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1.
For 77 main-sequence F–G stars in the solar neighborhood with published iron, magnesium, and europium abundances determined from high-dispersion spectra and with the ages estimated from theoretical isochrones, we calculated the spatial velocities using Hipparcos data and the Galactic orbital elements. A comparison with the orbital elements of the globular clusters that are known to have been accreted by our Galaxy in the past reveals stars of extragalactic origin. We show that the abundance ratios of r-and α-elements in all the accreted stars differ sharply from those in the stars that are genetically associated with the Galaxy. According to current theoretical models, europium is produced mainly in low-mass type-II supernovae (SNe II), while magnesium is synthesized in large quantities in high-mass SN II progenitors. Since all the old accreted stars of our sample exhibit a significant Eu overabundance relative to Mg, we conclude that the maximum masses of the SN II progenitors outside the Galaxy were much lower than those inside it. On the other hand, only a small number of young accreted stars exhibit low negative ratios [Eu/Mg]<0. This can be explained by the delay of primordial star formation and the explosions of high-mass SNe II in a relatively small part of extragalactic space. We provide evidence that the interstellar medium was weakly mixed at the early evolutionary stages of the Galaxy formed from a single protogalactic cloud, and that the maximum mass of the SN II progenitors increased in it with time simultaneously with the increase in mean metallicity. 相似文献
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M. I. Wilkinson A. Vallenari C. Turon U. Munari D. Katz G. Bono M. Cropper A. Helmi N. Robichon F. Thévenin S. Vidrih T. Zwitter F. Arenou M.-O. Baylac G. Bertelli A. Bijaoui F. Boschi F. Castelli F. Crifo M. David A. Gomboc A. Gómez M. Haywood U. Jauregi P. de Laverny Y. Lebreton P. Marrese T. Marsh S. Mignot D. Morin S. Pasetto M. Perryman A. Pr&#;a A. Recio-Blanco F. Royer A. Sellier A. Siviero R. Sordo C. Soubiran L. Tomasella Y. Viala 《Monthly notices of the Royal Astronomical Society》2005,359(4):1306-1335
4.
Ge-Ying Xie Bo ZhangHong-Jie Li Jin-Jiang HuChen-Pu Li 《Chinese Astronomy and Astrophysics》2011,35(1):29
Based on a large amount of observed data of element abundances in metal-poor stars, taking the abundance distribution of heavy elements in the solar system as a standard, and selecting Sr, Ba and Eu as the typical elements of the three nucleosynthetic processes in metal-poor stars, namely the weak sprocess, main s-process and r-process, we have studied the contributions of the three kinds of neutron-capture processes to the abundance distribution of heavy elements in metal-poor stars, with the parameterization method. It is found that the higher the metal abundance, the greater the contributions of the weak s-process and the chief s-process to the abundances of lighter neutron-capture elements. The heavier neutron-capture elements are mainly produced by the r-process and the chief s-process; and that at low metallicity, the abundances of heavy neutron-capture elements are mainly produced by the r-process. In the early Galaxy, the weak s-process has almost no contribution to the element abundance. 相似文献
5.
确定银河系的年龄是现代天体物理学的一项基本任务。其方法之一是核纪年法，即通过恒星中某一长寿命放射性元素的丰度随时间的变化来确定恒星的年龄，并以此作为银河系年龄的下限，其中目前的观测丰度来自恒星的光谱分析，恒星形成时的初始丰度来自理论模型的预言。这种方法最初是利用元素对Th／Nd来确定G矮星的年龄，近年来开始利用元素对Th／Eu和U／Th来确定晕族场星和球状星团内恒星的年龄。简要介绍了核纪年法确定银河系年龄的原理，回顾了恒星中Th和U的观测研究，其中着重介绍了极贫金属星的研究。详细讨论了用核纪年法估计银河系年龄的不确定性。作为与核纪年法的比较，简单介绍了确定银河系年龄的其他方法。提出了今后需要进一步研究的几项工作。 相似文献
6.
