly r-process enriched stars, classified as r-II stars ([Eu/Fe]>+1). In parallel, halo stars show that the ratio of a light r-process element (Y) to Eu is tightly correlated with [Eu/Fe], giving the lowest [Y/Eu] ratio that levels off at r-II stars. On the other hand, recent hydrodynamical simulations of coalescing double neutron stars (cNSNSs) have suggested that r-process sites may be separated into two classes providing different electron-fraction distributions: tidally-driven dynamical ejecta and (dynamical or postmerger) non-tidal ejecta. Here, we show that a widely spanning feature of [Eu/Fe] can be reproduced by models that consider the different masses of tidally-driven dynamical ejecta from both cNSNSs and coalescing black hole/neutron star binaries (cBHNSs). In addition, the observed [Y/Eu] trend is explained by the combined nucleosynthesis in two kinds of ejecta with varying mass asymmetry in double NS systems. Our scenario suggests that massive tidally-driven dynamical ejecta accompanied by massive non-tidal part from cNSNSs or cBHNSs could alone accommodate r-II abundances, including an actinide boost in some cases. The event rate for cNSNSs estimated from our study agrees with the latest result of ~1000 (90% confidence interval of 110-3840) Gpc$^{-3}$yr$^{-1}$ by gravitational-wave detection, and a few events per Gpc$^3$ per year of cBHNSs associated with r-process production are predicted to emerge.

astro-ph.GA↗

arXiv2019

Identification of Absorption Lines of Heavy Metals in the Wavelength Range 0.97--1.32 Micron

Stellar absorption lines of heavy elements can give us various insights into the chemical evolution of the Galaxy and nearby galaxies. Recently developed spectrographs for the near-infrared wavelengths are becoming more and more powerful for producing a large number of high-quality spectra, but identification and characterization of the absorption lines in the infrared range remain to be fulfilled. We searched for lines of the elements heavier than the iron group, i.e., those heavier than Ni, in the Y (9760--11100 AA) and J (11600--13200 AA) bands. We considered the lines in three catalogs, i.e., Vienna Atomic Line Database (VALD), the compilation by R. Kurucz, and the list published in 1999 by Melendez and Barbuy. Candidate lines were selected based on synthetic spectra and the confirmation was done by using WINERED spectra of 13 supergiants and giants within FGK spectral types (spanning 4000--7200 K in the effective temperature). We have detected lines of Zn I, Sr II, Y II, Zr I, Ba II, Sm II, Eu II, and Dy II, in the order of atomic number. Although the number of the lines is small, 23 in total, they are potentially useful diagnostic lines of the Galactic chemical evolution, especially in the regions for which interstellar extinction hampers detailed chemical analyses with spectra in shorter wavelengths. We also report the detection of lines whose presence was not predicted by the synthetic spectra created with the above three line lists.

astro-ph.SR↗

arXiv2019

A new formation model for omega Centauri: the crossroad of astrophysical processes

We investigate the formation processes of the Galactic globular cluster (GC) omega Cen with multiple stellar populations based on our original hydrodynamical simulations with chemical enrichment by Type II supernovae (SNe II), asymptotic giant branch (AGB) stars, and neutron star mergers (NSMs). The principal results are as follows. Multiple stellar populations with a wide range of [Fe/H] can be formed from rather massive and compact molecular cloud with a mass of 2 * 10^7 M_sun in the central region of its dwarf galaxy within less than a few hundred Myr. Gas ejected from SNe II and AGB stars can mix well to form new stars with higher He abundances (Y) and higher [Fe/H]. The He-rich stars are strongly concentrated in the GC's central region so that the GC can show a steep negative gradient of Y. Relative ratios of light elements to Fe show bimodal distributions for a given [Fe/H] owing to star formation from original gas and AGB ejecta. [La/Fe] and [Ba/Fe] can rapidly increase until [Fe/H]~-1.5 and then decrease owing to Fe ejection from SNe II. Although AGB ejecta can be almost fully retained in intra-cluster medium, NSM ejecta can be retained only partially. This difference in the retention capability is responsible for the observed unique [Eu/Fe]-[Fe/H] and [La/Eu]-[Fe/H] relations in omega Cen. Some observational results such as the [O/Na]$-$[Fe/H] relation and radial [Fe/H] gradient are yet to be well reproduced in the present model.

