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1.
We present our recently developed 3-dimensional chemodynamical code for galaxy evolution. This code follows the evolution of different galactic components like stars, dark matter and different components of the interstellar medium (ISM), i.e. a diffuse gaseous phase and the molecular clouds. Stars and dark matter are treated as collisionless N-body systems. The ISM is numerically described by a smoothed particle hydrodynamics (SPH) approach for the diffuse gas and a sticky particle scheme for the molecular clouds. Additionally, the galactic components are coupled by several phase transitions like star formation, stellar death or condensation and evaporation processes within the ISM. As an example we show the dynamical and chemical evolution of a star forming dwarf galaxy with a total baryonic mass of 2 ċ 109 M⊙. After a moderate collapse phase the stars and the molecular clouds follow an exponential radial distribution, whereas the diffuse gas shows a central depression as a result of stellar feedback. The metallicities of the galactic components behave quite differently with respect to their temporal evolution as well as their radial distribution. Especially, the ISM is at no stage well mixed. This revised version was published online in September 2006 with corrections to the Cover Date.  相似文献   

2.
Numerical simulations predict that metal-poor gas accretion from the cosmic web fuels the formation of disk galaxies. This paper discusses how cosmic gas accretion controls star formation, and summarizes the physical properties expected for the cosmic gas accreted by galaxies. The paper also collects observational evidence for gas accretion sustaining star formation. It reviews evidence inferred from neutral and ionized hydrogen, as well as from stars. A number of properties characterizing large samples of star-forming galaxies can be explained by metal-poor gas accretion, in particular, the relationship among stellar mass, metallicity, and star-formation rate (the so-called fundamental metallicity relationship). They are put forward and analyzed. Theory predicts gas accretion to be particularly important at high redshift, so indications based on distant objects are reviewed, including the global star-formation history of the universe, and the gas around galaxies as inferred from absorption features in the spectra of background sources.  相似文献   

3.
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4.
恒星尘埃的实验室研究--实验天体物理学   总被引：1，自引：0，他引：1  
原始球粒陨石含有来自恒星的微小固体颗粒(微米级),这些尘埃的同位素组成与太阳系物质截然不同,它们是目前唯一能直接获得的恒星固体样品．已发现的恒星尘埃有金刚石、石墨、碳化硅、刚玉、尖晶石、氮化物、和硅酸盐等,它们的母体恒星包括红巨星,AGB恒星、新星和超新星．对恒星尘埃的研究,使得更深入地了解星系的化学演化历史、恒星内部的核反应和湍流机制、恒星大气中尘埃的形成、星际介质物理现象等．恒星尘埃把天体物理领域延伸到了微观世界,它有机地结合了地球化学实验技术和天体物理理论,开辟了一门崭新的天文学分支实验天体物理学．  相似文献   

5.
Primitive meteorites contain microscopic pre-solar stardust grains that originated from stellar outflows and supernova ejecta. Identified phases include nano-diamond, graphite, silicon carbide, corundum, spinel, hibonite, nitride, and silicates. Their stellar origin was manifested by their enormous isotopic ratio variations compared to solar system materials. They are solid samples from various stellar sources, including red giant stars, AGB stars, novae, and supernovae. Laboratory isotopic analyses of these grains provide unique insights into stellar evolution, nucleosynthesis and mixing processes, dust formation in stellar envelopes, and galactic chemical evolution. Pre-solar grains open a new observational window for astrophysical researches.  相似文献   

