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1.
L. S. Lyubimkov 《Kinematics and Physics of Celestial Bodies》2010,26(4):169-180
Helium, which was first discovered on the sun with the help of spectral analysis, plays, together with hydrogen, a principal
role in astrophysics. We consider here two fundamental quantities: primordial helium abundance formed during Big Bang nucleosynthesis
and the current initial helium abundances in nearby stars. It is shown that stellar atmospheres are enriched in helium during
the main-sequence stage. Observational evidence for helium contamination in close OB-binaries is discussed. Stars with strong
abundance anomalies are considered, such as chemically peculiar Ap and Bp helium-deficient stars and some types of objects
with helium atmospheres. 相似文献
2.
R. Cayrel 《Astronomy and Astrophysics Review》1996,7(3):217-242
Summary. Up to a decade ago, searches for population III stars (i.e. with strictly the chemical composition left by the Big Bang) had
led to the results that (1) no such star had been found, (2) stars with metallicities significantly below [Fe/H] = were exceedingly rare. Thanks to a major survey, undertaken by Beers, Preston and Shectman 18 years ago, covering about 7500
square degrees in the sky, and down to magnitude =16.0, the situation has drastically changed. The observational limit towards the lowest metallicities is now about [Fe/H]
= , i.e. 4 dex below the solar metallicity , (a level of pollution by supernova ejecta of only a few ppm), and over 100 stars are known with metallicities [Fe/H] in
the range to . The study of this sample, and of a few stars found more serendipitously, has allowed a number of new conclusions: (i) The
cosmological element Li stays constant (prolongation of the Spite's plateau) down to the lowest metallicities, a great observational gift to the
hot Big Bang cosmology (ii) All heavier elements show a roughly linear increase with the abundance of O (or even Fe if the
metallicity is below [Fe/H] = ), including the other light elements, Be and B. This last point has led to a reappraisal of the current view that they were
produced by spallation of interstellar nuclei by galactic cosmic rays, because the rise of those elements with metallicity
should then have been more quadratic than linear. An alternative new perspective is that these elements are produced by spallation
of the primary nuclei ejected by SNe ii against protons of the interstellar medium. (iii) The ratio of the alpha elements (O, Si, Mg,...) to iron also stays constant
down to the lowest metallicities, at about 3 times the solar value. (iv) Significant deviations to a lockstep variation of
the various elements within the iron-peak start to appear below [Fe/H] = . The strongest are a decrease of [Cr/Fe] and an increase of [Co/Fe] when [Fe/H] decreases from to . These trends are not explained by the current status of explosive nucleosynthesis. (v) A great scatter of the abundances
of the neutron capture elements relative to iron appears at very low metallicities. Similar scatter is seen for [Al/Fe].
A remarkable star with [Fe/H] = , CS 22892-052, has been found, with a superb spectrum of the -elements, involving over-abundances of those with respect to iron by factors ranging between 10 and 50. (vi) The kinematics
of the very metal-poor stars is similar to that of other halo stars, with a complete lack of systemic rotation in an inertial
frame, if not a small amount of counter-rotation in the Galaxy. Evidence exists that the velocity ellipsoid is radially elongated
for stars within 10 kpc from the galactic center, whereas it is more spherical or even radially contracted at 20 kpc from
the galactic center. (vii) The low metallicity stars were likely formed at an early cosmological epoch ( if H km/s), before the Galaxy had developed a disk. The new views concerning the sizes of the Ly clouds open the possibility that the low-metallicity Ly systems are large halos having the right metallicity for being protogalaxies, just forming early stellar generations. (viii)
One may wonder why, if more than 100 stars are known with metallicities between [Fe/H] = to no pop. III has been found, or even not one star near [Fe/H] = . Different kinds of explanations have been proposed, with none conclusive at present. Either we have already observed a pop.
III star, but its pristine Big Bang composition has been corrupted by a small amount of interstellar matter accreted during
its 10 Gyr of orbiting in an already-enriched gas, or the collective process of star formation has polluted the medium before
it has produced the low-mass stars we can still observe now, or, simpler, pop. III stars exist, but are sufficiently rare
that we have not yet observed a volume large enough to have found one.
Received: April 3, 1996 相似文献
3.