astro-ph.GA↗

arXiv2019

Galactic r-process abundance feature shaped by radial migration

Growing interests in the chemical feature of r-process elements among nearby disk stars represented by the [Eu/Fe] vs. [Fe/H] diagram have sprouted since it can assess the origin of r-process elements through the comparison with theoretical models, including a test as to if neutron star mergers can be the major site of r-process nucleosynthesis. On the other hand, recent studies reveal that local chemistry is strongly coupled with the dynamics of Galactic disk, which predicts that stars radially move on the disk where the observed elemental feature is different at various Galactocentric distances. Here, we show that radial migration of stars across the Galactic disk plays a crucial role in shaping the r-process abundance feature in the solar vicinity. In this proposed scenario, we highlight the importance of migration from the outer disk where [r-process/Fe] of some old stars is predicted to be enhanced to the level beyond the expectation from the observed Galactic Fe and Eu radial gradient, which results in a large span of [r-process/Fe] among nearby disk stars. The variation in the [r-process/Fe] ratio seen across the Galactic disk as well as in dwarf galaxies may be an outcome of different stellar initial mass functions which change the occurrence frequency between supernovae leaving behind neutron stars and ones ending with black holes. Here we propose that enhancement in [Eu/Fe] is attributed to the initial mass function lacking high-mass stars such as > 25 solar masses in the scheme for which neutron star mergers are a major source of r-process elements.

astro-ph.GA↗

arXiv2019

FeI lines in 0.91-1.33 ${\rm μ}$m spectra of red giants for measuring the microturbulence and metallicities

For a detailed analysis of stellar chemical abundances, high-resolution spectra in the optical have mainly been used, while the development of near-infrared (NIR) spectrograph has opened new wavelength windows. Red giants have a large number of resolved absorption lines in both the optical and NIR wavelengths, but the characteristics of the lines in different wave passbands are not necessarily the same. We present a selection of FeI lines in the $z^{\prime}$, $Y$, and $J$ bands (0.91-1.33 $μ$m). On the basis of two different lists of lines in this range, the Vienna Atomic Line Database (VALD) and the catalog published by Meléndez & Barbuy in 1999 (MB99), we selected sufficiently strong lines that are not severely blended and compiled lists with 107 FeI lines in total (97 and 75 lines from VALD and MB99, respectively). Combining our lists with high-resolution ($λ/Δλ= 28,000$) and high signal-to-noise ($>500$) spectra taken with a NIR spectrograph, WINERED, we present measurements of the iron abundances of two prototype red giants: Arcturus and $μ$ Leo. A bootstrap method for determining the microturbulence and abundance together with their errors is demonstrated. The standard deviations of $\logε_{\rm Fe}$ values from individual FeI lines are significantly smaller when we use the lines from MB99 instead of those from VALD. With the MB99 list, we obtained $ξ=1.20\pm0.11\ {\rm km~s^{-1}}$ and $\logε_{\rm Fe}=7.01\pm0.05$ dex for Arcturus, and $ξ=1.54\pm0.17\ {\rm km~s^{-1}}$ and $\logε_{\rm Fe}=7.73\pm0.07$ dex for $μ$ Leo. These final values show better agreements with previous values in the literature than the corresponding values we obtained with VALD.

astro-ph.SR↗

arXiv2018

Early chemical evolution of Zn driven by magnetorotational supernovae and the pathway to the solar Zn composition

The site of Zn production remains an elusive and challenging problem in astrophysics. A large enhancement of the [Zn/Fe] ratios of very metal-poor stars in the Galactic halo suggests the death of short-lived massive stars, i.e., core-collapse supernovae (CCSNe), as one major site for Zn production. Previous studies have claimed that some specific CCSNe can produce Zn in sufficient quantities. However, it remains unclear which models can withstand the critical test of observations. Using a Zn abundance feature similar to that of r-process elements in faint satellite galaxies, we find evidence that Zn production took place through much rarer events than canonical CCSNe. This finding can be unified with the implied decrease in the rate of Zn production with an increasing metallicity for Galactic halo stars, which narrows down the major site of Zn production in the early galaxy to magneto-rotational SNe (MR-SNe). On the other hand, in the later phase of galactic evolution, we predict that the major Zn-production site switched from MR-SNe to thermonuclear SNe (SNe Ia). According to this scenario, an accumulation of the contributions from two types of SNe eventually led to the solar isotope composition of Zn which mainly owes 66,68Zn to MR-SNe and 64Zn to SNe Ia triggered by He-detonation. The requirement of Zn production in SNe Ia sheds a new light on the hot debate on the scenario for SN Ia progenitors, suggesting that a He-detonation model might be one major channel for SNe Ia.

astro-ph.GA↗

arXiv2017

Enrichment in r-process elements from multiple distinct events in the early Draco dwarf spheroidal galaxy