6.
A two-dimensional chemodynamical model of the Milky Way Galaxy is presented that can account for the structural, kinematical, and chemical pecularities of the galactic components in a self-consistent way. The dynamics of three stellar components and the multi-phase interstellar medium consisting of clouds and intercloud gas are followed in detail. Mass interchange and energetic interaction processes between the stars and the gas phases are treated simultaneously according to the astrophysical experience including star formation, supernovae type I and II, planetary nebulae, stellar winds, evaporation and condensation, drag, cloud collisions, heating and cooling, and stellar nucleosynthesis. These processes are coupling large ranges on temporal and spatial scales, and allow for feedback and self-regulation mechanisms, which play a significant role in galactic evolution. In comparison with observations the capability of the chemodynamical treatment is convincingly proved by the excellent agreement with various observations. In addition, also well-known problems (G-dwarf problem, the discrepancy between local effective yields, etc.), which so far could be only explained by artificial constraints, are solved in the global scenario. Here we wish also to focus on temporal behaviours of the radial abundance gradient and abundance ratios in order to stimulate further more specific observations and to make particular predictions which can test the validity of used model ingredients like stellar yields. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

7.
The abundance patterns of the most metal‐poor stars in the Galactic halo and small dwarf galaxies provide us with a wealth of information about the early Universe. In particular, these old survivors allow us to study the nature of the first stars and supernovae, the relevant nucleosynthesis processes responsible for the formation and evolution of the elements, early star‐ and galaxy formation processes, as well as the assembly process of the stellar halo from dwarf galaxies a long time ago. This review presents the current state of the field of “stellar archaeology” – the diverse use of metal‐poor stars to explore the high‐redshift Universe and its constituents. In particular, the conditions for early star formation are discussed, how these ultimately led to a chemical evolution, and what the role of the most iron‐poor stars is for learning about Population III supernovae yields. Rapid neutron‐capture signatures found in metal‐poor stars can be used to obtain stellar ages, but also to constrain this complex nucleosynthesis process with observational measurements. Moreover, chemical abundances of extremely metal‐poor stars in different types of dwarf galaxies can be used to infer details on the formation scenario of the halo and the role of dwarf galaxies as Galactic building blocks. I conclude with an outlook as to where this field may be heading within the next decade. A table of ~ 1000 metal‐poor stars and their abundances as collected from the literature is provided in electronic format (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

8.
We reviewed the recent progress in the field of stellar/galactic archeology, which is a study of the relics from the early galaxy. The oldest and most pristine objects that can be observed in the galaxy are the low mass metal poor stars of the Milky Way. They were formed during the early phases, when the ISM might have been polluted only by the Pop-III supernovae. With the recent large spectroscopic surveys (e.g. HK survey by Beers and collaborators, the Hamburg-ESO survey by Christlieb and collaborators and Sloan Digital Sky Survey) it has been possible to get clues on the nature of the first stars that has contributed to the heavy elements. Most of these metal-poor low mass stars also retain their signature of the early dynamical evolution of the galaxy, which can be studied through their orbits around the galaxy and spatial distribution. Here, we discuss the connection between the chemical and the kinematical properties of metal-poor stars in order to probe the early galaxy formation. We also discuss about the globular clusters, the satellite galaxies around the Milky Way and its possible contribution to the formation of the galaxy halo.  相似文献   

9.
In binary stellar systems, exoplanet searches have revealed planetary mass companions orbiting both in circumstellar and in circumbinary orbits. Modelling studies suggest increased dynamical complexity around the young stars that form such systems. Circumstellar and circumbinary disks likely exhibit different physical conditions for planet formation, which also depends on the stellar separation. Although binaries and higher order multiple stars are relatively common in nearby star-forming regions, surprisingly few systems with circumbinary distributions of proto-planetary material have been found. With its spectacular ring of dust and gas encircling the central triple star, one such system, GG Tau A, has become a unique laboratory for investigating the physics of circumsystem gas and dust evolution. We review here its physical properties.  相似文献   

10.
In this paper, I present a brief overview of the observational evidence for the interaction between the winds of hot stars and their surrounding interstellar medium (ISM). As the star evolves, it will impinge on the gas in different ways depending on the nature of its wind and on the physical state of the interstellar gas. The three main evolutionary phases of massive stars, the main sequence O phase, the intermediate red supergiant or luminous blue variable phase and finally the Wolf-Rayet phase are discussed. The study of the ISM in the vicinity of hot stars is shown to be a powerful tool to study the history of the gas and gain more insight into the evolution of the star. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

