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The impact of asymmetric neutrino emissions on nucleosynthesis in core-collapse supernovae II – progenitor dependences –

Shin-ichiro Fujimoto1 , Hiroki Nagakura2
1National Institute of Technology, Kumamoto College, Kumamoto 861-1102, Japan
2Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA
E-mail: [email protected]
(Accepted 2021 Jan. 16. Received 2020 Dec. 8; in original form 2021 Jan. 16)
Abstract

We investigate the impact of asymmetric neutrino-emissions on explosive nucleosynthesis in core-collapse supernovae (CCSNe) of progenitors with a mass range of 9.5 to 25MM_{\odot}. We perform axisymmetric, hydrodynamic simulations of the CCSN explosion with a simplified neutrino-transport, in which anti-correlated dipolar emissions of νe\nu_{\rm e} and ν¯e{\bar{\nu}}_{\rm e} are imposed. We then evaluate abundances and masses of the CCSN ejecta in a post-processing manner. We find that the asymmetric ν\nu-emission leads to the abundant ejection of pp- and nn-rich matter in the high-νe\nu_{\rm e} and -ν¯e{\bar{\nu}}_{\rm e} hemispheres, respectively. It substantially affects the abundances of the ejecta for elements heavier than Ni regardless of progenitors, although those elements lighter than Ca are less sensitive. Based on these results, we calculate the IMF-averaged abundances of the CCSN ejecta with taking into account the contribution from Type Ia SNe. For masy=10/3%m_{\rm asy}=10/3\% and 10%10\%, where masym_{\rm asy} denotes the asymmetric degree of the dipole components in the neutrino emissions, the averaged abundances for elements lighter than Y are comparable to those of the solar abundances, whereas those of elements heavier than Ge are overproduced in the case with masy30%m_{\rm asy}\geq 30\%. Our result also suggests that the effect of the asymmetric neutrino emissions is imprinted in the difference of abundance ratio of [Ni/Fe] and [Zn/Fe] between the high-νe\nu_{\rm e} and -ν¯e{\bar{\nu}}_{\rm e} hemispheres, indicating that the future spectroscopic X-ray observations of a CCSN remnant will bring evidence of the asymmetric neutrino emissions if exist.

keywords:
stars: supernova: general – nuclear reactions, nucleosynthesis, abundances – neutrinos
pubyear: 2021pagerange: The impact of asymmetric neutrino emissions on nucleosynthesis in core-collapse supernovae II – progenitor dependences –B

1 Introduction

A core-collapse supernova (CCSN) is an energetic explosion of a massive star with a mass of 8M\geq 8M_{\odot}, whose explosion energy reaches to 1051erg10^{51}\rm erg for a typical CCSN. Although the explosion mechanism is still uncertain, decades of modelling efforts with improving the input physics and numerical techniques made significant progress in our understanding of CCSN dynamics; indeed, we have witnessed that many multi-dimensional(D) simulations by modern CCSN codes in both 2D (Summa et al., 2016; Vartanyan et al., 2018; Nagakura et al., 2018; Pajkos et al., 2019; Harada et al., 2020; Burrows & Vartanyan, 2020) and 3D (Lentz et al., 2015; Roberts et al., 2016; Kuroda et al., 2018; Vartanyan et al., 2019a; Nagakura et al., 2019a; Müller et al., 2019; Glas et al., 2019; Walk et al., 2019; Burrows et al., 2020; Nagakura et al., 2020; Pan et al., 2020; Bollig et al., 2020) have demonstrated successful explosions. Although those results should be still considered provisional, the community has reached the consensus that the CCSN dynamics has strongly influenced on the dimension, indicating that their observational consequences such as nucleosynthesis in the ejecta need to be also analysed in multi-D, otherwise, the qualitative trend may be missed. In this paper, we consider the explosive nucleosynthesis under the multi-D treatments of both matter-dynamics and neutrino transport, albeit approximately.

CCSNe are also known as a cosmic factory of heavy elements synthesized via various channels of hydrostatic nuclear burning and explosive nucleosynthesis during the development of CCSN explosions. Extensive studies on the nucleosynthesis in massive stars and CCSNe have been performed, based on spherically symmetric models. The nucleosynthetic computations are carried out on hydrostatic stellar evolution codes from the main sequence to the pre-SN stage, and then they are taken over by hydrodynamic simulations for the subsequent CCSN explosion phase, in which a thermal bomb- or a piston-prescription has been usually adapted to trigger the explosion (Hashimoto, 1995; Woosley & Weaver, 1995; Thielemann et al., 1996; Woosley et al., 2002; Rauscher et al., 2002; Limongi & Chieffi, 2006; Heger & Woosley, 2010; Limongi & Chieffi, 2018). More systematic and elaborated studies in spherical symmetry have been initiated for large numbers of massive stars with a spherical hydrodynamic code with approximate neutrino transport and a simplified neutrino-core model for the explosion phase (Ugliano et al., 2012; Perego et al., 2015; Sukhbold et al., 2016). Multi-D effects on explosive nucleosynthesis in CCSN have been studied with a 2D hydrodynamic code in a thermal bomb manner (Nagataki et al., 1997, 1998; Nagataki, 2000) and examined for a more energetic, aspherical SN, or hypernovae (see, e.g., Maeda & Nomoto (2003); Nagataki et al. (2006)). r-process nucleosynthesis in magneto-hydrodynamic (MHD)-driven CCSNe and collapsar-jets, which would be rare events, has been investigated with multi-D MHD codes (see, e.g., Nishimura et al. (2006); Fujimoto et al. (2007, 2008); Ono et al. (2009); Winteler et al. (2012); Nishimura et al. (2015, 2017); Mösta et al. (2018); Halevi & Mösta (2018); Siegel et al. (2019); Miller et al. (2020); Reichert et al. (2020)). In the context of neutrino-driven CCSNe, the explosive nucleosynthesis has been also examined for multi-D simulations of CCSNe with approximated neutrino transport (Pruet et al., 2005, 2006; Fujimoto et al., 2011; Wanajo et al., 2011, 2013b, 2013a; Harris et al., 2017; Wongwathanarat et al., 2017; Eichler et al., 2018; Wanajo et al., 2018; Sieverding et al., 2020).

Asymmetric neutrino emissions accompanied by lepton-emission self-sustained asymmetry (LESA) have been observed in recent multi-D CCSN simulations with detailed neutrino transport among different groups (Tamborra et al., 2014; O’Connor & Couch, 2018; Vartanyan et al., 2019a) and the emissions with the NS kick are also revealed in a 2D axisymmetric CCSN simulation with full Boltzmann neutrino transport (Nagakura et al., 2019b). Those coherent asymmetric neutrino emissions have a potential influence on the explosive nucleosynthesis in the CCSN ejecta; hence, we studied them for an SN1987A-like progenitor with a mass of 19.4MM_{\odot} in Fujimoto & Nagakura (2019). Employed with an axisymmetric hydrodynamic code with a simplified neutrino transport and a neutrino-core model (see also Ugliano et al., 2012; Sukhbold et al., 2016), we have shown that the asymmetric emissions of ν\nu tend to yield larger amounts of nn- and pp-rich ejecta in the hemisphere of the higher ν¯e{\bar{\nu}}_{\rm e} and νe\nu_{\rm e} emissions, respectively. For small magnitudes of the neutrino asymmetry, or 10/3%10/3\% and 10%10\%, abundances of elements heavier than Zn are comparable to or slightly larger than those of the solar abundances. On the other hand, for larger asymmetric cases (30%\geq 30\%), the ejecta have too many elements compared to the solar abundances due to the larger amounts of the nn-rich ejecta.

In our previous study, however, we focused on the case with a single progenitor, which is not enough to make a detailed comparison to the solar abundances. To this end, we need to cover a wide range of progenitors on our nucleosynthetic computations, with which we calculate IMF-averaged abundances. It should be also mentioned that the comprehensive study of the progenitor dependence is inevitable to compare our results with future observations of SN remnants (SNRs). In this paper, we perform 2D hydrodynamic simulations of the CCSN explosion and nucleosynthetic calculations for six progenitors with a mass range from 9.5 to 25MM_{\odot}, in addition to the 19.4MM_{\odot} progenitor of our previous work. Note that we change the asymmetric degree of neutrino emissions in each progenitor, thus we have 35 models in total. We confirm that the overall trend is qualitatively in line with that found in our previous paper, although there are some diversities among progenitors, which we delve into in this study.

