Geometrodynamical distances to the Galaxy's hydrogen streams     

Shoko Jin  D. Lynden-Bell 《Monthly notices of the Royal Astronomical Society》2008,383(4):1686-1694

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AMI limits on 15-GHz excess emission in northern H ii regions     

A. M. M. Scaife  N. Hurley-Walker  M. L. Davies  P. J. Duffett-Smith  F. Feroz  K. J. B. Grainge  D. A. Green  M. P. Hobson  T. Kaneko  A. N. Lasenby  G. G. Pooley  R. D. E. Saunders  P. F. Scott  D. J. Titterington  E. M. Waldram  J. Zwart 《Monthly notices of the Royal Astronomical Society》2008,385(2):809-822

We present observations between 14.2 and 17.9 GHz of 16 Galactic H  ii regions made with the Arcminute Microkelvin Imager. In conjunction with data from the literature at lower radio frequencies we investigate the possibility of a spinning dust component in the spectra of these objects. We conclude that there is no significant evidence for spinning dust towards these sources and measure an average spectral index of  α= 0.15 ± 0.07  (where   S ∝ν^−α  ) between 1.4 and 17.9 GHz for the sample.  相似文献   

河北一次层状云系降水的微物理机制数值模拟与分析   总被引：1，自引：0，他引：1  

王秀娟  姜忠宝  杨洁帆  夏葳  张超 《气象与环境学报》2014,30(1):1-7

利用一维层状云模式,详细分析了2009年5月1日中国中东部地区一次层状云系的微物理结构和降水过程。结果表明：此次降水为层状云系降水,云系垂直结构符合顾震潮三层概念模型和“播种云-供给云”机制,其中第一层（上层：4.7-7.0 km）存在冰雪晶,雪主要通过冰晶自动转化和凝华增长。第二层（中层：2.6-4.6 km）有冰晶、雪、霰、云水、雨滴,此层贝吉龙过程作用明显。第三层（下层：1.3-2.5 km）主要粒子为云滴、雨滴、从上层融化的雪和霰,霰的融化对于雨滴的形成贡献最大。云体发展成熟时,各层之间存在一定的播种－供应关系,如第一层向第二层顶部播撒雪和冰晶,第二层向第三层顶部播撒霰和雪。  相似文献   

东北冷涡积层混合云系形成条件的个例分析          下载免费PDF全文

王涛  齐彦斌  王东  王贵军 《气象与环境学报》2010,26(5):7-12

利用2003年7月7—8日NCEP/NCAR资料和地面高空常规探测资料,利用客观分析的方法,研究了东北冷涡积层混合云系形成的环境条件。结果表明：这一冷涡天气系统是由北部和南部两个低压系统组成,而且均比较深厚,有明显低温区配合。东北地区的降水主要受南部的低压系统影响。该系统有气旋性环流配合,在气旋中心区、气流辐合区和气旋东南侧西南气流中均存在相对湿度高于80%的湿度区。湿度区中含有湿度高于90%的区域,积层混合云系产生在这个区域内,而且降水区与系统的不稳定区和动力场辐合区配合一致。研究表明,东北冷涡天气系统中积层混合云系是在冷涡系统东南部的西南气流中形成的,水汽输送条件较好,而且有高湿不稳定区配合,对研究其生成和发展有指示意义。  相似文献   

降水性层状云系中过冷水的分布特征     

王俊 《山东气象》2003,23(2):9-11

统计分析了降水性层状云系中过冷水出现的频率、过冷水区尺度、云水含量的分布特征,以及云中总的粒子尺度谱,为人工增雨作业提供重要依据。  相似文献   

H i and optical spectroscopy towards the M15 intermediate-velocity cloud     

J. V. Smoker  N. Lehner  F. P. Keenan  E. J. Totten  E. Murphy  K. R. Sembach  R. D. Davies  B. Bates 《Monthly notices of the Royal Astronomical Society》2001,322(1):13-21

We present single-dish Arecibo 21-cm H  i observations, covering a 0675×0625 RA–Dec. grid, of the intermediate-velocity cloud (IVC) centred upon the M15 globular cluster. The velocity and positional structure of the IVC gas at V _LSR=70 km s⁻¹ are investigated; it is found to be clumpy and has a peak surface density N _{H  i} ∼8×10¹⁹ cm⁻². Additionally, we have performed a long H  i integration towards HD 203664, a Galactic halo star some 31 from M15, in which optical IVC absorption has previously been detected. No H  i with a velocity exceeding 60 km s⁻¹ was found to a brightness temperature limit of 0.05 K. However, additional pointings did detect IVC gas approximately mid-way between HD 203664 and M15. Finally, we present both Arecibo H  i pointings and low-resolution spectra in the Ca  ii H and K lines towards 15 field stars in the general field towards M15, in an attempt to obtain the distance to the IVC. Intermediate-velocity H  i is detected towards seven sightlines. Stellar spectral types are derived for 12 of the sample. Assuming that these stars lie on the main sequence, their distances are estimated to lie in the range 150≤ d ≤1350 pc. No Ca  ii absorption is observed, either because the IVC is further away than ∼1350 pc or more likely because the gas along these sightlines is of too low a density to be detected by the current observations.  相似文献   

Star formation in unbound giant molecular clouds: the origin of OB associations?     