11.
We explore how the expulsion of gas from star-forming cores due to supernova explosions affects the shape of the initial cluster mass function, that is, the mass function of star clusters when cluster infant weight-loss triggered by gas expulsion is over. We demonstrate that if the radii of cluster-forming gas ‘cores’ are roughly constant over the core mass range, as supported by observations, then more massive cores undergo slower gas expulsion. Therefore, for a given star-formation efficiency, more massive cores retain a larger fraction of stars after gas expulsion. The initial cluster mass function may thus substantially differ from the core mass function, with the final shape depending on the star-formation efficiency. A mass-independent star-formation efficiency of about 20% turns a power-law core mass function into a bell-shaped initial cluster mass function, while mass-independent efficiencies of order 40% preserve the shape of the core mass function.  相似文献   

12.
It is well known that galaxies accumulating large quantities of gas undergo violent bursts of star formation. This is believed to be due to tidal interactions of galaxies leading to the infall of gas into their central regions. Bursts of star formation in this scenario are transitory phenomena and can be induced only by external sources.However, in some cases there is no direct evidence of tidal interactions in starburst galaxies.We discuss another possibility of bursting phenomena in galaxies connected with nonlinear feedback processes in mass-exchange between components of star-forming region. We consider a three-component model including cold clouds, warm gas and massive stars and take into account the delay processes in the transformation of hot gas ejected by massive stars and evaporated from cold phase, into the warm phase. Self-regulating mechanism of phase transition of small clouds into warm gas due to heating radiation of massive stars is also taken into account.The analysis of stability of the system shows that it could be unstable even in case of a small efficiency in the birth of massive stars. The evolution of unstable nonlinear perturbations leads to the development of self-sustained nonlinear oscillations of star formation.  相似文献   

13.
We use the Cambridge stellar evolution code stars to model the evolution of 5 and  7 M⊙  zero-metallicity stars. With enhanced resolution at the hydrogen- and helium-burning shell in the asymptotic giant branch (AGB) phases, we are able to model the entire thermally pulsing AGB (TP-AGB) phase. The helium luminosities of the thermal pulses are significantly lower than in higher metallicity stars so there is no third dredge-up. The envelope is enriched in nitrogen by hot-bottom burning of carbon that was previously mixed in during second dredge-up. There is no s -process enrichment owing to the lack of third dredge-up. The thermal pulses grow weaker as the core mass increases and they eventually cease. From then on the star enters a quiescent burning phase which lasts until carbon ignites at the centre of the star when the CO core mass is  1.36 M⊙  . With such a high degeneracy and a core mass so close to the Chandrasekhar mass, we expect these stars to explode as type 1.5 supernovae, very similar to type Ia supernovae but inside a hydrogen-rich envelope.  相似文献   

14.
As low- and intermediate-mass stars reach the asymptotic giant branch (AGB), they have developed into intriguing and complex objects that are major players in the cosmic gas/dust cycle. At this stage, their appearance and evolution are strongly affected by a range of dynamical processes. Large-scale convective flows bring newly-formed chemical elements to the stellar surface and, together with pulsations, they trigger shock waves in the extended stellar atmosphere. There, massive outflows of gas and dust have their origin, which enrich the interstellar medium and, eventually, lead to a transformation of the cool luminous giants into white dwarfs. Dust grains forming in the upper atmospheric layers play a critical role in the wind acceleration process, by scattering and absorbing stellar photons and transferring their outward-directed momentum to the surrounding gas through collisions. Recent progress in high-angular-resolution instrumentation, from the visual to the radio regime, is leading to valuable new insights into the complex dynamical atmospheres of AGB stars and their wind-forming regions. Observations are revealing asymmetries and inhomogeneities in the photospheric and dust-forming layers which vary on time-scales of months, as well as more long-lived large-scale structures in the circumstellar envelopes. High-angular-resolution observations indicate at what distances from the stars dust condensation occurs, and they give information on the chemical composition and sizes of dust grains in the close vicinity of cool giants. These are essential constraints for building realistic models of wind acceleration and developing a predictive theory of mass loss for AGB stars, which is a crucial ingredient of stellar and galactic chemical evolution models. At present, it is still not fully possible to model all these phenomena from first principles, and to predict the mass-loss rate based on fundamental stellar parameters only. However, much progress has been made in recent years, which is described in this review. We complement this by discussing how observations of emission from circumstellar molecules and dust can be used to estimate the characteristics of the mass loss along the AGB, and in different environments. We also briefly touch upon the issue of binarity.  相似文献   