《New Astronomy Reviews》2000,44(4-6):303-313
We review the yields of intermediate mass elements (from C to Zn) from massive stars and their associated uncertainties, in the light of recent theoretical results. We consider the role of those yields for our understanding of the chemical evolution of the solar neighbourhood and of the halo of our Galaxy. Current yields reproduce in a satisfactory way the solar system composition, but several problems remain concerning abundance ratios in halo stars. 相似文献
7.
Data from our compiled catalog of spectroscopically determined magnesium abundances in dwarfs and subgiants with accurate parallaxes are used to select Galactic halo stars according to kinematic criteria and to identify presumably accreted stars among them. Accreted stars are shown to constitute the majority in the Galactic halo. They came into the Galaxy from disrupted dwarf satellite galaxies. We analyze the relations between the relative magnesium abundances, metallicities, and Galactic orbital elements for protodisk and accreted halo stars. We show that the relative magnesium abundances in protodisk halo stars are virtually independent of metallicity and lie within a fairly narrow range, while presumably accreted stars demonstrate a large spread in relative magnesium abundances up to negative [Mg/Fe]. This behavior of protodisk halo stars suggests that the interstellar matter in the early Galaxy mixed well at the halo formation phase. The mean metallicity of magnesium-poor ([Mg/Fe] < 0.2 dex) accreted stars has been found to be displaced toward the negative values when passing from stars with low azimuthal velocities (|Θ| < 50 km s?1) to those with high ones at Δ[Fe/H] ≈ ?0.5 dex. The mean apogalactic radii and inclinations of the orbits also increase with increasing absolute value of |Θ|, while their eccentricities decrease. As a result, negative radial and vertical gradients in relative magnesium abundances are observed in the accreted halo in the absence of correlations between the [Mg/Fe] ratios and other orbital elements, while these correlations are found at a high significance level for genetically related Galactic stars. Based on the above properties of accreted stars and our additional arguments, we surmise that as the masses of dwarf galaxies decrease, the maximum SN II masses and, hence, the yield of α-elements in them also decrease. In this case, the relation between the [Mg/Fe] ratios and the inclinations and sizes of the orbits of accreted stars is in complete agreement with numerical simulations of dynamical processes during the interaction of galaxies. Thus, the behavior of the magnesium abundance in accreted stars suggests that the satellite galaxies are disrupted and lose their stars en masse only after dynamical friction reduces significantly the sizes of their orbits and drags them into the Galactic plane. Less massive satellite galaxies are disrupted even before their orbits change appreciably under tidal forces. 相似文献
8.
H. J. Rocha-Pinto L. I. Arany-Prado W. J. Maciel 《Astrophysics and Space Science》1994,211(2):241-251
A model is presented for the chemical evolution of the solar neighbourhood which takes into account three families of galactic objects, according to their condensation states: stars, refuses and gas. Stars are defined as all condensed objects with masses greater than or equal to the minimum mass which ignites hydrogen and which will give rise to an evolutionary track on the HR diagram to the left of Hayashi's limit; refuses include the remnants, which are compact objects resulting from stellar deaths, and the residues, which have masses not large enough to ignite hydrogen; gas is defined as the mass which can be condensed to form stars and/or residues. We have developed equations for the mass evolution of each family, and have studied the gas metallicity distribution within the framework of the instantaneous recycling approximation, adopting different initial conditions. In order to constrain the model parameters we have also used preliminary evaluations of comet cloud masses to investigate the role of the residues as sinks of heavy elements in the Galaxy. 相似文献
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The actual knowledge on Horizontal Branch stars of the metal-poor Globular Clusters of our Galaxy suggests that these stars evolve along the Horizontal Branch from the very blue stars towards lower effective temperature.From this behaviour it follows that a substantial mass loss occurs at the He-flash. On the basis of a gravitational thermalization of Horizontal Branch stars any recent observational data concerning NGC 7006 are re-examined. It is pointed out that today it is not necessary to leave the hypothesis of a strong correlation between the metal and helium content for our Galaxy.At present at the Osservatorio Astronomico, Bologna. 相似文献
11.