This paper is organized as follows. In section 2, we describe the methods and models for numerical simulations of 2D axisymmetric simulations of CCSN explosions and nucleosynthetic calculations. In section 3, we first summarize the essential trend found in our previous study for the 19.4M19.4M_{\odot} progenitor and then present the results of 2D CCSN simulations and nucleosynthetic computations for the other progenitors. We then discuss asymmetric distributions of the ejecta with particularly focusing on compositional differences in section 4, which provides an important insight towards spectroscopic X-ray observations on SNRs. Finally, we will summarize our conclusion in section 5.

2 Method & model

2.1 Axisymmetric simulations of CCSN explosion

As in our previous study (Fujimoto & Nagakura, 2019), we perform hydrodynamic simulations from core collapse to SN explosion of massive stars, employed with two codes, the GR1D code (O’Connor, 2015) and a modified Zeus 2D code (Stone & Norman, 1992a, b; Ohnishi et al., 2006, 2007; Fujimoto et al., 2011), in which a simplified ν\nu transport, or a light-bulb scheme, is adopted and appropriate weak interactions are taken into account (see detailed in Appendix of Fujimoto et al. (2011)). We follow matter evolution from the core collapse to the shock stole with the GR1D code in spherical symmetry, and then the results are mapped in the polar coordinate (r,θ)(r,\theta) on the Zeus 2D code. The computational domain in 2D simulations is from 5050\rm to 50,000km50,000{\,\rm km} in rr and 0θπ0\leq\theta\leq\pi, covered with (500,128)(500,128) meshes. The central part (50km\leq 50{\,\rm km}) of the proto-neutron star (proto-NS) is excised and is treated as the central point source with a mass of MM, which continuously increases due to mass accretion through the inner boundary (50km50{\,\rm km}) of the computational domain of our 2D simulations. At the time of remapping from 1D to 2D, the mass of a central point source is 1.2M1.2M_{\odot}, and we add non-radial (l=1l=1) perturbations on radial velocities, whose magnitudes are 1%1\%, to break the spherical symmetry.

In the present study, neutrinos are assumed to be emitted from the neutrinospheres with a thermal spectrum. We assume that the neutrino temperatures are spherical symmetric but luminosities have dipole components;

Lνe\displaystyle L_{\nu_{\rm e}} =\displaystyle= Lνe,ave(1+masycosθ),\displaystyle L_{\nu_{\rm e},\rm ave}(1+m_{\rm asy}\cos\theta), (1)
Lν¯e\displaystyle L_{{\bar{\nu}}_{\rm e}} =\displaystyle= Lν¯e,ave(1masycosθ),\displaystyle L_{{\bar{\nu}}_{\rm e},\rm ave}(1-m_{\rm asy}\cos\theta), (2)

where LνeL_{\nu_{\rm e}} and Lν¯eL_{{\bar{\nu}}_{\rm e}} are luminosity of νe\nu_{\rm e} and ν¯e{\bar{\nu}}_{\rm e} and Lνe,aveL_{\nu_{\rm e},\rm ave} and Lνe,aveL_{\nu_{\rm e},\rm ave} are angular-averaged LνeL_{\nu_{\rm e}} and Lν¯eL_{{\bar{\nu}}_{\rm e}}, respectively. Here the evolution of Lνe,aveL_{\nu_{\rm e},\rm ave}, Lν¯e,aveL_{{\bar{\nu}}_{\rm e},\rm ave} and ν\nu temperatures are evaluated with a ν\nu-core model from the mass accretion rate at the inner boundary of the computational domain, as in Ugliano et al. (2012); Sukhbold et al. (2016) but with some modifications (Appendix A). As in our previous work (Fujimoto & Nagakura, 2019), we have tuned two parameters of the ν\nu-core model so that the 19.4M19.4M_{\odot} progenitor explodes as SN1987A-like (the explosion energy, EexpE_{\rm exp}, 1051erg\sim 10^{51}\rm erg and the ejected mass of Ni56{}^{56}\rm Ni, MM(Ni56{}^{56}\rm Ni), of (0.070.08)M(0.07-0.08)M_{\odot}; ) for cases of spherical ν\nu emission (masy=0%m_{\rm asy}=0\%). We run the simulations by fixing those tunned parameters but changing masym_{\rm asy} as 0%, 10/3%, 10%, 30%, and 50%. We note that the typical asymmetry of neutrino emissions may be masy10%m_{\rm asy}\leq 10\% (see e.g., Tamborra et al., 2014; Nagakura et al., 2019b; Vartanyan et al., 2019b), although it is not definitive; hence, we investigate cases with higher asymmetric, neutrino-emissions in this study.

In the present study, we investigate the impact of neutrino asymmetry on explosive nucleosynthesis in CCSNe of progenitors (Woosley & Heger, 2015; Woosley et al., 2002) with a mass of 9.5M,11.2M,13.0M,15.0M,17.0M9.5M_{\odot},11.2M_{\odot},13.0M_{\odot},15.0M_{\odot},17.0M_{\odot}, and 25.0M25.0M_{\odot} in addition to the progenitor of 19.4M19.4M_{\odot}, whose detailed nucleosynthetic results are presented in our previous study (Fujimoto & Nagakura, 2019) (see also §3.1).

Refer to caption
Figure 1: Mass-density profiles as a function of a mass-coordinate MrM_{r} for the progenitors adopted in this study.

Figure 1 shows density profiles of the progenitors of Mms=9.5M,11.2M,13.0M,15.0M,17.0M,19.4MM_{\rm ms}=9.5M_{\odot},11.2M_{\odot},13.0M_{\odot},15.0M_{\odot},17.0M_{\odot},19.4M_{\odot}, and 25.0M25.0M_{\odot} as a function of a mass-coordinate MrM_{r}. The 9.5(15.0) MM_{\odot} progenitor has a density profile very similar to that of the 11.2(19.4)MM_{\odot} progenitor, but shallower at the mass coordinate, MrM_{r}, 1.5M\geq 1.5M_{\odot} (2.0M2.0M_{\odot}). At Mr1.5MM_{r}\geq 1.5M_{\odot}, the density of the 13.0 and 17.0MM_{\odot} progenitors lie between those of the light (9.5 and 11.2MM_{\odot}) progenitors and those of the 15.0 and 19.4MM_{\odot} progenitors, while the density is much higher for the 25.0MM_{\odot} progenitor. As we shall discuss below, the accretion component of neutrino luminosity is directly associated with the density profile at the onset of collapse, indicating that the difference of the density profile is responsible for the progenitor dependence of the impact of asymmetric neutrino emissions on the explosive nucleosynthesis (see Sec. 3 for more details).

It should be mentioned that a mass cut-off due to a black hole formation needs to be assumed in order to compute the IMF-averaged abundances. We refer Brown & Woosley (2013); Suzuki & Maeda (2018) which suggest that the mass cut-off with 2540M\geq 25-40M_{\odot} is appropriate. A maximum mass of 25M25M_{\odot}, therefore, seems to be adequate for the current study. It should be mentioned that the IMF-averaged [X/Fe] is insensitive to the mass cut-off in spherically symmetric neutrino emission models as long as it is 2540M\geq 25-40M_{\odot} (see Fujimoto et al. (2021) in preparation); hence our choice of the mass cut-off would give fewer impacts on our present results. We also note that the seven progenitors employed in this study are enough to compute the averaged IMF abundances, since the relative contribution of the averaged [X/Fe] over the seven progenitors are comparable to that over more than 20 progenitors from 9.5M9.5M_{\odot} to 25.0M25.0M_{\odot} in our spherically symmetric neutrino emission models (Fujimoto et al. (2021) in preparation).

