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1.
星系中的巨分子云(GMCs)是恒星形成的主要区域,因此它的形成和演化对于星系的演化是至关重要的。本文中将介绍分子云的基本特性、分子云之间的碰撞和巨分子云的形成、碎裂和寿命以及其他环境因素,如旋臂扰动、较差自转等在巨分子云的形成和演化中的作用。同时也探讨在采用数值模拟研究巨分子云演化时所取分子云数目的影响。 相似文献
2.
宋国玄 《中国天文和天体物理学报》1989,(1)
巨分子云的碰撞造成了大质量恒星在碰撞分子云中的形成,这些大质量恒星的形成产生了膨胀的HII区域,从而使巨分于云碎裂成小质量的分子云。这是本文提出的巨分子云碎裂机制。因此巨分子云的寿命也主要由区分子云间的碰撞几率所决定。我们的分析表明,巨分子云的寿命有赖于巨分子云所在的旋涡星系中的不同位置。寿命的最大可能存在区间为8.18×10~7yr与2.45×10~8yr。利用我们提出的机制可以在分子云研究的数值计算与数值模拟中得到应用。 相似文献
3.
本文分析了在巨分子云聚合形成机制下旋臂扰动的影响 .结果表明 ,在巨分子云聚合形成过程中 ,当不考虑恒星形成引起的巨分子云的碎裂时 ,旋臂的存在使分子云在绕星系中心作自转运动时 ,在旋臂区域分子云的密度大大增加而使较多的大质量分子云由于碰撞而形成 ,特别能促使一些质量更大的巨分子云形成 .但当这些聚合形成的大质量分子云走出旋臂区域进入臂间区域时 ,它们又会自动瓦解 .因此在整个星系盘上 ,与没有旋臂扰动情况相比 ,F(M )∝logM的曲线只是相应地往上有一平移 ,而对形成的中间质量的巨分子云的数量基本没有影响 相似文献
4.
分析在聚合形成机制下,巨分子云在刚体自转盘中的形成过程。研究结果表明,形成的巨分子云主要由其附近的分子云组成,由于速度弥散的作用,非弹性碰撞才自引力使分子云聚合在一起,以这种方式形成的巨分子云是小质量的,如果较差自转存在这些小质量的巨分子云便有更多的机会聚合在一起形成更大质量的巨分子云,这进一步说明,较差自转在巨分子云的形成中起了很大的积极作用。 相似文献
5.
分析在聚合形成机制下,巨分子云在刚体自转盘中的形成过程．研究结果表明,形成的巨分子云主要由其附近的分子云组成．由于速度弥散的作用,非弹性碰撞和自引力使分子云聚会在一起,以这种方式形成的巨分子云是小质量的．如果较差自转存在,这些小质量的巨分子云便有更多的机会聚合在一起形成更大质量的巨分子云．这进一步说明,较差自转在巨分子云的形成中起了很大的积极作用． 相似文献
6.
本分析了在巨分子云聚合形成机制下旋臂扰动的影响，结果表明，在巨分子云聚合形成过程中，当不考虑恒星形成引起的巨分子云的破裂时，旋臂的存在使分子云在绕星系中心作自转运动时，在旋臂区域分子云的密度大大增加而使大质量分子云由于碰撞而形成，特别能促使一些质量更大的巨分子云形成。但当这些聚合形成的大质量分子云走出旋臂区域进入臂间区域时，它们又会自动瓦解。因此在整个星系盘上，与没有旋臂扰动情况相比，Ｆ（Ｍ）〈 相似文献
7.
为了研究有大质量恒星形成的分子云与其它分子云之间的差异,对北天的59个作为大质量恒星形成区的Spitzer延展绿色天体(Extended Green Objects,简称EGOs)视线方向进行了分子云~(12)CO J=2-1和J=3-2频谱观测,并与文献中对同一批天体方向观测得到的~(12)CO J=1-0频谱数据合并进行分析.对与EGO天体成协的分子云(简称EGO分子云)和其它non-EGO分子云进行了CO多跃迁谱线强度和宽度的统计比较分析.在数据统计的基础上,讨论了这两类分子云的气体温度分布、密度分布、速度场分布对观测数据统计特征的影响.分析结果表明,直接决定是否有大质量恒星形成的关键因素可能并不是巨分子云的质量是否足够大,而是巨分子云的引力塌缩程度足否充分(即分子云团块的体积填充因子是否足够大). 相似文献
8.
