Takuji Tsujimoto

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.

The history of the cosmic supernova rate derived from the evolution of the host galaxies

We make a prediction of the cosmic supernova rate history as a composite of the supernova rates in spiral and elliptical galaxies. We include the metallicity eUect on the evolution of Type Ia supernova (SN Ia) progenitors and construct detailed models for the evolutions of spiral and elliptical galaxies in clus- ters and the —eld to meet the latest observational constraints. In the cluster environment, the synthesized cosmic star formation rate (SFR) has an excess at corresponding to the early starburst in ellipticals z Z 3 and a shallower slope from the present to the peak at the redshift of z D 1.4 compared with Madaus plot. In the —eld environment, we assume that ellipticals form over a wide range of redshifts as 1 ( z ( The synthesized cosmic SFR has a broad peak around z D 3, which is in good agreement with the 4. observed one. The resultant cosmic SFRs lead to the following predictions for the cosmic SN Ia rate: (1) the SN Ia rate in spirals has a break at z D 2 because of the low-metallicity inhibition of SNe Ia, regard- less of whether the galaxies are in clusters or in the —eld; (2) at high redshifts, the SN Ia rate has a strong peak around z D 3 in clusters, whereas in the —eld much lower rate is expected, re—ecting the diUerence in the formation epochs of ellipticals. Subject headings: cosmology: theorygalaxies: abundancesgalaxies: evolution ¨ supernovae: general

The Evolution of Dwarf Galaxies with Star Formation in an Outward-propagating Supershell

We simulate the dynamical and chemical evolution of a dwarf galaxy embedded in a dark matter halo, using a three-dimensional N-body/smoothed particle hydrodynamics (SPH) simulation code combined with stellar population synthesis. The initial condition is adopted in accord with a 1010 M☉ virialized sphere in a 1 σ cold dark matter perturbation which contains 10% baryonic mass. A supersonic spherical outflow is driven by the first starburst near the center of the galaxy and produces an expanding supershell in which stars are subsequently formed. Consecutive formation of stars in the expanding shell makes the stellar system settled, with the exponential brightness profile, the positive metallicity gradient, and the inverse color gradient in agreement with observed features of dwarf galaxies. We therefore propose that the energy feedback via stellar winds and supernovae is a decisive mechanism for formation of less compact, small systems like dwarf galaxies.

A New Approach to Determine the Initial Mass Function in the Solar Neighborhood

Oxygen to iron abundance ratios of metal-poor stars provide information on nucleosynthesis yields from massive stars that end in Type II supernova (SN II) explosions. Using a standard model of chemical evolution of the Galaxy we have reproduced the solar neighborhood abundance data and estimated the oxygen and iron yields of genuine SN II origin. The estimated yields are compared with the theoretical yields to derive the relation between the lower and upper mass limits in each generation of stars and the initial mass function (IMF) slope. Independent of this relation, we furthermore derive the relation between the lower mass limit and the IMF slope from the stellar mass-to-light ratio in the solar neighborhood. These independent relations unambiguously determine the upper mass limit of mu = 50 ± 10 M☉ and the IMF slope index of 1.3-1.6 above 1 M☉. This upper mass limit corresponds to the mass beyond which stars end as black holes without ejecting processed matter into the interstellar medium. We also find that the IMF slope index below 0.5 M☉ cannot be much shallower than 0.8.

Low-mass helium star models for Type Ib supernovae : light curves, mixing, and nucleosynthesis

The applicability of theoretical models of He-star explosions to type Ib SN explosions is explored. Particular attention is given to light curves and mixing, Rayleigh-Taylor instabilities and mixing, and nucleosynthesis and the mass of Ni-56. Typical numerical results are presented in graphs, and it is concluded that the explosions of SN 1983N and SN 1983I can be accurately represented in terms of explosions of He stars with M(alpha) of 3-4 solar mass. A strong M(alpha) dependence of light-curve shape, photospheric velocity, and Ni-56 mass is found. 44 refs.

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 γ-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 Myr −1 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.

The Absolute Magnitude of RR Lyrae Stars Derived from the Hipparcos Catalogue

The present determination of the absolute magnitude MV(RR) of RR Lyrae stars is twofold, relying upon Hipparcos proper motions and trigonometric parallaxes separately. First, applying the statistical parallax method to the proper motions, we find MV(RR) = 0.69±0.10 for 99 halo RR Lyrae stars with [Fe/H] = -1.58. Second, applying the Lutz-Kelker correction to the RR Lyrae HIP 95497 with the most accurately measured parallax, we obtain MV(RR) = (0.58–0.68)+0.28−0.31 at [Fe/H] = -1.6. Furthermore, allowing full use of low-accuracy and negative parallaxes, as well 125 RR Lyrae stars with -2.49 ≤ [Fe/H] ≤ 0.07, the maximum likelihood estimation yields the relation MV(RR) = (0.59 ± 0.37) + (0.20 ± 0.63)([Fe/H] + 1.60), which formally agrees with the recent preferred relation. The same estimation again yields MV(RR) = 0.65±0.33 for the 99 halo RR Lyrae stars. Although the formal errors in the latter three parallax estimates are rather large, all four results suggest the fainter absolute magnitude, MV(RR) ≈ 0.6-0.7 at [Fe/H] = -1.6. The present results still provide the lower limit on the age of the universe that is inconsistent with a flat, matter-dominated universe and current estimates of the Hubble constant.