We terminate our CCSN simulations when the shock wave reaches to 10,000km\sim 10,000{\,\rm km}. Figure 2 shows the time evolution of angular-averaged shock radii, rshr_{\rm sh} (left panel), Lνe,aveL_{\nu_{\rm e},\rm ave} (top right panel), and Lν¯e,aveL_{{\bar{\nu}}_{\rm e},\rm ave} (bottom right panel) for case with symmetric neutrino emission (masy=0%m_{\rm asy}=0\%) and for progenitors of 9.5M,11.2M,13.0M,15.0M,17.0M,19.4M9.5M_{\odot},11.2M_{\odot},13.0M_{\odot},15.0M_{\odot},17.0M_{\odot},19.4M_{\odot}, and 25.0M25.0M_{\odot} as a function of the time from the core bounce (tpbt_{\rm pb}). Note that the evolution of rshr_{\rm sh}, neutrino luminosities, and temperatures weakly depend on masym_{\rm asy}; hence, we display only the results with masy=0m_{\rm asy}=0 in these panels. For the light progenitors: 9.5M9.5M_{\odot}, 11.2M11.2M_{\odot}, 13.0M13.0M_{\odot} and, 17.0M17.0M_{\odot}, shock revivals occur at relatively early time (\sim 200  ms), while the shock wave for the other progenitors (15.0M15.0M_{\odot}, 19.4M19.4M_{\odot}, and 25.0M25.0M_{\odot}) revives at a later phase \sim 300  ms. Neutrino luminosities are low for the light progenitors of 9.5M9.5M_{\odot} and 11.2M11.2M_{\odot}, whereas they are higher for heavier progenitors due to larger mass accretion rates. In particular, for progenitors with high compactness (Mms=15.0MM_{\rm ms}=15.0M_{\odot}, 19.4M19.4M_{\odot}, and 25.0M25.0M_{\odot}), the luminosities and temperatures are remarkably higher than other models.

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Figure 2: Time evolution of the angular-averaged shock radii rshr_{\rm sh} (left panel), and Lνe,aveL_{\nu_{\rm e},\rm ave} (top right panel), and Lν¯e,aveL_{{\bar{\nu}}_{\rm e},\rm ave} (bottom right panel) as a function of tpbt_{\rm pb}. We note that they are less sensitive to masym_{\rm asy}; hence, we only display the results with masy=0%m_{\rm asy}=0\%.

2.2 Nucleosynthetic calculation

We follow Lagrangian thermodynamic histories of SN ejecta for nucleosynthetic computation with a tracer particle method (Nagataki et al., 1997; Seitenzahl et al., 2010; Harris et al., 2017), in which tracer particles evolve in accordance with the fluid-velocity of the CCSN simulations while storing their physical quantities such as density, temperature, and electron fraction, YeY_{e}. We distribute 6,000 tracer particles in the regions from 1,0001,000km to 10,00010,000km (or an O-rich layer) at the beginning of the simulations, with being adaptively weighted to the mass of a layer where the particle is located. By virtue of the adaptive mass of tracer particles, the highest resolutions in the particle mass become (105\sim(10^{-5}-104)M10^{-4})M_{\odot} in the present study. We made a sensitivity study of nuclear abundances to the particle numbers in (Fujimoto et al., 2011), and a reasonable convergence is achieved at 6,000 particles; indeed, we found that the differences in ejected masses of nuclei between 3,000 and 6,000 particles cases are less than \sim 1%, hence, we adopted 6,000 particles in this study. In our models, \sim 3,000-4,500 particles are ejected by SN explosions, whose masses weakly depend on masym_{\rm asy} and are evaluated to be 0.09MM_{\odot}, (0.20-0.22)MM_{\odot}, (0.35-0.36)MM_{\odot}, (0.44-0.47)MM_{\odot}, (0.53-0.56)MM_{\odot}, (0.52-0.58)MM_{\odot}, and (0.78-0.81)MM_{\odot}, for progenitors with (9.5,11.2,13.0,15.0,17.0,19.4(9.5,11.2,13.0,15.0,17.0,19.4, and 25.0)M25.0)M_{\odot}.

We follow the abundance evolution of 2,488 nuclides from nn, pp to Nd196{}^{196}\rm Nd in the CCSN ejecta by employing the Lagrangian thermodynamic histories of the ejecta and a nuclear reaction network as in our previous study (Fujimoto & Nagakura, 2019). Note that, when the ejecta become hotter than 9×109K9\times 10^{9}{\,\rm K}, we set the abundances of the nuclides to be the chemical composition in nuclear statistical equilibrium. We also take into account abundance change via neutrino interactions for He and nuclei from C to Kr as in Fujimoto et al. (2011). In the nucleosynthetic computations, we recompute YeY_{e} following the weak interactions employed in the nuclear reaction network, albeit possessing YeY_{e} data from the hydrodynamic simulations. This is mainly because more elaborate weak reactions are incorporated in the nuclear reaction network than those used in 2D hydrodynamic simulations (Fujimoto et al., 2011), and the recomputation is necessary in particular for reversed-LνL_{\nu} cases (see below) in nucleosynthetic computations.

We note that the deviation of the neutrino spectra from a Fermi-Dirac distribution has a negligible effect on the composition of the SN ejecta via ν\nu-processes. The impact was well investigated by the two simulations in Sieverding et al. (2019); a simulation of a piston-driven CCSN explosion of a 27M27M_{\odot} progenitor and Eexp=1.2×1051ergE_{\rm exp}=1.2\times 10^{51}\,{\rm erg} with a quasi-thermal energy distribution of ν\nu and a spherically symmetric, artificially exploded CCSN simulation of the 27M27M_{\odot} progenitor, whose ν\nu luminosities and spectra evaluated with a hydrodynamic code, in which an approximated neutrino transport is taken into account. We, hence, ignore the effect of deviations from the thermal spectrum in neutrino spectra in this study.

We find that in the region, rcc10,000kmr_{\rm cc}\leq 10,000{\,\rm km}, the peak temperature of the SN shock wave becomes higher than 1×109K\sim 1\times 10^{9}{\,\rm K}, where nuclei heavier than C are abundantly synthesized. Here rccr_{\rm cc} is the radius of the ejecta at the onset of the core collapse. We therefore conduct nucleosynthetic computation only for the tracer particles, which are located rcc10,000kmr_{\rm cc}\leq 10,000{\,\rm km}. For the initial abundances of the particles, we take abundances of 20 nuclei in the progenitors at the onset of the core collapse. The abundances are taken from the results of stellar evolution in Woosley et al. (2002). For 15.0M15.0M_{\odot} and 25.0M25.0M_{\odot} progenitors, we also adopt alternative abundance data of more than 1,300 nuclei (Rauscher et al., 2002), which consist of those with Z83Z\leq 83 including odd ZZ nuclei as well as ss-process nuclei produced via the weak ss-process. The chemical composition of the outer ejecta with rcc>10,000kmr_{\rm cc}>10,000{\,\rm km} is set to be the abundances in the progenitors at the onset of the core collapse, ignoring the γ\gamma-processes in pp-process layers (Rayet et al., 1995) and the ν\nu-processes in the outer layers (Sieverding et al., 2019). We note that the ejected masses from the region r>rccr>r_{\rm cc} are 7.73M7.73M_{\odot}, 9.30M9.30M_{\odot}, 9.49M9.49M_{\odot}, 10.5M10.5M_{\odot}, 11.7M11.7M_{\odot}, 13.0M13.0M_{\odot}, and 9.75M9.75M_{\odot} for progenitors with 9.5M,11.2M,13.0M,15.0M,17.0M9.5M_{\odot},11.2M_{\odot},13.0M_{\odot},15.0M_{\odot},17.0M_{\odot}, 19.4M19.4M_{\odot}, and 25.0M25.0M_{\odot}.

Recent multi-D CCSN simulations reveal the correlation between the directions of stronger shock and the hemisphere with high-ν¯e{\bar{\nu}}_{\rm e} emissions (the high-ν¯e{\bar{\nu}}_{\rm e} hemisphere) (see, e.g., Tamborra et al. (2014); Nagakura et al. (2019b); Vartanyan et al. (2019b)). In some of our models, however, the direction of the high-ν¯e{\bar{\nu}}_{\rm e} hemisphere does not correlate with that of the stronger shock expansion. It is attributed to the fact that the shock revival takes place in stochastic directions, meanwhile we set a priori the dipole direction of neutrino asymmetry. We find that ten models are mis-correlated; (Mms/MM_{\rm ms}/M_{\odot}, masy/%m_{\rm asy}/\%) = (9.5,10)(9.5,10), (9.5,30)(9.5,30), (13.0,10)(13.0,10), (13.0,30)(13.0,30), (13.0,50)(13.0,50), (15.0,30)(15.0,30), (17.0,10)(17.0,10), (17.0,30)(17.0,30), (17.0,50)(17.0,50), and (25.0,10)(25.0,10). For the nucleosynthetic calculation in these mis-correlated models, we retain the matter data but reverse the direction of ν\nu asymmetry in nucleosynthetic computations. We discuss the impact of the reversed-LνL_{\nu} procedure in Appendix B, and confirm that it is a reasonable prescription to obtain qualitatively accurate results.