Paul C. Clark  Ian A. Bonnell  Hans Zinnecker  Matthew R. Bate 《Monthly notices of the Royal Astronomical Society》2005,359(3):809-818

We investigate the formation of star clusters in an unbound giant molecular cloud, where the supporting kinetic energy is twice as large as the cloud's self-gravity. This cloud manages to form a series of star clusters and disperse, all within roughly two crossing times (10 Myr), supporting recent claims that star formation is a rapid process. Simple assumptions about the nature of the star formation occurring in the clusters allows us to place an estimate for the star formation efficiency at about 5–10 per cent, consistent with observations. We also propose that unbound clouds can act as a mechanism for forming OB associations. The clusters that form in the cloud behave as OB subgroups. These clusters are naturally expanding from one another due to the unbound nature of the flows that create them. The properties of the cloud we present here are consistent with those of classic OB associations.  相似文献   

Dense cores in the L1630 molecular cloud: discovering new protostars with SCUBA     

Robin R. Phillips †  y G. Gibb ‡  Leslie T. Little 《Monthly notices of the Royal Astronomical Society》2001,326(3):927-936

Maps of the 450- and 850-μm dust continuum emission from three star-forming condensations within the Lynds 1630 molecular cloud, made with the SCUBA bolometer array, reveal the presence of four new submillimetre sources, each of a few solar masses (two of which are probably class I and two of which are class 0), as well as several sources the existence of which was previously known. The sources are located in filaments and appear elongated when observed at 450 μm. They probably have dust temperatures in the range 10 to 20 K, in good agreement with previous ammonia temperature estimates. Attempts to fit their structures with power-law and Gaussian density distributions suggest that the central distribution is flatter than expected for a simple singular isothermal sphere.
Although the statistics are poor, our results suggest that the ratio of 'protostellar core' mass to total virial mass may be similar for both large and small condensations.  相似文献   

Protostellar evolution during time-dependent anisotropic collapse     

Mahmoud Aburihan †  Jason D. Fiege  Richard N. Henriksen  Thibaut Lery 《Monthly notices of the Royal Astronomical Society》2001,326(4):1217-1227

The formation and collapse of a protostar involves the simultaneous infall and outflow of material in the presence of magnetic fields, self-gravity and rotation. We use self-similar techniques to self-consistently model the anisotropic collapse and outflow by using a set of angle-separated self-similar equations. The outflow is quite strong in our model, with the velocity increasing in proportion to radius, and material formally escaping to infinity in the finite time is required for the central singularity to develop.
Analytically tractable collapse models have been limited mainly to spherically symmetric collapse, with neither magnetic field nor rotation. Other analyses usually employ extensive numerical simulations, or either perturbative or quasistatic techniques. Our model is unique as an exact solution to the non-stationary equations of self-gravitating magnetohydrodynamics (MHD), which features co-existing regions of infall and outflow.
The velocity and magnetic topology of our model is quadrupolar, although dipolar solutions may also exist. We provide a qualitative model for the origin and subsequent evolution of such a state. However, a central singularity forms at late times, and we expect the late-time behaviour to be dominated by the singularity, rather than depend on the details of its initial state. Our solution may, therefore, have the character of an attractor among a much more general class of self-similarity.  相似文献   

Interstellar chemistry recorded by nitrogen isotopes in Solar System organic matter     

Jérôme Aléon  François Robert 《Icarus》2004,167(2):424-430

The relationship between the organic and D/H ratios in small Solar System bodies (meteorites, interplanetary dust and comets) suggests that isotopic exchange reactions taking place at various temperatures are at the origin of the observed variations. These relationships are used to determine the exothermicity (ΔE) of ion-molecule reactions that fractionated the nitrogen isotopic ratio in the presolar molecular cloud; that is ΔE=43±10 K. Comparison with current models of interstellar chemistry suggests that such a value could be achieved by condensation of ¹⁵N-rich gas-phase precursors onto grain surfaces and their further isolation from the gas by incorporation into large macromolecular structures.  相似文献