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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. 相似文献
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Abstract— A wide range of stellar nucleosynthetic sources has been analyzed to derive their contributions of short‐lived and stable nuclei to the presolar cloud. This detailed study is required to infer the most plausible source(s) of short‐lived nuclei through a critical comparison among the various stellar sources that include AGB stars, novae, supernovae II, Ia, and Wolf‐Rayet stars that evolved to supernovae Ib/c. In order to produce the canonical value of 26Al/27Al in the early solar system, almost all stellar sources except low‐mass AGB stars imply large isotopic anomalies in Ca‐Al‐rich inclusions (CAIs). This is contrary to the observed isotopic compositions of CAIs. The discrepancy could impose stringent constraints on the formation and thermal evolution of CAIs from different chondrites. Among the various stellar scenarios, the injection of short‐lived nuclei into the previously formed solar protoplanetary disc by a massive star of an ad hoc chosen high‐injection mass cut is a possible scenario. There is a possibility of the contribution of short‐lived nuclides by a 1.5–3 M⊙ AGB star as it implies the smallest shift in stable isotopes. A low‐mass AGB star of relatively low metallicity would be even a better source of short‐lived nuclei. However, this scenario would require independent gravitational collapse of the presolar cloud coupled with ambipolar diffusion of magnetic flux. Alternatively, numerous scenarios can be postulated that involve distant (≥10 pc) massive stars can contribute 60Fe to the presolar cloud and can trigger its gravitational collapse. These scenarios would require production of 26Al and 41Ca by irradiation in the early solar system. Significant production of 26Al and 60Fe can be explained if massive, rotating Wolf‐Rayet stars that evolved to supernovae Ib/c were involved. 相似文献
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We show that the explosive transition of the neutron star(NS)to a quark star(QS)(a Quark Nova)in Cassiopeia A(Cas A)a few days following the supernova(SN)proper can account for several of the puzzling kinematic and nucleosynthetic features that are observed.The observed decoupling between Fe and44Ti and the lack of Fe emission within44Ti regions is expected in the QN model owing to the spallation of the inner SN ejecta by relativistic QN neutrons.Our model predicts the44Ti to be more prominent to the NW of the central compact object(CCO)than in the SE and little of it along the NE-SW jets,in agreement with Nu Star observations.Other intriguing features of Cas A are addressed,such as the lack of a pulsar wind nebula and the reported few percent drop in the CCO temperature over a period of 10 yr. 相似文献
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Vikram V. Dwarkadas 《Astrophysics and Space Science》2007,307(1-3):153-158
Core-Collapse supernovae arise from stars greater than 8 M⊙. These stars lose a considerable amount of mass during their lifetime, which accumulates around the star forming wind-blown
bubbles. Upon the death of the star in a spectacular explosion, the resulting SN shock wave will interact with this modified
medium. We study the evolution of the shock wave, and investigate the properties of this interaction. We concentrate on the
evolution of the SN shock wave in the medium around a 35 solar mass star. We discuss the hydrodynamics of the resulting interaction,
the formation and growth of instabilities, and deviations from sphericity. 相似文献
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This paper has two parts: one about observational constraints related to the empirical differential oxygen abundance distribution
(EDOD), and the other about inhomogeneous models of chemical evolution, in particular the theoretical differential oxygen
abundance distribution (TDOD). In the first part, the EDOD is deduced from subsamples related to two different samples involving
(i) N=532 solar neighbourhood (SN) stars within the range, −1.5<[Fe/H]<0.5, for which the oxygen abundance has been determined
both in presence and in absence of the local thermodynamical equilibrium (LTE) approximation (Ramirez et al. in Astron. Astrophys.