15.
It is a truth universally acknowledged, that a galaxy in possession of a good quantity of gas must want to form stars. It is the details of how and why that baffle us all. The simplest theories either would have this process a carefully self-regulated affair, or one that goes completely out of control and is capable of wrecking the galaxy which hosts it. Of course the majority of galaxies seem to amble along somewhere between these two extremes, and the mean properties tend to favour a quiescent self-regulated evolutionary scenario. But there area variety of observations which require us to invoke transitory ‘bursts’ of star-formation at one time or another in most galaxy types. Several nearby dwarf spheroidal galaxies have clearly determined star-formation histories with apparent periods of zero star formation followed by periods of fairly active star formation. If we are able to understand what separated these bursts we would understand several important phenomena in galaxy evolution. Were these galaxies able to clear out their gas reservoir in a burst of star formation? How did this gas return? or did it? Have these galaxies receieved gas from the IGM instead? Could stars from these types of galaxy contribute significantly to the halo population in our Galaxy? To answer these questions we need to combine accurate stellar photometry and Colour-Magnitude Diagram interpretation with detailed metal abundances to combine a star-formation rate versus time with a range of element abundances with time. Different elements trace different evolutionary process (e.g., relative contributions of type I and II supernovae). We often aren't even sure of the abundance spread in these galaxies. We have collected detailed high resolution UVES spectra of four nearby dwarf spheroidal galaxies (Sculptor, Fornax, Leo I &; Carina) to begin to answer these questions. This is a precursor study to a more complete study with FLAMES. We presented at this meeting the initial results for the Sculptor and Fornax dwarf spheroidal galaxies which have been previously had single element (low resolution) calcium abundance studies (Tolstoy et al., 2001). See Figures 1 and 2.  相似文献   

16.
We analyse two recent computations of Type II supernova nucleosynthesis by Woosley & Weaver (hereafter WW95) and Thielemann, Nomoto & Hashimoto (hereafter TNH96), focusing on the ability to reproduce the observed [Mg/Fe] ratios in various galaxy types. We show that the yields of oxygen and total metallicity are in good agreement. However, TNH96 models produce more magnesium in the intermediate and less iron in the upper mass range of Type II supernovae than WW95 models. To investigate the significance of these discrepancies for chemical evolution, we calculate simple stellar population yields for both sets of models and different initial mass function slopes. We conclude that the Mg yields of WW95 do not suffice to explain the [Mg/Fe] overabundance either in giant elliptical galaxies and bulges or in metal-poor stars in the solar neighbourhood and the Galactic halo. Calculating the chemical evolution in the solar neighbourhood according to the standard infall model, we find that, using WW95 and TNH96 nucleosynthesis, the solar magnesium abundance is underestimated by 29 and 7 per cent, respectively.   We include the relaxation of the instantaneous mixing approximation in chemical evolution models by splitting the gas component into two different phases. In additional simulations of the chemical evolution in the solar neighbourhood, we discuss various time-scales for the mixing of the stellar ejecta with the interstellar medium. We find that a delay of the order of 108 yr leads to a better fit of the observational data in the [Mg/Fe]–[Fe/H] diagram without destroying the agreement with solar element abundances and the age–metallicity relation.  相似文献   