该文利用高精度的大尺度N体数值模拟,研究了WMAP(Wilkinson Microwave Anisotropy Probe)第3年的观测结果对星系团中的巨弧数目的影响,发现预言的巨弧数目比主流的ACDM模型预言的要低80%多。加入绝热气体几乎不会改变巨弧的发生概率,而引入恒星形成和超新星反馈后得到的概率不会大于原来的1.3倍。如此小的贡献是因为虽然有恒星的数值模拟得到的星系团具有更高的中心密度,但同时星系团的形状更趋球对称,这是影响巨弧概率的2个相反的因素。以前基于ACDM模型预言的巨弧概率还不能很好地与观测相符,WMAP第3年的观测结果将更难以解释。 相似文献
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分子云间的自引力在巨分子云聚合形成中的作用 总被引：1，自引：0，他引：1
宋国玄 《中国科学院上海天文台年刊》1996,(17):234-239
在巨分子云聚合形成过程中，一般都包括了分子云间的非弹性磁撞与分子云间的自引力。本主要对分子云间的自引力在这一形成过程中的作用进行分析，得到的结论是分子云间自引力对分子去的成团是至关重要的。 相似文献
11.
We investigate the conditions under which a self-gravitating system of particles of different masses may be gravitationally unstable if there is a systematic correlation between the random velocity of a particle and its mass. For an isotropic uniform medium without rotation but with mass spectrum and velocity depending on the particle mass, a situation arises where the Jeans length for such a system may be significantly smaller than for the case when some mean values are used instead of mass and velocity spectra. For a differentially rotating medium, representing a spiral galaxy, we obtain the analogue of the Toomre parameter for a heterogeneous (multi-component) system. We demonstrate that the gas system in spirals represented by an ensemble of giant molecular clouds may be considerably less stable in the case of random velocity–mass correlation than for a system with unique velocity dispersion. 相似文献
12.
Richard P. Nelson 《Monthly notices of the Royal Astronomical Society》1998,298(3):657-676
We present the results of collapse calculations for uniformly rotating, prolate clouds performed using the numerical method: smoothed particle hydrodynamics (SPH). The clouds considered are isothermal, prolate spheroids with different axial ratios ( a/b ), and with different values of β, the ratio of the rotational to gravitational energy. Small density perturbations are added to the clouds, and different initial perturbation spectra are studied. All of the clouds considered are strongly unstable to gravitational contraction, and so collapse to form a spindle configuration. Such a linear structure is unstable to fragmentation, so that the clouds break up into a number of subcondensations. The long-term evolution of the system is then determined by the angular momentum possessed by these fragments.
It is found that a number of the calculations performed result in the formation of orbitally stable binary systems, composed of two rotationally supported discs in orbit about their common centre of mass. Tidal interactions during closest approach, close three-body interactions and the continued accretion of material with high specific angular momentum are all found to increase the orbital separation during these calculations, ensuring that the systems do not merge at later times. The calculations are therefore relevant to the problem of binary star formation, though the systems produced tend to have large orbital separations and periods. One of the strong points of the models presented, however, is their ability to produce systems with a range of mass ratios and orbital eccentricities, without the explicit inclusion of biases in the initial conditions. 相似文献
It is found that a number of the calculations performed result in the formation of orbitally stable binary systems, composed of two rotationally supported discs in orbit about their common centre of mass. Tidal interactions during closest approach, close three-body interactions and the continued accretion of material with high specific angular momentum are all found to increase the orbital separation during these calculations, ensuring that the systems do not merge at later times. The calculations are therefore relevant to the problem of binary star formation, though the systems produced tend to have large orbital separations and periods. One of the strong points of the models presented, however, is their ability to produce systems with a range of mass ratios and orbital eccentricities, without the explicit inclusion of biases in the initial conditions. 相似文献
13.
If X , which follows a power-law distribution, is observed subject to Gaussian measurement error e , then X + e is distributed as the convolution of the power-law and Gaussian distributions. Maximum-likelihood estimation of the parameters of the two distributions is considered. Large-sample formulae are given for the covariance matrix of the estimated parameters, and implementation of a small-sample method (the jackknife) is also described. Other topics dealt with are tests for goodness of fit of the posited distribution, and tests whether special cases (no measurement errors or an infinite upper limit to the power-law distribution) may be preferred. The application of the methodology is illustrated by fitting convolved distributions to masses of giant molecular clouds in M33 and the Large Magellanic Cloud (LMC), and to H i cloud masses in the LMC. 相似文献
14.
Rowan J. Smith Paul C. Clark Ian A. Bonnell 《Monthly notices of the Royal Astronomical Society》2009,396(2):830-841
We present an analysis of star-forming gas cores in a smooth particle hydrodynamics simulation of a giant molecular cloud. We identify cores using their deep potential wells. This yields a smoother distribution with clearer boundaries than density. Additionally, this gives an indication of future collapse, as bound potential cores (p-cores) represent the earliest stages of fragmentation in molecular clouds. We find that the mass function of the p-cores resembles the stellar initial mass function and the observed clump mass function, although p-core masses (∼0.7 M⊙ ) are smaller than typical density clumps. The bound p-cores are generally subsonic, have internal substructure and are only quasi-spherical. We see no evidence of massive bound cores supported by turbulence. We trace the evolution of the p-cores forward in time, and investigate the connection between the original p-core mass and the stellar mass that formed from it. We find that there is a poor correlation, with considerable scatter suggesting accretion on to the core is dependent on more factors than just the initial core mass. During the accretion process the p-cores accrete from beyond the region first bound, highlighting the importance of the core environment to its subsequent evolution. 相似文献
15.