The stellar record of elemental abundances in satellite galaxies is important to identify the origin of r-process because such a small stellar system could have hosted a single r-process event, which would distinguish member stars that are formed before and after the event through the evidence of a considerable difference in the abundances of r-process elements, as found in the ultra-faint dwarf galaxy Reticulum II (Ret II). However, the limited mass of these systems prevents us from collecting information from a sufficient number of stars in individual satellites. Hence, it remains unclear whether the discovery of a remarkable r-process enrichment event in Ret II explains the nature of r-process abundances or is an exception. We perform high-resolution spectroscopic measurements of r-process abundances for twelve metal-poor stars in the Draco dwarf galaxy in the metallicity range of -2.5<[Fe/H]<-2. We found that these stars are separated into two groups with r-process abundances differing by one order of magnitude. A group of stars with high abundances of r-process elements was formed by a single r-process event that corresponds to the event evidenced in Ret II. On the other hand, the low r-process abundance group was formed by another sporadic enrichment channel producing a far fewer r-process elements, which is clearly identified for the first time. Accordingly, we identified two populations of stars with different r-process abundances, which are built by two r-process events that enriched gases at levels that differ by more than one order of magnitude.

astro-ph.SR↗

arXiv2017

Formation of globular clusters with internal abundance spreads in r-process elements: strong evidence for prolonged star formation

Several globular clusters (GCs) in the Galaxy are observed to show internal abundance spreads in r-process elements (e.g., Eu). We here propose a new scenario which explains the origin of these GCs (e.g., M5 and M15). In this scenario, stars with no/little abundance variations first form from a massive molecular cloud (MC). After all of the remaining gas of the MC is expelled by numerous supernovae, gas ejected from asymptotic giant branch stars can be accumulated in the central region of the GC to form a high-density intra-cluster medium (ICM). Merging of neutron stars then occurs to eject r-process elements, which can be efficiently trapped in and subsequently mixed with the ICM. New stars formed from the ICM can have r-process abundances quite different from those of earlier generations of stars within the GC. This scenario can explain both (i) why r-process elements can be trapped within GCs and (ii) why GCs with internal abundance spreads in r-process elements do not show [Fe/H] spreads. Our model shows that (i) a large fraction of Eu-rich stars can be seen in Na-enhanced stellar populations of GCs, as observed in M15, and (ii) why most of the Galactic GCs do not exhibit such internal abundance spreads. Our model demonstrates that the observed internal spreads of $r$-process elements in GCs provide strong evidence for prolonged star formation (~10^8 yr).

astro-ph.GA↗

arXiv2017

Chemical evolution of 244Pu in the solar vicinity and its implication for the properties of r-process production

Meteoritic abundances of r-process elements are analyzed to deduce the history of chemical enrichment by r-process from the beginning of disk formation to the present time in the solar vicinity, by combining the abundance information from short-lived radioactive nuclei such as 244Pu with that from stable r-process nuclei such as Eu. These two types of nuclei can be associated with one r-process event and cumulation of events till formation of the solar system, respectively. With help of the observed local star formation history, we deduce the chemical evolution of 244Pu and obtain three main results: (i) the last r-process event occurred 130-140 Myr before formation of the solar system, (ii) the present-day low 244Pu abundance as measured in deep sea reservoirs results from the low recent star formation rate compared to ~4.5 - 5 Gyr ago, and (iii) there were ~15 r-process events in the solar vicinity from formation of the Galaxy to the time of solar system formation and ~30 r-process events to the present time. Then, adopting a reasonable hypothesis that a neutron star merger is the r-process production site, we find that the ejected r-process elements are extensively spread out and mixed with interstellar matter with a mass of ~3.5 million solar masses, which is about 100 times larger than that for supernova ejecta. In addition, the event frequency of r-process production is estimated to be one per about 1400 core-collapse supernovae, which is identical to the frequency of neutron star mergers estimated from the analysis of stellar abundances.

astro-ph.GA↗

arXiv2016

Formation of anomalous globular clusters with metallicity spreads: A unified picture

Recent observations have revealed that at least 8 globular clusters (GCs) in the Galaxy show internal abundance spreads in [Fe/H]. We investigate the origin of these `anomalous' GCs using numerical simulations of GCs in the dwarfs orbiting around the Galaxy and chemical evolution model of dwarfs hosting the GCs. The principal results are as follows. GCs formed in a host dwarf galaxy with a total mass of ~ 10^10 M_sun can merge to form a single nuclear GC before the host is completely destroyed by the Galaxy, if they are massive (> 3*10^5 M_sun) and if they are formed in the inner region (R<400 pc). The GC merger remnants can capture field stars during its spiral-in to nuclear regions. If two GCs are formed from star formation events separated by ~300 Myr in their host dwarf, then the new GC formed from GC merging can have [Fe/H] spread of 0.2 dex and [Ba/Fe] spread of 0.3 dex. GCs formed from GC merging can show variety of internal abundance spreads depending on the details of their hosts' chemical evolution. We suggest that anomalous GCs were formed from GC merging that occurred before the destruction of GC host dwarfs yet after self-enrichment processes responsible for the observed anti-correlations between chemical abundances of light elements. We also suggest that the observed no/little dependence of [Eu/Fe] on [Fe/H] in the Galactic GC M22 is evidence of massive dwarf galaxies hosting these anomalous GCs.

