银河系不同星族恒星的α元素丰度分布

发布时间：2018-07-02 14:22

本文选题：银河系 + 恒星　；参考：《河北师范大学》2011年硕士论文

【摘要】：太阳附近长寿命的F和G型矮星被称为银河系演化的活化石,其恒星大气的元素丰度直接反映了银河系不同演化阶段的化学成分,为研究银河系的结构和化学演化提供了强有力的探针。因此,这类恒星一直是天文学的主要观测目标。我们的工作主要是大样本恒星丰度的统计分析,我们的统计数据包括两部分:第一部分是我们使用国家天文台兴隆观测站2.16米望远镜及其附属的折轴摄谱仪,得到的102颗F和G型矮星的高分辨率、高信噪比光谱,利用MIDAS软件包进行光谱数据处理得到谱线的等值宽度,利用恒星大气模型和原子谱线参数,得到了O、Mg、Si、Ca、Ti等α元素的化学丰度。第二部分是大样本的恒星元素统计,选取了17篇文献。两部分样本共得到了1161颗F和G型矮星(去除重复的恒星),根据纯运动学判据确定了每颗恒星的星族成分,其中薄盘星为672颗,厚盘星为281颗,晕星为60颗,其余48个样本星为过渡恒星。我们的目的是通过大样本的统计工作,分析α元素(Mg、Si、Ca、Ti)丰度和O元素丰度在不同星族成分中的分布趋势,进而研究不同星族恒星的化学演化,为银河系的形成和化学演化模型提供可靠的观测约束。论文的第一部分首先简单介绍了样本的选取标准和选取结果,并详细分析了的样本的完备性,讨论了不同样本之间丰度弥散。第二部分介绍了恒星运动学的有关知识和星族的划分方法,计算了样本恒星的空间速度并确定了样本的星族成分。第三部分详细讨论了不同星族恒星α元素丰度[α/Fe]的演化趋势,并讨论分析了银盘恒星α元素的丰度梯度。最后一部分是结论和讨论。通过对太阳附近1113颗矮星的高分辨率光谱分析得到的元素丰度的统计分析,获得到如下一些重要结果: (1)金属丰度分布:薄盘恒星的金属丰度范围为-1.0[Fe/H]+0.5;厚盘恒星的金属丰度范围为-3.0[Fe/H]+0.5,覆盖整个样本的金属丰度范围;晕族恒星的金属丰度范围为-3.0[Fe/H]-0.7。因此,厚盘与薄盘恒星、厚盘与晕族恒星以及薄盘与晕族恒星的金属丰度都存在交叉重叠区域,不能从金属丰度上严格区分三个不同的星族成分。 (2)平均银心距分布:薄盘恒星的R~m= 5.5~10.0 kpc,主要分布在7.0 ~9.0kpc范围内;厚盘恒星的R~m= 4.0 ~ 11.0 kpc,主要分布在5.5 ~7.5kpc范围内;晕族恒星的R~m= 4.0 ~ 10.0 kpc,主要分布在4.5 ~6.0kpc范围内。可见,薄盘、厚盘和晕族恒星的平均轨道半径分布也是相互重叠的,不能严格区分。 (3)最大法向距分布:薄盘恒星在垂直银道面方向的最大法向距Z_(max)1kpc;厚盘恒星的Z_(max)=0.1 ~ 4kpc;晕族恒星的Z_(max)1kpc。因此,薄盘与厚盘之间、厚盘与晕族恒星之间的最大法向距也存在部分重叠,不能严格区分。 (4)α元素(Mg、Si、Ca、Ti)的丰度[α/Fe]在贫金属星中相对超丰,但4种α元素的演化趋势并不完全一致,而且[α/Fe]随金属丰度[Fe/H]的演化趋势在薄盘和厚盘恒星中也是不同的。薄盘恒星中4种α元素的丰度[α/Fe]随[Fe/H]的增加呈线性下降趋势,到达太阳金属丰度[Fe/H]=0时达到太阳系的丰度即[α/Fe]=0。厚盘恒星中[α/Fe]随[Fe/H]的增加并非呈单调下降趋势,而且[Mg/Fe]的演化趋势与其它3种α元素Si、Ca、Ti的演化也不完全一致:其中[Mg/Fe]在[Fe/H]-1.5时随[Fe/H]的增加呈线性下降趋势,在[Fe/H] = -2.0～-0.7之间[Mg/Fe]几乎呈平台分布,但当[Fe/H]-0.7时[Mg/Fe]又随[Fe/H]的增加迅速下降直到太阳系的丰度值;而其它3种α元素Si、Ca、Ti的丰度[X/Fe]一直随[Fe/H]的增加呈线性下降趋势,但在整个金属丰度范围内并不是单一斜率下降,而是在[Fe/H] = -0.7处存在一个突然下降的“膝点”,即[α/Fe]随金属丰度[Fe/H]增加到达[Fe/H] = -0.7附近时有个突然的加速下降。 (5)在厚盘和薄盘恒星金属丰度相互重叠区域-1.0[Fe/H]+0.5内,厚盘恒星与薄盘恒星之间的[α/Fe]是混合在一起的,不能依据α元素丰度如[Mg/Fe]的大小严格地区分厚盘和薄盘恒星,但厚盘恒星[α/Fe]丰度的平均值明显高于薄盘恒星。 (6)对富金属的薄盘恒星([Fe/H]0)来说,4种α元素Mg、Si、Ca、Ti的相对丰度[α/Fe]随金属丰度[Fe/H]的演化趋势各不相同:[Mg/Fe]随[Fe/H]的增加继续下降到低于太阳系的丰度;[Si/Fe]和[Ca/Fe]在[Fe/H]0时保持太阳系丰度值不变;[Ti/Fe]随着[Fe/H]的升高有微弱的增加趋势。 (7)[O/Fe]随[Fe/H]的演化趋势与[Mg/Fe]一致,但薄盘恒星[O/Fe]在[Fe/H]≈-0.4处存在一个膝状下降。而[α/O]随[O/H]的增加而线性下降,薄盘和厚盘的[α/O]也存在混合,不能严格区分,且厚盘星的丰度弥散较薄盘星大。 (8)无论是厚盘、薄盘还是晕族恒星的[α/Fe]丰度都与恒星形成时的平均轨道半径R~m、以及离开银道面的最大距离Z_(max)分布无关,即不存在α元素丰度梯度。薄盘和厚盘的α元素丰度[α/Fe]在R~m=7kpc时是勉强能够分开的,但是还是有一部分的厚盘星与薄盘星混合;在银盘纵向上,在Z_(max)=1kpc时薄盘和厚盘的α元素丰度[α/Fe]可以分开,但是同样有部分厚盘星混合在薄盘区域内。在Z_(max)=4kpc时,厚盘和晕星的[α/Fe]能够明显分开。 (9)薄盘、厚盘和晕族恒星的运动学性质明显不同,其中绕银心旋转的轨道速度VLSR的区分更加明显,薄盘恒星:V_(LSR)-60km/s,且V_(total)90km/m;厚盘恒星:-180km/sV_(LSR)-60km/s,且250V_(total) 90km/m;晕族恒星:V_(LSR)-180km/s,且V_(total) 200 km/s。此外,三个星族成分的速度弥散也不同,由薄盘到厚盘再到晕星的速度弥散依次增大。但恒星的α元素丰度[α/Fe]与运动学速度之间没有相关性,且既存在一些具有典型薄盘恒星[α/Fe]丰度而运动学上属于厚盘的恒星,也存在一些具有典型厚盘恒星[α/Fe]丰度而运动学上属于薄盘的恒星。 (10)晕族恒星的[α/Fe]丰度也呈现随[Fe/H]增加而下降的趋势,但晕族恒星[α/Fe]丰度的弥散远大于丰度的分析误差,且与厚盘恒星的α元素丰度存在严重的混合,存在一些[α/Fe]丰度低于薄盘的所谓贫α元素晕星。这些统计结果表明,虽然平均来讲厚盘恒星的[α/Fe]丰度高于薄盘恒星,但在金属丰度重合区域内[α/Fe]也是相互交叉混合的,不能严格区分。这与近年来单个样本的观测结果不同。因此,关于薄盘和厚盘恒星的α元素丰度是否区分,还需要更多的大样本观测数据来的确认。
[Abstract]:The long-lived F and G dwarf stars in the vicinity of the sun are known as the living fossil of the Milky way evolution. The element abundance of the star atmosphere directly reflects the chemical composition of the different evolution stages of the Milky way, providing a powerful probe for the study of the structure and chemical evolution of the Milky way.
