恒星中子俘获元素丰度的定量分解研究
摘要
恒星的元素丰度为我们理解恒星与星系的形成及演化提供了非常重要的信息，所以探索恒星元素丰度的起源是核天体物理学的重要内容。元素的天体物理来源多种多样，过程也极其复杂，因此恒星的观测丰度通常不能用单一的核合成过程来解释。在这种情况下，我们用分量模型对恒星的元素丰度进行了分解，并对结果进行了分析和讨论。
贫金属星的元素丰度，尤其是中子俘获元素的丰度，为我们研究星系形成早期的天体演化提供了重要线索。本文利用两颗主要r-过程星CS22892-052、CS31082-001和两颗弱r-过程星HD122563、HD88609的观测丰度，得到了主要r-过程和弱r-过程分量的丰度分布。贫金属星的观测丰度表明轻元素和铁族元素的增丰与弱r-过程元素增丰相关联，我们将弱r-过程分量的丰度分布推广到了轻元素和铁族元素。利用主要r-过程分量和弱r-过程分量，我们对30颗贫金属星的观测丰度进行了分解。通过对分量系数的分析，发现其中的两颗星是弱r-过程星。由于星系形成早期的大质量星的贡献（P分量）没有考虑在内，[Sr/Fe]的观测值很低的“低Sr星”BD-185550的观测丰度不能用这两个分量拟合，其轻元素和铁族元素的观测丰度明显大于计算结果，这说明该星的P分量贡献不可忽略。用BD-185550的观测丰度减去计算值，我们得到了P分量的丰度分布。用主要r-过程分量、弱r-过程分量和P分量重新拟合30颗星的丰度之后，发现P分量的贡献随[Fe/H]的增加迅速减小。
对贫金属星流恒星的研究表明其弱r-过程分量系数近似为常数，这意味着尽管弱r-过程元素与Fe元素产生于不同的过程，但是在整个星流污染的历史中弱r-过程元素随着Fe的增加而增加，且以近似固定的产量比例由II型超新星产生。我们发现星流恒星中有一颗弱r-过程星，HD237846，其金属丰度（[Fe/H]=-3.29）比已知的弱r-过程星HD88609的金属丰度（[Fe/H]=-3.07）还要低，这说明弱r-过程产生的丰度分布在较宽的金属丰度范围内比较稳定。在[Fe/H]＞-2.2时，C_(r,m)与C_(r,m)接近，意味着主要r-过程和弱r-过程分量比例在[Fe/H]～-2.2时达到太阳系的值。CS29513-032是星流恒星中的一颗s-rich星，它的s-过程丰度来自低质量AGB星的污染，其r-过程分量系数与同金属度下的星流恒星的r-过程分量系数接近，暗示CS29513-032与其它星流恒星有着共同的天体物理起源。
根据CS22892-052、CS31082-001和HD122563、HD88609观测丰度的比较结果，我们估算了弱r-过程和主要r-过程前身星的质量范围和产量。计算结果显示，弱r-过程产生于前身星质量大约为11-26M⊙的超新星爆发阶段，其中前身星质量在15M⊙＜M＜26M⊙范围内的超新星的贡献大于80%。
球状星团M22中恒星的丰度可以分解为主要r-过程、弱r-过程和主要s-过程分量的贡献，且它们的主要r-过程分量系数和弱r-过程分量系数都接近常数，其s-过程元素丰度来自低质量的AGB星的污染。尽管金属丰度比较低，M22恒星的s-过程丰度分布已经达到了太阳系的主要s-过程丰度分布。s-过程分量系数与r-过程分量系数随金属度的变化趋势明显不同。s-过程分量系数随[Fe/H]增长的趋势意味着s-过程对M22丰度的贡献是逐渐增加的。这一现象可能是星团内部演化的结果，并且可以用低质量AGB星具有较长寿命来解释。
我们利用五分量模型对银河系较高金属丰度恒星的丰度进行分解，给出了各分量贡献随金属丰度的变化趋势。利用该方法，我们对天炉座矮星系的恒星丰度进行了研究，并与具有相似金属丰度的银河系恒星的丰度进行比较，得到了矮星系演化的初始质量函数“bottom-heavy”的重要证据。
The elemental abundances provide some important information about the formation andevolution of the stars and galaxies for us. It is the important content of nuclear astrophysics tostudy the origin of the elemental abundances. The astrophysical origins of elements arevarious and the processes of production are very complex. This indicates that the observedabundances of stars usually can not be explained by a single process. In this case, wedecompose the stellar abundances and discuss the results.
The elemental abundances of metal-poor stars, especially their neutron-capture elementabundances, provide important clues to study the evolution of the stars during the earlyGalaxy for us. In this paper, based on the abundances of main r-process CS22892-052, CS31082-001and weak r-process stars HD122563, HD88609, we derived the abundancepatterns of main r-process and weak r-process. The observed abundances of metal-poor starsshow that the light elements and iron-group elements should be coupled with the weakr-process elements, so the weak r-process pattern is derived to the light elements andiron-group elements. Using main r-process component and weak r-process component, wedecompose the observed abundances of30metal-poor stars. We find two of them are weakr-process stars by analyzing the component coefficients. The results suggest the observedabundances of “low-[Sr/Fe] stars” BD-185550can not be fitted, because the contribution of Pcomponent from the massive stars in the early Galaxy is not contained. For light elements andiron group elements, the calculated results are larger than their observed abundances, thismeans that the P component contribution the star should not be ignored. We obtain theabundance pattern of P component through substracting the calculated values from observedabundances. Using main r-process component, weak r-process component and P component,we refit the abundances of these stars. We find the contributions of P component decreaserapidly with the increasing metallicities.
