共查询到20条相似文献，搜索用时 31 毫秒
1.
银河系的银盘、银晕、核球的平均金属丰度为－０．２,－１．６,－０．２。年龄－金属丰度关系给出了银河系形成和演化的线索。为了解释观测的年龄－金属丰度关系及其弥散,银河系化学演化模型必须考虑恒星轨道运动、非瞬时混合等机制。 相似文献
2.
In this paper we review the chemical evolution models for the Galactic bulge: in particular, we discuss the predictions of
models as compared with the available abundance data and infer the mechanism as well as the time scale for the formation of
the Galactic bulge. We show that good chemical evolution models reproducing the observed metallicity distribution of stars
in the bulge predict that the [α/Fe] >0 over most of the metallicity range. This is a very important constraint indicating
that the bulge of our Galaxy formed at the same time and even faster than the inner Galactic halo. We also discuss predictions
for the evolution of light elements such as D and 7Li and conclude that the D astration should be maximum due to the high star formation rate required for the bulge whereas
the evolution of the abundance of Li should be similar to that observed in the solar neighbourhood, but with an higher Li
abundance in the interstellar medium at the present time.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
3.
Cristina Chiappini 《Astrophysics and Space Science》2002,281(1-2):253-256
According to the two-infall model for the chemical evolution of the Galaxy the halo and bulge formed on a relatively short
timescale (0.8–1.0 Gyr) out of the first infall episode, whereas the disk accumulated much more slowly and ‘inside-out’ during
a second independent infall episode. We explored the effects of a threshold in the star formation process, during both the
halo and disk phases. In the comparison between model predictions and available data, we have focused our attention on abundance
gradients as well as gas, stellar and star formation rate distributions along the disk. We suggest that the mechanism for
the formation of the halo leaves detectable imprints on the chemical properties of the outer regions of the disk, whereas
the evolution of the halo and the inner disk are almost completely disentangled. This is due to the fact that the halo and
disk densities are comparable at large Galactocentric distances and therefore the gas lost from the halo can substantially
contribute to building up the outer disk. We predict that the abundance gradients along the Galactic disk have increased in
time during the first billion years of the disk evolution and remained almost constant in the last ~5Gyrs.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
4.
The chemical evolution of the 3-component system of halo-bulge-disc is calculated. If the bulge accretes primordial halo matter quickly and forms stars rapidly before the gas is ejected by a galactic wind after 109 yr, the metallicity distribution of the bulge K-giants (Rich, 1988) is reproduced. The metal-enriched matter in a wind from the bulge is mixed with the halo gas which is accreted into the disc. The metallicity distribution of the G-dwarfs and Twarog's age-metallicity relatin in the solar neighbourhood can be well reproduced by assuming reasonable bulge-to-disc mass ratio. 相似文献
5.
Previous models for the chemical evolution of the Magellanic Clouds have assumed either a steepened IMF compared to the solar
neighbourhood or preferential expulsion of oxygen and α-particle elements by selective galactic winds. These assumptions were
largely motivated by a belief that the O/Fe ratio in the Clouds is substantially lower than in the Galaxy, but the difference
appears to have been exaggerated: Galactic supergiants have a similar O/Fe ratio as Cloud supergiants, there is no corresponding
effect in Mg and other α-elements and a combination of data from planetary nebulae, H II regions and supernova remnants indicates
an O/Fe ratio more or less equal to solar. Consequently new analytical models for the chemical evolution of the Magellanic
Clouds have been developed, assuming chemical yields and time delays identical to those we previously assumed for the solar
neighbourhood, but assuming (in addition to infall) non-selective galactic winds and burst-like modes of star formation represented
by discontinuous variations in the star formation rate per unit gas mass. We find adequate agreement with age-metallicity
relations and element:element ratios within their substantial uncertainties, whereas our LMC model turns out to give an excellent
fit to the anomalous Galactic halo stars discovered by Nissen and Schuster (1997). It also gives an enhanced SNIa/SNII ratio
compared to the solar neighbourhood, due to the assumption that the SFR has declined in the past 1 to 2 Gyr.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
6.
