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1.
恒星作为宇宙的基础组成元素,其形成过程一直是天文学中的重要研究对象。人们已基本了解中小质量恒星(质量小于8M_⊙)的形成和演化过程;受到数量少、嵌埋深、演化快和反馈剧烈等因素的影响,大质量恒星(质量大于8M_⊙)的形成过程依然谜团重重。介绍了小质量恒星形成的基本理论,以及吸积盘、竞争吸积、并合三种主流的大质量恒星形成模型;回顾了以往使用红外或射电望远镜对大质量恒星形成区的观测和分析,以及现阶段使用多波段巡天观测手段对大质量恒星形成区的研究成果;着重介绍了目前公认的大质量恒星形成的示踪物——分子外向流的理论和观测现状,以及大质量外向流的优秀候选体——绿色延展天体的发现、理论及观测研究情况。最后,对大质量恒星形成的理论和观测研究进行了总结和展望。 相似文献
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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
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恒星形成于分子云之中, 分子外向流是恒星形成正在进行的重要动力学特征, 也是研究和认识恒星形成的重要契入点. 利用紫金山天文台青海观测站德令哈13.7m毫米波望远镜, 采用5种分子谱线探针(包括12CO、13CO、C18O、HCO$^+$ $J=1-0$和CS $J=2-1$, J为角动量量子数), 对一个包含IRAS 19230+1506、IRAS 19232+1504和G050.3179--00.4186这3个源的大质量恒星形成复合体进行了成图观测研究. 通过对以上分子谱线数据并结合红外波段巡天数据的分析, 在这3个源中首次探测到了分子外向流活动, 并确定了分子外向流的中心驱动源. 最后对这3个源进行了分子外向流相关物理量参数的计算, 分析了这些物理量参数之间的关系, 结果表明分子外向流的性质与中心驱动源的性质息息相关. 相似文献
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给出恒星形成区GGD12-15的宽波段JHK和窄波段H2v=1-0S(1)近红外成像观测．观测图像揭示了致密的年轻红外星团和与红外源成协的红外星云，并发现了以H2发射结表征的星团外流活动．大多数红外点源在光学波段不可见；对76颗红外点源的JHK′测光结果显示，有32颗具有红外超，其中5颗表现原恒星特征，表明分子云中的恒星形成活动很活跃．以B8光谱型作为大质量星分界，由色星等图估计出大质量星所占星团比例为-10％～26％．GGD12—15星团的K′星等分布的峰值位于15.0mag，并在13．0mag-16．0mag平坦分布；[H—K′]色分布的峰值出现在-0．7mag，在此以上更红的星团成分占70％．在GGD12-15区新发现的氢分子发射结集中在星团中心领域，其空间分布明显与剧烈的恒星形成活动相关；有5个发射结位于分子外流的中心区域，暗示其激发可能与分子外流同源． 相似文献
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《天文学报》2017,(2)
"气尘比"(Gas to Dust Ratio,GDR)是星际气体与星际尘埃的质量之比.广泛认同的银河系气尘比值是100-150.气尘比值的大小不仅取决于星际环境,也与所考虑的尘埃成分相关.恒星形成区是恒星形成的致密分子云区域,不同的分子云,其GDR也可能不同于普遍采用的数值.此工作选择3个典型的恒星形成区进行气尘比的研究,它们分别是:大质量恒星形成活跃的猎户座(Orion)分子云,小质量恒星形成区的代表金牛座(Taurus)分子云,极少或者无恒星形成活动的Polaris分子云,对这3个天区的研究有利于了解不同辐射环境恒星形成区的气尘比变化.在此对CO谱线积分强度与氢分子柱密度之间的转换系数X_(CO)取常数,以统计的方法计算了3个分子云的气尘比N(H)/A_V,其值在Orion天区、Taurus天区和Polaris天区分别为25、38和55(单位:10~(20)cm~(-2).mag~(-1)),明显高于之前人们给出的银河系平均值.根据星际尘埃模型,将N(H)/A_v转换成气体尘埃的质量比.采用被广泛接受的WD01尘埃模型(V波段的选择性消光比R_v=3.1的情况),得到3个恒星形成区的气尘比分别为:160(Orion分子云)、243(Taurus分子云)、354(Polaris分子云),显著高于普遍采用的弥漫星际介质中100-150的取值范围.恒星形成区的N(H)/A_v值高于平均值的另外一个可能的原因是,恒星形成区的尘埃由于吸积或者碰撞增长变大,降低了V波段的单位质量消光效率,而不是气尘质量比本身的增加. 相似文献
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《天文学报》2013,(4)
通过对恒星形成区AFGL 5157进行了24′×24′(12 pc×12 pc)的成图观测,得到了该分子云的~(13)CO(J=1-0)和C~(18)O(J=1-0)云核各自的分布结构和平均物理参数.在云核的边缘位置,同位素丰度比X[(~(13)CO)/(C~(18)O)]约为10,接近于巨分子云的比值.~(13)CO和C~(18)O云核的维里质量小于云核质量,具有引力不稳定性,且C~(18)O云核更易塌缩.C~(18)O分子云核的东北方向和西南方向的分子云柱密度分布分别为1.1×10~(23)×z~(-0.43)和4.6×10~(25)×z~(-0.58),z表示到云核中心的距离.由~(12)CO(J=1-0)高速线翼成份的测量,估计了外流源的质量损失率,求得外向流的质量和速度的关系近似为m∝v~(-1.8).~(13)CO分子云核的恒星形成率为23%,该区域可能受反射星云NGC 1985的触发而正在形成中大质量恒星或者星团. 相似文献
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具有不同质量的恒星在耗尽其热核能源后,最终可能会坍缩成为性质完全不同的致密天体,如白矮星、中子星或者黑洞。从20世纪30年代起,黑洞的观测及其证认一直是天体物理学的研究热点之一。首先简要地回顾了恒星级黑洞的形成及其候选天体的研究历史;然后介绍了如何从观测上证认恒星级黑洞:接着详细讨论了恒星级黑洞的质量和自转参数的测量方法;最后介绍恒星级黑洞观测及其证认的最新研究进展,并做出结论:目前已经有充分的证据宣告在部分吸积X射线双星中存在恒星级黑洞。 相似文献
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目前,对大质量恒星形成的初始条件不很清楚,特别是大质量恒星是否形成于星团中心仍有争议.有人从时标上考虑,认为嵌埋星团的质量分层现象意味着大质量恒星只能在星团中心诞生.利用Monte Carlo方法对嵌埋星团的动力学演化进行了数值模拟,并与观测进行了比较.假设初始时刻大质量恒星随机分布,一定比例的嵌埋星团因为大质量恒星的随机运动,在演化的某个时刻会呈现暂态质量分层,其中一部分相当明显.这说明,大质量恒星在中心形成并非嵌埋星团质量分层现象的唯一解释.此外,气体的动力学摩擦能有效地减小动力学质量分层的时标,从而增大暂态质量分层的概率. 相似文献
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对于大质量的形成，由于区域的遥远和结构的复杂以及过程的特殊，研究相对迟缓，对可能形成大质量得的云核和大质量年轻星体的活动，进行多波段搜寻和研究，取得了相应的进展。 相似文献
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R.D. Oudmaijer M.E. van den Ancker D. Baines P. Caselli J.E. Drew M.G. Hoare S.L. Lumsden B. Montesinos S. Sim J.S. Vink H.E. Wheelwright W.J. de Wit 《Astronomische Nachrichten》2011,332(3):238-241