465:271, 2007); and (ii) N=64 SN thick disk, SN thin disk, and bulge K-giant stars within the range, −1.7<[Fe/H]<0.5, for which the oxygen abundance
has been determined (Melendez et al. in Astron. Astrophys. 484:L21, 2008). A comparison is made with previous results implying use of [O/H]–[Fe/H] empirical relations (Caimmi in Astron. Nachr. 322:241,
2001b; New Astron. 12:289, 2007) related to (iii) 372 SN halo subdwarfs (Ryan and Norris in Astron. J. 101:1865, 1991); and (iv) 268 K-giant bulge stars (Sadler et al. in Astron. J. 112:171, 1996). The EDOD of the SN thick + thin disk is determined by weighting the mass, for assumed SN thick to thin disk mass ratio
within the range, 0.1–0.9.
In the second part, inhomogeneous models of chemical evolution for the SN thick disk, the SN thin disk, the SN thick + thin
disk, the SN halo, and the bulge, are computed assuming the instantaneous recycling approximation. The EDOD data are fitted,
to an acceptable extent, by their TDOD counterparts with the exception of the thin or thick + thin disk, where two additional
restrictions are needed: (i) still undetected, low-oxygen abundance thin disk stars exist, and (ii) a single oxygen overabundant
star is removed from a thin disk subsample. In any case, the (assumed power-law) stellar initial mass function (IMF) is universal
but gas can be inhibited from, or enhanced in, forming stars at different rates with respect to a selected reference case.
Models involving a strictly universal IMF (i.e. gas neither inhibited from, nor enhanced in, forming stars with respect to
a selected reference case) can also reproduce the data to an acceptable extent.
Our main conclusions are (1) different models are necessary to fit the (incomplete) halo sample, which is consistent with
the idea of two distinct halo components: an inner, flattened halo in slow prograde rotation, and an outer, spherical halo
in net retrograde rotation (Carollo et al. in Nature 450:1020, 2007); (2) the oxygen enrichment within the inner SN halo, the SN thick disk, and the bulge, was similar and coeval within the
same metallicity range, as inferred from observations (Prochaska et al. in Astron. J. 120:2513, 2000); (3) the fit to thin disk data implies an oxygen abundance range similar to its thick disk counterpart, with the extension
of conclusion (2) to the thin disk, and the evolution of the thick + thin disk as a whole (Haywood in Mon. Not. R. Astron.
Soc. 388:1175, 2008) cannot be excluded; (4) leaving outside the outer halo, a fit to the data related to different environments is provided
by models with a strictly universal IMF but different probabilities of a region being active, which implies different global
efficiencies of the star formation rate; (5) a special case of stellar migration across the disk can be described by models
with enhanced star formation, where a fraction of currently observed SN stars were born in situ and a comparable fraction is due to the net effect of stellar migration, according to recent results based on high-resolution
N-body + smooth particle hydrodynamics simulations (Roškar et al. in Astrophys. J. Lett. 684:L79, 2008). 相似文献
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S.L. Ellison S.G. Ryan J.X. Prochaska 《Monthly notices of the Royal Astronomical Society》2001,326(2):628-636
The study of elemental abundances in damped Lyman alpha systems (DLAs) at high redshift represents one of our best opportunities to probe galaxy formation and chemical evolution at early times. By coupling measurements made in high- z DLAs with our knowledge of abundances determined locally and with nucleosynthetic models, we can start to piece together the star formation histories of these galaxies. Here, we discuss the clues to galactic chemical evolution that may be gleaned from studying the abundance of Co in DLAs. We present high resolution echelle spectra of two quasi-stellar objects (QSOs), Q2206−199 and Q1223+17, both already known to exhibit intervening damped systems. These observations have resulted in the first ever detection of Co at high redshift, associated with the z abs =1.92 DLA in the sightline towards Q2206−199. We find that the abundance of Co is approximately 1/4 solar and that there is a clear overabundance relative to iron, [Co/Fe]=+0.31±0.05 . From the abundance of Zn, we determine that this is a relatively metal-rich DLA, with a metallicity of approximately 1/3 Z⊙ . Therefore, this first detection of Co is similar to the marked overabundance relative to Fe seen in Galactic bulge and thick-disc stars. 相似文献