James Binney 《New Astronomy Reviews》2013,57(3-4):29-51
Our Galaxy is a complex machine in which several processes operate simultaneously: metal-poor gas is accreted, is chemically enriched by dying stars, and then drifts inwards, surrendering its angular momentum to stars; new stars are formed on nearly circular orbits in the equatorial plane and then diffuse through orbit space to eccentric and inclined orbits; the central stellar bar surrenders angular momentum to the surrounding disc and dark halo while acquiring angular momentum from inspiralling gas; the outer parts of the disc are constantly disturbed by satellite objects, both luminous and dark, as they sweep through pericentre. We review the conceptual tools required to bring these complex happenings into focus. Our first concern must be the construction of equilibrium models of the Galaxy, for upon these hang our hopes of determining the Galaxy’s mean gravitational field, which is required for every subsequent step. Ideally our equilibrium model should be formulated so that the secular evolution of the system can be modelled with perturbation theory. Such theory can be used to understand how stars diffuse through orbit space from either the thin gas disc in which we presume disc stars formed, or the debris of an accreted object, the presumed origin of many halo stars. Coupling this understanding to the still very uncertain predictions of the theory of stellar evolution and nucleosynthesis, we can finally extract a complete model of the chemodynamic evolution of our reasonably generic Galaxy. We discuss the relation of such a model to cosmological simulations of galaxy formation, which provide general guidance but cannot be relied on for quantitative detail. 相似文献
12.
First generation stars are the oldest stars that were formed in post-big bang, primitive gas, containing no elements heavier than carbon, with ages greater than 14 Gyr and having undergone no evolution so far. Observations over a long time have confirmed that, up to now, no stars with zero metallicity ([Fe/H]) or with [Fe/H]≤ −6 have been found in the Galaxy. To explain this absence, we shall make a theoretical calculation of the probability of detecting first generation stars using Tsuiimoto et al.'s model of chemical evolution of the galactic halo and assuming an initial mass function of the Miller-Scalo form. We use all the observational data on the halo stars to constrain the parameters. Our result is that, if the mass of the cloud that formed the stars is 106–107 M, then the probability of detecting first generation stars is 6.14×10−4–6.14×10−5. 相似文献
13.
Teruaki Ohnishi 《Astrophysics and Space Science》1974,28(2):325-350
The chemical evolution of the Galaxy with a pulsating active nucleus is investigated. The surface densities of gas, stellar remnants, stars and chemical species such as helium and heavy elements inZ6 are calculated as functions of the position in the Galaxy and of the evolutional time of the Galaxy. According to this model, the entire luminosity of the galactic disk becomes almost constant at some 2×109 yr after the galactic formation, but the nuclear bulge, whose dimensions gradually diminishes, becomes more and more luminous with time. On the other hand, the abundance depletion of helium and heavy elements appears in the inner region of the disk after some 6×109 yr of the galactic formation. It also becomes clear that the activity for the nucleosynthesis in the nucleus is limited only in the early history of the Galaxy and has been reduced rapidly with time. Using this model, we can account for the observed phenomena such as the smooth dependence of the elemental abundance in the halo population on the distance from the galactic center, the high abundance of heavy elements in quasar spectra and etc. 相似文献
14.
P.E. Nissen 《Astrophysics and Space Science》1999,265(1-4):249-256
Recent determinations of precise abundance ratios for nearby halo and thick disk stars in the metallicity range −1.3 < [Fe/H]
< −0.5 have revealed a significant cosmic spread in the abundances of oxygen, magnesium, sodium, nickel, s-process and r-process elements relative to iron. Possible explanations of these variations are reviewed. In particular, it is discussed
if the differences in abundance ratios are correlated with the kinematics of the stars, and hence can be used to identify
stellar populations in the Galaxy.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
15.
