A new estimation of manganese distribution for local dwarf spheroidal galaxies

The distribution of abundance for iron-peak elements in dwarf spheroidal galaxies(d Sphs) is important for galaxy evolution and supernova(SN) nucleosynthesis. Nowadays, manganese(Mn) is one of the most observed iron-peak elements in local d Sphs. Studies of its distributions allow us to derive and understand the evolution history of these d Sphs. We improve a phenomenological model by a two-curve model including a new initial condition, that includes detailed calculations of SN explosion rates and yields. We compare the results with the observed Mn distribution data for three d Sphs: Fornax, Sculpture and Sextans.We find that the model can describe the observed Fe and Mn distributions well simultaneously for the three d Sphs. The results also indicate that the initial conditions should be determined by the low metallicity samples in the beginning time of the galaxies and the previous assumption of metellicity-dependant Mn yield of SNIa is not needed when a wide mass range of core-collapse SNe is included. Our method is applicable to the chemical evolution of other iron-peak elements in d Sphs and can be modified to provide more detailed processes for the evolution of dSphs.     

Isotopic r-process abundances produced by supernova explosions

The rapid neutron capture process (r-process) is one of the major nucleosynthesis processes responsible for the synthesis of heavy nuclei beyond iron. Isotopes beyond Fe are most exclusively formed in neutron capture processes and more heavier ones are produced by the r-process. Approximately half of the heavy elements with mass number A>70 and all of the actinides in the solar system are believed to have been produced in the r-process. We have studied the r-process in supernovae for production of heavy elements beyond A=40 with the newest mass values available. The supernovae envelopes at a temperature >10⁹ K and neutron density of 10²⁴ cm⁻³ are considered to be one of the most potential sites for the r-process. We investigate the r-process in a site-independent, classical approach which assumes a chemical equilibrium between neutron captures and photodisintegrations followed by a β-flow equilibrium. We have studied the r-process path corresponding to temperatures ranging from 1.0×10⁹ K to 3.0×10⁹ K and neutron density ranging from 10²⁰ cm⁻³ to 10³⁰ cm⁻³. The primary goal of the r-process calculations is to fit the global abundance curve for solar system r-process isotopes by varying time dependent parameters such as temperature and neutron density. This method aims at comparing the calculated abundances of the stable isotopes with observation. The abundances obtained are compared with supernova explosion condition and found in good agreement. The elements obtained along the r-process path are compared with the observed data at all the above temperature and density range.     

Spectroscopic studies of southern-hemisphere Cepheids: Six objects in Centaurus (V Cen, V737 Cen) and Sagittarius (BB Sgr, W Sgr, X Sgr, Y Sgr)

This paper is the next one in the series of our works aimed at determining the atmospheric parameters and chemical composition of southern-hemisphere Cepheids. We present the results of our study for six bright Cepheids: V Cen, V737 Cen, BB Sgr, W Sgr, X Sgr, and Y Sgr. We have analyzed 14 high-resolution spectra taken with the 1.9-m telescope at the South African Astronomical Observatory. In addition to determining the chemical composition and atmospheric parameters, we point out and discuss several features in the spectra of individual Cepheids. In particular,we have detected emission in the cores of the Hβ and Hα lines forWSgr near its maximum light, while X Sgr shows the splitting of metal absorption lines into individual components without any change of the hydrogen lines. These peculiarities can be explained by different manifestations of shock waves in the Cepheid atmospheres and by the presence of circumstellar envelopes around X Sgr and W Sgr. The chemical composition has been estimated for V737 Cen, BB Sgr, and X Sgr for the first time. On the whole, our abundance estimates for α-elements, iron-peak elements, and r- and s-process elements are close to the solar ones for all objects, carbon is underabundant, the oxygen abundance is nearly solar, the “odd” elements, Na and Al, are overabundant (except X Sgr), magnesium is underabundant for V Cen and X Sgr and overabundant for the remaining objects. Such a chemical composition is typical of yellow supergiants after the first dredge-up. Keywords: Cepheids, spectra, atmospheric parameters, chemical composition.     