The study of metal-poor stellar stream suggests that the weak r-process coefficients ofstream stars are almost constant. This means that the abundances of the weak r-processelements have increase along with those of element Fe over the polluted history of the stellarstream and that the weak r-process elements and Fe are produced by SNe II in a nearly constant mass fraction. We find there is a weak r-process star HD237846in the stellar stream.Its metallicity ([Fe/H]=3.29) is even lower than that of HD88609. This implies that theweak r-process pattern is very stable in a wider metallicity range. At the [Fe/H]＞-2.2,Cr,mis close toCr,w, which means that the ratio between the contributions of weak r-process andmain r-process to the stream stars are close to the ratio of solar system. CS29513-032is as-rich steam star whose s-procss elements are a result of pollution from the low-mass AGBstar. We find that its r-process coefficients are close to other stream stars with similarmetallicity, which implies that the astrophysical origin of CS29513-032and other streamstars is similar.
Based on the comparision between observed abundances of CS22892-052, CS31082-001and HD122563，HD88609, we estimate progenitor mass and yields of weakr-process and main r-process. We find that the weak r-process occurs in the supernova withthe progenitor mass range of～11-26M⊙and that the SNe with progenitor mass range about15M⊙ The abundances of M22stars can be decomposed by the weak r-process, main r-process,and main s-process components. The main r-process and weak r-process componentcoefficients are almost constants for the stars. Their s-process element abundances come fromthe pollution of low-mass AGB stars. Although the stars of M22have low metallicities, theirs-process abundance pattern has reached to the main s-process pattern in the solar system. Thecomponent coefficient trends of s-process and r-processes in M22are different obviously. Theincrease trend of the s-process component coefficients with [Fe/H] means that thecontributions of main s-process to the abundances of M22stars gradually increase, which canbe explained by the longer lifetime of low-mass AGB stars. This is maybe the results ofinter-evolution.
Using five components, we decompose the abundances of higher metallicities stars in theMilky Way and obtain the trends of individual components with metallicities. Using the samemethod, we study the stellar abundances of Fornax dwarf galaxy and compare the results withthose of the Milky Way stars with similar metallicities. Both observational results and calculated results are significant evidences that the IMF of the Fornax dSph is bottom heavierthan that of the Milky Way.
贫金属星的元素丰度，尤其是中子俘获元素的丰度，为我们研究星系形成早期的天体演化提供了重要线索。本文利用两颗主要r-过程星CS22892-052、CS31082-001和两颗弱r-过程星HD122563、HD88609的观测丰度，得到了主要r-过程和弱r-过程分量的丰度分布。贫金属星的观测丰度表明轻元素和铁族元素的增丰与弱r-过程元素增丰相关联，我们将弱r-过程分量的丰度分布推广到了轻元素和铁族元素。利用主要r-过程分量和弱r-过程分量，我们对30颗贫金属星的观测丰度进行了分解。通过对分量系数的分析，发现其中的两颗星是弱r-过程星。由于星系形成早期的大质量星的贡献（P分量）没有考虑在内，[Sr/Fe]的观测值很低的“低Sr星”BD-185550的观测丰度不能用这两个分量拟合，其轻元素和铁族元素的观测丰度明显大于计算结果，这说明该星的P分量贡献不可忽略。用BD-185550的观测丰度减去计算值，我们得到了P分量的丰度分布。用主要r-过程分量、弱r-过程分量和P分量重新拟合30颗星的丰度之后，发现P分量的贡献随[Fe/H]的增加迅速减小。
对贫金属星流恒星的研究表明其弱r-过程分量系数近似为常数，这意味着尽管弱r-过程元素与Fe元素产生于不同的过程，但是在整个星流污染的历史中弱r-过程元素随着Fe的增加而增加，且以近似固定的产量比例由II型超新星产生。我们发现星流恒星中有一颗弱r-过程星，HD237846，其金属丰度（[Fe/H]=-3.29）比已知的弱r-过程星HD88609的金属丰度（[Fe/H]=-3.07）还要低，这说明弱r-过程产生的丰度分布在较宽的金属丰度范围内比较稳定。在[Fe/H]＞-2.2时，C_(r,m)与C_(r,m)接近，意味着主要r-过程和弱r-过程分量比例在[Fe/H]～-2.2时达到太阳系的值。CS29513-032是星流恒星中的一颗s-rich星，它的s-过程丰度来自低质量AGB星的污染，其r-过程分量系数与同金属度下的星流恒星的r-过程分量系数接近，暗示CS29513-032与其它星流恒星有着共同的天体物理起源。
根据CS22892-052、CS31082-001和HD122563、HD88609观测丰度的比较结果，我们估算了弱r-过程和主要r-过程前身星的质量范围和产量。计算结果显示，弱r-过程产生于前身星质量大约为11-26M⊙的超新星爆发阶段，其中前身星质量在15M⊙＜M＜26M⊙范围内的超新星的贡献大于80%。
球状星团M22中恒星的丰度可以分解为主要r-过程、弱r-过程和主要s-过程分量的贡献，且它们的主要r-过程分量系数和弱r-过程分量系数都接近常数，其s-过程元素丰度来自低质量的AGB星的污染。尽管金属丰度比较低，M22恒星的s-过程丰度分布已经达到了太阳系的主要s-过程丰度分布。s-过程分量系数与r-过程分量系数随金属度的变化趋势明显不同。s-过程分量系数随[Fe/H]增长的趋势意味着s-过程对M22丰度的贡献是逐渐增加的。这一现象可能是星团内部演化的结果，并且可以用低质量AGB星具有较长寿命来解释。
我们利用五分量模型对银河系较高金属丰度恒星的丰度进行分解，给出了各分量贡献随金属丰度的变化趋势。利用该方法，我们对天炉座矮星系的恒星丰度进行了研究，并与具有相似金属丰度的银河系恒星的丰度进行比较，得到了矮星系演化的初始质量函数“bottom-heavy”的重要证据。
The elemental abundances provide some important information about the formation andevolution of the stars and galaxies for us. It is the important content of nuclear astrophysics tostudy the origin of the elemental abundances. The astrophysical origins of elements arevarious and the processes of production are very complex. This indicates that the observedabundances of stars usually can not be explained by a single process. In this case, wedecompose the stellar abundances and discuss the results.