R. Caimmi 《Astronomische Nachrichten》1998,319(5):285-307
The current paper investigates how the empirical, G-dwarf metallicity distribution constrains simple, comoving models of chemical evolution. In doing this, the application of the models to a data sample, performed in a previous paper, is refined and extended. The key idea is that (i) different star formation rates with different mass spectra take place in different phases of evolution, i.e. contraction and equilibrium, and (ii) disk formation begins at a time t = Td and ends at t = Tc, which marks the transition from contraction to equilibrium. In this view, the lowest-metallicity point of the empirical, differential distribution, consistent with a linear fit, is related to the beginning of disk formation, and an apparent discontinuity point to the transition from contraction to equilibrium. In addition, different linear fits hold on the left (early distribution) and on the right (late distribution) of the discontinuity point. Models consistent with the empirical, G-dwarf metallicity distribution are related to linear fits on the early and late side. Homologous solutions during the equilibrium phase are analysed in detail with respect to changes in Tc and Ta, the age of the Galaxy. Then we are left with a single free parameter which is relevant to the chemical evolution, i.e. the mass spectrum exponent during the equilibrium phase. The allowed values for the other parameters, thought as a function of the above mentioned one, are plotted for each case. A Salpeter mass spectrum exponent, p = −2.35, is ruled out by the theoretical, lower stellar mass limit, contrary to a Scalo mass spectrum exponent, p = −2.90, in contrast with previous literature. The reasons for this discrepancy are discussed. Our results are marginally consistent with a same initial mass function during the contraction and equilibrium phase, but in this case the disk mass fraction is of the is same order, or less, than the halo mass fraction. It is also investigated how the empirical age-metallicity relation constrains the duration of the contraction phase, for a reasonable upper limit of Ta. Keeping in mind that the empirical, G-dwarf metallicity distribution has not been corrected for the large cosmic scatter shown by the empirical, age-metallicity relation, we find a duration of disk formation, Tc — Td = 1.07–1.5 Gyr, by a factor 3–5 less than it is found by use of simple infall models. The reasons of this difference are explained. The idea of a massive, white dwarf halo, which seems to be indicated by microlensing experiments, is ruled out by the empirical, G-dwarf metallicity distribution, in the light of the current model and provided the solar neighbourhood is a typical region of the Galaxy. More refined models involving e.g., the relax of instantaneous recycling would change our results, but the trend is expected to be only slightly altered. 相似文献
7.
A three-component chemical evolution model of the Galaxy is presented, which we believe will cast a new light on the G-dwarf problem. The model is based on a scenario of the Galaxy consisting of three major evolutionary phases: halo, thick disk and thin disk, separated by two short interludes of rapid collapse. The evolution of different stellar populations are treated separately, the combination of which yields an overall metallicity distribution function for the solar neighbourhood. We tested three different models using the same set of basic equations: the “prompt initial enrichment” (PIE) model, the “proportional yield” (PPY) model and the “collapse” (CLP) model. Best-fit parameters are derived. The results show that the different populations have remarkably different IMFs, while mass exchange has only minimally affected the chemical evolution in the solar vicinity, so that the solar vicinity can be regarded as a closed system, at least in the late stage of the Galactic evolution. 相似文献
8.