Time-dependent three-dimensional spectrum synthesis for Type Ia supernovae

A Monte Carlo code ( artis ) for modelling time-dependent three-dimensional spectral synthesis in chemically inhomogeneous models of Type Ia supernova ejecta is presented. Following the propagation of γ-ray photons, emitted by the radioactive decay of the nucleosynthesis products, energy is deposited in the supernova ejecta and the radiative transfer problem is solved self-consistently, enforcing the constraint of energy conservation in the comoving frame. Assuming a photoionization-dominated plasma, the equations of ionization equilibrium are solved together with the thermal balance equation adopting an approximate treatment of excitation. Since we implement a fully general treatment of line formation, there are no free parameters to adjust. Thus, a direct comparison between synthetic spectra and light curves, calculated from hydrodynamic explosion models, and observations is feasible. The code is applied to the well-known W7 explosion model and the results tested against other studies. Finally, the effect of asymmetric ejecta on broad-band light curves and spectra is illustrated using an elliptical toy model.     

Rapid neutron capture process in supernovae and chemical element formation

The rapid neutron capture process (r-process) is one of the major nucleosynthesis processes responsible for the synthesis of heavy nuclei beyond iron. Isotopes beyond Fe are most exclusively formed in neutron capture processes and more heavier ones are produced by the r-process. Approximately half of the heavy elements with mass number A > 70 and all of the actinides in the solar system are believed to have been produced in the r-process. We have studied the r-process in supernovae for the production of heavy elements beyond A = 40 with the newest mass values available. The supernova envelopes at a temperature >109 K and neutron density of 10²⁴ cm^?3 are considered to be one of the most potential sites for the r-process. The primary goal of the r-process calculations is to fit the global abundance curve for solar system r-process isotopes by varying time dependent parameters such as temperature and neutron density. This method aims at comparing the calculated abundances of the stable isotopes with observation. We have studied the r-process path corresponding to temperatures ranging from 1.0 × 10⁹ K to 3.0 × 10⁹ K and neutron density ranging from 10²⁰ cm^?3 to 10³⁰ cm^?3. With temperature and density conditions of 3.0 × 10⁹ K and 10²⁰ cm^?3 a nucleus of mass 273 was theoretically found corresponding to atomic number 115. The elements obtained along the r-process path are compared with the observed data at all the above temperature and density range.     

Delayed Detonation at a Single Point in Exploding White Dwarfs

Delayed detonation in an exploding white dwarf, which propagates from an off-center transition point rather than from a spherical transition shell, is described and simulated. The differences between the results of two-dimensional simulations and the one-dimensional case are presented and discussed. The two-dimensional effects become significant in transition density below 3rho7, where the energetics, the production of Fe group elements, and the symmetry of the explosion are all affected. In the two-dimensional case, the explosion is less energetic and less Ni is produced in the detonation phase of the explosion. For low transition density, the reduction in Ni mass can reach 20%-30%. The asymmetry in abundances between regions close to the transition point and regions far from that point is large and could be a source of polarization patterns in the emitted light. We conclude that the spatial and temporal distribution of transition locations is an important parameter that must be included in delayed detonation models for Type Ia supernovae.     

Primordial nucleosynthesis

Nucleosynthesis is the process by which chemical elements and their isotopes are formed. The heavy elements (carbon and heavier ones) are thought to be the result of thermonuclear burning in stars, and especially the relatively rare stars that become supernovae. Big Bang nucleosynthesis generated few elements: only hydrogen, deuterium, some of the helium and lithium, traces (if any) of beryllium and boron. After a brief overview of the physical processes involved therein, we present the predictions of the primordial nucleosynthesis in the standard Big Bang model and compare them to the abundances of the primordial light elements as derived from observational data.     