The elemental abundances of metal-poor stars, especially their neutron-capture elementabundances, provide important clues to study the evolution of the stars during the earlyGalaxy for us. In this paper, based on the abundances of main r-process CS22892-052, CS31082-001and weak r-process stars HD122563, HD88609, we derived the abundancepatterns of main r-process and weak r-process. The observed abundances of metal-poor starsshow that the light elements and iron-group elements should be coupled with the weakr-process elements, so the weak r-process pattern is derived to the light elements andiron-group elements. Using main r-process component and weak r-process component, wedecompose the observed abundances of30metal-poor stars. We find two of them are weakr-process stars by analyzing the component coefficients. The results suggest the observedabundances of “low-[Sr/Fe] stars” BD-185550can not be fitted, because the contribution of Pcomponent from the massive stars in the early Galaxy is not contained. For light elements andiron group elements, the calculated results are larger than their observed abundances, thismeans that the P component contribution the star should not be ignored. We obtain theabundance pattern of P component through substracting the calculated values from observedabundances. Using main r-process component, weak r-process component and P component,we refit the abundances of these stars. We find the contributions of P component decreaserapidly with the increasing metallicities.
The study of metal-poor stellar stream suggests that the weak r-process coefficients ofstream stars are almost constant. This means that the abundances of the weak r-processelements have increase along with those of element Fe over the polluted history of the stellarstream and that the weak r-process elements and Fe are produced by SNe II in a nearly constant mass fraction. We find there is a weak r-process star HD237846in the stellar stream.Its metallicity ([Fe/H]=3.29) is even lower than that of HD88609. This implies that theweak r-process pattern is very stable in a wider metallicity range. At the [Fe/H]＞-2.2,Cr,mis close toCr,w, which means that the ratio between the contributions of weak r-process andmain r-process to the stream stars are close to the ratio of solar system. CS29513-032is as-rich steam star whose s-procss elements are a result of pollution from the low-mass AGBstar. We find that its r-process coefficients are close to other stream stars with similarmetallicity, which implies that the astrophysical origin of CS29513-032and other streamstars is similar.
Based on the comparision between observed abundances of CS22892-052, CS31082-001and HD122563，HD88609, we estimate progenitor mass and yields of weakr-process and main r-process. We find that the weak r-process occurs in the supernova withthe progenitor mass range of～11-26M⊙and that the SNe with progenitor mass range about15M⊙
Using five components, we decompose the abundances of higher metallicities stars in theMilky Way and obtain the trends of individual components with metallicities. Using the samemethod, we study the stellar abundances of Fornax dwarf galaxy and compare the results withthose of the Milky Way stars with similar metallicities. Both observational results and calculated results are significant evidences that the IMF of the Fornax dSph is bottom heavierthan that of the Milky Way.
引文
[1] Beers T C, Christlieb N. The discovery and analysis of very metal-poor starsin the Galaxy[J]. ARA&A,2005,43:531-80
[2] Cowan J J, Sneden C. Heavy element synthesis in the oldest stars and theearly Universe[J]. Nature,2006,440:1151-1156
[3]黄润乾,恒星物理,北京：科学出版社,1998,299.
[4] Iben I Jr. The life and times of an intermediate mass star-In isolation/ina close binary[J]. QJRAS,1985,26:1-39.
[5] Busso M, Gallino R, Wasserburg G J. Nucleosynthesis in asymptotic giantbranch stars: Relevance for Galactic enrichment and solar systemformation[J]. ARA&A,1999,37:239-309.
[6] Burbidge E M, Burbidge G R, Fowler W A, Hoyle F. Synthesis of the elementsin stars[J].Rev ModPhys.1957,29:547-650.
[7] Raiteri C M, Busso M, Gallino R, et al. S-process nucleosynthesis in massivestars and the weak component. I-Evolution and neutron captures in a25solar mass star[J]. ApJ,1991,367:228-238.
[8] Raiteri C M, Busso M, Gallino R, et al. s-process nucleosynthesis in massivestars and the weak component. II. Carbon burning and glactic enrichment[J].ApJ,1991,371:665-672.
[9] Raiteri C M, Gallino R, Busso M. S-process in massive stars as a functionof metallicity and interpretation of observational trends[J]. ApJ,1992,387:263-275.
[10] Raiteri C M, Gallino R, Busso M, et al. The weak s-component andnucleosynthesis in massive stars[J]. ApJ,1993,419:207-223.
[11] Lamb S,Howard W M,Truran J W, Iben I J. Neutron-capture nucleosynthesis inthe helium-burning cores of massive stars[J]. ApJ,1977,217:213-221.
[12] The L S, El Eid M F, Meyer B S. A new study of s-process nucleosynthesisin massive stars[J]. ApJ,2000,533:998-1015.
[13] Arlandini C, K ppeler F, Wisshak K, et al. Neutron capture in low-massAsymptotic Giant Branch stars: Cross sections and abundance signatures [J].ApJ,1999,525:886-900.
[14] Cowan J J, Thielemann F K, Truran J W. The r-process and nucleochronology[J].Phys Rep,1991,208:267-394.
[15] Sneden C, Cowan J J, Gallino R. Neutron-capture elements in the earlyGalaxy[J]. ARA&A,2008,46:241-288.
[16] McWilliam A,Preston G W,Sneden C, Searle L. A spectroscopic analysis of33of the most metal-poor stars. I[J]. AJ,1995,109:2736-2756
[17] McWilliam A, Preston G W, Sneden C, Searle L.Spectroscopic Analysis of33of the most metal poor stars. II[J]. AJ,1995,109:2757-2799.