Our work is mainly the statistical analysis of the abundance of large sample stars. Our statistics include two parts. The first part is the high resolution, high signal to noise ratio spectrum of 102 F and G dwarf stars, and the spectrum of the high signal to noise ratio, using the MIDAS software package. The chemical abundances of O, Mg, Si, Ca, Ti and other alpha elements are obtained by using the stellar atmosphere model and the parameters of the atomic spectral line. The second part is the stellar element statistics of large samples, and 17 articles are selected. The two parts of the samples have obtained 1161 F and G Dwarfs (removal of repeated stars), according to the pure kinematic criteria. The star composition of each star is determined, of which 672 are thin disk stars, 281 thick disk stars, 60 halo stars, and the remaining 48.
The sample star is a transition star. Our aim is to analyze the distribution trend of the abundance of alpha elements (Mg, Si, Ca, Ti) and the abundance of O elements in different star composition by large sample statistics, and then study the chemical evolution of different star stars and provide reliable observation constraints for the formation and chemical evolution model of the galaxy. Firstly, the criteria and results of sample selection are briefly introduced, and the completeness of the samples is analyzed in detail. The abundance dispersion between different samples is discussed. The second part introduces the related knowledge of the stellar kinematics and the division method of the star family, calculates the space velocity of the sample star and determines the Star composition of the sample. The third part of the sample is determined. The evolution trend of alpha element abundances (alpha /Fe]) in different star stellar stars is discussed in detail, and the abundance gradient of alpha elements in a silver disk star is discussed and analyzed. The last part is the conclusion and discussion.
Based on the statistical analysis of elemental abundance obtained by high-resolution spectral analysis of 1113 dwarfs near the sun, the following important results are obtained:
(1) distribution of metallicity: the metallicity of a thin disk star is -1.0[Fe/H]+0.5; the metal abundance of the thick disk star is -3.0[Fe/H]+0.5, covering the metal abundance of the whole sample; the metallicity of the halo star is -3.0[Fe/H]-0.7., the thick disk and Bo Panheng star, the thick disk and the halo star, and the gold of the thin disk and the halo star. There are overlapping regions in the genus abundance, and three different star components can not be strictly distinguished from metallicity.
(2) average distribution of silver heart distance: R~m= 5.5~10.0 KPC of thin disk stars, mainly distributed in 7 ~9.0kpc; R~m= 4 ~ 11 KPC in thick disk stars, mainly distributed in the 5.5 ~7.5kpc range; R~m= 4 ~ 10 KPC of halo stars, mainly in the range of 4.5 ~6.0kpc. It also overlaps each other and can't be strictly differentiated.
(3) the maximum normal distance distribution: the maximum normal direction distance of the thin disk star in the vertical silver channel direction Z_ (max) 1kpc, the Z_ (max) =0.1 ~ 4kpc of the thick disk star, the Z_ (max) 1kpc. of the halo star, and the maximum normal distance between the thick disk and the thick disk, and the maximum normal distance between the thick disk and the halo star, and can not be strictly distinguished.
(4) the abundance of alpha elements (Mg, Si, Ca, Ti) [alpha /Fe] is relatively super abundant in poor metal stars, but the evolution trend of the 4 alpha elements is not completely consistent, and [the evolution trend of alpha /Fe] with metallicity [Fe/H] is also different in thin plates and thick disk stars. The abundance of 4 alpha elements in the thin disk stars [alpha /Fe] increases linearly with the increase of [Fe/H], to the increase of [Fe/H]. When the metal abundance of [Fe/H]=0 reaches the abundance of the solar system, [the increase of [alpha /Fe]] in the thick disk stars [alpha /Fe]=0.] is not monotonically decreasing, and the evolution of [Mg/Fe] is not exactly the same as the evolution of the other 3 kinds of alpha elements Si, Ca, Ti, and [Mg/Fe] in [Fe/H] -1.5 decreases linearly with the increase of [Fe/H]. ] = [Mg/Fe] between -2.0 and -0.7 is almost a platform distribution, but at the time of [Fe/H]-0.7, the [Mg/Fe] decreases rapidly with the increase of [Fe/H] to the abundance of the solar system, while the other 3 kinds of alpha elements, Si, Ca, and Ti [X/Fe] have been decreasing linearly with the increase of [Fe/H], but not in the entire metallicity range, but in [F. E/H] = -0.7 has a sudden drop in the "knee point", that is, [alpha /Fe] has a sudden acceleration drop with the increase of metallicity [Fe/H] to [Fe/H] = -0.7.