3 results

3.1 Summary of our results in previous paper

We briefly summarize the essential results of our previous study (Fujimoto & Nagakura, 2019): the impact of asymmetric neutrino emissions on explosive nucleosynthesis for a CCSN of the 19.4M19.4M_{\odot} progenitor. As shown in Sec. 2.1, the progenitor is tuned to reveal the SN1987A-like explosion with Eexp1051ergE_{\rm exp}\sim 10^{51}\rm erg and M(M(Ni56{}^{56}\rm Ni)(0.070.08)M)\sim(0.07-0.08)M_{\odot}. In our model, the shock wave is revived at 0.3\sim 0.3 s after the core bounce (Fig. 2) and then it expands quasi-spherically. The explosion exhibits the aspherical distribution of the density, temperature, and entropy of neutrino-driven inner ejecta. Although the fluid-dynamics is less sensitive to masym_{\rm asy}, the distribution of YeY_{e} of SN ejecta strongly depends on masym_{\rm asy} due to a different amount of ν\nu absorption through the different degree of the dipolar and anti-correlated ν\nu emissions (Eqs. (1) and (2)); In the hemisphere of π/2<θπ\pi/2<\theta\leq\pi (the high-ν¯e{\bar{\nu}}_{\rm e} hemisphere), the asymmetric emissions of ν\nu tend to yield larger amounts of nn-rich ejecta (Ye<0.49Y_{e}<0.49), while pp-rich matter (Ye>0.51Y_{e}>0.51) are ejected in the opposite hemisphere of the higher νe\nu_{\rm e} emissions (0θ<π/20\leq\theta<\pi/2; the high-νe\nu_{\rm e} hemisphere). See also Fig. 1 in Fujimoto & Nagakura (2019) for more details.

For larger asymmetric cases with masy30%m_{\rm asy}\geq 30\%, the larger amounts of the nn-rich ejecta are produced in the high-ν¯e{\bar{\nu}}_{\rm e} hemisphere. As a result, there are too many elements heavier than Zn compared to the solar abundances (Fig. 4 in Fujimoto & Nagakura (2019)). For cases with small masym_{\rm asy} (10/3%10/3\% and 10%10\%), abundances of the elements are comparable to or slightly larger than those of the solar abundances. We also observed the characteristic features in elemental distribution; abundances lighter than Ca are insensitive to masym_{\rm asy} and the production of Ni, Zn, and Ge is much larger in the nn-rich ejecta in the high-ν¯e{\bar{\nu}}_{\rm e} hemisphere than those in the pp-rich ejecta in the high-νe\nu_{\rm e} hemisphere even for smaller asymmetric cases with 10%\leq 10\%. With these results in mind, let us delve into the progenitor dependence, which is the main subject in this paper.

3.2 Abundances of SN ejecta

Figure 3 shows mass profiles of dMejdM_{\rm ej} in Ye,1Y_{\rm e,1} of the ejecta from the inner region (rcc10,000kmr_{\rm cc}\leq 10,000{\,\rm km}) for the progenitors with Mms=9.5M,11.2M,13.0M,15.0M,17.0MM_{\rm ms}=9.5M_{\odot},11.2M_{\odot},13.0M_{\odot},15.0M_{\odot},17.0M_{\odot}, and 25.0M25.0M_{\odot}, and for cases with masy=m_{\rm asy}= 0%, 10/3%, 10%, 30%, and 50%. Here Ye,1Y_{\rm e,1} is the electron fraction evaluated when the temperature is equal to 1×109K1\times 10^{9}\rm K during the ejection and dMejdM_{\rm ej} is a mass of ejecta integrated with a bin of dYe,1=0.005dY_{\rm e,1}=0.005. We note that YeY_{e} of the ejecta chiefly changes through the neutrino absorption near the proto-NS during the ejection, implying that it freezes out at the place with YeYe,1Y_{e}\sim Y_{\rm e,1} where it is far away from proto-NS in this phase (see, e.g., Fig.4 in Fujimoto et al., 2011). The ejecta goes through the adiabatic cooling with expansions but YeY_{e} re-increases from a certain point due to subsequent β\beta-decays of unstable nuclei in the ejecta. For larger masym_{\rm asy}, Ye,1Y_{\rm e,1} distributions are widely varying, indicating that mass fractions of nn- and pp-rich ejecta become larger due to larger ν¯e{\bar{\nu}}_{\rm e} and νe\nu_{\rm e} absorptions on nucleon in the ejecta. We also find that masses of nn-rich ejecta tend to be larger with larger amounts of the inner ejecta and with larger ν¯e{\bar{\nu}}_{\rm e} luminosities. It should be mentioned, however, that the mass of nn-rich ejecta is not a monotonic function of the progenitor mass; indeed, they are smaller in 17.0M17.0M_{\odot} (11.2M11.2M_{\odot}) than in 15.0M15.0M_{\odot} (9.5M9.5M_{\odot}). It is attributed to the fact that the mass density profile of progenitors just prior to the onset of gravitational collapse is a non-monotonic order to the zero age main-sequence mass. This can be seen in Fig. 1; there are regions where the density becomes lower in 17.0M17.0M_{\odot} (11.2M11.2M_{\odot}) than 15.0M15.0M_{\odot} (9.5M9.5M_{\odot}), which results in decreasing accretion components of neutrino luminosity (see also Fig.2). As a result, the impact of asymmetric neutrino emissions becomes weak, implying that the production of nn-rich ejecta is less efficient.

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Figure 3: Progenitor dependence of mass profiles of dMejdM_{\rm ej} in Ye,1Y_{\rm e,1} of the ejecta from the inner region (rcc10,000kmr_{\rm cc}\leq 10,000{\,\rm km}). We display the results of all progenitors except for Mms=19.4MM_{\rm ms}=19.4M_{\odot} (see Fig. 3 in Fujimoto & Nagakura, 2019, for Mms=19.4MM_{\rm ms}=19.4M_{\odot} model). The asymmetric degree of the neutrino emissions, masym_{\rm asy}, is increased from top to bottom in each panel.

Figure 4 shows the composition of the ejecta for our models in terms of [X/Fe] 111[A/B] = log[(XA/XA,)/(XB/XB,)]\log\left[(X_{\rm A}/X_{\rm A,\odot})/(X_{\rm B}/X_{\rm B,\odot})\right], where XiX_{\rm i} and Xi,X_{\rm i,\odot} are a mass fraction of an element i\rm i and its solar value (Anders & Grevesse, 1989), respectively. of the ejecta as a function of an atomic number ZZ for the progenitors with Mms=9.5M,11.2M,13.0M,15.0M,17.0MM_{\rm ms}=9.5M_{\odot},11.2M_{\odot},13.0M_{\odot},15.0M_{\odot},17.0M_{\odot}, and 25.0M25.0M_{\odot} and for cases with masy=m_{\rm asy}= 0%, 10/3%, 10%, 30%, and 50%. We present [X/Fe] for cases with initial abundances of 20 nuclei in the stellar evolution code (Woosley et al., 2002) (solid-lines with filled-squares) and those for cases with the alternative initial abundances of more than 1,300 nuclei (Rauscher et al., 2002) (dashed-lines with open-squares) only for the 15.0M15.0M_{\odot} and 25.0M25.0M_{\odot} progenitors. We find that [X/Fe] of odd-ZZ elements and elements with Z27Z\geq 27 are largely underproduced if we adopt the initial abundances of the 20 nuclei, while [X/Fe] of even-ZZ elements with Z26Z\leq 26 are comparable. Hence, the pronounced odd-even patterns of [X/Fe] for elements lighter than Fe would be due to the small number of initial abundances (20 nuclei) in our simulations. The comparison suggests that the odd-even patterns of [X/Fe] found in other progenitor models may disappear if we employ initial abundances computed with larger nuclear reaction network. Keeping in mind the systematic error, let us study the progenitor dependence of the impact of asymmetric neutrino emissions on the abundance pattern in the ejecta.