We investigate one mechanism of the change in the isotopic composition of cosmologically distant clouds of interstellar gas whose matter was subjected only slightly to star formation processes. According to the standard cosmological model, the isotopic composition of the gas in such clouds was formed at the epoch of Big Bang nucleosynthesis and is determined only by the baryon density in the Universe. The dispersion in the available cloud composition observations exceeds the errors of individual measurements. This may indicate that there are mechanisms of the change in the composition of matter in the Universe after the completion of Big Bang nucleosynthesis. We have calculated the destruction and production rates of light isotopes (D, 3He, 4He) under the influence of photonuclear reactions triggered by the gamma-ray emission from active galactic nuclei (AGNs). We investigate the destruction and production of light elements depending on the spectral characteristics of the gamma-ray emission. We show that in comparison with previous works, taking into account the influence of spectral hardness on the photonuclear reaction rates can increase the characteristic radii of influence of the gamma-ray emission from AGNs by a factor of 2?C8. The high gamma-ray luminosities of AGNs observed in recent years increase the previous estimates of the characteristic radii by two orders of magnitude. This may suggest that the influence of the emission from AGNs on the change in the composition of the medium in the immediate neighborhood (the parent galaxy) has been underestimated. 相似文献
4.
《Chinese Astronomy and Astrophysics》2022,46(2):1-21
Lithium (Li) is one of the most important light elements that was primordially synthesized in the Big Bang Nucleosynthesis (BBN). It is also an element that confused astrophysicists for decades, as its observed abundance often contradicts with the theoretical prediction in many different types of celestial objects. Li-rich giant stars are such objects. Their atmospheres contain anomaly high Li abundance than that expected by the standard stellar evolution model. Although the first Li-rich giant star was discovered almost 40 years ago, their origin is still being debated. With the launch of massive spectroscopic survey program such as the Large Sky Area Multi-Object Fiber Spectroscopy Telescope (LAMOST) survey, the extending of available asteroseismology data from space satellites (such as Kepler), and the developments of data-driven techniques, breakthroughs have been archived in the field of Li-rich studies. In this paper, we review the progress that was made during the past four decades, and present our up-to-date understanding to Li-rich giant stars. 相似文献
5.
Wenyuan Cui Jianrong Shi Yuanyuan Geng Caixia Zhang Xiaoying Meng Lang Shao Bo Zhang 《Astrophysics and Space Science》2013,346(2):477-492
In order to get a broader view of the s-process nucleosynthesis we study the abundance distribution of heavy elements of 35 barium stars and 24 CEMP-stars, including nine CEMP-s stars and 15 CEMP-r/s stars. The similar distribution of [Pb/hs] between CEMP-s and CEMP-r/s stars indicate that the s-process material of both CEMP-s and CEMP-r/s stars should have a uniform origin, i.e. mass transfer from their predominant AGB companions. For the CEMP-r/s stars, we found that the r-process should provide similar proportional contributes to the second s-peak and the third s-peak elements, and also be responsible for the higher overabundance of heavy elements than those in CEMP-s stars. Which hints that the r-process origin of CEMP-r/s stars should be closely linked to the main r-process. The fact that some small r values exist for both barium and CEMP-s stars, implies that the single exposure event of the s-process nucleosynthesis should be general in a wide metallicity range of our Galaxy. Based on the relation between C r and C s, we suggest that the origin of r-elements for CEMP-r/s stars have more sources. A common scenario is that the formation of the binary system was triggered by only one or a few supernova. In addition, accretion-induced collapse(AIC) or SN 1.5 should be the supplementary scenario, especially for these whose pre-AGB companion with higher mass and smaller orbit radius, which support the higher values of both C r and C s. 相似文献
6.
Zurab Berezhiani Paolo Ciarcelluti Santi Cassisi Adriano Pietrinferni 《Astroparticle Physics》2006,24(6):495-510
There can exist a hidden sector of the Universe in the form of parallel “mirror” world which has the same particle physics as the observable world and interacts with the latter only gravitationally. Big Bang nucleosynthesis bounds demand that the mirror sector should have a smaller temperature than the ordinary one. This implies that the mirror matter could play a role of dark matter, and in addition its chemical content should be dominated by helium. Here we study the evolutionary and structural properties of the mirror stars which essentially are similar to that of the ordinary stars but with higher helium contents. Being invisible in terms of photons, they could be observed only as MACHOs in the microlensing experiments. Using a numerical code, we compute evolution of stars with large helium abundances (Y = 0.30–0.80) and a wide range of masses, from 0.5 to 10 M. We found that helium dominated mirror star should have much faster evolutionary time (up to a factor 30) than the ordinary star with the same mass. In addition, we show the diagrams of luminosities, effective temperatures, central temperatures and densities, and compute the masses of the He-core at ignition and the minimum mass for carbon ignition, for different chemical compositions. The general conclusion is that mirror stars evolve faster as compared to ordinary ones, and explode earlier as type II supernovae, thus enriching the galactic halo of processed mirror gas with higher metallicity, with implications for MACHO observations and galaxy evolution. 相似文献
7.