Yin Jia Jiang Zhi-bo Yang Ji Chen Zhi-wei Wang Min 《Chinese Astronomy and Astrophysics》2011,35(2):511
By using the 13.7 m millimeter wave telescope of the Qinghai Station of Purple Mountain Observatory at Delingha, we have performed the mapping observations simultaneously at the (J = 1-0) lines of 12CO, 13CO and C18O towards respectively the 17 star forming regions associated with clusters. All of them show rather strong C18O emission, except IRAS 04547+4753. Because of the different sizes of molecular clouds, there are 13 regions being observed to the half maximum of 13CO integrated intensity, and the large-area mapping observation has not been made for the other 4 regions with rather large extents. Based on the observed data, the physical properties of molecular cores are calculated, such as the line width, brightness temperature, size, density and mass. The averaged ratios of the virial mass Mvir and local thermodynamic equilibrium mass MLTE of the 13CO and C18O cores are 0.66 and 0.74, respectively, suggesting that these cores are nearly at the virial equilibrium state. In order to compare the cores and clusters in morphologies, the contour maps of the integrated intensities of 13CO and C18O are overlaid on the K-band images of 2MASS. At the same time, the sizes and masses of the clusters associated with cores are calculated by adopting the photometric results of the near-infrared point sources in 2MASS database. Based on the derived masses of the molecular cores and clusters, the star formation efficiency (SFE) is calculated for the molecular clouds, and we find that it varies in the range from 10% to 30%. 相似文献
16.
K. Ferrire 《Astronomische Nachrichten》2010,331(1):27-33
We review the current observational knowledge of the interstellar magnetic field within ∼150 pc ofthe Galactic center. We also discuss the various theoretical scenarios that have been put forward to explain the existing observations. Our critical overview leads to two important conclusions: (1) The interstellar magnetic field near the GC is approximately poloidal on average in the diffuse intercloud medium and approximately horizontal in dense interstellar clouds. (2) In the general intercloud medium, the field is relatively weak and probably close to equipartition with cosmic rays (B ∼ (6–20) μ G), but there exist a number of localized filaments where the field is much stronger (some filaments could possibly have B ≳ 1 mG). In dense interstellar clouds, the field is probably rather strong, with typical values ranging between a few 0.1 mG and a few mG (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
17.
Martin A. Zwaan † 《Monthly notices of the Royal Astronomical Society》2001,325(3):1142-1148
Five galaxy groups with properties similar to those of the Local Group have been surveyed for H i clouds with the Arecibo Telescope. In total 300 pointings have been observed on grids of approximately 2.5×1.5 Mpc2 centred on the groups. The 4.5 σ detection limit on the minimal detectable H i masses is approximately 7×106 M⊙ ( H 0 =65 km s−1 Mpc−1 ) . All detections could be attributed to optical galaxies; no significant detections of H i clouds have been made. This null result leads to the conclusion that the total H i mass of intragroup clouds must be less than 10 per cent of the total H i mass of galaxy groups and less than 0.05 per cent of the dynamical mass. The recent hypothesis that Galactic high-velocity clouds are Local Group satellite galaxies is highly inconsistent with these observations. 相似文献
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We present model results for the chemistry in an expanding cloud or clump in which molecules are injected into the gas phase from grain surfaces when the clump reaches a certain visual extinction A
v during the expansion. We consider separately injection at two different values of A
v, and include a representative large hydrocarbon, C6H, and sulphur-bearing molecule, H2SO4, as well as H2O and CO. We examine the abundances of certain molecules which have been observed in diffuse and translucent clouds, and compare the results obtained for these abundances with and without an injection during expansion. We also compare our results withpublished observations, and conclude that in most clouds an injection of molecules has occurred. 相似文献
20.
J.E. Pringle Ronald J. Allen S.H. Lubow 《Monthly notices of the Royal Astronomical Society》2001,327(2):663-668
In a recent paper, Elmegreen has made a cogent case, from an observational point of view, that the lifetimes of molecular clouds are comparable to their dynamical time-scales. If so, this has important implications for the mechanisms by which molecular clouds form. In particular, we consider the hypothesis that molecular clouds may form not by in situ cooling of atomic gas, but rather by the agglomeration of the dense phase of the interstellar medium, much, if not most, of which is already in molecular form. 相似文献