[18] Ryan, S G, Norris, J E,&Beers, T C. Extremely metal-poor stars. II.Elemental abundances and the early chemical enrichment of the Galaxy[J].ApJ,1996,471:254-278.
[19] Helmi A, White Simon D M, de Zeeuw P T, Zhao Hongsheng. Debris streams inthe solar neighbourhood as relicts from the formation of the Milky Way [J].Nature,1999,402:53-55.
[20] Letarte B, Hill V, Jablonka P, Tolstoy E, Fran ois P, Meylan G. VLT/UVESspectroscopy of individual stars in three globular clusters in the Fornaxdwarf spheroidal galaxy[J]. A&A,2006,453:547-554.
[21] Cowan J J, Pfeiffer B, Kratz K L, et al. R-process Abundances andchronometers in metal-poor stars[J]. ApJ,1999,521:194-205.
[22] Sneden C, Cowan J J, Lawler J E, et al.Europium isotopic abundances in verymetal poor stars[J]. ApJ,2002,566: L25-L28.
[23] Truran J W, Cowan J J, Pilachowski C A, Sneden C. Probing the neutron-capturenucleosynthesis history of galactic matter [J].Publ. Astron. Soc. Pac.2002,114:1293-1308.
[24] Sneden C, Cowan J J, Lawler J E, et al. The extremely metal-poor, neutroncapture-rich star CS22892-052: A comprehensive abundance analysis[J]. ApJ,2003,591:936-953.
[25] Sneden C, Cowan J J,Ivans I I, et al. Evidence of multiple r-process sitesin the early Galaxy: New observations of CS22892-052[J]. ApJ,2000,533,L139-L142.
[26] Hill V, Plez B, Cayrel R,et al. First stars. I. The extreme r-element rich,iron-poor halo giant CS31082-001. Implications for the r-process site(s)and radioactive cosmochronology[J]. A&A,2002,387:560-579.
[27] Travaglio C. Galactic Evolution of Sr, Y, And Zr: A multiplicity ofnucleosynthetic processes[J]. ApJ,2004,601:864-884.
[28] Ishimaru Y, Wanajo S, Aoki W, Ryan S G,&Prantzos N. First enrichment ofr-process elements in our Galaxy[J]. Nuclear Physics A,2005,758:603-606.
[29] Westin J, Sneden C, Gustafsson B, et al. The r-Process-enrichedLow-Metallicity Giant HD115444[J]. ApJ,2000,530:783-799.
[30] Johnson J. Abundances of30Elements in23Metal-Poor Stars[J]. ApJS,2002,139:219-247.
[31] Honda S, Aoki W,Ishimaru Y,Wanajo S.Neutron-Capture Elements in the verymetal-poor Star HD88609: Another star with excesses of lightneutron-capture elements[J].ApJ,2007,666:1189-1197.
[32] Montes F, Beers T C, Cowan J,et al. Nucleosynthesis in the Early Galaxy[J].ApJ,2007,671:1685-1695.
[33] Izutani N,Umeda H,Tominaga N. Explosive nucleosynthesis of weak r-processelements in extremely metal-poor core-collapse supernovae[J]. ApJ,2009,692:1517-1531.
[34] Arcones A,&Montes F. Production of light-element primary process nucleiin neutrino-driven winds[J]. ApJ,2011,731:5A-15A.
[35] Qian Y-Z, Wasserburg G J. Where, oh where has the r-process gone?[J]. PhyRep,2007,442:237-268.
[36] Fields B D,Truran J W, Cowan J J.A simple model for r-process scatter andhalo evolution[J].ApJ,2002,575:845-854.
[37] Hansen C J, et al. Silver and palladium help unveil the nature of a secondr-process[J]. A&A,2012,545:31-58.
[38] Anders E,&Grevesse N. Abundances of the elements:meteoritic and solar[J].Geochim. Cosmochim. Acta,1989,53:197-214.
[39] Lodders K, Palme H,&Gail H-P. Abundances of the elements in the solarsystem[J]. Landolt-B nstein—Group VI Astronomy and AstrophysicsNumerical Data and Functional Relationships in Science and TechnologyVolume,2009, arXiv:0901.1149.
[40] Cowan J J, Sneden C，Burles S, et al. The chemical composition and age ofthe metal-poor halo star BD+17°3248[J]. ApJ,2002,572:861-879.
[41] Johnson J,&Bolte M. The r-process in the early Galaxy[J]. ApJ,2002,579:616-625.
[42] Honda S, Aoki W, Kajino T, et al. Spectroscopic studies of extremelymetal-poor stars with the subaru high dispersion spectrograph. II. Ther-process elements, including thorium[J]. ApJ,2004,607:474-498.
[43] Honda S, Aoki W, Ishimaru Y, et al. Neutron-capture elements in the verymetal poor star HD122563[J]. ApJ,2006,643:1180-1189.
[44] Barklem P S, Christlieb N,Beers T C.et al. The Hamburg/ESO r-processenhanced star survey (HERES). II. Spectroscopic analysis of thesurvesample[J]. A&A,2005,439:129-151.
[45] Ivans I. Near-ultraviolet observations of HD221170: New insights into thenature of r-process-rich stars[J]. ApJ,2006,645:613-633.
[46] Christlieb N, Sch rck T, Frebel A, Beers T C, Wisotzki L, Reimers D. Thestellar content of the Hamburg/ESO survey. IV. Selection of candidatemetal-poor stars[J]. A&A,2008,484:721-732.
[47] Hayek W, et al. The Hamburg/ESO r-process enhanced star survey (HERES).IV. Detailed abundance analysis and age dating of the strongly r-processenhanced stars CS29491-069and HE1219-0312[J]. A&A,2009,504:511-524.