As in case with the 19.4M19.4M_{\odot} progenitor (see Fig.4 in Fujimoto & Nagakura, 2019), [X/Fe] for elements lighter than Ca are insensitive to masym_{\rm asy}. For symmetric LνL_{\nu} cases (masy=0m_{\rm asy}=0), elements heavier than Cu (Z=29Z=29) other than Sr, Y, and Zr are underproduced except for the progenitor of 9.5M9.5M_{\odot}. The exceptional trend found in 9.5M9.5M_{\odot} model is mainly due to the fact that the fractions of nn-rich ejecta are large even in the case with masy=0m_{\rm asy}=0 (see top left panel in Fig.3). It should be mentioned that isotopes of Sr, Y, and Zr with the neutron number N=50N=50 are abundantly produced in nn-rich ejecta (Ye,10.49Y_{\rm e,1}\leq 0.49). which appear appreciably in 9.5, 15.0, and 17.0MM_{\odot} models even in the case with masy=0m_{\rm asy}=0. For larger masym_{\rm asy}, on the other hand, [X/Fe] for elements heavier than Zn (Z=30Z=30) become larger due to larger fractions of nn-rich elements. In particular, for cases with masy30%m_{\rm asy}\geq 30\%, these elements are overproduced compared with the solar abundances regardless of progenitors. We also note that Zn is synthesized in the p-rich ejecta but the contribution to Zn in the ejecta is minor because of the small abundances (mass fraction <0.01<0.01; see e.g., Fig.10 in Wanajo et al. (2018)).

Refer to caption
Refer to caption
Figure 4: Same as Fig. 3 but for [X/Fe]. masym_{\rm asy} is 0%, 10/3%, 10%, 30%, and 50% from top to bottom in each panel. Solid-lines with filled-squares and dashed-lines with open-squares show [X/Fe] for cases with the initial abundances of 20 nuclei in a stellar evolution code (Woosley et al., 2002) and of more than 1,300 nuclei (Rauscher et al., 2002) (only for the 15.0M15.0M_{\odot} and 25.0M25.0M_{\odot} progenitors), respectively. We note that the pronounced odd-even patterns of [X/Fe] for elements lighter than Fe would be a numerical artifact due to the small number of the initial abundances. See the text for more details.

It is also interesting to be mentioned that [Zn/Fe] for the case with masy=10/3%m_{\rm asy}=10/3\% in 9.5MM_{\odot} model is less than those for masy=0m_{\rm asy}=0, indicating that the abundance ratio is not always monotonic function with masym_{\rm asy}. This peculiar trend can be understood as follows. Let us first point out that Zn is chiefly composed by Zn64{}^{64}\rm Zn and Zn66{}^{66}\rm Zn and abundantly synthesized in the ejecta with Ye,10.450.48Y_{e,1}\sim 0.45-0.48 and 0.43\leq 0.43 (see, e.g., Fig.10 in Wanajo et al. (2018)), i.e., there is a hole between the two Ye,1Y_{e,1} regions. For the 9.5MM_{\odot} model, Zn is mainly produced in the region of Ye,10.450.48Y_{e,1}\sim 0.45-0.48 in the case with masy=0m_{\rm asy}=0. With increasing masym_{\rm asy} to 10/3%10/3\%, the ejecta tends to be shifted to the region of the lower Ye,1Y_{e,1} due to the higher ν¯e{\bar{\nu}}_{\rm e} absorptions, and some of them get into the hole (see in the top, left figure on Fig. 3), results in decreasing [Zn/Fe].

We now turn our attention to the comparison between IMF-averaged abundances of our results and the solar one. We average abundances of the ejecta with the Salpeter IMF weighted by their masses over the seven progenitors of 9.5M,11.2M,13.0M,15.0M,17.0M,19.4M9.5M_{\odot},11.2M_{\odot},13.0M_{\odot},15.0M_{\odot},17.0M_{\odot},19.4M_{\odot}, and 25.0M25.0M_{\odot}, setting the minimum and maximum masses of CCSNe to be 9.5M9.5M_{\odot} and 25M25M_{\odot}, respectively. The abundances of the other progenitors are computed by the linear interpolation of the results of our models with the progenitors of nearby MmsM_{\rm ms}. Figure 5 shows IMF-averaged [X/Fe] of the ejecta for cases with masy=m_{\rm asy}= 0%, 10/3%, 10%, 30%, and 50% in panels from top to bottom. Solid-lines with filled-squares show averaged [X/Fe] for cases with the initial abundances of 20 nuclei in a stellar evolution code (Woosley et al., 2002), while dashed-lines with open-squares portray those with more than 1,300 nuclei (Rauscher et al., 2002) only for the 15.0M15.0M_{\odot} and 25.0M25.0M_{\odot} progenitors. We find that IMF-averaged [X/Fe] for elements with 30Z4030\leq Z\leq 40 in cases with the smaller asymmetry of masy=10/3%m_{\rm asy}=10/3\% and 10%10\% are roughly consistent with the solar abundance, although Rb and Zr are slightly underproduced and overproduced, respectively. On the other hand, those for the elements with 30Z4030\leq Z\leq 40 with masy30%m_{\rm asy}\geq 30\% are remarkably overproduced compared with the solar abundances.

It should be mentioned that the contribution to [X/Fe] for elements with 30Z4030\leq Z\leq 40 of the weak ss-process, which is taken into account in cases with the initial abundances of more than 1,300 nuclei (dashed-lines with open-squares), dominates over that of the nn-rich ejecta. We also emphasize that for the symmetric ν\nu-emission (masy=0m_{\rm asy}=0; top panel) elements heavier than Se (Z=34Z=34) are underproduced compared with the solar abundances, even if we consider the contribution of the weak ss-process (dashed-lines with open-squares); hence the nn-rich ejecta synthesized via the asymmetric neutrino emissions play a complementary role to approach the solar abundance.

Refer to caption
Figure 5: IMF-averaged [X/Fe] of total ejecta for cases with masy=m_{\rm asy}= 0%, 10/3%, 10%, 30%, and 50% in panels from top to bottom. Solid-lines with filled-squares and dashed-lines with open-squares represent the cases with the initial abundances of 20 nuclei in a stellar evolution code (Woosley et al., 2002) and of more than 1,300 nuclei (Rauscher et al., 2002) only for the 15.0M15.0M_{\odot} and 25.0M25.0M_{\odot} progenitors, respectively.

In short, we conclude that ν\nu asymmetry with masy30%m_{\rm asy}\geq 30\% is excluded in light of the solar abundances. We must mention a caveat, however; masym_{\rm asy} for the light progenitors of 9.5M9.5M_{\odot} and 11.2M11.2M_{\odot} cannot be constrained from the investigation due to the small masses of the nn-rich ejecta (top, left and center panels of Fig. 3) and thus a small contribution to elements with Z31Z\geq 31.

There is another caveat in the above analysis; we need to take into account the contribution from Type Ia SN (SNIa) since it is the dominant contributor to the solar abundances of the iron-group elements. Hence, we add the contribution of SNIa to the IMF-averaged [X/Fe] of our CCSN ejecta so that [O/Fe] becomes zero. We note that the resultant [X/Fe] with the solar abundances, which correspond to, by definition, is set to be zero following the convention in the literature. Figure 6 shows [X/Fe] of the IMF-averaged CCSN ejecta added by the contribution of the SNIa ejecta of the W7 model (Leung & Nomoto, 2018), in which a Ni-overproduction problem is resolved (Kobayashi et al., 2020), for cases with masy=m_{\rm asy}= 0%, 10/3%, 10%, 30%, and 50% in panels from top to bottom. We find that IMF-averaged [X/Fe] with the SNIa contribution well reproduce the solar abundances of elements from Z=639Z=6-39 except for F for cases with the smaller asymmetry of masy=10/3%m_{\rm asy}=10/3\% and 10%10\% and cases with the initial abundances of more than 1,300 nuclei. We note that the underproduction of F would not be a problem, since it can be complemented by the AGB stars (Spitoni et al., 2018; Olive & Vangioni, 2019) and rotating massive stars (Limongi & Chieffi, 2018; Choplin et al., 2018) through ss-processes.

Refer to caption
Figure 6: Same as Fig. 4 but adding the contribution from SNIa based on W7 model (Leung & Nomoto, 2018). masym_{\rm asy} is 0%, 10/3%, 10%, 30%, and 50% from top to bottom in each panel.