J. Meléndez I. Ramírez L. Casagrande M. Asplund B. Gustafsson D. Yong J. D. do Nascimento Jr. M. Castro M. Bazot 《Astrophysics and Space Science》2010,328(1-2):193-200
We review three Li problems. First, the Li problem in the Sun, for which some previous studies have argued that it may be Li-poor compared to other Suns. Second, we discuss the Li problem in planet hosting stars, which are claimed to be Li-poor when compared to field stars. Third, we discuss the cosmological Li problem, i.e. the discrepancy between the Li abundance in metal-poor stars (Spite plateau stars) and the predictions from standard Big Bang Nucleosynthesis. In all three cases we find that the “problems” are naturally explained by non-standard mixing in stars. 相似文献
8.
9.
Eric J. Lerner 《Astrophysics and Space Science》1995,227(1-2):145-149
Supporters of the standard Big Bang theory point to the abundances of light elements, predicted by Big Bang Nucleosynthesis (BBN) as one of the main observational supports of the theory. However, current data no longer confirm BBN. Instead, measurements of the abundances of He3, He4, and D clearly contradict BBN at more than a 3 level, eliminating a key support of the Big Bang. 相似文献
10.
在提出的贫金属星中子俘获元素丰度的计算模型基础上研究1999年新发表的21颗贫金属星的中子俘获元素丰度分布。结果表明，对较重的中子俘获元素理论预测曲线与观测值符合得很好，而对较轻的中子俘获元素二者有所偏离。这表明在贫金属环境下，对较重的中子俘获元素各核合成过程产生的丰度分布与太阳系中相应过程的丰度分布相似，但贡献比例与太阳系不同；而对较轻的中子俘获元素丰度分布与太阳系的丰度分布有所偏离；这也说明较轻的和较重的中子俘获元素的核合成场所不同，即具有不同的核合成机制。同时还特别讨论了丰度观测误差对表征各核合成过程的分量系数的影响。 相似文献
11.
锂(Li)元素最初诞生于大爆炸核合成,是最重要的轻元素之一.但锂元素丰度在很多类天体中均表现出观测与理论不符的现象,这一问题困扰了天体物理学家数十年.富锂巨星就是这样的一类天体,它们大气中的Li丰度超过了标准恒星演化模型的理论值.虽然富锂巨星早在约四十年前就被发现,但其起源依然是未解之谜.随着以郭守敬望远镜(LAMOST)巡天等为代表的大型光谱巡天项目的开展、以开普勒(Kepler)卫星为代表的星震学观测数据的产出以及数据驱动类方法和技术的飞速发展,针对富锂巨星的研究取得了一系列重要的突破.在此将回顾富锂巨星近四十年来的研究进展,并总结对于富锂巨星最新的认知. 相似文献
12.
13.
14.
We report the detection of the Pb i lambda4057.8 line in the very metal-poor (&sqbl0;Fe&solm0;H&sqbr0;=-2.7), carbon-rich star, LP 625-44. We determine the abundance of Pb (&sqbl0;Pb&solm0;Fe&sqbr0;=2.65) and 15 other neutron-capture elements. The abundance pattern between Ba and Pb agrees well with a scaled solar system s-process component, while the lighter elements (Sr-Zr) are less abundant than Ba. The enhancement of s-process elements is interpreted as a result of mass transfer in a binary system from a previous asymptotic giant branch (AGB) companion, an interpretation strongly supported by radial velocity variations of this system. The detection of Pb makes it possible, for the first time, to compare model predictions of s-process nucleosynthesis in AGB stars with observations of elements between Sr and Pb. The Pb abundance is significantly lower than the prediction of recent models (e.g., Gallino et al.), which succeeded in explaining the metallicity dependence of the abundance ratios of light s-elements (Sr-Zr) to heavy ones (Ba-Dy) found in previously observed s-process-enhanced stars. This suggests that one should either (1) reconsider the underlying assumptions concerning the (13)C-rich s-processing site ((13)C pocket) in the present models or (2) investigate alternative sites of s-process nucleosynthesis in very metal-poor AGB stars. 相似文献
15.
综述了近扯为ＡＧＢ星核合成的理论研究情况，包括轻，重核素核合成理论，ＡＧＢ星的分类，ＡＧＢ星的演化特征，ＡＧＢ星内的元素核合成理论的研究及外赋ＭＳ，Ｓ 双星吸积机制的研究情况。 相似文献
16.
D. C. V. Mallik 《Journal of Astrophysics and Astronomy》1981,2(2):171-185
The current rate of nucleosynthesis in the solar neighbourhood is re-evaluated on the basis of Arnett’s (1978) stellar yields,
the mass loss models of Chiosi, Nasi and Sreenivasan (1978) and the initial mass function determined by Lequeux (1978). If
massive stars are held responsible for most of the metals we observe, a higher birthrate of these stars in the past is indicated
in view of the low current rate of nucleosynthesis. The intermediate mass stars may not supply the bulk of the metals unless
total disruption of their carbon core takes place.