astro-ph.GA↗

arXiv2015

The r-process in Magnetorotational Supernovae

One of the hottest open issues involving the evolution of r-process elements is fast enrichment in the early Universe. Clear evidence for the chemical enrichment of r-process elements is seen in the stellar abundances of extremely metal poor stars in the Galactic halo. However, small-mass galaxies are the ideal testbed for studying the evolutionary features of r-process enrichment given the potential rarity of production events yielding heavy r-process elements. Their occurrences become countable and thus an enrichment path due to each event can be found in the stellar abundances. We examine the chemical feature of Eu abundance at an early stage of ${\rm[Fe/H]} \lesssim -2$ in the Draco and Sculptor dwarf spheroidal (dSph) galaxies. Accordingly, we constrain the properties of the Eu production in the early dSphs. We find that the Draco dSph experienced a few Eu production events, whereas Eu enrichment took place more continuously in the Sculptor dSph due to its larger stellar mass. The event rate of Eu production is estimated to be about one per $100$-$200$ core-collapse supernovae, and a Eu mass of $\sim (1-2) \times 10^{-5}$\ms per single event is deduced by associating this frequency with the observed plateau value of ${\rm [Eu/H]} \sim -1.3$ for ${\rm [Fe/H]} \gtrsim -2$. The observed plateau implies that early Eu enrichment ceases at ${\rm [Fe/H]} \approx -2$. Such a selective operation only in low-metallicity stars supports magnetorotational supernovae, which require very fast rotation, as the site of early Eu production. We show that the Eu yields deduced from chemical evolution agree well with the nucleosynthesis results from corresponding supernovae models.

astro-ph.GA↗

arXiv2015

The origin of dust extinction curves with or without the 2175 A bump in galaxies: The case of the Magellanic Clouds

The Large and Small Magellanic Clouds (LMC and SMC, respectively) are observed to have characteristic dust extinction curves that are quite different from those of the Galaxy (e.g., strength of the 2175 A bump). Although the dust composition and size distribution of the Magellanic Clouds (MCs) that can self-consistently explain their observed extinction curves have been already proposed, it remain unclear whether and how the required dust properties can be achieved in the formation histories of the MCs. We therefore investigate the time evolution of the dust properties of the MCs and thereby derive their extinction curves using one-zone chemical evolution models with formation and evolution of small and large silicate and carbonaceous dust grains and dusty winds associated with starburst events. We find that the observed SMC extinction curve without a conspicuous 2175 A bump can be reproduced well by our SMC model, if the small carbon grains can be selectively lost through the dust wind during the latest starburst about 0.2 Gyr ago. We also find that the LMC extinction curve with a weak 2175 A bump can be reproduced by our LMC model with less efficient removal of dust through dust wind. We discuss possible physical reasons for different dust wind efficiencies between silicate and graphite and among galaxies.

astro-ph.GA↗

arXiv2015

Chemical Feature of Eu abundance in the Draco dwarf spheroidal galaxy

Chemical abundance of r-process elements in nearby dwarf spheroidal (dSph) galaxies is a powerful tool to probe the site of r-process since their small-mass scale can sort out individual events producing r-process elements. A merger of binary neutron stars is a promising candidate of this site. In faint, or less massive dSph galaxies such as the Draco, a few binary neutron star mergers are expected to have occurred at most over the whole past. We have measured chemical abundances including Eu and Ba of three red giants in the Draco dSph by Subaru/HDS observation. The Eu detection for one star with [Fe/H]=-1.45 confirms a broadly constant [Eu/H] of ~-1.3 for stars with [Fe/H]>-2. This feature is shared by other dSphs with similar masses, i.e., the Sculptor and the Carina, and suggests that neutron star merger is the origin of r-process elements in terms of its rarity. In addition, two very metal-poor stars with [Fe/H]=-2.12 and -2.51 are found to exhibit very low Eu abundances such as [Eu/H]<-2 with an implication of a sudden increase of Eu abundance by more than 0.7 dex at [Fe/H] ~ -2.2 in the Draco dSph. In addition, the detection of Ba abundances for these stars suggests that the r-process enrichment initiated no later than the time when only a few % of stars in the present-day Draco dSph was formed. Though an identification of the origin of an early Eu production inside the Draco dSph should be awaited until more abundance data of stars with [Fe/H]<-2 in the Draco as well as other faint dSphs become available, the implied early emergence of Eu production event might be reconciled with the presence of extremely metal-poor stars enriched by r-process elements in the Galactic halo.

