Early Chemical Evolution Research Articles

Fossil Records of Cosmic Reionization in Galactic Stellar Halos

Galactic stellar halos have long been considered to contain fossil information on early dynamical and chemical evolution of galaxies. We propose that the surface brightness distributions of old stellar halos contain the influence of reionization on early formation histories of galaxies. By assuming that reionization significantly suppresses star formation in small subgalactic clumps virialized after reionization redshift ($z_{\rm reion}$), we first numerically investigate how structural and kinematical properties of stellar halos formed from merging of these subgalactic clumps depend on $z_{\rm reion}$. We then discuss what observable properties of galactic stellar halos offer us the fossil records of reionization influence on hierarchical formation of halos based on the current results of numerical simulations. We particularly suggest that both the half-light radius of stellar halos and the slope of their surface brightness profile contain useful information on when star formation in subgalactic clumps were significantly influenced by reionization. By using the simulated surface brightness distributions of galactic stellar halos for models with different $z_{\rm reion}$, we also discuss how wide-field imaging studies of extragalactic halos will help us to elucidate the epoch of cosmic reionization.

Early chemical evolution of the Milky Way

Early chemical evolution of the Milky Way

CH stars at high Galactic latitudes

In the present work we report on several CH stars identified in a sample of Faint High Latitude Carbon stars from Hamburg survey and discuss their medium resolution spectra covering a wavelength range 4000 - 6800 \AA . Estimation of the depths of bands (1,0) $^{12}$C$^{12}$C ${\lambda}$4737 and (1,0) $^{12}$C$^{13}$C ${\lambda}$4744 in these stars indicate isotopic ratio $^{12}$C/$^{13}$C ${\sim}$ 3, except for a few exceptions; these ratios are consistent with existing theories of CH stars evolution. The stars of Hamburg survey, a total of 403 objects were reported to be carbon star candidates with strong C$_{2}$ and CN molecular bands. In the first phase of observation, we have acquired spectra of ninety one objects. Inspection of the objects spectra show fifty one objects with C$_{2}$ molecular bands in their spectra of which thirteen stars have low flux below about 4300 \AA . Twenty five objects show weak or moderate CH and CN bands, twelve objects show weak but detectable CH bands in their spectra and there are three objects which do not show any molecular bands due to C$_{2}$, CN or CH in their spectra. Objects with C$_{2}$ molecular bands and with good signals bluewards of 4300 \AA which show prominent CH bands in their spectra are potential candidate CH stars. Thirty five such candidates are found in the present sample of ninty one objects observed so far. The set of CH stars identified could be the targets of subsequent observation at high resolution for a detail and comprehensive analysis for understanding their role in early Galactic chemical evolution.

The Nature of Lyα Blobs: Supernova-dominated Primordial Galaxies

We consider a forming galaxy undergoing multitudinous supernova (SN) explosions, as a possible model of \lya blobs (LABs). For this purpose, an ultra--high resolution hydrodynamic simulation is performed using $1024^3$ grid points, where SN remnants are resolved with sufficient accuracy. It is found that multiple SN explosions produce kpc--size expanding hot bubbles, which drive cool, dense shells by strong shock. The colliding high--density cooling shells radiate intensive \lya emission, resulting in a high \lya luminosity of $\sim10^{43}$ erg s$^{-1}$, comparable to the observed level in LABs. Also, recently discovered bubbly features in some LABs are quite similar to the structure predicted in the present simulation. Furthermore, the result demonstrates that LABs are representative of evolving primordial galaxies; they could hold direct information on the early chemical enrichment of galaxies, contrary to present--day galaxies which have undergone intense recycling of interstellar matter, thus erasing most of the early chemical history. It turns out that the metal mixing proceeds in a very inhomogeneous fashion, so that there appears a large spread of metallicity, that is,$[{\rm Fe/H}] \approx 0 {\rm ~to} -5$ or $[{\rm O/H}] \approx 1 {\rm ~to} -4$. Hence, the early galactic chemical evolution may have proceeded in a different manner from that hitherto considered in one--zone models.

