銀河系化學(xué)演化及球狀星團多星族問題研究

王悅

(中國科學(xué)院國家天文臺 北京 100101)

恒星的化學(xué)元素豐度特征能夠反映其形成和演化歷史.以化學(xué)元素豐度為手段,研究了銀河系中恒星的徑向遷移對銀河系化學(xué)演化的影響,以及球狀星團中漸進巨星支(AGB)恒星的多星族問題.

近年的觀測和理論研究表明:恒星在銀盤里有徑向遷移.基于詳細的銀河系化學(xué)演化模型,再采用分布函數(shù)模擬恒星徑向遷移過程,研究了恒星的徑向遷移對銀盤徑向元素豐度梯度的影響.結(jié)果顯示:太陽鄰域的恒星誕生于很大的銀心距范圍內(nèi).恒星的徑向移動使某一銀心距處恒星的金屬豐度彌散到一定范圍,使恒星的年齡與金屬豐度不再是單調(diào)關(guān)系.星際介質(zhì)中鐵元素和氧元素豐度沿銀盤徑向分布不會因為恒星的徑向遷移而產(chǎn)生明顯差異,即恒星的徑向遷移不會明顯改變元素豐度在銀盤徑向的梯度,但該梯度會因此變得平緩.年老恒星的元素豐度梯度比年輕恒星的更平緩.

球狀星團是銀河系的重要組成部分.光譜和測光的研究都揭示出球狀星團中存在多個星族,利用歐洲南方天文臺甚大望遠鏡(ESO/VLT)上的多目標(biāo)中高分辨率光纖光譜儀FLAMES (Fibre Large Array Multi Element Spectrograph),對4個銀河系球狀星團(NGC 104、NGC 2808、NGC 6121、NGC 6809)的AGB和紅巨星支(RGB)恒星樣本進行了觀測,從光譜數(shù)據(jù)得到了樣本星的鈉豐度.與RGB星相比,星團的AGB星的鈉豐度也都呈現(xiàn)出顯著的彌散,說明在這4個星團中均存在富鈉的第2星族AGB星.K-S檢驗表明,NGC 104、NGC 2808和NGC 6809的AGB和RGB星的鈉豐度可認為遵從同一分布,而NGC 6121的AGB和RGB星的鈉豐度是不同的分布,明顯缺乏高鈉的AGB星.各星團樣本中第1和第2星族恒星的數(shù)量及所占比例也可以得到.從文獻中收集了另外4個銀河系球狀星團(NGC 5904、NGC 6205、NGC 6266、NGC 6752)的AGB和RGB星樣本的鈉豐度數(shù)據(jù),對其中的NGC 6752的光譜數(shù)據(jù)進行了再分析.綜合分析8個球狀星團的AGB和RGB星樣本的鈉豐度分布情況以及第1和第2星族占比,我們發(fā)現(xiàn)AGB星呈現(xiàn)出比較復(fù)雜的情況:一些星團(NGC 6752和NGC 6266)中基本沒有富鈉的第2星族AGB星,一些星團(NGC 2808、NGC 5904和NGC 6809)的AGB星具有與RGB星相似的鈉豐度彌散和第2星族占比,其他星團的情況則介于此兩者之間.由此不能確定在第2星族AGB星的占比和所考慮的球狀星團特征參數(shù)之間存在明確的依賴關(guān)系.

最后,對球狀星團的星族劃分方法進行了研究.首先檢驗了基于銀河系場星鈉豐度分布的區(qū)分球狀星團第1和第2星族的鈉豐度標(biāo)準(zhǔn)([Fe/H]的函數(shù)).通過比較由場星標(biāo)準(zhǔn)以及文獻中分別單獨考慮每個星團樣本得到的17個球狀星團樣本的第1星族占比,發(fā)現(xiàn)場星標(biāo)準(zhǔn)會高估第1星族恒星所占的比例.考慮到球狀星團的多樣性,單獨分析每個球狀星團樣本的星族劃分方法更顯合理.利用場星作為參考來劃分球狀星團的星族需要謹慎.對球狀星團的星族和場星之間的聯(lián)系還需要更深入的研究.

The chemical feature of stars is an indicator of its formation and evolution history.Taking advantage of the elemental abundances,we study the influence of the radial stellar migration in the Galactic disk on chemical evolution,and the multiple stellar populations of asymptotic giant branch (AGB)stars in globular clusters.

Stellar radial migration has been found in the disk of the Milky Way.We modeled the radial stellar migration in the Galactic disk with an analytical method,and added it to a detailed Galactic chemical evolution model to study the influence of the radial stellar migration on the chemical evolution of the Milky Way,especially on the abundance gradients.We find that stars currently located in the solar vicinity were born in a large range of Galactocentric radius.The radial stellar migration can scatter the stellar abundance at a given Galactocentric radius,leading to a large dispersion around the classic simple agemetallicity relation.After migration,the abundance gradients along the Galactic radius would not be changed significantly by the radial stellar migration.However,the migration can flatten the radial gradients of the mean chemical abundance of stars,and older stars possess flatter abundance gradients than younger stars.

Globular clusters are important components of the Milky Way.Large amount of spectroscopic and photometric studies have shown that there are multiple stellar populations in globular clusters,which has been accepted as a general feature of globular clusters.In this work,with the high-resolution spectra obtained with the multi-object high-resolution fibre spectrograph FLAMES(Fibre Large Array Multi Element Spectrograph)on the Very Large Telescope of European Southern Observatory (ESO/VLT),we derived accurate Na abundances for the AGB and RGB (Red Giant Branch)sample stars of four Galactic globular clusters,i.e.NGC 104,NGC 2808,NGC 6121,and NGC 6809.Through the Na abundance distributions of the four globular clusters,we find that,compared with the RGB stars,the Na abundance of the AGB stars also show significant dispersions indicating that there are Na-rich second-population AGB stars in all of the four clusters.A K-S test shows that the Na abundance of the AGB and RGB stars of NGC 104,NGC 2808,and NGC 6809 can be regarded following the same distribution,while it is not the case for NGC 6121,and this cluster lacks the most Na-rich AGB star.Moreover,according to the Na abundance,we also determined the numbers and fractions of first- and second-population stars in both AGB and RGB samples of the four clusters.

In addition,we gathered from literature the Na abundances of AGB and RGB samples of another four Galactic globular clusters,i.e.NGC 5904,NGC 6205,NGC 6266,and NGC 6752,among which we re-analyzed the spectral data of the sample of NGC 6752 with our own method.The Na abundance distribution of their AGB and RGB samples and the fractions of their first- and second-population stars of these eight clusters (NGC 104,NGC 2808,NGC 5904,NGC 6121,NGC 6205,NGC 6266,NGC 6752,NGC 6809)show a complex picture:some globular clusters(NGC 6752 and NGC 6266)lack the Na-rich secondpopulation AGB star,and in some ones (NGC 2808,NGC 5904,and NGC 6809)the AGB stars show similar Na abundance dispersions and second-population fractions compared with the corresponding RGB stars,while the cases of others are in between.Due to large scatters and outliers,there are no dependencies between the AGB second-population fraction and those globular cluster parameters.

Last but not least,we investigated the methods to classify different populations in globular clusters.By comparing the first population fractions of 17 globular clusters estimated by the field star criterion with those in the literature derived by individual-cluster-concerned methods,we find that the field star criterion tends to overestimate the first population fractions.The population separation methods for individual globular cluster are recommended.Caution should be paid if one wants to take field stars as a reference for the globular cluster population identification.Further study on the interplay between field stars and globular cluster populations is desired.