R. Caimmi 《Astronomische Nachrichten》1997,318(6):339-364
Recent data on the empirical metallicity distribution of G dwarfs in the disk solar neighbourhood are fitted in two different ways. We use an extended Poisson distribution in the limit where the probability of star formation is small, and a Gauss distribution in the limit where a large number of physical variables is required to determine stellar metal abundance. Both are found to reproduce the data at the same (acceptable) extent, with a slight preference for the former. The emprirical, differential metallicity distribution of G dwarfs in the disk solar neighbourhood is compared with its theoretical counterpart, in the picture of a closed, comoving model of chemical evolution. The limits of the currently used infall models are discussed and a scenario of galactic formation and evolution is presented. The Galactic history is thought as made of two main phases: contraction (which produces the extended component) and equilibrium (which gives the disk). In this view, the stars observed within the solar cylinder did not necessarily arise from the primordial gas which later collapsed into the disk solar neighbourhood. It is found that the G-dwarf problem is strongly alleviated, with the possible exception of the low-metallicity and high-metallicity tail of the distribution. The best choice of parameters implies: (i) a metal yield in the contraction phase which is larger by a factor of about 5 with respect to the equilibrium phase; (ii) a model halo mass fraction of about 0.3; (iii) a model disk mass fraction of about 0.6. It provides additional support to the idea of a generalized Schmidt star formation law, which is different in different phases of evolution. The model, cumulative, G-dwarf metallicity distribution in the disk solar neighbourhood is found to predict too may low-metallicity stars with respect to its empirical counterpart, related to a Poissonian or Gaussian fit. The main resons for the occurrence of a G-dwarf problem are discussed. Finally, a stochastic process of star formation, related to a Poisson distribution, is briefly outlined. 相似文献
9.
为解释著名的G矮星问题,提出银河系化学演化的三成分模型,即由银晕、厚盘和薄盘所构成的演化模型．相邻演化阶段间隔着一个快速坍缩过程．对不同星族成分的演化过程分别进行模拟,并在总体上得到一个太阳附近区域的G矮星丰度分布函数．检验了三种不同的模型：初始富化模型、比例生成模型和坍缩模型．利用最小二乘拟合得到最佳模型的参数．结果表明,太阳附近区域的化学演化受物质交换的影响较小,至少在银河系演化的晚期,可将太阳附近区域看作封闭系统．同时,单位质量中新合成的重元素比例对三种恒星成分可分别近似为常数,其差别则说明不同星族恒星的初始质量函数存在着显著差异． 相似文献
10.
恒星的Al元素丰度可以为探索星团和星系的化学演化提供重要线索.通过系统分析银河系薄盘、厚盘、核球、银晕以及M4、M5等球状星团中恒星的[Al/Fe]随恒星金属丰度[Fe/H]的变化趋势,得出银河系薄盘、厚盘和核球恒星的[Al/Fe]随着[Fe/H]的增加而缓慢下降,而球状星团M4和M5恒星的[Al/Fe]随[Fe/H]增加没有下降趋势,这暗示Ia超新星对M4和M5恒星元素丰度的贡献比较小.详细研究了银河系恒星[Al/Fe]与[Mg/Fe]、[Na/Fe]的相关性,结果表明银河系场星的[Al/Fe]与[Mg/Fe]正相关,但在球状星团M4和M5恒星中未见此相关性;银河系盘星及M4和M5等球状星团恒星的[Al/Fe]与[Na/Fe]都存在正相关. 相似文献
11.
《天文和天体物理学研究(英文版)》2015,(8)
Hypervelocity stars are believed to be ejected out from the Galactic center through dynamical interactions between(binary) stars and the central supermassive black hole(s). In this paper, we report 19 low mass F/G/K type hypervelocity star candidates from over one million stars found in the first data release of the LAMOST regular survey. We determine the unbound probability for each candidate using a MonteCarlo simulation by assuming a non-Gaussian proper-motion error distribution, and Gaussian heliocentric distance and radial velocity error distributions. The simulation results show that all the candidates have unbound possibilities over 50% as expected,and one of them may even exceed escape velocity with over 90% probability. In addition, we compare the metallicities of our candidates with the metallicity distribution functions of the Galactic bulge, disk, halo and globular clusters, and conclude that the Galactic bulge or disk is likely the birth place for our candidates. 相似文献
12.
Markus Samland 《Astrophysics and Space Science》2002,281(1-2):305-308
The formation of a disk galaxy within a slowly growing dark halo is simulated with a new chemo-dynamical model. The model
describes the evolution of the stellar populations, the multi-phase ISM and all important interaction. I find, that the galaxy
forms radially from inside-out and vertically from top-to-bottom. The derived stellar age distributions show that the inner
halo is the oldest component, followed by the outer halo, the triaxial bulge, the halo-disk transition region and the disk.