A comparison of the observed distribution of absolute proper motions with a kinematical model of the Galaxy is presented. Proper motions with respect to galaxies were obtained for about 40 000 stars along the main Galactic meridian and in two fields near the North Galactic pole (programme MEGA). The Galaxy is considered as composed of the disk (main sequence and disk red giants), the thick disk and spheroid populations. For each subsystem, spatial velocity components and their dispersions were computed. The distribution of kinematical parameters were modelled for stars located in different directions of the Galaxy. 相似文献
16.
K.C. Freeman 《Astrophysics and Space Science》1999,267(1-4):129-138
Away from the young disk, several classes of early type stars are found. They include (i) the old, metal-poor blue horizontal branch stars of the halo and the metal-poor tail of the thick disk; (ii) metal-rich young A stars in a rapidly rotating subsystem but with a much higher velocity dispersion than the A stars of the
young disk, and (iii) a newly discovered class of metal-poor young main sequence A stars in a subsystem of intermediate galactic rotation (Vrot ≈ 120 km s−1). The existence and kinematics of these various classes of early type stars provide insight into the formation of the metal-poor
stellar halo of the Galaxy and into the continuing accretion events suffered by our Galaxy.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
17.
The main topic of long-term researches by G.A. Shajn is the nature of diffuse matter, its distribution in the Galaxy and extragalactic systems, interaction with the interstellar medium and hot stars,and the formation of emission and reflection nebulae and stars. Based on the analysis of experimental data, mainly photographic observations of nebulae in the Milky Way and extragalactic systems, he made conclusions and suggested well-founded hypotheses on a wide range of considered problems, including those related to cosmogony. The structure of nebulae, and their masses and sizes give reasons behind the conclusion that most of them are formed not in the process of ejection of matter from the stars, but rather they are objects which are born and evolve, and quite often are comprised of giant conglomerates of gas, dust and stars. The distribution of OB-type stars and nebulae in spiral branches points to their genetic relation and the fundamental role of the interstellar medium as the source of their formation. The structural features of nebulae are determined by the action of magnetohydrodynamic forces. Magnetic fields in a galaxy control the motion of diffuse gas-dust matter and ensure the maintenance of its spiral structure. These ideas continue being developed in modern directions of astrophysics. 相似文献
18.
The relative space motions of the Sun and nearby stars are considered. The coordinates and velocities of the stars are taken from the Catalogue of Nearby Stars by Gliese and Jahreiss (1991). The minimum space separation between the Sun and every star as well as the corresponding moment of time are calculated by two ways. Firstly, the straight line motions are considered. Secondly, the effect of the Galaxy potential is taken into account. The Galaxy model proposed by Kutuzov and Ossipkov (1989) is used. Twenty five stars approaching the Sun closer than two parsecs are selected. The effects of the uncertainties in the observational data are studied. The influence of the encounters to the Oort cloud is discussed. 相似文献
19.
It has been suggested by Cameron that a cloud of comets containing a mass of condensable elements, comparable to the mass of such elements in the sun, formed on the outskirts of the solar system. If the formation of such comet clouds is a general feature of star formation, they constitute a significant sink of elements heavier than helium. It is shown here that this process provides a possible explanation for the very slow rate at which the mean metal abundance of disk stars has increased during the lifetime of the Galaxy. 相似文献
20.
Inverse Compton (IC) scattering by relativistic electrons produces a major component of the diffuse emission from the Galaxy.
The photon fields involved are the cosmic microwave background and the interstellar radiation field (ISRF) from stars and
dust. Calculations of the inverse Compton distribution have usually assumed a smooth ISRF, but in fact a large part of the
Galactic luminosity comes from the most luminous stars, which are rare. Therefore we expect the ISRF, and hence the inverse
Compton emission, to be clumpy at some level, which could be detectable by instruments such as GLAST. Even individual nearby
luminous stars could be detectable assuming just the normal cosmic-ray electron spectrum. We present the basic formalism required
and give possible candidate stars to be detected and make predictions for GLAST. Then we apply the formalism to the OB associations
and the Sun, showing that the IC emission produced is not negligible compared to the sensitivity of current or coming detectors.
We estimate that the gamma-ray flux from the halo around the Sun contributes to the diffuse background emission at the few
percent level. 相似文献