The Herbig Ae/Be stars are intermediate mass pre‐main sequence stars that bridge the gap between the low mass T Tauri stars and the Massive Young Stellar Objects. In this mass range, the acting star forming mechanism switches from magnetically controlled accretion to an as yet unknown mechanism, but which is likely to be direct disk accretion onto the star. We observed a large sample of Herbig Ae/Be stars with X‐shooter to address this issue from a multi‐wavelength perspective. It is the largest such study to date, not only because of the number of objects involved, but also because of the large wavelength coverage from the blue to the near‐infrared. This allows many accretion diagnostics to be studied simultaneously. By correlating the various properties with mass, temperature and age, we aim to determine where and whether the magnetically controlled mass accretion mechanism halts and the proposed direct disk accretion takes over. Here, we will give an overview of the background, present some observations and discuss our initial results. We will introduce a new accretion diagnostic for the research of Herbig Ae/Be stars, the HeI 1.083 μm line (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
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Richard B. Larson 《Monthly notices of the Royal Astronomical Society》2002,332(1):155-164
Nearly all of the initial angular momentum of the matter that goes into each forming star must somehow be removed or redistributed during the formation process. The possible transport mechanisms and the possible fates of the excess angular momentum are discussed, and it is argued that transport processes in discs are probably not sufficient by themselves to solve the angular momentum problem, while tidal interactions with other stars in forming binary or multiple systems are likely to be of very general importance in redistributing angular momentum during the star formation process. Most, if not all, stars probably form in binary or multiple systems, and tidal torques in these systems can transfer much of the angular momentum from the gas around each forming star to the orbital motions of the companion stars. Tidally generated waves in circumstellar discs may contribute to the overall redistribution of angular momentum. Stars may gain much of their mass by tidally triggered bursts of rapid accretion, and these bursts could account for some of the most energetic phenomena of the earliest stages of stellar evolution, such as jet-like outflows. If tidal interactions are indeed of general importance, planet-forming discs may often have a more chaotic and violent early evolution than in standard models, and shock heating events may be common. Interactions in a hierarchy of subgroups may play a role in building up massive stars in clusters and in determining the form of the upper initial mass function (IMF) . Many of the processes discussed here have analogues on galactic scales, and there may be similarities between the formation of massive stars by interaction-driven accretion processes in clusters and the buildup of massive black holes in galactic nuclei. 相似文献
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K. R. Covey T. P. Greene G. W. Doppmann C. J. Lada B. A. Wilking 《Astronomische Nachrichten》2005,326(10):886-890