The early chemical evolution of nova outflows

The chemistry of the early stages of the evolution of a nova wind is examined. The new models include an extended chemistry and, in the light of current model atmosphere calculations, a careful treatment of the photoreactions. The results indicate that the chemistry may be somewhat richer than previously determined and, contrary to the findings of previous studies, is essentially not photon-dominated. The abundance of species such as CN and CO match observations well and the ratio of their abundances may be a useful density diagnostic. However, except at very early times, CO formation does not saturate; this has important implications for the subsequent dust-formation epoch. In particular it provides theoretical backing for those observations which suggest that carbon-rich and silicate dusts can be formed concurrently.

Hypernovae: Their Properties and Gamma-Ray Burst Connection

Natures of very energetic supernovae, i.e., hypernovae, are discussed. They are the explosive deaths of stars more massive than ∼ 20-25M, probably being linked to enigmatic Gamma-Ray Bursts: Optical properties of hypernovae indicate that they are significantly aspherical, while so far no detailed modeling of such an explosion is available. We present light curves and late-phase spectra of aspherical supernova/hypernova models, showing that the optical observations of SNe 1998bw and 2002ap can be well accounted for. The abundance patterns of hypernovae are characterized by large ratios (Zn, Co)/Fe and small ratios (Mn, Cr)/Fe, indicating significant contribution to the early Galactic chemical evolution.

Bipolar Supernova Explosions: Nucleosynthesis and Implications for Abundances in Extremely Metal‐Poor Stars

Hydrodynamics and explosive nucleosynthesis in bipolar supernova explosions are examined to account for some peculiar properties of hypernovae as well as peculiar abundance patterns of metal-poor stars. The explosion is assumed to be driven by bipolar jets that are powered by accretion onto a central remnant. The energy injection rate by the jets is assumed to be proportional to the accretion rate, i.e., Ėjet = αc2. We explore the features of the explosions with varying progenitors' masses and jet properties. The outcomes are different from conventional spherical models. (1) In the bipolar models, Fe-rich materials are ejected at high velocities along the jet axis, while O-rich materials occupy the central region, whose density becomes very high as a consequence of continuous accretion from the side. This configuration can explain some peculiar features in the light curves and the nebular spectra of hypernovae. (2) Production of 56Ni tends to be smaller than in spherical thermal bomb models. To account for a large amount of 56Ni observed in hypernovae, the jets should be initiated when the compact remnant mass is still smaller than 2-3 M☉, or they should be very massive and slow. (3) Ejected isotopes are distributed as follows in order of decreasing velocities: 64Zn, 59Co, 56Fe, 44Ti, and 4He at the highest velocities; 55Mn, 52Cr, 32S, and 28Si at the intermediate velocities; and 24Mg and 16O at the lowest velocities. (4) The abundance ratios (Zn, Co)/Fe are enhanced while the ratios (Mn, Cr)/Fe are suppressed. This can account for the abundance pattern of extremely metal-poor stars. These agreements between the models and observations suggest that hypernovae are driven by bipolar jets and have significantly contributed to the early Galactic chemical evolution.

Bipolar explosion models for hypernovae

Bipolar explosion models for hypernovae (very energetic supernovae) are presented. These models provide a favorable situation to explain some unexpected features in observations of hypernovae, e.g., high velocity matter dominated by Fe and low velocity matter dominated by O. The overall abundance of these models gives a good fit, at least qualitatively, to abundances in extremely metal-poor stars. We suggest hypernovae be driven by bipolar jets and contribute significantly to the early Galactic chemical evolution.

Nucleosynthesis and early chemical evolution in galaxies

Nucleosynthesis and galactic chemical evolution are inter-linked topics that merge various fields in astronomy and physics. Speakers at the RAS discussion meeting of January 2003 combined theory and observation in understanding these fields. Stelios A Tsangarides, Enrico Arnone and Sean G Ryan report.

Jets and Black Holes in Hypernova Explosions

Bipolar explosion models for hypernovae (very energetic supernovae), associated with the black hole formation, are presented. The model features are as follows: (1) Fe-peak elements are ejected at high velocities to reachthe surface region, while dense O-rich materials occupy the central region at low velocities. (2) The abundance ratios (Zn, Co)/Fe are enhanced while the ratios (Mn, Cr)/Fe are suppressed. The overall abundances are consistent with those observed in extremely metal-poor stars formed at the very early phase of the Galaxy, suggesting significant contribution of such explosions to the early Galactic chemical evolution. (3) Certain relations among the optical luminosity, the chemical composition, and the property of the central remnant are predicted. Such relations may serve for identifying possible gravitational wave emitters in the future.