To see the importance of each progenitor to the IMF-averaged [X/Fe] of the CCSN ejecta quantitatively, we compute the relative contribution to the IMF-averaged [X/Fe] of the progenitors of as a function of ZZ for cases with masy=m_{\rm asy}= 0% and 10% (see Figure 7). In the computation, we adopt abundances of ejecta for cases with the full initial compositions as the abundances of ejecta for the progenitors of 15M15M_{\odot} and 25M25M_{\odot} and thus the contribution of ss-process elements is partially included. We evaluate the relative contribution of a progenitor with Mms/M=mi=[9.5,11.2,13.0,15.0,17.0,19.4,25.0]M_{\rm ms}/M_{\odot}=m_{i}=[9.5,11.2,13.0,15.0,17.0,19.4,25.0] for i=1,2,,7i=1,2,...,7. as the IMF-averaged [X/Fe] over progenitors from MlM_{l} to MuM_{u}, where we set {Ml/M,Mu/M}\{M_{l}/M_{\odot},M_{u}/M_{\odot}\} to {(mi1+mi)/2,(mi+mi+1)/2}\{(m_{i-1}+m_{i})/2,(m_{i}+m_{i+1})/2\} for i=2,3,,6i=2,3,...,6, {9.5,(mi+mi+1)/2}\{9.5,(m_{i}+m_{i+1})/2\} for i=1i=1, and {(mi1+mi)/2,25}\{(m_{i-1}+m_{i})/2,25\} for i=7i=7. We omit the contribution of odd-ZZ elements with Z<30Z<30 in Figure 7. This is because the contribution is strongly inherited from the initial compositions adopted in each model and is incorrectly dominant over that for the progenitors of 15M15M_{\odot} and 25M25M_{\odot} in the current evaluation. We find that the relative contribution to elements lighter than Ni is insensitive to masym_{\rm asy}. On the contrary, it is very sensitive to masym_{\rm asy} for each element of Z=3040Z=30-40. Our result suggests that the appropriate multi-D treatments are required in both CCSN and nucleosynthesis computations to determine the primary CCSN progenitor producing each element in the range of Z=3040Z=30-40.

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Figure 7: Relative contribution to IMF-averaged [X/Fe] of the mass ranges (see text for more details) of the progenitors around 9.5M,11.2M,13.0M,15.0M,17.0M,19.4M9.5M_{\odot},11.2M_{\odot},13.0M_{\odot},15.0M_{\odot},17.0M_{\odot},19.4M_{\odot}, and 25.0M25.0M_{\odot} (from bottom to top) as a function of ZZ for cases with masy=m_{\rm asy}= 0% (left panel) and masy=m_{\rm asy}= 10% (right panel). We omit the contribution of odd-ZZ elements with Z<30Z<30, which strongly depends on the initial compositions of each model. For the progenitors of 15M15M_{\odot} and 25M25M_{\odot}, we adopt abundances of ejecta for cases with the full initial compositions as the abundances of ejecta.

4 Asymmetric distributions of heavy elements in CCSN ejecta

Next, we look into the compositional differences between high-νe\nu_{\rm e} and ν¯e{\bar{\nu}}_{\rm e} hemispheres, where pp-rich ejecta (Ye,1>0.51Y_{\rm e,1}>0.51) and nn-rich ejecta (Ye,1<0.49Y_{\rm e,1}<0.49) abundantly exist, respectively (Fig.1 in Fujimoto & Nagakura (2019)). This study would provide valuable insight into the analysis of SNR observations (see below). Hereafter, we distinguish the two hemispheres geometrically for the case with masy=0m_{\rm asy}=0 (spherical ν\nu emission) in order to tentatively compare the cases with asymmetric neutrino emissions; we assign 0θπ/20\leq\theta\leq\pi/2 and π/2<θπ\pi/2<\theta\leq\pi as northern and southern hemispheres, respectively.

In our previous work, we showed that for the 19.4MM_{\odot} progenitor, the compositional differences between the two hemispheres are small for elements lighter than Ca but appreciable for elements heavier than Cu (Fig.5 in Fujimoto & Nagakura (2019)). The trend can be seen in the other progenitors but the 15.0MM_{\odot} one is an exception. The peculiarity of the 15.0MM_{\odot} progenitor can be seen in Figure 8, which shows northern- or southern- averaged [X/Fe] of the total ejecta (top figure) and the inner ejecta (bottom figure), respectively, for cases with masy=m_{\rm asy}= 0% (top panels in each figure), 10/3% (middle panels), and 10% (bottom panels). We note that abundances in the high-ν¯e{\bar{\nu}}_{\rm e} hemisphere are very similar to those of all the ejecta (dotted lines with open-squares in the bottom left panel of Fig. 4) and that the similarity is common for all progenitor models. On the other hand, the differences in [X/Fe] for elements lighter than Ca between high-νe\nu_{\rm e} (or northern) and ν¯e{\bar{\nu}}_{\rm e} (or southern) hemispheres emerge only in 15M15M_{\odot} model due to the large asymmetric explosion. The differences for Sr, Y, and Zr can be clearly seen even for the spherical ν\nu emission (the top panels in each figure) and those for Ni, Zn, Ge, Se, and Mo become more prominent in the cases with larger masym_{\rm asy}. Focusing on the inner ejecta (bottom figure), in which amounts of ss-process elements are smaller than the total ejecta, we find that the differences for Ni, Zn, Ge, Se, and Mo are further enhanced.

Refer to caption
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Figure 8: [X/Fe] of the total ejecta (top figure) and the inner one (rcc10,000kmr_{\rm cc}\leq 10,000{\,\rm km}; bottom figure) for cases with 15.0M15.0M_{\odot} model. The solid and dotted lines denote the results in the high-νe\nu_{\rm e} and high-ν¯e{\bar{\nu}}_{\rm e} (dotted lines) hemispheres, respectively. masym_{\rm asy} is 0%, 10/3%, and 10% from top to bottom in each panel.

Figure 9 shows the progenitor dependence of [Ni/Fe] and [Zn/Fe] of the ejecta in the high-νe\nu_{\rm e} (or northern) and the high-ν¯e{\bar{\nu}}_{\rm e} (or southern) hemispheres for masy=m_{\rm asy}= 0%, 10/3%, and 10%. Filled and open circles indicate [Ni/Fe] in the high-ν¯e{\bar{\nu}}_{\rm e} (or southern) hemispheres and the high-νe\nu_{\rm e} (or northern), respectively, while filled and open triangles show [Zn/Fe] in the high-ν¯e{\bar{\nu}}_{\rm e} (or southern) hemispheres and the high-νe\nu_{\rm e} (or northern), respectively. For cases with the symmetric ν\nu emission (top panel), the differences of both [Ni/Fe] and [Zn/Fe] between the two hemispheres are small except in [Zn/Fe] for the light progenitors of 9.5M9.5M_{\odot} and 11.2M11.2M_{\odot}, in which Zn is abundantly produced (top left and center panels in Fig. 4). The differences in these progenitors are chiefly caused by asymmetric and early-phase shock revival, which results in producing the asymmetric nn-rich ejecta. We note that it is the similar case as SN explosion of a super-AGB progenitor (Wanajo et al., 2011).

With increasing masym_{\rm asy}, on the other hand, the differences of [Ni/Fe] and [Zn/Fe] between the two hemispheres becomes larger regardless of progenitors, and they are more prominent if we remove the contribution of the O-rich ejecta. However, We note that, although the asymmetric distribution of [Zn/Fe] is larger than that in [Ni/Fe], the ejected masses of Zn are smaller by an order of magnitude compared with those of Ni, indicating that the asymmetry of [Ni/Fe] may be easier to be measured than that of [Zn/Fe] in real observations (but see below). We also find that the asymmetric distributions of both [Ni/Fe] and [Zn/Fe] are significant for the lighter progenitors with a mass of 13M\leq 13M_{\odot}.

Our result suggests that the asymmetric distributions of [Ni/Fe] and [Zn/Fe] in the ejecta would be a sign of the existence of the asymmetric neutrino emissions in a CCSN core except for very light progenitors. We also note that, although the mass of Zn is much smaller than Ni, the ejecta mass tends to be larger for the heavier progenitors (15M\geq 15M_{\odot}), indicating that asymmetric distributions of [Zn/Fe] may be directly resolved by the future observations (see below). For those observations, it would be crucial to spatially resolve the ejecta, since the asymmetry would be imprinted more vividly in the inner region.

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Figure 9: Progenitor dependence of [Ni/Fe] and [Zn/Fe] of the total ejecta. Filled and open circles (triangles) indicate [Ni/Fe] ([Zn/Fe]) in the high-ν¯e{\bar{\nu}}_{\rm e} (or southern) hemispheres and the high-νe\nu_{\rm e} (or northern), respectively. We show the results with masy=m_{\rm asy}= 0%, 10/3%, and 10% from top to bottom. For a display purpose, [Ni/Fe] (circles) slightly move to the horizontal direction.