(5) in the overlapping region -1.0[Fe/H]+0.5 of thick disk and thin disk stellar metallicity, the [alpha /Fe]] between the thick disk star and the thin disk star is mixed together, and the thick disk and thin disk stars are not strictly divided according to the size of the alpha element abundances such as [Mg/Fe], but the average value of the thick disk stars [alpha /Fe] abundances is obviously higher than that of the thin disk stars.
(6) for the metal rich thin disk star ([Fe/H]0), the relative abundance of the 4 alpha elements, Si, Ca, Ti, [the evolution trend of the alpha /Fe] with the metallicity [Fe/H] is different: [Mg/Fe] continues to fall below the abundance of the solar system with the increase of [Fe/H]; [Si/Fe] and [Ca/Fe] are kept unchanged in the solar system when [Fe/H]0. A slight increase in the trend.
(7) [O/Fe] is consistent with the evolution trend of [Fe/H] with [Mg/Fe], but the [O/Fe] of thin disk star has a geniculate decline at [Fe/H] -0.4. [alpha /O] decreases linearly with the increase of [O/H], and [alpha /O] also exists in thin plates and thick plates, and can not be strictly differentiated, and the abundance of thick disc stars is larger than that of thin disc stars.
(8) the [alpha /Fe] abundances of the thick disk, the thin disk or the halo star are all related to the average orbital radius R~m of the star formation and the maximum distance Z_ (max) away from the silver channel, that is, there is no alpha abundance gradient. The alpha element abundances of the thin plates and the thick discs [alpha /Fe] are barely separated at R~m= 7kpc, but there are still some parts. The thick disk stars are mixed with thin disc stars; in the longitudinal direction of the silver disk, the alpha element abundances of the thin plates and the thick discs are separated at Z_ (max) =1kpc, but the same part of the thick disk star is also mixed in the thin disk region. The [alpha] /Fe] of the thick disk and the halo star can be distinctly separated in Z_ (max) =4kpc.
(9) the kinematic properties of the thin disk, the thick disk and the halo star are distinctly different, in which the difference in the velocity of the orbital velocity VLSR around the core is more obvious, the thin disk Star: V_ (LSR) -60km/s, and V_ (total) 90km/m; the thick disk Star: -180km/sV_ (LSR) -60km/s, and 250V_ (total) The velocity dispersion of the composition of the family is also different, and the velocity diffusion from the thin plate to the halo star increases in turn, but there is no correlation between the alpha abundance of the stars [alpha /Fe] and the kinematic velocity, and there are some stars with typical thin disk stars [alpha /Fe] abundance and kinematic thick disk, but also some typical thick disk stars [alpha]. /Fe] abundance in kinematics belongs to a thin disc star.
(10) the [alpha /Fe] abundance of the halo stars also decreases with the increase of [Fe/H], but the dispersion of the halo star is far greater than the analysis error of the abundance, and there is a serious mixture with the abundance of the alpha element in the thick disk star, and there are some [alpha /Fe] abundances below the thin plates as the poor alpha element halo stars. The [alpha /Fe] abundance of a thick disk star is higher than that of a thin disk star, but in the area of the metallicity overlap [alpha /Fe] is also intersecting with each other. It is not strictly differentiated. This is different from the observations of a single sample in recent years. Therefore, more large sample observations are needed for the distinction of the alpha element abundances of thin plates and thick disk stars. Confirmation.
【学位授予单位】：河北师范大学
【学位级别】：硕士
【学位授予年份】：2011
【分类号】：P144

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