While a declining birthrate is in conflict with the result obtained from the age-metallicity relation of stars, it is supported
by some galactic evolution models which interpret successfully the white dwarf mass distribution data. If the constraint of
a nearly time-invariant birthrate were strictly accepted, then models of the prompt initial enrichment type are required to
explain the observed abundances in terms of nucleosynthesis in massive stars. 相似文献
17.
C. Charbonnel 《Astrophysics and Space Science》2002,281(1-2):161-171
Stellar nucleosynthesis is the corner-stone of many astrophysical problems. Its understanding, which can be tested by countless observations, leads to insights into the stellar structure and evolution and provides crucial clues to the physics of galaxies and of the Universe. Precise answers can be given to the questions ‘When, where and how are the chemical elements synthesized in stars?’ However, in spite of the observational confirmation of many predictions, important weaknesses remain in many crucial details of the global view. We discuss here some of the theoretical developments which are required in order to improve the nucleosynthesis predictions for low and intermediate mass stars. 相似文献
18.
Jiang Zhang Fang Zhao Yanping Chen Wenyuan Cui Bo Zhang 《Journal of Astrophysics and Astronomy》2013,34(4):373-391
CEMP-r/s stars at low metallicity are known as double-enhanced stars that show enhancements of both r-process and s-process elements. The chemical abundances of these very metal-poor stars provide us a lot of information for putting new restraints on models of neutron-capture processes. In this article, we put forward an accreted scenario in which the double enrichment of r-process and s-process elements is caused by a former intermediate-mass Asymptotic Giant Branch (AGB) companion in a detached binary system. As the AGB superwind is only present at the ultimate phase of AGB stars, there is thus a lot of potential that the degenerate-core mass of an intermediate-mass AGB star reaches the Chandrasekhar limit before the AGB superwind. In these circumstances, both s-process elements produced in the AGB shell and r-process elements synthesized in the subsequent explosion would be sprayed contemporaneously and accreted by its companion. Despite similarity to physical conditions of a core-collapse supernova, a major focus in this scenario is the degenerate C–O core surrounded by an envelope of a former intermediate-mass AGB donor that may collapse and explode. Due to the existence of an outer envelope, r-process nucleosynthesis is expected to occur. Hypothesizing the material-rich europium (Eu) accreted by the secondary via the wind from the supernova to be in proportion to the geometric fraction of the companion with respect to the exploding donor star, we find that the estimated yield of Eu (as representative of r-process elements) per AGB supernova event is about 1 × 10?9 M ⊙ ～ 5 × 10?9 M ⊙. Using the yields of Eu, the overabundance of r-process elements in CEMP-r/s stars can be accounted for. The calculated results show that the value of parameter f , standing for efficiency of wind pollution from the AGB supernova, will reach about 104, which means that the enhanced factor is much larger than unity due to the impact of gravity of the donor and the result of the gravitational focusing effect of the companion. 相似文献
19.
Two R-Process Components in Ultra-Metal-Poor Stars: The Neutron-Capture Element Distribution of CS 22892-052 总被引：1，自引：0，他引：1
Bo Zhang Ji Li Yue-Xiang Wang Yan-Xia Zhang Jun-Hong Liu Qiu-He Peng 《Astrophysics and Space Science》2002,280(4):325-336
The abundance patterns of neutron-capture elements in very metal-poor halo stars play a crucial role in guiding and constraining
theoretical models of nucleosynthesis. Many studies have suggested that the abundance patterns of the heavier (Z≥ 56) stable neutron-capture elements in very metal-poor halo stars are consistent with the solar system r-process abundance distribution, but this concordance breaks down for the lighter neutron-capture elements in the range of
40<Z<56. Some studies argue that there are two separate r-processes respectively responsible for the productions of the heavier and lighter neutron-capture elements. The new observed
data of the lighter n-capture elements in the 40<Z<56 domain (Nb, Ru, Rh, Pd, Ag and Cd) in CS 22892-052 makes it available to examine whether or not there are two different
r-processes. Based upon these observed abundances of n-capture elements in ultra metal-poor star CS22892-052, we present a phenomenological model to identify the characters of
the different nucleosynthesis processes in very metal-poor stars. The results show that the model predictions can well match
the observations in CS 22892-052, which truly means that there are different r-processes for the lighter and heavier neutron-capture elements, and the stellarr-process patterns are similar to the solar system r-process abundance distribution.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
20.
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