5 Conclusion

This paper is a sequel to Fujimoto & Nagakura (2019), in which we investigated the impacts of the asymmetric neutrino emissions on explosive nucleosynthesis for the 19.4MM_{\odot} progenitor. In the present study, we extend the investigation to seven progenitors with a mass range from 9.5 to 25MM_{\odot} to see the progenitor dependence and evaluate the IMF-averaged abundances. We performed 35 axisymmetric, hydrodynamic simulations of the CCSN explosion in total, employed with a 2D hydrodynamic code with a simplified neutrino transport. To measure the impacts of asymmetric neutrino emissions, we systematically change the amplitude of the dipole component of neutrino luminosities (masym_{\rm asy}) from 0%0\% to 50%50\% (see Eqs. 1 and 2). The 2D hydrodynamic data is post-processed with tracer particles and then we have evaluated abundances and masses of the SN ejecta with the nuclear reaction network of about 2,500 nuclei from nn, pp to Nd. The IMF-averaged abundances are computed with taking into account the contribution from type Ia SNe, and we find that the even-ZZ elements lighter than Fe is less sensitive to masym_{\rm asy} (see Fig.6). However, some remarkable dependence on masym_{\rm asy} emerges in our analyses, which are briefly summarized as follows;

  1. 1.

    The asymmetric ν\nu emissions lead to the abundant ejection of pp- and nn-rich matter in the high-νe\nu_{\rm e} and -ν¯e{\bar{\nu}}_{\rm e} hemispheres, respectively.

  2. 2.

    The asymmetric ejection of the nn-rich matter in the high-ν¯e{\bar{\nu}}_{\rm e} hemisphere gives a strong impact on the abundances of elements heavier than Ni regardless of progenitors (see Fig.4).

  3. 3.

    For masy=10/3%m_{\rm asy}=10/3\% and 10%10\%, which are comparable to those appeared in recent multi-D simulations of CCSNe (Tamborra et al., 2014; Nagakura et al., 2019b; Vartanyan et al., 2019b), the averaged abundances for elements lighter than Y are comparable to those of the solar abundances (see Fig.6).

  4. 4.

    For masy30%m_{\rm asy}\geq 30\%, however, the averaged abundances for elements heavier than Ge are clearly overproduced compared with the solar abundances (see Fig.6).

  5. 5.

    The compositional differences of elements heavier than Ni emerge, in general, between the two hemispheres in the case with high masym_{\rm asy} regardless of progenitors. However, the detail is a bit complex; indeed, the strong asymmetric explosions (as the 15.0M15.0M_{\odot} models) lead to some peculiar trends in the compositional difference (see Fig. 9).

Last but not least, we briefly discuss the implications of our results for future observations of SNR. The proper motion of NS triggered by the asymmetric CCSN explosion is expected in the opposite direction to stronger shock expansion (see, e.g., Scheck et al., 2006; Nordhaus et al., 2010, 2012; Wongwathanarat et al., 2013; Nagakura et al., 2019b). The temperature in the post-shock matter becomes higher in the stronger shock wave, which leads to the preferential synthesis of Fe and intermediate-mass elements (IME), such as Si, S, Ar and Ca, through the explosive Si- and O-burning, respectively. Hence, the NS would reside in the opposite hemisphere to the one with IME-rich ejecta; indeed, some recent observations (Holland-Ashford et al., 2017; Katsuda et al., 2018; Holland-Ashford et al., 2020) suggest that there is a correlation between the flight-direction of NS and the asymmetric distributions of ejecta compositions. We confirmed that if the asymmetric neutrino emissions exit, the nn-rich ejecta tend to appear in the high-ν¯e{\bar{\nu}}_{\rm e} hemisphere, which is the same direction of the stronger shock expansion (Tamborra et al., 2014; Nagakura et al., 2019b; Vartanyan et al., 2019b), regardless of progenitors. Therefore, the nn-rich ejecta, which are abundant in Ni, Zn, and much heavier elements, would be located in the same direction as the IME-rich ejecta and opposed to that of the NS kick. Our result suggests that Ni (Zn) would be preferable targets to measure the asymmetry for low (high) mass progenitors, although more detailed studies are called to assess the detectability quantitatively to each instrument. This may be an urgent task since the spectroscopic X-ray observations by XRISM (XRISM Science Team, 2020) is around the corner.

Acknowledgements

We are grateful to S. Katsuda for useful comments on observational aspects of Zn in SNRs. We also thank the anonymous referee for constructive comments that improved the content of this paper. This work is partly supported by JSPS KAKENHI Grant Number 20K03957.

DATA AVAILABILITY

The data underlying this article will be shared on reasonable request to the corresponding author.

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Appendix A Neutrino-core model

In our CCSN simulations, the neutrino emissions are evolved with a neutrino-core model (Ugliano et al., 2012; Sukhbold et al., 2016). In the model, the total neutrino luminosity is evolved as,

Lν(t)=E˙c+SR˙cRc,L_{\nu}(t)=-\dot{E}_{c}+S\frac{\dot{R}_{c}}{R_{c}}, (3)

where EcE_{c} and RcR_{c} are the total energy and radius of a proto-NS core, respectively. SS denotes the surface term  (see Ugliano et al., 2012, for the definition), which can be expressed as

S=ζGMcmaccRc,S=-\zeta\frac{GM_{c}m_{\rm acc}}{R_{c}}, (4)

where ζ(>0)\zeta(>0) is a dimensionless parameter with the order of unity; McM_{c} is the mass of the proto-NS core, which increases with time through the mass accretion. maccm_{\rm acc} denotes the mass of an accretion layer enveloping the proto-NS. It should be mentioned that SS is evaluated through the gravitational energy of proto-NS, Eg=2GMc2/(5Rc)E_{g}=-2GM_{c}^{2}/(5R_{c}), in our model, by introducing a new dimensionless parameter, fsurff_{\rm surf}, as

S=fsurfEg.S=f_{\rm surf}E_{g}. (5)

This simplification allows us to evaluate SS without specifying ζ\zeta and maccm_{\rm acc}.

By using the virial theorem, EcE_{c} can be expressed in terms of EgE_{g} and SS (see Eq. 1 in Ugliano et al., 2012); thus, the time derivative of EcE_{c} can be written as,

E˙c=3Γ43(Γ1)E˙g+13(Γ1)S˙,\dot{E}_{c}=\frac{3\Gamma-4}{3(\Gamma-1)}\dot{E}_{g}+\frac{1}{3(\Gamma-1)}\dot{S}, (6)

where Γ\Gamma is the polytropic index, which is assumed to be constant in the model.

In the present study, we have tuned the two parameters, Γ\Gamma and fsurff_{\rm surf}, of the neutrino-core model so that the 19.4M19.4M_{\odot} progenitor explodes as SN1987A-like (the explosion energy 1051erg\sim 10^{51}\rm erg and the ejected mass of Ni56{}^{56}\rm Ni of (0.070.08)M(0.07-0.08)M_{\odot}). The tuning results in Γ=1.7\Gamma=1.7 and fsurf=0.5f_{\rm surf}=0.5 of the neutrino-core model.

Thus, the time evolution of the total neutrino luminosity can be written as

Lν(t)=3Γ43(Γ1)(Eg+S)Rc˙Rc+S2(3Γ4)Eg3(Γ1)M˙accMcL_{\nu}(t)=\frac{3\Gamma-4}{3(\Gamma-1)}(E_{g}+S)\frac{\dot{R_{c}}}{R_{c}}+\frac{S-2(3\Gamma-4)E_{g}}{3(\Gamma-1)}\frac{\dot{M}_{\rm acc}}{M_{c}} (7)

where we assume that M˙c\dot{M}_{c} equal to the mass accretion rate, M˙acc\dot{M}_{\rm acc}, at the inner boundary of the computational domain in our CCSN simulations. For the radius of the neutrino-core, we follow a model of Scheck et al. (2006), as

Rc(t)=Rc,b1+(Rc,bRc,f1)[1exp(ttdelayτ)],R_{c}(t)=\frac{R_{c,b}}{1+\left(\frac{R_{c,b}}{R_{c,f}}-1\right)\left[1-\exp\left(\frac{t-t_{\rm delay}}{\tau}\right)\right]}, (8)

where Rc,b=80kmR_{c,b}=80\rm km, Rc,f=10kmR_{c,f}=10\rm km, tdelay=50mst_{\rm delay}=50\rm{\,\rm ms}, and τ=1.1s\tau=1.1\,{\rm s} are adopted in this study, which reproduce reasonably well the time evolution of Rc(t)R_{c}(t) in GR1D simulations.