[48] Mashonkina L, Christlieb N, Barklem, P S, Hill V, Beers, T C,&VelichkoA. The Hamburg/ESO r-process enhanced star survey (HERES). V. Detailedabundance analysis of the r-process enhanced star HE2327-5642[J]. A&A,2010,516:46-67.
[49] Roederer I U, Sneden C, Lawler J,&Cowan J J. New abundance determinationsof cadmium,lutetium,and osmium in the r-process enriched star BD+173248[J].ApJ,2010,714: L123-L127.
[50] Travaglio C, Gallino R, Busso M，et al.Lead: Asymptotic Giant Branchproduction and Galactic chemical evolution[J].ApJ,2001,549:346-352.
[51] Cowan J J, Sneden C, Beers T C,et al. Hubble Space Telescope Observationsof heavy elements in metal-poor Galactic halo stars[J]. ApJ,2005,627:238-250.
[52] Cescutti G. An inhomogeneous model for the Galactic halo: a possibleexplanation for the spread observed in s-and r-process elements[J]. A&A,2008,481:691-699.
[53] Qian Y-Z,&Wasserburg G J. A model for abundances in metal-poor stars[J].ApJ,2001,559:925-941
[54] Eggen O J, Lynden-Bell D,&Sandage A R. Evidence from the motions of oldstars that the Galaxy collapsed[J]. ApJ,1962,136:748-767.
[55] Searle L&Zinn R. Compositions of halo clusters and the formation of thegalactic halo[J]. ApJ,1978,225:357-379.
[56] Kepley A A, et al. Halo Star Streams in the Solar Neighborhood[J]. AJ,2007,134:1579-1595.
[57] Roederer I U, Sneden C, Thompson I B, Preston G W,&Shectman S A.Characterizing the chemistry of the Milky Way stellar halo:Detailedchemicakl analysis of a metal-poor stellar stream[J]. ApJ,2010,711,573-596.
[58] Busso M, Gallino R, Lambert D L, Travaglio C,&Smith V V. Nucleosynthesisand mixing on the asymptotic giant branch.III.predicted and observeds-process abundances[J]. ApJ,2001,557:802-821.
[59] Aoki W, et al. Neutron capture elements in s-process-rich, very metal-poorstars[J]. ApJ,2001,561:346-363.
[60] Zhang B, Ma K,&Zhou G，Zhang B, Ma K, Zhou G D. Neutron-capture elementsin the s-and r-process-rich stars: Constraints on neutron-capturenucleosynthesis processes[J]. ApJ,2006,642:1075-1081.
[61] Heger A,&Woosley S E. Nucleosynthesis and evolution of massive metal-freestars[J]. ApJ,2010,724:341-373.
[62] David K Lai, Michael Bolte, Johnson Jennifer A, Sara Lucatello, HegerAlexander, Woosley S E. Detailed abundances for28metal-poor stars: Stellar relics in theMilky Way[J]. ApJ,2008,681:1524-1556.
[63] Webb N A, Serre B, Gendre B,et al. X-ray sources and their opticalcounterparts in the globular cluster M22[J]. A&A,2004,424:133-143.
[64] Gaudi B S. Interpreting the M22spike[J]. ApJ,2002,566:452-462.
[65] Bond I A, Abe F, Eguchi S. Multiple outbursts of a cataclysmic variablein the globular cluster M22[J]. ApJ,2005,620: L103-L106.
[66] Albrow M d, Marchi G D, Sahu K. The spatially resolved mass fuction of theglobular cluster M22[J]. ApJ,2002,579:660-670.
[67] Marino A F, Milone, A P, Piotto G, Villanova S, Bedin L R, Bellini A, RenziniA. A double stellar generation in the globular cluster NGC6656(M22).Two stellar groups with different iron and s-process element abundances[J]. A&A,2009,505:1099-1113.
[68] Marino A F, et al. The two metallicity groups of the globular cluster M22: a chemical perspective [J]. A&A,2011,532:8-31.
[69] Simmerer J, Sneden C, Ivans I I, Kraft R P, Shetrone M D, Smith V V. Acomparison of copper abundances in globular cluster and halo field giantstars [J]. AJ,2003,125:2018-2028.
[70] Allen D M, Porto de Mello G F. Mn, Cu and Zn abundances in barium starsand their correlations with neutron capture elements[j]. A&A,2011,525:63-78.
[71] Travaglio C, Galli D, Gallino R, Busso M, Ferrini F,&Straniero O.Galacticchemical evolution of heavy elements: From Barium to Europium[J]. ApJ,1999,521:691-702.
[72] Serminato A, Gallino R, Travaglio C, Bisterzo S, Straniero O. Galacticchemical evolution of the s-process from AGB stars [J]. PASA,2009,26:153-161.
[73] Timmes F X, Woosley S E&Weaver T A. Galactic chemical evolution: Hydrogenthrough zinc [J]. ApJS,1995,98:617-658.
[74] Mishenina T V, Kovtyukh V V, Soubiran C, Travaglio C, Busso M. Abundancesof Cu and Zn in metal-poor stars: Clues for Galaxy evolution[J]. A&A,2002,396:189-201.
[75] Masaomi Ono, Masa-aki Hashimoto, Shin-ichiro Fujimoto, Kei Kotake andShoichi Yamada. Explosive Nucleosynthesis in Magnetohydrodynamical Jetsfrom Collapsars II. Heavy-Element Nucleosynthesis of s, r, p-Processes[J].2012, arxiv:1203.6488.
[76] Mishenina T V, Kovtyukh V V. Analysis of neutron capture elements inmetal-poor stars [J]. A&A,2001,370:951-966.
[77] Reddy B E, Tomkin J, Lambert D L, Allende Prieto C. The chemical compositionsof Galactic disc F and G dwarfs[J]. MNRAS,2003,340:304-340.
[78] Reddy B E, Lambert D L, Allend Prieto C. Elemental abundance survey of theGalactic thick disc[J]. MNRAS,2006,367:1329-1366.
