Interacting ellipsoids: a minimal model for the dynamics of rubble-pile bodies     

F. Roig  R. Duffard  D. Lazzaro 《Icarus》2003,165(2):355-370

A simple mechanical model is formulated to study the dynamics of rubble-pile asteroids, formed by the gravitational re-accumulation of fragments after the collisional breakup of a parent body. In this model, a rubble-pile consists of N interacting fragments represented by rigid ellipsoids, and the equations of motion explicitly incorporate the minimal degrees of freedom necessary to describe the attitude and rotational state of each fragment. In spite of its simplicity, our numerical examples indicate that the overall behavior of our model is in line with several known properties of collisional events, like the energy and angular momentum partition during high velocity impacts. Therefore, it may be considered as a well defined minimal model.  相似文献   

The Kα complex of He-like iron with dielectronic satellites     

Justin Oelgoetz  Anil K. Pradhan 《Monthly notices of the Royal Astronomical Society》2001,327(4):L42-L46

It is shown that the dielectronic satellites (DES) dominate X-ray spectral formation in the 6.7-keV K α complex of Fe  xxv at temperatures below that of maximum abundance in collisional ionization equilibrium T _m. Owing to their extreme temperature sensitivity, the DES are excellent spectral diagnostics for     in photoionized, collisional or hybrid plasmas; whereas the forbidden, intercombination and resonance lines of Fe  xxv are not. A diagnostic line ratio GD ( T ) is defined including the DES and the lines, with parameters from new relativistic atomic calculations. The DES absorption resonance strengths may be obtained from differential oscillator strengths, possibly to yield the     column densities. The DES contribution to highly ionized Fe should be of interest for models of redward broadening of K α features, ionized accretion discs, accretion flows and K α temporal-temperature variability in AGN.  相似文献   

Sextet transitions in Fe ii     

M. P. Donnelly  A. Hibbert 《Monthly notices of the Royal Astronomical Society》2001,321(2):247-253

We have calculated oscillator strengths for the J – J ' levels of the a  ⁶D– z  ⁶P^o, the a  ⁶D– z  ⁶D^o and the a  ⁶D– z  ⁶F^o multiplets in Fe  ii , using configuration interaction wave functions. The importance of the mixing of the ⁶P^o, ⁶D^o and ⁶F^o states is shown, as is the effect of core polarization of both the 3 p and 3 s orbitals. Good agreement with recent experimental results is obtained.  相似文献   

Dust and hydrogen molecules in the extremely metal-poor dwarf galaxy SBS 0335–052     

HiroyukiHirashita †  Leslie K.Hunt  reaFerrara 《Monthly notices of the Royal Astronomical Society》2002,330(1):L19-L23

During the early stages of galaxy evolution, the metallicity is generally low and nearby metal-poor star-forming galaxies may provide templates for primordial star formation. In particular, the dust content of such objects is of great importance, because early molecular formation can take place on grains. To gain insight into primeval galaxies at high redshift, we examine the dust content of the nearby extremely low-metallicity galaxy SBS  0335–052  which hosts a very young starburst (≲10⁷ yr). In young galaxies, the dust formation rate in Type II supernovae governs the amount of dust, and by incorporating recent results on dust production in Type II supernovae we model the evolution of dust content. If the star-forming region is compact (≲100 pc), as suggested by observations of SBS  0335–052  , our models consistently explain the quantity of dust, far-infrared luminosity, and dust temperature in this low-metallicity object. We also discuss the H₂ abundance. The compactness of the region is important to H₂ formation, because the optical depth of dust for UV photons becomes large and H₂ dissociation is suppressed. We finally focus on implications for damped Ly α systems.  相似文献   

Silicon monosulphide radiative association     

C. M. Andreazza  E. P. Marinho 《Monthly notices of the Royal Astronomical Society》2007,380(1):365-368

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Radiative feedback from an early X-ray background     

S. C. O. Glover  P. W. J. L. Brand 《Monthly notices of the Royal Astronomical Society》2003,340(1):210-226

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The Rôle Of Astrochemistry In Low Mass Star Formation     

J. Rawlings 《Astrophysics and Space Science》2003,285(3-4):777-789

Molecular processes play both active and passive/diagnostic rôles in the process of star formation. Various molecular behaviours can be identified in star-forming regions with the result that different molecular species can be used to constain different aspects of the infall process, such as the density structures, the kinematics and the evolutionary history of star-forming cores. The main limitations in the chemical analysis of infall sources arise from poorly constrained boundary conditions; in particular the chemical and physical initial conditions are usually very uncertain. The most promising application of astrochemical modelling is probably in the analysis of the infall dynamics through combined chemical/radiative transfer modelling of molecular emission line profiles.  相似文献   

Electron impact excitation of S-like iron     

Kanti M. Aggarwal  Francis P. Keenan 《Monthly notices of the Royal Astronomical Society》2003,338(2):412-424

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膨胀石墨化学氧化法制备技术研究进展   总被引：3，自引：0，他引：3  

刘开平  周敬恩 《地球科学与环境学报》2003,25(4):85-91

膨胀石墨是石墨的重要深加工产品之一.目前膨胀石墨工业生产用的最多和最成熟的方法是化学氧化法,具有很多优点.由于这种工艺生产过程存在环境污染,产品在应用中对设备有腐蚀,高温抗氧化性及强度不高等原因,限制了其进一步发展.介绍了近年来围绕化学氧化法制备工艺及产品性能改进问题国内外研究的主要进展.讨论了利用低品位和细鳞片石墨原料,降低或消除生产过程中的NOX、重金属离子的污染,降低产品的含硫量和腐蚀性,提高产品的膨胀倍数、抗氧化性和强度,改进产品的膨化工艺等方面的技术措施及应用效果.指出了化学氧化法进一步发展的方向和应用前景.  相似文献   

Detection of sporadic impact flashes on the Moon: Implications for the luminous efficiency of hypervelocity impacts and derived terrestrial impact rates     

J.L. Ortiz  F.J. Aceituno  J. Aceituno  P. Santos-Sanz  J. Llorca  F.J. Martín-Torres  E. Pallé 《Icarus》2006,184(2):319-326

We present the first redundant detection of sporadic impact flashes on the Moon from a systematic survey performed between 2001 and 2004. Our wide-field lunar monitoring allows us to estimate the impact rate of large meteoroids on the Moon as a function of the luminous energy received on Earth. It also shows that some historical well-documented mysterious lunar events fit in a clear impact context. Using these data and traditional values of the luminous efficiency for this kind of event we obtain that the impact rate on Earth of large meteoroids (0.1-10 m) would be at least one order of magnitude larger than currently thought. This discrepancy indicates that the luminous efficiency of the hypervelocity impacts is higher than 10⁻², much larger than the common belief, or the latest impact fluxes are somewhat too low, or, most likely, a combination of both. Our nominal analysis implies that on Earth, collisions of bodies with masses larger than 1 kg can be as frequent as 80,000 per year and blasts larger than 15-kton could be as frequent as one per year, but this is highly dependent on the exact choice of the luminous efficiency value. As a direct application of our results, we expect that the impact flash of the SMART-1 spacecraft should be detectable from Earth with medium-sized telescopes.  相似文献