Recent observations of nearby star forming regions have offered evidence that young brown dwarfs undergo a period of mass accretion analogous to the T Tauri phase observed in young stars. Brown dwarf analogs to stellar protostars, however, have yet to be definitively observed. These young, accreting objects would shed light on the nature of the dominant brown dwarf formation process, as well as provide ideal laboratories to investigate the dependence of the accretion mechanism on protostellar mass. Recent near infrared surveys have identified candidate proto‐brown dwarfs and characterized low mass protostars in nearby star forming regions. These techniques allow near infrared spectra to diagnose the effective temperature, accretion luminosity, magnetic field strength and rotation velocity of young low mass stars across the stellar/substellar boundary. The lowest mass proto‐brown dwarfs (M < 40 MJup), however, will prove challenging to observe given current near IR observational capabilities. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
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A. L. Gyulbudaghian 《Astrophysics》2009,52(2):168-183
The hypothesis advanced by V. A. Ambartsumyan according to which stars are formed from prestellar superdense objects-- protostars--
was an alternative to the hypothesis of the 1950's (and even now, not much changed) according to which stars are formed by
accretion with subsequent collapse (in various modifications). Ambartsumyan's basic inferences were based on an analysis of
the observational data available at that time. This paper presents both Ambartsumyan's pioneering ideas and some modern hypotheses
of star formation. Some results from studies of molecular clouds and star formation regions are also discussed. One of the
distinctive features of young stellar objects (YSO) is the outflow of matter from these objects (molecular, in the form of
jets, etc.), a phenomenon whose importance for the evolution of stars was noted by Ambartsumyan as long ago as 1937. Radial
systems of dark globules are examined, as well as H-H objects associated with star formation regions, cometary nebulae, and
close Trapeziumtype systems (consisting of YSO).
Translated from Astrofizika, Vol. 52, No. 2, pp. 185–202 (May 2009). 相似文献
16.
《New Astronomy》2024
A very well-known property of close binary stars is that they usually rotate slowly than a similar type single star. Massive stars in close binary systems are supposed to experience an exchange of mass and angular momentum via mass transfer and tidal interaction, and thus the evolution of binary stars becomes more complex than that of individual stars. In recent times, it has become clear that a large number of massive stars interact with binary companions before they die. The observation also reveals that in close pairs the rotation tends to be synchronized with the orbital motion and the companions are naturally tempted to invoke tidal friction. We here introduce the effect of tidal angular momentum in the model of wind driven non-conservative mass transfer taking mass accretion rate as uniform with respect to time. To model the angular momentum evolution of a low mass main sequence companion star can be a challenging task. So, to make the present study more interesting, we have considered initial masses of the donor and gainer stars at the proximity of bottom-line main sequence stars and they are taken with lower angular momentum. We have produced a graphical profile of the rate of change of tidal angular momentum and the variation of tidal angular momentum with respect to time under the present consideration. 相似文献
17.