[79] Mikolaitis arūnas, Tautvai ienè, G, Gratton R, Bragaglia A, Carretta, E.Chemical composition of evolved stars in the open cluster IC4651[J]. MNRAS,2011,413:2199-2206.
[80] Trevisan M, Barbuy B K, Eriksson K, Gustafsson B, Grenon M, Pompéia L.Analysis of old very metal rich stars in the solar neighbourhood[J]. A&A,2011,535:42-67.
[81] Irwin M,&Atzidimitriou D. Structural parameters for the Galactic dwarfspheroidals[J]. MNRAS,1995,277:1354-1378.
[82] Letarte B, et al. A high-resolution VLT/FLAMES study of individual starsin the centre of the Fornax dwarf spheroidal galaxy[J]. A&A,2010,523:17-56.
[83] Tsujimoto T. Chemical signature indicating a lack of massive stars in dwarfgalaxies[J]. ApJ,2011,736:113-119.
[84] Dutton A A, Mendel J T, Simard L. Evidence for a non-universal stellarinitial mass function in low-redshift high-density early-type galaxies[J].MNRAS Letters,2012,422: L33-L37.
[85] Liang S, Li H J,Shen X J, Cui W Y, Zhang B. Abundance analysis of r+s stars:r-process abundance comparison between r+s stars and r-rich stars[J]. PASP,2012,124:304-315.
[86] Shen X J, Zhang B, Li H J, Liang S, Cui W Y. An explanation of the correlationsof abundance ratio between neutron-capture elements and iron group elementsin Ba star[J]. ApSS,2013,343:541-554.
[2] Cowan J J, Sneden C. Heavy element synthesis in the oldest stars and theearly Universe[J]. Nature,2006,440:1151-1156
[3]黄润乾,恒星物理,北京：科学出版社,1998,299.
[4] Iben I Jr. The life and times of an intermediate mass star-In isolation/ina close binary[J]. QJRAS,1985,26:1-39.
[5] Busso M, Gallino R, Wasserburg G J. Nucleosynthesis in asymptotic giantbranch stars: Relevance for Galactic enrichment and solar systemformation[J]. ARA&A,1999,37:239-309.
[6] Burbidge E M, Burbidge G R, Fowler W A, Hoyle F. Synthesis of the elementsin stars[J].Rev ModPhys.1957,29:547-650.
[7] Raiteri C M, Busso M, Gallino R, et al. S-process nucleosynthesis in massivestars and the weak component. I-Evolution and neutron captures in a25solar mass star[J]. ApJ,1991,367:228-238.
[8] Raiteri C M, Busso M, Gallino R, et al. s-process nucleosynthesis in massivestars and the weak component. II. Carbon burning and glactic enrichment[J].ApJ,1991,371:665-672.
[9] Raiteri C M, Gallino R, Busso M. S-process in massive stars as a functionof metallicity and interpretation of observational trends[J]. ApJ,1992,387:263-275.
[10] Raiteri C M, Gallino R, Busso M, et al. The weak s-component andnucleosynthesis in massive stars[J]. ApJ,1993,419:207-223.
[11] Lamb S,Howard W M,Truran J W, Iben I J. Neutron-capture nucleosynthesis inthe helium-burning cores of massive stars[J]. ApJ,1977,217:213-221.
[12] The L S, El Eid M F, Meyer B S. A new study of s-process nucleosynthesisin massive stars[J]. ApJ,2000,533:998-1015.
[13] Arlandini C, K ppeler F, Wisshak K, et al. Neutron capture in low-massAsymptotic Giant Branch stars: Cross sections and abundance signatures [J].ApJ,1999,525:886-900.
[14] Cowan J J, Thielemann F K, Truran J W. The r-process and nucleochronology[J].Phys Rep,1991,208:267-394.
[15] Sneden C, Cowan J J, Gallino R. Neutron-capture elements in the earlyGalaxy[J]. ARA&A,2008,46:241-288.
[16] McWilliam A,Preston G W,Sneden C, Searle L. A spectroscopic analysis of33of the most metal-poor stars. I[J]. AJ,1995,109:2736-2756
[17] McWilliam A, Preston G W, Sneden C, Searle L.Spectroscopic Analysis of33of the most metal poor stars. II[J]. AJ,1995,109:2757-2799.
[18] Ryan, S G, Norris, J E,&Beers, T C. Extremely metal-poor stars. II.Elemental abundances and the early chemical enrichment of the Galaxy[J].ApJ,1996,471:254-278.
[19] Helmi A, White Simon D M, de Zeeuw P T, Zhao Hongsheng. Debris streams inthe solar neighbourhood as relicts from the formation of the Milky Way [J].Nature,1999,402:53-55.
[20] Letarte B, Hill V, Jablonka P, Tolstoy E, Fran ois P, Meylan G. VLT/UVESspectroscopy of individual stars in three globular clusters in the Fornaxdwarf spheroidal galaxy[J]. A&A,2006,453:547-554.
[21] Cowan J J, Pfeiffer B, Kratz K L, et al. R-process Abundances andchronometers in metal-poor stars[J]. ApJ,1999,521:194-205.
[22] Sneden C, Cowan J J, Lawler J E, et al.Europium isotopic abundances in verymetal poor stars[J]. ApJ,2002,566: L25-L28.
[23] Truran J W, Cowan J J, Pilachowski C A, Sneden C. Probing the neutron-capturenucleosynthesis history of galactic matter [J].Publ. Astron. Soc. Pac.2002,114:1293-1308.
[24] Sneden C, Cowan J J, Lawler J E, et al. The extremely metal-poor, neutroncapture-rich star CS22892-052: A comprehensive abundance analysis[J]. ApJ,2003,591:936-953.
[25] Sneden C, Cowan J J,Ivans I I, et al. Evidence of multiple r-process sitesin the early Galaxy: New observations of CS22892-052[J]. ApJ,2000,533,L139-L142.