Cosmic evolution of the biogenic elements and compounds

Summary This paper traces the evolution of the biogenic elements H, C, N, O, P and S from their creation by cosmic nucleosynthesis to their inclusion in living systems on the surface of the Earth. Evidence for the presence of significant prebiotic molecules in interstellar clouds and in primitive meteorites is reviewed. The possible relevance of this discovery to the origin of life on Earth is assessed in the light of evidence suggesting that such molecules could not easily be synthesized in a primitive CO₂-dominated terrestrial atmosphere.     

Effects of possible deviations of fundamental physical constants on primordial nucleosynthesis

We study the effects of possible deviations of fundamental physical constants on the yields of light nuclides, ²D, ³He, ⁴He, ⁷Li, and others during primordial nucleosynthesis. The deviations of fundamental constants from their current values are considered in the low-energy approximation of string theories; the latter predict the existence of a scalar field, which, apart from the tensor gravitational field, determines the space geometry. A two-parameter (η, δ) model is constructed for primordial nucleosynthesis: η = n _B /n _γ is the baryon-to-photon density ratio, and Ω is the relative deviation of fundamental physical constants at the epoch of primordial nucleosynthesis from their current values. A dependence of η on the deviation of coupling constants Ω has been derived on condition that the primordial helium abundance is Y _p = f(η, δ) = const, where const corresponds to experimental values. We thus showed that the relative baryonic density (and hence Ω_B could vary over a much wider range than allowed by the standard nucleosynthesis model. Considering this result, we discuss the recently found mismatch between Ω_B obtained from an analysis of CMBR anisotropy and from the standard primordial nucleosynthesis model.     

A hydrodynamic model for asymmetric explosions of rapidly rotating collapsing supernovae with a toroidal atmosphere

We numerically solved the two-dimensional axisymmetric hydrodynamic problem of the explosion of a low-mass neutron star in a circular orbit. In the initial conditions, we assumed a nonuniform density distribution in the space surrounding the collapsed iron core in the form of a stationary toroidal atmosphere that was previously predicted analytically and computed numerically. The configuration of the exploded neutron star itself was modeled by a torus with a circular cross section whose central line almost coincided with its circular orbit. Using an equation of state for the stellar matter and the toroidal atmosphere in which the nuclear statistical equilibrium conditions were satisfied, we performed a series of numerical calculations that showed the propagation of a strong divergent shock wave with a total energy of ～0.2×10⁵¹ erg at initial explosion energy release of ～1.0×10⁵¹ erg. In our calculations, we rigorously took into account the gravitational interaction, including the attraction from a higher-mass (1.9M_⊙) neutron star located at the coordinate origin, in accordance with the rotational explosion mechanism for collapsing supernovae. We compared in detail our results with previous similar results of asymmetric supernova explosion simulations and concluded that we found a lower limit for the total explosion energy.     

AGB星核合成理论的研究进展

综述了近扯为ＡＧＢ星核合成的理论研究情况，包括轻，重核素核合成理论，ＡＧＢ星的分类，ＡＧＢ星的演化特征，ＡＧＢ星内的元素核合成理论的研究及外赋ＭＳ，Ｓ 双星吸积机制的研究情况。     

The origin and abundances of the chemical elements revisited

Summary The basic scheme of nucleosynthesis (building of heavy elements from light ones) has held up very well since it was first proposed more than 30 years ago by E.M. Burbidge, G.R. Burbidge, A.G.W. Cameron, W.A. Fowler, and F. Hoyle. Significant advances in the intervening years include (a) observations of elemental and a few isotopic ratios in many more extrasolar-system sites, including metal-poor dwarf irregular galaxies, where very little has happened, and supernovae and their remnants, where a great deal has happened, (b) recognition of the early universe as good for making all the elements up to helium, (c) resolution of heavy element burning in stars into separate carbon, neon, oxygen, and silicon burning, with fine tuning of the resulting abundances by explosive nucleosynthesis in outgoing supernova shock waves, (d) clarification of the role of Type I supernovae, (e) concordance between elements produced in short-lived and long-lived stars with those that increased quickly and slowly over the history of the galaxy, and (f) calibration of calculations of the evolution and explosion of massive stars against the detailed observations of SN 1987A. The discussion presupposes a reader (a) with some prior knowledge of astronomy at the level of recognizing what is meant by an A star and an AGB star and (b) with at least a mild interest in how we got to where we currently are.     