Despite the still idealized model, the final galaxy resembles present-day disk galaxies in many aspects. In particular, the
stellar metallicity distribution in the halo of the model resembles the one of M31. The bulge in the model shows, at least
two stellar subpopulations, an early collapse population and a population that formed later out of accreted disk mass. In
the stellar metallicity distribution of the disk, I find a pronounced ‘G-dwarf problem’ which is the result of a pre-enrichment
of the disk ISM with metal-rich gas from the bulge.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
13.
Agostino Renda Daisuke Kawata Yeshe Fenner Brad K. Gibson 《Monthly notices of the Royal Astronomical Society》2005,356(3):1071-1078
The chemical evolution history of a galaxy hides clues about how it formed and has been changing through time. We have studied the chemical evolution history of the Milky Way (MW) and Andromeda (M31) to find which are common features in the chemical evolution of disc galaxies as well as which are galaxy-dependent. We use a semi-analytic multizone chemical evolution model. Such models have succeeded in explaining the mean trends of the observed chemical properties in these two Local Group spiral galaxies with similar mass and morphology. Our results suggest that while the evolution of the MW and M31 shares general similarities, differences in the formation history are required to explain the observations in detail. In particular, we found that the observed higher metallicity in the M31 halo can be explained by either (i) a higher halo star formation efficiency (SFE), or (ii) a larger reservoir of infalling halo gas with a longer halo formation phase. These two different pictures would lead to (i) a higher [O/Fe] at low metallicities, or (ii) younger stellar populations in the M31 halo, respectively. Both pictures result in a more massive stellar halo in M31, which suggests a possible correlation between the halo metallicity and its stellar mass. 相似文献
14.
In order to understand the forming mechanism of the radial abun- dance gradient of the Galactic disk and the evolution of cold gas, we have con- structed a chemical evolution model of the Galactic disk, in which the star for- mation law concerned with molecular hydrogens is adopted, and the evolution of mass surface density is calculated for the molecular and atomic hydrogens separately, then the model predictions and the observed radial distributions of some physical quantities are compared. The result indicates that the model prediction is sensitive to the adopted infall timescale, the model which adopts the star formation law concerned with the molecular hydrogens can agree well with the major observed properties of the Galactic disk, especially can obtain naturally the radial oxygen abundance gradient of the Galactic disk, and the radial surface density profile of cold gas. The assumption of instantaneous or non-instantaneous recycling approximation has a small effect on the evolution of cold gas, especially in the case of rather low gas density. 相似文献
15.
银河系的形成与演化是天体物理学研究的重大前沿课题,银河系的化学演化在其中更具有极其重要的地位。随着观测资料的不断积累和理论工作的不断深入,银河系化学演化的研究取得了一系列进展。在观测方面,从太阳附近区域,整个银盘,银晕和核球等方面简要回顾了银河系化学演化模型主要观测约束的近期结果;在化学演化模型方面,回顾了银河系化学演化研究的发展历程和近期进展,并对未来的研究进行了展望。 相似文献
16.
17.
Open Clusters as a Record of the Past 总被引：1，自引：0，他引：1
E.D. Friel 《Astrophysics and Space Science》1999,265(1-4):271-278
The Galactic open cluster population has long been used as a probe of the structure of the Galactic disk and a timeline for
studying its evolution. With ages that range up to 12 billion years and positions that span a large range of Galactocentric
distances, the open clusters provide a broad sample with which to investigate issues such as the history of star formation
in the Galaxy, the chemical evolution of the disk, and the competing influences of cluster formation and disruption that mold
the properties of the current cluster population.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
18.