Excavation of the First Stars

The external pollution of the first stars in the Galaxy is investigated. The first stars were born in clouds composed of the pristine gas without heavy elements. These stars accreted gas polluted with heavy elements while they still remained in the cloud. As a result, it is found that they exhibit a distribution with respect to the surface metallicity. We have derived the actual form of this distribution function. This metallicity distribution function strongly suggests that the recently discovered most metal-deficient star HE 0107-5240 with [Fe/H] = -5.3 was born as a metal-free star and accreted gas polluted with heavy elements. Thus, the heavy elements such as Fe present in HE 0107-5240 must have been supplied from supernovae of later generations exploding inside the cloud in which the star had been formed. The elemental abundance pattern on the surface of stars suffering from such an external pollution should not be diverse but exhibit the average pattern of numerous supernovae. Future observations for a number of metal-deficient stars with [Fe/H] < -5 will be able to prove or disprove this external pollution scenario. Other possibilities to produce a star with this metallicity are also discussed.

Probing the Site for r-Process Nucleosynthesis with Abundances of Barium and Magnesium in Extremely Metal-poor Stars

We suggest that if the astrophysical site for r-process nucleosynthesis in the early Galaxy is confined to a narrow mass range of Type II supernova (SN II) progenitors, with a lower mass limit of Mms=20 M middle dot in circle, a unique feature in the observed distribution of [Ba/Mg] versus [Mg/H] for extremely metal-poor stars can be adequately reproduced. We associate this feature, a bifurcation of the observed elemental ratios into two branches in the Mg abundance interval -3.7</=&sqbl0;Mg&solm0;H&sqbr0;</=-2.3, with two distinct processes. The first branch, which we call the y-branch, is associated with the production of Ba and Mg from individual massive supernovae. The derived mass of Ba synthesized in SNe II is 8.5x10-6 M middle dot in circle for Mms=20 M middle dot in circle and 4.5x10-8 M middle dot in circle for Mms=25 M middle dot in circle. We conclude that SNe II with Mms approximately 20 M middle dot in circle are the dominant source of r-process nucleosynthesis in the early Galaxy. An SN-induced chemical evolution model with this Mms-dependent Ba yield creates the y-branch, reflecting the different nucleosynthesis yields of [Ba/Mg] for each SN II with Mms greater, similar20 M middle dot in circle. The second branch, which we call the i-branch, is associated with the elemental abundance ratios of stars which were formed in the dense shells of the interstellar medium swept up by SNe II with Mms<20 M middle dot in circle that do not synthesize r-process elements, and it applies to stars with observed Mg abundances in the range &sqbl0;Mg&solm0;H&sqbr0;<-2.7. The Ba abundances in these stars reflect those of the interstellar gas at the (later) time of their formation. The existence of a [Ba/Mg] i-branch strongly suggests that SNe II that are associated with stars of progenitor mass Mms</=20 M middle dot in circle are infertile sources for the production of r-process elements. We predict the existence of this i-branch for other r-process elements, such as europium (Eu), to the extent that their production site is in common with Ba.

ORIGIN OF CHEMICAL AND DYNAMICAL PROPERTIES OF THE GALACTIC THICK DISK

We adopt a scenario in which the Galactic thick disk was formed by minor merging between the first generation of the Galactic thin disk (FGTD) and a dwarf galaxy about ~9 Gyr ago and thereby investigate chemical and dynamical properties of the Galactic thick disk. In this scenario, the dynamical properties of the thick disk have long been influenced both by the mass growth of the second generation of the Galactic thin disk (i.e., the present thin disk) and by its non-axisymmetric structures. On the other hand, the early star formation history and chemical evolution of the thin disk was influenced by the remaining gas of the thick disk. Based on N-body simulations and chemical evolution models, we investigate the radial metallicity gradient, structural and kinematical properties, and detailed chemical abundance patterns of the thick disk. Our numerical simulations show that the ancient minor merger event can significantly flatten the original radial metallicity gradient of the FGTD, in particular, in the outer part, and also can be responsible for migration of inner metal-rich stars into the outer part (R > 10 kpc). The simulations show that the central region of the thick disk can develop a bar due to dynamical effects of a separate bar in the thin disk. Whether or not rotational velocities (V ) can correlate with metallicities ([Fe/H]) for the simulated thick disks depends on the initial metallicity gradients of the FGTDs. The simulated orbital eccentricity distributions in the thick disk for models with higher mass ratios (~0.2) and lower orbital eccentricities (~0.5) of minor mergers are in good agreement with the corresponding observations. The simulated V -|z| relation of the thick disk in models with low orbital inclination angles of mergers are also in good agreement with the latest observational results. The vertical metallicity gradient of the simulated thick disk is rather flat or very weakly negative in the solar neighborhood. Our Galactic chemical evolution models show that if we choose two distinctive timescales for star formation in the thin and thick disks, then the models can explain both the observed metallicity distribution functions and correlations between [Mg/Fe] and [Fe/H] for the two disks in a self-consistent manner. We discuss how the early star formation history and chemical evolution of the Galactic thin disk can be influenced by the pre-existing thick disk.