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An alternative non-infall model for the chemical evolution in the solar neighbourhood is proposed. The evolution of the disk is divided into two phases. In phaseA, the magnetic field and the gas viscosity produced an outward flux of gas, forming and maintaining the ring observed today. This flux balanced the star formation in the ring. The number of stars increased until the beginning of phaseB, during which stellar viscosity generated an inward flux of stars towards the inner disk, while the magnetic fields continued supplying gas to the ring. The combination of these two effects brought the ring to a quasi-steady state, with a constant mass of gas and stars which we assume has continued till the present. A coherent picture is obtained in which the observational restrictions are explained without introducing any arbitrary hypothesis. The inward flux of stars in phaseB has transported the metal-poor G-dwards to the inner region, thus explaining their absence in the solar neighbourhood. 相似文献
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Data from our compiled catalog of spectroscopically determined magnesium abundances in stars with accurate parallaxes are used to select thin-disk dwarfs and subgiants according to kinematic criteria. We analyze the relations between the relative magnesium abundances in stars, [Mg/Fe], and their metallicities, Galactic orbital elements, and ages. The [Mg/Fe] ratios in the thin disk at any metallicity in the range ?1.0 dex <[Fe/H] < ?0.4 dex are shown to be smaller than those in the thick disk, implying that the thin-disk stars are, on average, younger than the thick-disk stars. The relative magnesium abundances in such metal-poor thin-disk stars have been found to systematically decrease with increasing stellar orbital radii in such a way that magnesium overabundances ([Mg/Fe] > 0.2 dex) are essentially observed only in the stars whose orbits lie almost entirely within the solar circle. At the same time, the range of metallicities in magnesium-poor stars is displaced from ?0.5 dex < [Fe/H] < +0.3 dex to ?0.7 dex < [Fe/H] < +0.2 dex as their orbital radii increase. This behavior suggests that, first, the star formation rate decreases with increasing Galactocentric distance and, second, there was no star formation for some time outside the solar circle, while this process was continuous within the solar circle. The decrease in the star formation rate with increasing Galactocentric distance is responsible for the existence of a negative radial metallicity gradient (grad R[Fe/H] = ?0.05 ± 0.01 kpc?1) in the disk, which shows a tendency to increase with decreasing age. At the same time, the relative magnesium abundance exhibits no radial gradient. We have confirmed the existence of a steep negative vertical metallicity gradient (grad Z[Fe/H] = ?0.29 ± 0.06 kpc?1) and detected a significant positive vertical gradient in relative magnesium abundance (grad Z[Mg/Fe] = 0.13 ± 0.02 kpc?1); both gradients increase appreciably in absolute value with decreasing age. We have found that there is not only an age-metallicity relation, but also an age-magnesium abundance relation, in the thin disk. We surmise that the thin disk has a multicomponent structure, but the existence of a negative trend in the star formation rate along the Galactocentric radius does not allow the stars of its various components to be identified in the immediate solar neighborhood. 相似文献
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The eclipsing and double-lined spectroscopic binary system V453 Cygni consists of two early B-type stars, one of which is nearing the terminal age main sequence and one which is roughly halfway through its main-sequence lifetime. Accurate measurements of the masses and radii of the two stars are available, which makes a detailed abundance analysis both more interesting and more precise than for isolated stars. We have reconstructed the spectra of the individual components of V453 Cyg from the observed composite spectra using the technique of spectral disentangling. From these disentangled spectra, we have obtained improved effective temperature measurements of 27 900 ± 400 and 26 200 ± 500 K , for the primary and secondary stars, respectively, by fitting non local thermodynamic equilibrium theoretical line profiles to the hydrogen Balmer lines. Armed with these high-precision effective temperatures and the accurately known surface gravities of the stars we have obtained the abundances of helium and metallic elements. A detailed abundance analysis of the primary star shows a normal (solar) helium abundance if the microturbulence velocity derived from metallic lines is used. The elemental abundances show no indication that CNO-processed material is present in the photosphere of this high-mass terminal age main-sequence star. The elemental abundances of the secondary star were derived by a differential study against a template spectrum of a star with similar characteristics. Both the primary and secondary components display elemental abundances which are in the ranges observed in the Galactic OB stars. 相似文献