Primordial nucleosynthesis and Ω B ∼1 cosmologies with interacting radiation and matter

The constraints on the present baryon density from primordial nucleosynthesis in universes with interacting radiation and matter are investigated. For illustration, a class of exact cosmological models is studied in which two separate, interacting fluids act as the source of the gravitational field, a radiative perfect fluid modelling the cosmic microwave background and a second perfect fluid modelling the observed material content of the Universe. Althought the two fluid models under consideration are found to predict primordial element abundances similar to those predicted in the standard model (and consequently in general accord with observed values), the upper limit on the present baryon density inferred from the observed abundances of the light elements is found to be greater than that in the standard model due to the different evolution of the baryon density in the models. From this result, and using the fact that the upper limit on _B (the ratio of the present value of the baryon density to the value of the critical density) is further weakened in inhomogeneous cosmological models, it is found that unlike the situation in the standard model, cosmologies with _B 1 are permitted without violating the constraints of nucleosynthesis, thereby allowing the possibility that the Universe could be closed by baryonic matter alone.     

The Basic Building Blocks of Galaxies

Current cold dark matter models of structure formation make a clear prediction for cosmic structures in the Dark Ages. We discuss the formation and nature of the first collapsed and first luminous objects in the universe arising in these theories. The first virialized objects are dark matter halos at the free streaming length which depends on the mass and nature of the assumed weakly interacting massive particle. The first objects that also contain significant fractions of gas have masses of the cosmological Jeans scale ∼ 10^4M _⊙ at the redshifts of interest (z ∼ 30). The first pre-galactic objects that host stars have masses of 10⁶ M _⊙. This mass scale is given by the requirement of a sufficiently high virial temperature to enable the chemical reactions necessary to form molecular hydrogen which subsequently allows the gas to dissipate its gravitational energy and to collapse to form a star. An individual massive star is formed per such object and explodes in a supernova within a few Myrs. All these stages of the formation of the first objects are illustrated by fully resolved three dimensional cosmological hydrodynamic simulations. This revised version was published online in September 2006 with corrections to the Cover Date.     

Chemical evolution of the galactic halo

The chemical enrichment in the galactic halo is studied, on the basis of the numerical model developed in Paper I, with paricular attention to the overabundances of O and light elements with respect to Fe shown by metal poor stars. Some representative nucleosynthesis pictures for stars of both Population I and Population II are considered and their yields are compared with observations of relative abundances in the Sun and in the halo, to identify the possible reasons of the observed compositional differences. It is found that solar elemental ratios can be reproduced if intermediate mass stars are allowed to give some contribution to the production of Fe by type-I supernovae, while the ratios of abundances observed in the halo are more similar to the relative yields produced by massive stars. These features are shared by all the nucleosynthesis schemes which have been considered. Using the best model of Paper I, we show that the steep star formation induced by the collapse has a decisive effect in maintaining the overabundances of light elements during the whole evolution of the halo. The relevance of this conclusion is discussed also in the light of a possible interpretation of the differences between the two abundance scales for globular clusters.     

Binary progenitors of Supernovae

Supernovae of both Type I (hydrogen-poor) and Type II (hydrogen-rich) can be expected to occur among binary stars. Among massive stars (>10 M•), the companion makes it more difficult for the primary to develop an unstable core of >1.4.M• while still retaining the extended, hydrogen-rich envelope needed to make a typical Type II light curve. Among 1–10 M• stars, on the other hand, a companion plays a vital role in currently popular models for Type I events, by transferring material to the primary after it has become a stable white dwarf, and so driving it to conditions where either core collapse or explosive nuclear burning will occur. Several difficulties (involving nucleosynthesis, numbers and lifetimes of progenitors, the mass-transfer mechanism,etc.) still exist in these models. Some of them are overcome by a recent, promising scenario in which the secondary also evolves to a degenerate configuration, and the two white dwarfs spiral together to produce a hydrogen-free explosion, long after single stars of the same initial masses have ceased to be capable of fireworks.     