Chemical evolution of the Magellanic Clouds: analytical models 总被引：1，自引：0，他引：1
B. E. J. Pagel & G. Tautvaisviene 《Monthly notices of the Royal Astronomical Society》1998,299(2):535-544
We have extended our analytical chemical evolution modelling ideas for the Galaxy to the Magellanic Clouds. Unlike previous authors (Russell &38; Dopita, Tsujimoto et al. and Pilyugin), we assume neither a steepened initial mass function nor selective galactic winds, since among the α-particle elements only oxygen shows a large deficit relative to iron and a similar deficit is also found in Galactic supergiants. Thus we assume yields and time delays identical to those that we previously assumed for the solar neighbourhood. We include inflow and non-selective galactic winds and consider both smooth and bursting star formation rates, the latter giving a better fit to the age–metallicity relations. We predict essentially solar abundance ratios for primary elements and these seem to fit most of the data within their substantial scatter. Our model for the Large Magellanic Cloud also gives a remarkably good fit to the anomalous Galactic halo stars discovered by Nissen &38; Schuster. Our models predict current ratios of Type Ia supernova to core-collapse supernova rates enhanced by 50 and 25 per cent respectively relative to the solar neighbourhood, in fair agreement with ratios found by Cappellaro et al. for Sdm–Im relative to Sbc galaxies, but these ratios are sensitive to detailed assumptions about the bursts and a still higher enhancement in the Large Magellanic Cloud has been deduced from X-ray studies of remnants by Hughes et al. The corresponding ratios integrated over time up to the present are slightly below 1, but they exceed 1 if one compares the Magellanic Clouds with the Galaxy at times when it had the corresponding metallicities. 相似文献
19.
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
20.
R. Caimmi 《Astronomische Nachrichten》2001,322(4):241-263
The oxygen abundance distribution in solar neighbourhood halo subdwarfs is deduced, using two alternative, known empirical relationships, involving the presence or the absence of [O/Fe] plateau for low [Fe/H] values, from a sample of 372 kinematically selected halo stars, for which the iron abundance distribution has been determined by Ryan & Norris (1991). The data are interpreted by a simple, either homogeneous or inhomogeneous model of chemical evolution, using an updated value of the solar oxygen abundance. The effect of changing the solar oxygen abundance, the power‐law exponent in the initial mass function, and the rate of oxygen nucleosyntesis, keeping the remaining input parameters unchanged, is investigated, and a theorem is stated. In all cases, part of the gas must necessarily be inhibited from forming stars, and no disk contamination has to be advocated for fitting the empirical oxygen abundance distribution in halo subdwarfs of the solar neighbourhood (EGD). Then a theorem is stated, which allows a one‐to‐one correspondence between simple, homogeneous models with and without inhibited gas, related to same independent parameters of chemical evolution, except lower stellar mass limit, real yield, and inhibition parameter. The mutual correlations between the latter parameters are also specified. In addition the starting point, and the point related to the first step, of the theoretical distribution of oxygen abundance (TGD) predicted by simple, inhomogeneous models, is calculated analytically. The mean oxygen abundance of the total and only inhibited gas, respectively, are also determined. Following the idea of a universal, initial mass function (IMF), a power‐law with both an exponent p = 2.9, which is acceptably close to Scalo IMF for m ≳ m⊙, and an exponent p = 2.35, i.e. Salpeter IMF, have been considered. In general, both the age‐metallicity relationship and the empirical distribution of oxygen abundance in G dwarfs of the disk solar neighbourhood, are fitted by power‐law IMF exponents in the range 2.35 ≤ p ≤ 2.9. Acceptable models predict about 15% of the total mass in form of long‐lived stars and remnants, at the end of halo evolution, with a mean gas oxygen abundance which is substantially lower than the mean bulge and initial disk oxygen abundance. To avoid this discrepancy, either the existence of a still undetected, baryonic dark halo with about 15% of the total mass, or an equal amount of gas loss during bulge and disk formation, is necessary. The latter alternative implies a lower stellar mass limit close to 0.2 m⊙, which is related to a power‐law IMF exponent close to 2.77. Acceptable models also imply a rapid halo formation, mainly during the first step, Δt = 0.5 Gyr, followed by a period (three steps) where small changes occur. Accordingly, statistical fluctuations are found to produce only minor effects on the evolution. 相似文献