Having the total neutrino luminosity, we decompose it into that of each flavor. The total neutrino luminosity is the sum of those of νe\nu_{\rm e}, ν¯e{\bar{\nu}}_{\rm e} and νx\nu_{x} (μ\mu, μ¯\bar{\mu}, τ\tau, and τ¯\bar{\tau}), i.e.,

Lν(t)=Lνe(t)+Lν¯e(t)+4Lνx(t).L_{\nu}(t)=L_{\nu_{e}}(t)+L_{\bar{\nu}_{e}}(t)+4L_{\nu_{x}}(t). (9)

Following Ugliano et al. (2012); Sukhbold et al. (2016), we assume constant fractions of the luminosities;

Lνe,ave=fνeLν(t),Lν¯e,ave=fν¯eLν(t),Lνx=fνxLν(t).L_{\nu_{\rm e},{\rm ave}}=f_{\nu_{\rm e}}L_{\nu}(t),\,L_{{\bar{\nu}}_{\rm e},{\rm ave}}=f_{{\bar{\nu}}_{\rm e}}L_{\nu}(t),\,L_{\nu_{x}}=f_{\nu_{x}}L_{\nu}(t). (10)

Here fνe,fν¯ef_{\nu_{e}},f_{\bar{\nu}_{e}}, and fνxf_{\nu_{x}} are set to be 0.25, 0.25, and 0.125, guided with the spherical simulation the core collapse of the 15.0M15.0M_{\odot} progenitor.

Regarding neutrino spectra, we assume that they are all thermal with zero chemical potential. The temperatures, Tνi(t)T_{\nu_{i}}(t), are flavor-dependent, which are evaluated as below. The total neutrino luminosity can be written as

Lν=716σSB4π[Tνe4Rνe2+Tν¯e4Rν¯e2+4Tνx4Rνx2],L_{\nu}=\frac{7}{16}\sigma_{\rm SB}4\pi[T_{\nu_{e}}^{4}R_{\nu_{e}}^{2}+T_{\bar{\nu}_{e}}^{4}R_{\bar{\nu}_{e}}^{2}+4T_{\nu_{x}}^{4}R_{\nu_{x}}^{2}], (11)

where σSB\sigma_{\rm SB} is the Stephan-Boltzmann constant and RνiR_{\nu_{i}} denotes the neutrino sphere. We assume that RcR_{c} corresponds to the representative neutrino sphere, hence we rewrite Eq. 11 as

Lν(t)=716σSB[Tνe4(t)+Tν¯e4(t)+4Tνx4(t)]4πRc2(t),L_{\nu}(t)=\frac{7}{16}\sigma_{\rm SB}[T_{\nu_{e}}^{4}(t)+T_{\bar{\nu}_{e}}^{4}(t)+4T_{\nu_{x}}^{4}(t)]4\pi R_{c}^{2}(t), (12)

where σSB\sigma_{\rm SB} is the Stephan-Boltzmann constant. The temperature ratio among flavors is set as;

fT,νe:fT,ν¯e:fT,νx=0.3:0.35:0.35.f_{T,\nu_{e}}:f_{T,\bar{\nu}_{e}}:f_{T,\nu_{x}}=0.3:0.35:0.35. (13)

The ratios are determined based on the results of the spherical simulation of the core collapse of the 15.0M15.0M_{\odot} progenitor, in which we refer the average energy of each species of neutrinos. As a result, Tνi(t)T_{\nu_{i}}(t) can be obtained through Lν(t)L_{\nu}(t) and Rc(t)R_{c}(t); for instance, Tνe(t)T_{\nu_{e}}(t) can be obtained as

Tνe(t)=[4Lν(t)7πRc2(t)σSB(1+fE,ν¯e4fE,νe4+4fE,νx4fE,νe4)]1/4.T_{\nu_{e}}(t)=\left[\frac{4L_{\nu}(t)}{7\pi R_{c}^{2}(t)\sigma_{\rm SB}\left(1+\frac{f_{E,\bar{\nu}_{e}}^{4}}{f_{E,\nu_{e}}^{4}}+4\frac{f_{E,\nu_{x}}^{4}}{f_{E,\nu_{e}}^{4}}\right)}\right]^{1/4}. (14)

Appendix B Reversed-LνL_{\nu} procedure

As described in Sec. 3.2, the direction of the ν¯e{\bar{\nu}}_{\rm e}-hemisphere in our CCSN simulations is not always the same as that with the stronger shock expansion. We note, however, that more elaborate multi-D CCSN simulations (Tamborra et al., 2014; Nagakura et al., 2019b; Vartanyan et al., 2019b) have shown that they are positively correlated with each other, indicating that the anti-correlation would lead to unrealistic outcomes; hence, if the anti-correlation appears, we reverse the dipole direction of neutrino emissions for nucleosynthesis computations in this study, albeit ad-hoc procedure. In this appendix, we study the impact of the procedure, which is also important to understand the limitation of our results.

Figure 10 shows mass profiles of dMejdM_{\rm ej} in Ye,1Y_{\rm e,1} of the ejecta from the inner region (rcc10,000kmr_{\rm cc}\leq 10,000{\,\rm km}) for the progenitor with Mms=15.0MM_{\rm ms}=15.0M_{\odot}. The top and second panels show the mass profiles for cases with masy=0%m_{\rm asy}=0\% and masy=30%m_{\rm asy}=30\%, respectively, in which the direction of ν\nu-asymmetry for the hydrodynamic simulations is set to be the same as that in the nucleosynthetic calculation. The direction of the ν¯e{\bar{\nu}}_{\rm e}-hemisphere for the case with masy=30%m_{\rm asy}=30\%, however, does not correlate with that of stronger shock. The third panel displays the result with the reversed dipole direction in the nucleosynthesis computation to recover the positive correlation between the ν¯e{\bar{\nu}}_{\rm e}-hemisphere and that of the stronger shock expansion. By comparing the second and third panels, we find that the strong pp-rich ejecta in the former disappears in the latter, but more nn-rich ejecta appears, instead. To understand the trend more clearly, we study another case in which the asymmetric degree and dipole direction are the same as those used in the case with the second panel, but we employ a CCSN model with masy=0%m_{\rm asy}=0\% as a background; the result is displayed in the fourth panel. We confirm that the strong pp-rich ejecta is observed in both the second and fourth panels, indicating that it is an artifact due to the negative correlation between the ν¯e{\bar{\nu}}_{\rm e}-hemisphere and that of the stronger shock expansion. In other words, the νe\nu_{\rm e}-hemisphere becomes the same direction as that of the stronger shock expansion, implying that the νe\nu_{\rm e} absorption is artificially enhanced and then YeY_{e} of the ejecta is increased accordingly. To strengthen this argument, we study another model, in which we carry out the nucleosynthesis computation with the reversed neutrino asymmetry with masy=30%m_{\rm asy}=30\% under the CCSN simulation with masy=0%m_{\rm asy}=0\%; the result is displayed in the fifth (bottom) panel. As seen in the panel, the pp-rich ejecta is reduced, and the mass profile approaches that of the third panel. Our results suggest that the dipole direction of the neutrino emissions in nucleosynthesis computations is a more influential parameter than that in the CCSN simulation. Hence, we conclude that the adjustment of the dipole direction to hold the positive correlation between the ν¯e{\bar{\nu}}_{\rm e}-hemisphere and that of the stronger shock expansion in the nucleosynthesis computation is appropriate to capture the qualitative trend of the asymmetry of ejecta composition. We also confirm through this study that the uncertainty by the procedure is the same level as the differences between the third and bottom panels.

Refer to caption
Figure 10: Mass profiles of dMejdM_{\rm ej} in Ye,1Y_{\rm e,1} of the inner ejecta (rcc10,000kmr_{\rm cc}\leq 10,000{\,\rm km}) for the progenitor with Mms=15.0MM_{\rm ms}=15.0M_{\odot}. Each panel corresponds to the result with a different setup of the asymmetric neutrino emissions in either CCSN simulations, nucleosynthesis computations, or both (see the text for more details). masym_{\rm asy} used in the CCSN simulations is displayed at the leftmost position in all panels. The middle panel corresponds to the model, in which the asymmetric neutrino emissions are reversed in the nucleosynthesis computation from those used in the CCSN simulation. For the fourth and fifth (bottom) panels, the asymmetric degree of the neutrino emissions is the same (zero) for the CCSN simulations (i.e., their CCSN model is identical to that shown in the top panel), meanwhile, we set the asymmetric degree as 30% in the nucleosynthetic computations, and the dipole direction is the opposite between the two models.