R. Cesaroni 《Astrophysics and Space Science》2008,313(1-3):23-28
The advent of ALMA is bound to improve our knowledge of OB star formation dramatically. Here, we present an overview of this
topic outlining how high angular resolution and sensitivity may contribute to shed light on the structure of high-mass star
forming regions and hence on the process itself of massive star formation. The impact of this new generation instrument will
range from establishing the mass function of pre-stellar cores inside IR-dark clouds, to investigating the kinematics of the
gas from which OB stars are built up, to assessing or ruling out the existence of circumstellar accretion disks in these objects. 相似文献
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S. C. O. Glover P. W. J. L. Brand 《Monthly notices of the Royal Astronomical Society》2001,321(3):385-397
The first star formation in the Universe is expected to take place within small protogalaxies, in which the gas is cooled by molecular hydrogen. However, if massive stars form within these protogalaxies, they may suppress further star formation by photodissociating the H2 . We examine the importance of this effect by estimating the time-scale on which significant H2 is destroyed. We show that photodissociation is significant in the least massive protogalaxies, but becomes less so as the protogalactic mass increases. We also examine the effects of photodissociation on dense clumps of gas within the protogalaxy. We find that while collapse will be inhibited in low-density clumps, denser ones may survive to form stars. 相似文献
19.
Jun'ichi Sato † Masayuki Umemura Keisuke Sawada 《Monthly notices of the Royal Astronomical Society》2008,387(4):1517-1524
We consider the effect of a supernova (SN) explosion in a very massive binary that is expected to form in a portion of Population III stars with the mass higher than 100 M⊙ . In a Population III binary system, a more massive star can result in the formation of a black hole (BH) and a surrounding accretion disc. Such BH accretion could be a significant source of the cosmic reionization in the early Universe. However, a less massive companion star evolves belatedly and eventually undergoes a SN explosion, so that the accretion disc around a BH might be blown off in a lifetime of companion star. In this paper, we explore the dynamical impact of a SN explosion on an accretion disc around a massive BH, and elucidate whether the BH accretion disc is totally demolished or not. For the purpose, we perform three-dimensional hydrodynamic simulations of a very massive binary system, where we assume a BH of 103 M⊙ that results from a direct collapse of a very massive star and a companion star of 100 M⊙ that undergoes a SN explosion. We calculate the remaining mass of a BH accretion disc as a function of time. As a result, it is found that a significant portion of gas disc can survive through three-dimensional geometrical effects even after the SN explosion of a companion star. Even if the SN explosion energy is higher by two orders of magnitude than the binding energy of gas disc, about a half of disc can be left over. The results imply that the Population III BH accretion disc can be a long-lived luminous source, and therefore could be an important ionizing source in the early Universe. 相似文献
20.
Suzan Edwards 《Astrophysics and Space Science》2003,287(1-4):47-57
Energetic outflows provide a dramatic accompaniment to accretion disks in all stages of star formation. The low extinction toward Classical T Tauri stars offers an opportunity to probe the star-disk interface region to search for the launch site and acceleration region of accretion-driven winds. This search is complicated by the fact that the dominant sources of emission in the optical and ultraviolet are the funnel flows and accretion shocks associated with magnetospheric accretion. Thus the quest for inner wind diagnostics requires disentangling accretion and outflow processes from the same line profile. We discuss two tracers of a high velocity inner wind in stars with high disk accretion rates. One, a hot component, is traced by helium emission and must arise very close to the star. A second, cooler component, is traced by blueshifted absorption in strong resonance lines and arises further from the star, but still within about ten stellar radii. We present evidence that the character of both magnetospheric accretion and the inner wind may differ among stars with high and low disk accretion rates. 相似文献