Naohito Nakasato

Nucleosynthesis in type II supernovae

Abstract Among the major uncertainties involved in the Chandrasekhar mass models for Type Ia supernovae are the companion star of the accreting white dwarf (or the accretion rate that determines the carbon ignition density) and the flame speed after ignition. We present nucleosynthesis results from relatively slow deflagration (1.5 – 3 % of the sound speed) to constrain the rate of accretion from the companion star. Because of electron capture, a significant amount of neutron-rich species such as 54Cr, 50Ti, 58Fe, 62Ni, etc. are synthesized in the central region. To avoid the too large ratios of 54 Cr 56 Fe and 50 Ti 56 Fe , the central density of the white dwarf at thermonuclear runaway must be as low as ≲ 2 × 109 g cm−3. Such a low central density can be realized by the accretion as fast as M ≳ 1 × 10−7 M⊙ yr−1. These rapidly accreting white dwarfs might correspond to the super-soft X-ray sources.

Three-dimensional Simulations of the Chemical and Dynamical Evolution of the Galactic Bulge

A three-dimensional, hydrodynamical, N-body model for the formation of the Galaxy is presented, with special attention paid to the formation of the bulge component. Since none of the previous numerical models for the Galaxys formation have a proper treatment of the chemical evolution and/or sufficient spatial resolutions, we have constructed a detailed model of the chemical and dynamical evolution of the Galaxy using our GRAPE smoothed particle hydrodynamics (SPH) code. Our SPH code includes various physical processes related to the formation of stellar systems. Starting with cosmologically motivated initial conditions, we obtain a stellar system qualitatively similar to the Galaxy. Then we analyze the chemical and kinematic properties of the bulge stars in our model and find qualitative agreement with observational data. The early evolution of our model has revealed that most bulge stars form during the subgalactic merger (merger component of the bulge stars). Because of the strong starburst induced by the merger, the metallicity distribution function of such stars becomes as wide as observed. We find that another group of the bulge stars forms later in the inner region of the disk (nonmerger component of the bulge stars). Because of the difference in the formation epoch, the main source of iron for this group of stars is different from that for the merger component. Iron in the merger and nonmerger components comes mainly from Type II and Type Ia supernovae, respectively. Since a Type Ia supernova ejects ~10 times more iron than a Type II supernova, [Fe/H] of the nonmerger component tends to be higher than that of the merger component, which widens the metallicity distribution function. From these results, we suggest that the Galactic bulge consists of two chemically different components; one has formed quickly through the subgalactic clump merger in the proto-Galaxy, and the other has formed gradually in the inner disk.

Instabilities and Mixing in SN 1993J

Rayleigh-Taylor (R-T) instabilities in the explosion of SN 1993J are investigated by means of two-dimensional hydrodynamical simulations. It is found that the extent of mixing is sensitive to the progenitors core mass and the envelope mass. Because the helium core mass (3-4 M☉) is smaller than that of SN 1987A, R-T instabilities at the He/C+O interface develop to induce a large-scale mixing in the helium core, while the instability is relatively weak at the H/He interface as a result of the small envelope mass. The predicted abundance distributions, in particular the extent of the 56Ni mixing, are compared with those required in modeling the bolometric light curve and the late-time optical spectra. These comparisons provide significant constraints on the masses of the helium core and the envelope of the progenitor of SN 1993J.

Numerical Simulations of Globular Cluster Formation

We examine various physical processes associated with the formation of globular clusters by using the three-dimensional smoothed particle hydrodynamics (SPH) code. Our code includes radiative cooling of gases, star formation, energy feedback by stars including stellar winds and supernovae, and chemical enrichment by stars. We assume that, in the collapsing galaxy, isothermal cold clouds form through thermal condensations and become proto-globular clouds. We calculate the size of proto-globular clouds by solving the linearized equations for perturbation. We compute the evolution of the inner region of the protocloud with our SPH code for various initial radius and initial composition of gases. When the initial gases contain no heavy elements, the evolution of protoclouds sensitively depends on the initial radius. For a smaller initial radius, the initial starburst is so intense that the subsequent star formation occurs in the central regions to form a dense star cluster as massive as the globular cluster. When the initial gases contain some heavy elements, the metallicity of gases affects the evolution and the final stellar mass. If the initial radius of the proto-globular clouds was relatively large, the formation of a star cluster as massive as the globular clusters requires the initial metallicity as high as [Fe/H] ≥ -2. The self-enrichment of heavy elements in the star cluster does not occur in all cases.