astro-ph.GA↗

arXiv2014

The origins of light and heavy r-process elements identified by chemical tagging of metal-poor stars

Growing interests in neutron star (NS) mergers as the origin of r-process elements have sprouted since the discovery of evidence for the ejection of these elements from a short-duration gamma-ray burst. The hypothesis of a NS merger origin is reinforced by a theoretical update of nucleosynthesis in NS mergers successful in yielding r-process nuclides with A>130. On the other hand, whether the origin of light r-process elements are associated with nucleosynthesis in NS merger events remains unclear. We find a signature of nucleosynthesis in NS mergers from peculiar chemical abundances of stars belonging to the Galactic globular cluster M15. This finding combined with the recent nucleosynthesis results implies a potential diversity of nucleosynthesis in NS mergers. Based on these considerations, we are successful in the interpretation of an observed correlation between [light r-process/Eu] and [Eu/Fe] among Galactic halo stars and accordingly narrow down the role of supernova nucleosynthesis in the r-process production site. We conclude that the tight correlation by a large fraction of halo stars is attributable to the fact that core-collapse supernovae produce light r-process elements while heavy r-process elements such as Eu and Ba are produced by NS mergers. On the other hand, stars in the outlier, composed of r-enhanced stars ([Eu/Fe]>+1) such as CS22892-052, were exclusively enriched by matter ejected by a subclass of NS mergers that is inclined to be massive and consist of both light and heavy r-process nuclides.

astro-ph.GA↗

arXiv2014

Chemical evolution of galaxies with radiation-driven dust wind

We discuss how the removal of interstellar dust by radiation pressure of stars influences the chemical evolution of galaxies by using a new one-zone chemical evolution models with dust wind. The removal efficiency of an element (e.g., Fe, Mg, and Ca) through radiation-driven dust wind in a galaxy is assumed to depend both on the dust depletion level of the element in interstellar medium and the total luminosity of the galaxy in the new model. We particularly focus on the time evolution of [alpha/Fe] and its dependence on model parameters for dust wind in this study. The principal results are as follows. The time evolution of [Ca/Fe] is significantly different between models with and without dust wind in the sense that [Ca/Fe] can be systematically lower in the models with dust wind. The time evolution of [Mg/Fe], on the other hand, can not be so different between the models with and without dust wind owing to the lower level of dust depletion for Mg. As a result of this, [Mg/Ca] can be systematically higher in the models with dust wind. We compare these results with the observed elemental features of stars in the Large Magellanic Cloud (LMC), because a growing number of observational studies on [alpha/Fe] for the LMC have been recently accumulated for a detailed comparison. Based on the present new results, we also discuss the origins of [alpha/Fe] in the Fornax dwarf galaxy and elliptical galaxies in the context of radiation-driven dust wind.

astro-ph.GA↗

arXiv2014

Enrichment history of r-process elements shaped by a merger of neutron star pairs

The origin of r-process elements remains unidentified and still puzzles us. The recent discovery of evidence for the ejection of r-process elements from a short-duration gamma-ray burst singled out neutron star mergers (NSMs) as their origin. In contrast, core-collapse supernovae are ruled out as the main origin of heavy r-process elements (A>110) by recent numerical simulations. However, the properties characterizing NSM events - their rarity and high yield of r-process elements per event - have been claimed to be incompatible with the observed stellar records on r-process elements in the Galaxy. We add to this picture with our results, which show that the observed constant [r-process/H] ratio in faint dwarf galaxies and one star unusually rich in r-process in the Sculptor galaxy agree well with this rarity of NSM events. Furthermore, we found that a large scatter in the abundance ratios of r-process elements to iron in the Galactic halo can be reproduced by a scheme that incorporates an assembly of various protogalactic fragments, in each of which r-process elements supplied by NSMs pervade the whole fragment while supernovae distribute heavy elements only inside the regions swept up by the blast waves. Our results demonstrate that NSMs occurring at Galactic rate of 12-23 per Myr are the main site of r-process elements, and we predict the detection of gravitational waves from NSMs at a high rate with upcoming advanced detectors.

astro-ph.GA↗