Stellar Archaeology: A Keck Pilot Program on Extremely Metal-poor Stars from the Hamburg/ESO Survey. II. Abundance Analysis

We present a detailed abundance analysis of eight stars selected as extremely metal-poor candidates from the Hamburg/ESO Survey (HES). For comparison, we have also analyzed three extremely metal-poor candidates from the HK survey, and three additional bright metal-poor stars. With this work, we have doubled the number of extremely metal-poor stars ([Fe/H] ≤ 3.0 dex) with high-precision abundance analyses. Based on this analysis, our sample of extremely metal-poor candidates from the HES contains three stars with [Fe/H] ≤ -3.0 dex, three more with [Fe/H] ≤ -2.8 dex, and two stars that are only slightly more metal-rich. Thus, the chain of procedures that led to the selection of these stars from the HES successfully provides a high fraction of extremely metal-poor stars. We verify that our choices for stellar parameters, derived in Paper I and independently of the high-dispersion spectroscopic analysis, lead to acceptable ionization and excitation balances for Fe. Substantial non-LTE effects in Fe appear to be ruled out by the above agreement, even at these extremely low metallicities. For the α-elements Mg, Si, Ca, and Ti, the light element Al, the iron-peak elements Sc, Cr, and Mn, and the neutron-capture elements Sr and Ba, we find trends in abundance ratios [X/Fe] similar to those found by previous investigations. These trends appear to be identical for giants and for dwarfs. However, the scatter in most of these ratios, even at [Fe/H] ≤ -3.0 dex, is surprisingly small. Only Sr and Ba, among the elements we examined, show scatter larger than the expected errors. Future work (the 0Z Project) will provide much stronger constraints on the scatter (or lack thereof) in elemental abundances for a substantially greater number of stars. We discuss the implications of these results for the early chemical evolution of the Galaxy, including such issues as the number of contributing supernovae and the sizes of typical protogalactic fragments in which they were born. In addition, we have identified a very metal-poor star in our sample that appears to represent the result of the s-process chain, operating in a very metal-poor environment, and exhibits extremely enhanced C, Ba, and Pb and somewhat enhanced Sr.

実験室に原始太陽系をつくる 原始太陽系での蒸発・凝縮

Evaporation and condensation played a major role in producing chemical and/or isotopic variations in the early solar system, which have been recorded in primitive meteorites. Experimental studies on evaporation and condensation of various planetary materials have started in 1970's to reproduce chemical compositions and textures of constituents of primitive meteorites. In a recent decade, many evaporation experiments of minerals and melts have been carried out to obtain fundamental kinetic parameters and to comprehend the chemical (or isotopic) fractionations in the early solar system in terms of evolution. Experimental simulation of evaporation and condensation in the solar system should be combined with isotopic or chemical analyses of meteorites and observational and theoretical studies of the proto-planetary disk for complete understanding of the early solar system chemical evolution. The history of experimental studies of evaporation and condensation under the early solar system conditions and some recent progresses are reviewed in this paper.

Early ages shrinkage mechanisms of ultra-high-performance cement-based materials

Early age chemical and microstructural evolution of an ultra-high-performance cement-based material is analyzed by autogeneous shrinkage monitoring simultaneously with ultrasonic propagation measurements. The hydration progress is determined with a calorimetric equipment. The combined analysis with these complementary methods leads to the identification of five stages in this evolution: (1) a settling period with short-range reorganization without any cluster formation or shear resistance; (2) an aggregation period with unconnected clusters formation and growing leading to the percolation of solid particles; (3) a postpercolation bond development phase leading to a globally structured framework; (4) a progressive reinforcement by multiplication of connections within the framework leading to a fully hyperstatic state; (5) a hyperstatic phase where all the particles are connected, with the consolidation by pores filling.

The organic content of the Tagish Lake meteorite.

The Tagish Lake meteorite fell last year on a frozen lake in Canada and may provide the most pristine material of its kind. Analyses have now shown this carbonaceous chondrite to contain a suite of soluble organic compounds (approximately 100 parts per million) that includes mono- and dicarboxylic acids, dicarboximides, pyridine carboxylic acids, a sulfonic acid, and both aliphatic and aromatic hydrocarbons. The insoluble carbon exhibits exclusive aromatic character, deuterium enrichment, and fullerenes containing "planetary" helium and argon. The findings provide insight into an outcome of early solar chemical evolution that differs from any seen so far in meteorites.