17.
During the last three decades an observational approach has been applied at the Byurakan Astrophysical Observatory to the problems of the evolution of astronomical bodies and systems. In contradiction to the traditional point of view, assuming that the processes of condensation are dominant in the Universe, this approach makes use of the observed predominance of expansions, ejections, and explosions.In the past, the observational approach has led to the prediction of an expansion of some stellar associations confirmed later by the analysis of observations. It became clear that the stellar associations are very young systems where the star-formation process is still continuing. The new approach has also led to the concept of the activity of galactic nuclei. The observational approach considers as a phenomenon of primary importance the formation of nebulae as a consequence of the activity of dense bodies (nebulae surrounding the novae, planetary nebulae, supernova remnants, cometary nebulae, and the diffuse nebulae in OB-associations).The new approach in application to the early stages of stellar evolution is discussed. The T Tauri-stage is considered as a phase following the more dense protostellar state. The flare stars are regarded as the next phase of evolution. The phenomena of fuors (FU Ori-type brightenings) can be considered as an expression of the same tendency (the transformation of dense matter into a rarefied state).  相似文献   

18.
Elias Brinks   《New Astronomy Reviews》2004,48(11-12):1305
The SKA will revolutionise the study of the principles underlying star formation (SF), resolving interstellar cloud complexes which are the birthplaces of stars and answering such questions as which are the sufficient and necessary conditions for SF to commence. Also, massive SF is intimately related to stellar death. The SKA will be able to study the structure of the ISM at 100 pc resolution out to distances of up to 20 Mpc and will quantify the impact the demise of massive stars has on their environment. Importantly, the SKA will probe the transition region between ISM and IGM, linking star formation and stellar death in the disks of galaxies to faint HI structures further afield, such as “anomalous gas” and (Compact) High Velocity Clouds. Lastly, the superb sensitivity of the SKA will result in some hundred background sources per square degree against which HI absorption lines can be searched for, probing not only the relative importance of the different phases of the gas in galaxies but also the low density gas in the outskirts and between galaxies.  相似文献   

19.
Recent observations and hydrodynamical simulations of star formation inside a giant molecular cloud have revealed that, within a star-forming region, stars do not form evenly distributed throughout this region, but rather in small subclumps. It is generally believed that these subclumps merge and form a young star cluster. The time-scale of this merging process is crucial for the evolution and the possible survival of the final star cluster. The key issue is whether this merging process happens faster than the time needed to remove the residual gas of the cloud. A merging time-scale shorter than the gas-removal time would enhance the survival chances of the resulting star cluster. In this paper, we show by means of numerical simulations that the time-scale of the merging is indeed very fast. Depending on the details of the initial subclump distribution, the merging may occur before the gas is expelled from the newly formed cluster via either supernovae or the winds from massive stars. Our simulations further show that the resulting merger objects have a higher effective star formation efficiency than the overall star-forming region and confirm the results that mass-segregated subclumps form mass-segregated merger objects.  相似文献   

20.
Stars inject energy into the interstellar medium (ISM) by radiation, stellar winds, and supernova explosions. This energy injection causes the ISM to be inhomogeneous, which in turn alters the manner in which the energy is transferred through the ISM. A significant fraction of the energy is injected by massive stars, which formHii regions in the ISM. The structure and evolution ofHii regions in a cloudy medium deffers significantly from that in a homogeneous one. The strong stellar winds produced by massive stars form bubbles in the ISM, and the structure of these bubbles is often dominated by the structure of theHii region in which they are embedded. Finally, when the star explodes as a supernova, the evolution and appearance of the resulting remnant is determined by the structure of the bubble andHii region formed by the star during its lifetime.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984.  相似文献   

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