Abundances in Globular Clusters

Large inter- and intra-cluster abundance variations of several light elements are observed among Galactic globular cluster stars. Correlations among these abundances suggest that many of the chemical inhomogeneities have been caused by high temperature advanced proton-capture fusion reactions. The evidence for this proton-capture reshuffling is addressed, along with comments on possible sites for this nucleosynthesis. This revised version was published online in July 2006 with corrections to the Cover Date.     

Galactic kinematics from a sample of young massive stars

Based on published sources, we have created a kinematic database on 220 massive (> 10 M _⊙) young Galactic star systems located within ≤3 kpc of the Sun. Out of them, ≈100 objects are spectroscopic binary and multiple star systems whose components are massive OB stars; the remaining objects are massive Hipparcos B stars with parallax errors of no more than 10%. Based on the entire sample, we have constructed the Galactic rotation curve, determined the circular rotation velocity of the solar neighborhood around the Galactic center at R ₀ = 8kpc, V ₀ = 259±16 km s^?1, and obtained the following spiral density wave parameters: the amplitudes of the radial and azimuthal velocity perturbations f _R = ?10.8 ± 1.2 km s^?1 and f _θ = 7.9 ± 1.3 km s^?1, respectively; the pitch angle for a two-armed spiral pattern i = ?6.0° ± 0.4°, with the wavelength of the spiral density wave near the Sun being λ = 2.6 ± 0.2 kpc; and the radial phase of the Sun in χ _⊙ = ?120° ± 4°. We show that such peculiarities of the Gould Belt as the local expansion of the system, the velocity ellipsoid vertex deviation, and the significant additional rotation can be explained in terms of the density wave theory. All these effects decrease noticeably once the influence of the spiral density wave on the velocities of nearby stars has been taken into account. The influence of Gould Belt stars on the Galactic parameter estimates has also been revealed. Eliminating them from the kinematic equations has led to the following new values of the spiral density wave parameters: f _θ = 2.9 ± 2.1 km s^?1 and χ _⊙ = ?104° ± 6°.     

Supernova SN 1961v: An explosion of a very massive star

An investigation of the outburst of the unique supernova SN 1961v in the galaxy NGC 1058 is carried out. An analysis of hydrodynamic models of supernova outbursts and a comparison with a considerable body of observational data on SN 1961v clearly show that the SN 1961v phenomenon is an explosion of a very massive star (VMS) of a mass of 2000M and a radius of about 100R which results in expelling the envelope with a kinetic energy of 1.8×10⁵² erg. The light curve of SN 1961v (Figures 1, 7a, and 7b) furnishes direct evidence for the heterogeneity of the presupernova interior. The chemical composition profile produced during the evolution of the VMS and in the final explosion must have a number of the essential features (Figure 11). In particular, hydrogen has to be underabundant relative to the solar content and distributed in a specific manner throughout the star. At the late stages from February, 1963 to February, 1967, the light curve of SN 1961v (Figure 1) may be accounted for by the interaction of the expelled envelope with the stellar wind of the presupernova. The latest observations of SN 1961v in 1968 and 1970 are virtually those of a giantHii region created by the VMS before the explosion. Two astrophysical phenomena-the peculiar outburst of SN 1961v and the most luminous object R136a in the Large Magellacnic Cloud (LMC) which reveals a striking similarity with the presupernova-are evidence for the existence of VMSs. The evolution of VMSs similar to the object R136a may be terminated by explosions like the outburst of SN 1961v. Such explosions give rise to the formation of energetic supernova remnants whose examples may be the Cygnus superbubble and the supergiant shells in the LMC. A comparison of the internal structure of the presupernova with the available evolutionary calculations allows one to conclude that the influence of mass loss on the evolution of VMSs is negligible.