Halo star abundances and r-process synthesis

We review recent observational studies of heavy element abundances in low metallicity stars and explore some implications of these results for nucleosynthesis and early Galactic chemical evolution.

GAIA: Composition, formation and evolution of the Galaxy

The GAIA astrometric mission has recently been approved as one of thenext two ``cornerstones'' of ESA's science programme, with a launch datetarget of not later than mid-2012. GAIA will provide positional andradial velocity measurements with the accuracies needed to produce astereoscopic and kinematic census of about one billion stars throughoutour Galaxy (and into the Local Group), amounting to about 1 percent ofthe Galactic stellar population. GAIA's main scientific goal is toclarify the origin and history of our Galaxy, from a quantitative censusof the stellar populations. It will advance questions such as when thestars in our Galaxy formed, when and how it was assembled, and itsdistribution of dark matter. The survey aims for completeness to V=20mag, with accuracies of about 10 mu as at 15 mag. Combined withastrophysical information for each star, provided by on-boardmulti-colour photometry and (limited) spectroscopy, these data will havethe precision necessary to quantify the early formation, and subsequentdynamical, chemical and star formation evolution of our Galaxy.Additional products include detection and orbital classification of tensof thousands of extra-Solar planetary systems, and a comprehensivesurvey of some 105-106 minor bodies in our SolarSystem, through galaxies in the nearby Universe, to some 500 000 distantquasars. It will provide a number of stringent new tests of generalrelativity and cosmology. The complete satellite system was evaluated aspart of a detailed technology study, including a detailed payloaddesign, corresponding accuracy assesments, and results from a prototypedata reduction development. (Less)

Abundance Patterns in the Draco, Sextans, and Ursa Minor Dwarf Spheroidal Galaxies

The Keck I telescope has been used to obtain HIRES spectra for red giants belonging to the Draco, Sextans and Ursa Minor dwarf spheroidal (dSph) galaxies. An analysis of these spectra is presented, along with abundance ratios for more than 20 elements. The resulting database of element abundances for 17 stars is the most extensive yet assembled for stars in dSph environments. Our main findings are summarized as follows: (1) There is unambiguous evidence for a large internal spread in metallicity in all three galaxies: our program stars span a range of [Fe/H] = 1.53, 1.40 and 0.73 dex in Draco, Sextans and Ursa Minor, respectively. (2) The abundance patterns among the dSph stars are remarkably uniform, suggesting that all three galaxies have similar nucleosynthetic histories. (3) A comparison of the measured abundance ratios for our sample of dSph stars with published values for Galactic halo and disk field stars suggests that the dSph galaxies have 0.02 < [alpha/Fe] < 0.13 dex, whereas the halo field star sample has [alpha/Fe] ~ 0.28 dex over the same range in metallicity. (4) The most metal-rich dSph stars in our sample have [Y/Fe] abundances which are significantly lower than those measured for halo field stars of similar metallicity, while the measured [Ba/Eu] ratios for the dSph stars suggest that the early chemical evolution of these galaxies was dominated by the r-process. Taken together, these results suggest that the Galactic halo is unlikely to have assembled, in its entirety, through the disruption of dwarf galaxies similar to the low-luminosity dSphs studied here. (ABRIDGED).

Prebiotic methylation and the evolution of methyl transfer reactions in living cells.

An hypothesis is presented for the prebiotic origin of methyl groups and the evolution of methyl transfer reactions in living cells. This hypothesis, described in terms of prebiotic and early biotic chemical evolution, is based on experimental observations in our lab and in those of others, and on the mechanisms of enzymatic methyl transfer reactions that occur in living cells. Of particular interest is our demonstration of the reductive methylation of ethanolamine and glycine in aqueous solution by excess formaldehyde. These reactions, involving prebiotic compounds and conditions, are mechanistically analogous to the de novo origin of methyl groups in modern cells by reduction of methylene tetrahydrofolate. Furthermore, modern cellular methyl transfers from S-adenosylmethionine to amine nitrogen may involve formaldehyde as an intermediate and subsequent reductive methylation, analogous to the prebiotic chemistry observed herein.