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S. Hayakawa 《Earth, Moon, and Planets》1978,18(3):321-324
Rapid accretion of matter takes place while the solar system resides in a dark cloud of high density. The mass thus accreted may be comparable to the mass of the planetary system. Since the elemental abundances in a dark cloud are considered to be heterogeneous due to matter processed inside stars and then ejected, the continual accretion causes elemental heterogeneities in the solar system.Paper dedicated to Professor Hannes Alfvén on the occasion of his 70th birthday, 30 May 1978. 相似文献
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Takuji Tsujimoto Toshikazu Shigeyama Yuzuru Yoshii 《Astrophysics and Space Science》2002,281(1-2):221-222
Following our hypothesis that each supernova (SN) event triggers star formation in the swept-up gas, so that newly formed
stars inherit the elemental abundance pattern of individual SNe, we deduce the production sites and yields for r-process elements. We further show that a strong evidence for the origin of r-process nucleosynthesis products was just there in our backyard - supernova SN1987A -, and conclude that 20 M
⊙ SNe are the predominant production sites for r-process elements.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
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Abstract— A quantitative analysis is presented for the irradiation contributions of the short‐lived nuclides, specifically 26Al, by the X‐wind scenario in the early solar system. The analysis is based on the comprehensive numerical simulations of the scenario that involves thermal processing of protoCAIs during the decades long X‐wind cycle. It would be difficult to explain the canonical value of 26Al/27Al in Ca‐Al‐rich inclusions on the basis of its inferred irradiation yields. Hence, the bulk inventory of 26Al in the early solar system was not produced by the X‐wind scenario. We suggest the predominant occurrence of gradual flares compared to impulsive flares in the early solar system as in the case of the modern solar flares. One tenth of the bulk 26Al was only produced by irradiation in case the entire solar inventory of 10Be was produced by local irradiation. The bulk 26Al inventory along with 60Fe was probably synthesized by a massive star. We present a qualitative model of the astrophysical settings for the formation of the solar system on the basis of a survey of the presently active star forming regions. We hypothesize that the formation of the solar system could have occurred almost contemporaneously with the formation of the massive star within a single stellar cluster. As the massive star eventually exploded as supernova Ib/c subsequent to Wolf‐Rayet stages, the short‐lived nuclides were probably injected into the solar proto‐planetary disc. The dynamically evolving stellar cluster eventually dispersed within the initial ?10 million years prior to the major planetary formation episodes. 相似文献
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M. Haywood 《Monthly notices of the Royal Astronomical Society》2001,325(4):1365-1382
We present a revised metallicity distribution of dwarfs in the solar neighbourhood. This distribution is centred on solar metallicity. We show that previous metallicity distributions, selected on the basis of spectral type, are biased against stars with solar metallicity or higher. A selection of G-dwarf stars is inherently biased against metal-rich stars and is not representative of the solar neighbourhood metallicity distribution. Using a sample selected on colour, we obtain a distribution where approximately half the stars in the solar neighbourhood have metallicities higher than [Fe/H]=0 . The percentage of mid-metal-poor stars ([Fe/H]<−0.5) is approximately 4 per cent, in agreement with present estimates of the thick disc.