Metal Enrichment of the Primordial Interstellar Medium through Three-dimensional Hydrodynamical Evolution of the First Supernova Remnant

The long-term evolution of supernova (SN) remnants in the primordial interstellar medium (ISM) with an inhomogeneous structure is calculated to investigate metal enrichment of the primordial gas. For this purpose, we have constructed a parallel three-dimensional hydrodynamics code incorporating the radiative cooling and self-gravity. The self-gravity and radiative cooling develop the inhomogeneous structure of the ISM from a small perturbation with a power-law spectrum. The resultant density ranges from 0.5 to 106 cm -3. Calculations for an SN with the progenitor mass of 20 M☉ are performed as the first step of a series of our study. It is found from the results that a single SN distributes some of the newly synthesized heavy elements into a dense filament of the ISM with densities ranging from 100 to 104 cm-3 depending on where the SN explodes. Thus, the metallicity [Mg/H] of the dense filaments polluted by the SN ejecta becomes -2.7 ± 0.5. From these filaments, the first Population II stars will form. This value is in accordance with previous analytical work by Shigeyama & Tsujimoto with an accuracy of ~0.3 dex.

Type Ia Supernovae: Nucleosynthesis and Constraints on Progenitors

Among the major uncertainties involved in the Chandrasekhar mass models for Type Ia supernovae (SNe Ia) are the companion star of the accreting white dwarf (or the accretion rate that determines the carbon ignition density) and the flame speed after ignition. We present several new constraints on the companion star in the progenitor system, which includes: 1) nucleosynthesis results from relatively slow deflagration to constrain the rate of accretion from the companion star, and 2) Galactic chemical evolution models to constrain the main-sequence lifetime of the companion stars.

Satellite Survival in Cold Dark Matter Cosmology

We study the survival of substructures (clumps) within larger self-gravitating dark matter halos. Building on scaling relations obtained from N-body calculations of violent relaxation, we argue that the tidal field of galaxies and halos can only destroy substructures if spherical symmetry is imposed at formation. We explore other mechanisms that may tailor the number of halo substructures during the course of virialization. Unless the larger halo is built up from a few large clumps, we find that clump-clump encounters are unlikely to homogenize the halo on a dynamical timescale. Phase mixing would proceed faster in the inner parts and allow for the secular evolution of a stellar disk.

Initial Model Catalogue for Galaxy Evolution

We have computed full hydrochemodynamical evolution for 150 initial models of protogalaxies with our chemodynamical SPH code named GENSO. Various parameters for all models are identical except for a seed for a random number generator. In other words, all models have similar global properties but have the different merging history that leads to a different evolution in each model. Results of the series of computations have two main applications. Firstly, we have an initial model catalogue for subsequent modelling of galaxy evolution. Since the resulting evolution depends strongly on the initial phase of the particle distribution, it is crucial to find a suitable initial model when we model a specific real galaxy in the Universe, notably the Milky Way in our case. We will make a precise chemical and dynamical model of the Milky Way out of 150 models in our initial model catalogue. Secondly, we can obtain a large variety of global histories of physical values such as star formation, metallicity in the ISM and stellar components, and Type II and Ia supernova rates. For example, the resulting total star formation history shows the peak at a high redshift z ~ 6 and the peak value is ~280 Mo yr–1 Mpc–3. Also, the Type Ia rate obtained has a peak at z ~ 3.5. All of our results and model catalogue are publicly available from our website for those who wish to model galaxy evolution.

Type Ib-Ic-IIb-IIL Supernovae: Common Envelope Evolution, Instabilities, and Circumstellar Interaction

The origin of an observational diversity of supernova types, namely, Type Ia, Ib, Ic, II-P, II-L, IIb, and IIn, has not been well understood. We propose a new scenario that explains how such a diversity can result from the common envelope evolution of binary stars. The recent nearby supernovae SN 1993J and SN 1994I have provided particularly useful material to clarify the supernova-progenitor connection. For a progenitor of Type IIb supernova 1993J, we propose that merging of two stars in a close binary is responsible for the formation of a thin H-rich envelope. For a progenitor of Type Ic supernova 1994I, we propose a bare C+O star that has lost both H and He envelope after common envelope phase. By generalizing these scenarios, we show that common envelope evolution in massive close binary stars leads to various degree of stripping of the envelope mass of massive star. This can naturally lead to the explanation of the origin of Type II-L, IIn, IIb, Ib, and Ic in a unified manner. The binary hypothesis to explain the diversity of supernovae can be substantiated with new information on SN IIb 1993J and SN Ic 1994I. Hydrodynamical and nucleosynthesis models for these supernovae (including Rayleigh-Taylor instabilities) and their light curves are compared with observations. Based on these modeling, we suggest that the subluminous SN I 1993R might be a SN Ib originating from the smallest mass progenitor. Finally, circumstellar interactions of SN 1993J are examined to compare with X-ray observations and infer the density structure and instabilities of the ejecta.

Metal Enrichment History of the Proto-Galactic Interstellar Medium

To investigate the metal enrichment history of the primordial interstellar medium (ISM), we have studied the long-term evolution of supernova remnants (SNRs) and how SNRs distribute the heavy metals into the ISM when they explode. With the assumed IMF for massive stars, we have computed the multiple supernova explosions and evolution in an inhomogeneous ISM. We compare the predicted metallicity distribution of metal deficient halo stars with the observed one.