[26] Hill V, Plez B, Cayrel R,et al. First stars. I. The extreme r-element rich,iron-poor halo giant CS31082-001. Implications for the r-process site(s)and radioactive cosmochronology[J]. A&A,2002,387:560-579.
[27] Travaglio C. Galactic Evolution of Sr, Y, And Zr: A multiplicity ofnucleosynthetic processes[J]. ApJ,2004,601:864-884.
[28] Ishimaru Y, Wanajo S, Aoki W, Ryan S G,&Prantzos N. First enrichment ofr-process elements in our Galaxy[J]. Nuclear Physics A,2005,758:603-606.
[29] Westin J, Sneden C, Gustafsson B, et al. The r-Process-enrichedLow-Metallicity Giant HD115444[J]. ApJ,2000,530:783-799.
[30] Johnson J. Abundances of30Elements in23Metal-Poor Stars[J]. ApJS,2002,139:219-247.
[31] Honda S, Aoki W,Ishimaru Y,Wanajo S.Neutron-Capture Elements in the verymetal-poor Star HD88609: Another star with excesses of lightneutron-capture elements[J].ApJ,2007,666:1189-1197.
[32] Montes F, Beers T C, Cowan J,et al. Nucleosynthesis in the Early Galaxy[J].ApJ,2007,671:1685-1695.
[33] Izutani N,Umeda H,Tominaga N. Explosive nucleosynthesis of weak r-processelements in extremely metal-poor core-collapse supernovae[J]. ApJ,2009,692:1517-1531.
[34] Arcones A,&Montes F. Production of light-element primary process nucleiin neutrino-driven winds[J]. ApJ,2011,731:5A-15A.
[35] Qian Y-Z, Wasserburg G J. Where, oh where has the r-process gone?[J]. PhyRep,2007,442:237-268.
[36] Fields B D,Truran J W, Cowan J J.A simple model for r-process scatter andhalo evolution[J].ApJ,2002,575:845-854.
[37] Hansen C J, et al. Silver and palladium help unveil the nature of a secondr-process[J]. A&A,2012,545:31-58.
[38] Anders E,&Grevesse N. Abundances of the elements:meteoritic and solar[J].Geochim. Cosmochim. Acta,1989,53:197-214.
[39] Lodders K, Palme H,&Gail H-P. Abundances of the elements in the solarsystem[J]. Landolt-B nstein—Group VI Astronomy and AstrophysicsNumerical Data and Functional Relationships in Science and TechnologyVolume,2009, arXiv:0901.1149.
[40] Cowan J J, Sneden C，Burles S, et al. The chemical composition and age ofthe metal-poor halo star BD+17°3248[J]. ApJ,2002,572:861-879.
[41] Johnson J,&Bolte M. The r-process in the early Galaxy[J]. ApJ,2002,579:616-625.
[42] Honda S, Aoki W, Kajino T, et al. Spectroscopic studies of extremelymetal-poor stars with the subaru high dispersion spectrograph. II. Ther-process elements, including thorium[J]. ApJ,2004,607:474-498.
[43] Honda S, Aoki W, Ishimaru Y, et al. Neutron-capture elements in the verymetal poor star HD122563[J]. ApJ,2006,643:1180-1189.
[44] Barklem P S, Christlieb N,Beers T C.et al. The Hamburg/ESO r-processenhanced star survey (HERES). II. Spectroscopic analysis of thesurvesample[J]. A&A,2005,439:129-151.
[45] Ivans I. Near-ultraviolet observations of HD221170: New insights into thenature of r-process-rich stars[J]. ApJ,2006,645:613-633.
[46] Christlieb N, Sch rck T, Frebel A, Beers T C, Wisotzki L, Reimers D. Thestellar content of the Hamburg/ESO survey. IV. Selection of candidatemetal-poor stars[J]. A&A,2008,484:721-732.
[47] Hayek W, et al. The Hamburg/ESO r-process enhanced star survey (HERES).IV. Detailed abundance analysis and age dating of the strongly r-processenhanced stars CS29491-069and HE1219-0312[J]. A&A,2009,504:511-524.
[48] Mashonkina L, Christlieb N, Barklem, P S, Hill V, Beers, T C,&VelichkoA. The Hamburg/ESO r-process enhanced star survey (HERES). V. Detailedabundance analysis of the r-process enhanced star HE2327-5642[J]. A&A,2010,516:46-67.
[49] Roederer I U, Sneden C, Lawler J,&Cowan J J. New abundance determinationsof cadmium,lutetium,and osmium in the r-process enriched star BD+173248[J].ApJ,2010,714: L123-L127.
[50] Travaglio C, Gallino R, Busso M，et al.Lead: Asymptotic Giant Branchproduction and Galactic chemical evolution[J].ApJ,2001,549:346-352.
[51] Cowan J J, Sneden C, Beers T C,et al. Hubble Space Telescope Observationsof heavy elements in metal-poor Galactic halo stars[J]. ApJ,2005,627:238-250.
[52] Cescutti G. An inhomogeneous model for the Galactic halo: a possibleexplanation for the spread observed in s-and r-process elements[J]. A&A,2008,481:691-699.
[53] Qian Y-Z,&Wasserburg G J. A model for abundances in metal-poor stars[J].ApJ,2001,559:925-941
[54] Eggen O J, Lynden-Bell D,&Sandage A R. Evidence from the motions of oldstars that the Galaxy collapsed[J]. ApJ,1962,136:748-767.
[55] Searle L&Zinn R. Compositions of halo clusters and the formation of thegalactic halo[J]. ApJ,1978,225:357-379.
[56] Kepley A A, et al. Halo Star Streams in the Solar Neighborhood[J]. AJ,2007,134:1579-1595.