Hubble Space TelescopeObservations of Neutron‐Capture Elements in Very Metal Poor Stars

Using the Goddard High-Resolution Spectrograph (GHRS) of the Hubble Space Telescope (HST) we have detected the neutron-capture elements osmium, platinum, and lead in the very metal poor ([Fe/H] -2.8) Galactic halo star HD 115444. This star joins the metal-poor giant HD 126238 ([Fe/H] -1.7) as the second Galactic halo star for which neutron-capture elements have been detected. We have also determined upper limits on these same elements for the neutron-capture deficient star HD 122563 ([Fe/H] - 2.7). We have identified zirconium and germanium features in the HST spectra of all three stars; this marks the initial identification of the latter element in halo stars. We compare and contrast the spectra and abundances of the neutron-capture elements in these three stars. Combining the new HST observations of the third neutron-capture peak elements of HD 115444 with ground-based data, we find the stellar abundance distribution, over a wider atomic number range than ever before possible, is consistent with the solar system r-process abundance curve. Comparison of this star to the more metal-rich HD 126238 finds that the ratio of the third-peak element abundances to that of the pure r-process element europium is the same in both stars. Earlier ground-based studies have found that abundances of the lighter elements, such as barium and europium, in HD 115444 exceed those of HD 122563 by ~0.7 dex. Our osmium, platinum, and lead upper limits show a similar difference and are not inconsistent with either the scaled solar r-process or the solar total distribution. Thus, the second and third neutron-capture peak element (Z ≥ 56) abundances in all three stars are consistent with scaled solar system r-process abundances. However, the zirconium abundance is approximately the same in HD 122563 and HD 115444, so this element (near the first neutron-capture peak) is overabundant (somewhat) in HD 122563 and (slightly) in HD 115444 with respect to the solar r-process abundances. Germanium (synthesized in approximately equal amounts by the r- and the s-process in solar material) is underabundant in all three stars, but does seem to scale with metallicity—it is identical in HD 115444 and HD 126238, but significantly greater in the higher metallicity HD 126238. These new results support previous observations that demonstrate the operation of the r-process, including the synthesis of the heaviest such elements, early in the history of the Galaxy. Implications of these results for early Galactic chemical evolution are discussed.

Joint Discussion 1: Abundance Ratios in the Oldest Stars

Joint Discussion 1 was supported by Division IV (Stars) and Commission 29 (Stellar Spectra), and co-supported by Commissions 28 (Galaxies), 36 (Theory of Stellar Atmospheres) and 37 (Stellar Clusters and Associations). Members of the scientific organizing committee were: N. Arimoto (Japan), B. Barbuy (Brazil), T. Beers (USA), J. Bergeron (Germany), M. Bessell (Australia), R. Cayrel (France), G. Gilmore (UK), B. Gustafsson (Sweden), F. Matteucci (Italy), P. Nissen (Den-mark), and M. Rich (USA).The inspiration for this meeting was the growing overlap and connections between previously separate areas of astrophysical research, namely, studies of stellar abundances, the bulges of galaxies, the gaseous components of nearby galaxies and the clouds (some of which may be primordial) responsible for the narrow absorption lines in quasars.The signature of the early chemical evolution of our Galaxy is imprinted in the abundance ratios of the oldest stars. We recall that element ratios are determined by a mix of the relative rates of different types of supernovae, the stellar IMF, and the relative histories of star formation rates and gaseous flows, and thus encapsulate much of the history of star formation and ISM evolution in galaxies. Hence, abundance ratios in stars are a primary probe for testing theories of galaxy formation and evolution.We do not know how the Galaxy formed: both the Eggen, Lynden-Bell &amp; Sandage (1962) and the Searle &amp; Zinn (1978) scenarios may be accommodated in the recent proposal of van den Bergh (1993) where the inner Galaxy follows ELS, whereas the outer Galaxy formation conforms to the Searle-Zinn proposition. A combination of abundance ratios, ages derived from colour-magnitude diagrams, and kinematical properties, can give us the required information to trace the past history of our Galaxy. We note here, that although stellar evolution and model atmospheres are not discussed in the proceedings both topics are of fundamental underlying importance. Model atmospheres are used to derive temperatures, colors and bolometric corrections of stars that are used not only in abundance analyses but also in deriving the ages of stars by comparing CM diagrams with HR diagrams. This process is under close scrutiny because of the apparent difference between the ages of the oldest stars and the expansion age of the universe.