In order to have a metallicity distribution comparable to chemical evolution model predictions, we convert the star fraction to mass fraction, and show that another bias against metal-rich stars affects dwarf metallicity distributions, due to the colour (or spectral type) limits of the samples. Reconsidering the corrections resulting from the increasing thickness of the stellar disc with age, we show that the simple closed-box model with no instantaneous recycling approximation gives a reasonable fit to the observed distribution. Comparisons with the age–metallicity relation and abundance ratios suggest that the simple closed-box model may be a viable model of the chemical evolution of the Galaxy at solar radius. 相似文献
In order to have a metallicity distribution comparable to chemical evolution model predictions, we convert the star fraction to mass fraction, and show that another bias against metal-rich stars affects dwarf metallicity distributions, due to the colour (or spectral type) limits of the samples. Reconsidering the corrections resulting from the increasing thickness of the stellar disc with age, we show that the simple closed-box model with no instantaneous recycling approximation gives a reasonable fit to the observed distribution. Comparisons with the age–metallicity relation and abundance ratios suggest that the simple closed-box model may be a viable model of the chemical evolution of the Galaxy at solar radius. 相似文献
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A. Marcolini B. K. Gibson A. I. Karakas P. Sánchez-Blázquez 《Monthly notices of the Royal Astronomical Society》2009,395(2):719-735
We propose a new chemical evolution model aimed at explaining the chemical properties of globular clusters (GCs) stars. Our model depends upon the existence of (i) a peculiar pre-enrichment phase in the GC's parent galaxy associated with very low-metallicity Type II supernovae (SNe II) and (ii) localized inhomogeneous enrichment from a single Type Ia supernova (SN Ia) and intermediate-mass (4–7 M⊙ ) asymptotic giant branch field stars. GC formation is then assumed to take place within this chemically peculiar region. Thus, in our model the first low-mass GC stars to form are those with peculiar abundances (i.e. O-depleted and Na-enhanced), while 'normal' stars (i.e. O-rich and Na-depleted) are formed in a second stage when self-pollution from SNe II occurs and the peculiar pollution from the previous phase is dispersed. In this study, we focus on three different GCs: NGC 6752, 6205 (M 13) and 2808. We demonstrate that, within this framework, a model can be constructed which is consistent with (i) the elemental abundance anticorrelations, (ii) isotopic abundance patterns and (iii) the extreme [O/Fe] values observed in NGC 2808 and M 13, without violating the global constraints of approximately unimodal [Fe/H] and C+N+O. 相似文献
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Abstract– We review current observational and theoretical constraints on the galactic chemical evolution (GCE) of oxygen isotopes to explore whether GCE plays a role in explaining the lower 17O/18O ratio of the Sun, relative to the present‐day interstellar medium, or the existence of distinct 16O‐rich and 16O‐poor reservoirs in the solar system. Although the production of both 17O and 18O are related to the metallicity of progenitor stars, 17O is most likely produced in stars that evolve on longer timescales than those that produce 18O. Therefore, the 17O/18O ratio need not have remained constant over time, contrary to preconceptions and the simplest models of GCE. An apparent linear, slope‐one correlation between δ17O and δ18O in the ISM need not necessarily reflect an O isotopic gradient, and any slope‐one galactocentric gradient need not correspond to evolution in time. Instead, increasing 17O/18O is consistent both with observational data from molecular clouds and with modeling of the compositions of presolar grains. Models in which the rate of star formation has decelerated over the past few Gyr or in which an enhanced period of star formation occurred shortly before solar birth (“starburst”) can explain the solar‐ISM O‐isotopic difference without requiring a local input of supernova ejecta into the protosolar cloud. “Cosmic chemical memory” models in which interstellar dust is on average older than interstellar gas predict that primordial solar system solids should be 16O‐rich, relative to the Sun, in conflict with observations. However, scenarios can be constructed in which the 16O‐rich contribution of very massive stars could lead to 16O‐poor solids and a 16O‐rich bulk Sun, if the solar system formed shortly after a starburst, independent of the popular scenario of photochemical self‐shielding of CO. 相似文献
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Saul J. Adelman 《Monthly notices of the Royal Astronomical Society》1999,310(1):146-156
Elemental abundances of the moderately rotating B9–A3 stars λ UMa, 59 Her, 14 Cyg and 29 Cyg have been derived in a consistent manner with previous studies of this series from spectrograms obtained with Reticon and CCD detectors. The derived elemental abundances show that λ UMa is a mild Am star, while 59 Her is slightly metal-rich. Although 14 Cyg has values closer to solar than these stars, its subsolar Ca and Sc abundances indicate that it might be the hottest known hot-Am star. 29 Cyg is a metal-poor λ Boo star. 相似文献