[57] Roederer I U, Sneden C, Thompson I B, Preston G W,&Shectman S A.Characterizing the chemistry of the Milky Way stellar halo:Detailedchemicakl analysis of a metal-poor stellar stream[J]. ApJ,2010,711,573-596.
[58] Busso M, Gallino R, Lambert D L, Travaglio C,&Smith V V. Nucleosynthesisand mixing on the asymptotic giant branch.III.predicted and observeds-process abundances[J]. ApJ,2001,557:802-821.
[59] Aoki W, et al. Neutron capture elements in s-process-rich, very metal-poorstars[J]. ApJ,2001,561:346-363.
[60] Zhang B, Ma K,&Zhou G，Zhang B, Ma K, Zhou G D. Neutron-capture elementsin the s-and r-process-rich stars: Constraints on neutron-capturenucleosynthesis processes[J]. ApJ,2006,642:1075-1081.
[61] Heger A,&Woosley S E. Nucleosynthesis and evolution of massive metal-freestars[J]. ApJ,2010,724:341-373.
[62] David K Lai, Michael Bolte, Johnson Jennifer A, Sara Lucatello, HegerAlexander, Woosley S E. Detailed abundances for28metal-poor stars: Stellar relics in theMilky Way[J]. ApJ,2008,681:1524-1556.
[63] Webb N A, Serre B, Gendre B,et al. X-ray sources and their opticalcounterparts in the globular cluster M22[J]. A&A,2004,424:133-143.
[64] Gaudi B S. Interpreting the M22spike[J]. ApJ,2002,566:452-462.
[65] Bond I A, Abe F, Eguchi S. Multiple outbursts of a cataclysmic variablein the globular cluster M22[J]. ApJ,2005,620: L103-L106.
[66] Albrow M d, Marchi G D, Sahu K. The spatially resolved mass fuction of theglobular cluster M22[J]. ApJ,2002,579:660-670.
[67] Marino A F, Milone, A P, Piotto G, Villanova S, Bedin L R, Bellini A, RenziniA. A double stellar generation in the globular cluster NGC6656(M22).Two stellar groups with different iron and s-process element abundances[J]. A&A,2009,505:1099-1113.
[68] Marino A F, et al. The two metallicity groups of the globular cluster M22: a chemical perspective [J]. A&A,2011,532:8-31.
[69] Simmerer J, Sneden C, Ivans I I, Kraft R P, Shetrone M D, Smith V V. Acomparison of copper abundances in globular cluster and halo field giantstars [J]. AJ,2003,125:2018-2028.
[70] Allen D M, Porto de Mello G F. Mn, Cu and Zn abundances in barium starsand their correlations with neutron capture elements[j]. A&A,2011,525:63-78.
[71] Travaglio C, Galli D, Gallino R, Busso M, Ferrini F,&Straniero O.Galacticchemical evolution of heavy elements: From Barium to Europium[J]. ApJ,1999,521:691-702.
[72] Serminato A, Gallino R, Travaglio C, Bisterzo S, Straniero O. Galacticchemical evolution of the s-process from AGB stars [J]. PASA,2009,26:153-161.
[73] Timmes F X, Woosley S E&Weaver T A. Galactic chemical evolution: Hydrogenthrough zinc [J]. ApJS,1995,98:617-658.
[74] Mishenina T V, Kovtyukh V V, Soubiran C, Travaglio C, Busso M. Abundancesof Cu and Zn in metal-poor stars: Clues for Galaxy evolution[J]. A&A,2002,396:189-201.
[75] Masaomi Ono, Masa-aki Hashimoto, Shin-ichiro Fujimoto, Kei Kotake andShoichi Yamada. Explosive Nucleosynthesis in Magnetohydrodynamical Jetsfrom Collapsars II. Heavy-Element Nucleosynthesis of s, r, p-Processes[J].2012, arxiv:1203.6488.
[76] Mishenina T V, Kovtyukh V V. Analysis of neutron capture elements inmetal-poor stars [J]. A&A,2001,370:951-966.
[77] Reddy B E, Tomkin J, Lambert D L, Allende Prieto C. The chemical compositionsof Galactic disc F and G dwarfs[J]. MNRAS,2003,340:304-340.
[78] Reddy B E, Lambert D L, Allend Prieto C. Elemental abundance survey of theGalactic thick disc[J]. MNRAS,2006,367:1329-1366.
[79] Mikolaitis arūnas, Tautvai ienè, G, Gratton R, Bragaglia A, Carretta, E.Chemical composition of evolved stars in the open cluster IC4651[J]. MNRAS,2011,413:2199-2206.
[80] Trevisan M, Barbuy B K, Eriksson K, Gustafsson B, Grenon M, Pompéia L.Analysis of old very metal rich stars in the solar neighbourhood[J]. A&A,2011,535:42-67.
[81] Irwin M,&Atzidimitriou D. Structural parameters for the Galactic dwarfspheroidals[J]. MNRAS,1995,277:1354-1378.
[82] Letarte B, et al. A high-resolution VLT/FLAMES study of individual starsin the centre of the Fornax dwarf spheroidal galaxy[J]. A&A,2010,523:17-56.
[83] Tsujimoto T. Chemical signature indicating a lack of massive stars in dwarfgalaxies[J]. ApJ,2011,736:113-119.
[84] Dutton A A, Mendel J T, Simard L. Evidence for a non-universal stellarinitial mass function in low-redshift high-density early-type galaxies[J].MNRAS Letters,2012,422: L33-L37.
[85] Liang S, Li H J,Shen X J, Cui W Y, Zhang B. Abundance analysis of r+s stars:r-process abundance comparison between r+s stars and r-rich stars[J]. PASP,2012,124:304-315.
[86] Shen X J, Zhang B, Li H J, Liang S, Cui W Y. An explanation of the correlationsof abundance ratio between neutron-capture elements and iron group elementsin Ba star[J]. ApSS,2013,343:541-554.