Jet Deflection by a Quasi-Steady-State Side Wind in the Laboratory     

David J. Ampleford  Andrea Ciardi  Sergey V. Lebedev  Simon N. Bland  Simon C. Bott  Jeremy P. Chittenden  Gareth N. Hall  Adam Frank  Eric Blackman 《Astrophysics and Space Science》2007,307(1-3):29-34

We present experimental data on the steady state deflection of a highly supersonic jet by a side-wind in the laboratory. The use of a long interaction region enables internal shocks to fully cross the jet, leading to the development of significantly more structure in the jet than in previous work with a similar setup (Lebedev et al., 2004). The ability to control the length of the interaction region in the laboratory allows the switch between a regime representing a clumpy jet or wind and a regime similar to a slowly varying mass loss rate. The results indicate that multiple internal oblique shocks develop in the jet and the possible formation of a second working surface as the jet attempts to tunnel through the ambient medium.  相似文献   

Direct Propagation of Photospheric Acoustic <Emphasis Type="Italic">p</Emphasis> Modes into Nonmagnetic Solar Atmosphere     

C. Malins  R. Erdélyi 《Solar physics》2007,246(1):41-52

Solar p modes are one of the dominant types of coherent signals in Doppler velocity in the solar photosphere, with periods showing a power peak at five minutes. The propagation (or leakage) of these p-mode signals into the higher solar atmosphere is one of the key drivers of oscillatory motions in the higher solar chromosphere and corona. This paper examines numerically the direct propagation of acoustic waves driven harmonically at the photosphere, into the nonmagnetic solar atmosphere. Erdélyi et al. (Astron. Astrophys. 467, 1299, 2007) investigated the acoustic response to a single point-source driver. In the follow-up work here we generalise this previous study to more structured, coherent, photospheric drivers mimicking solar global oscillations. When our atmosphere is driven with a pair of point drivers separated in space, reflection at the transition region causes cavity oscillations in the lower chromosphere, and amplification and cavity resonance of waves at the transition region generate strong surface oscillations. When driven with a widely horizontally coherent velocity signal, cavity modes are caused in the chromosphere, surface waves occur at the transition region, and fine structures are generated extending from a dynamic transition region into the lower corona, even in the absence of a magnetic field.  相似文献   

High-performance direct gravitational N-body simulations on graphics processing units     

Simon F. Portegies Zwart  Robert G. Belleman  Peter M. Geldof 《New Astronomy》2007,12(8):641-650

We present the results of gravitational direct N-body simulations using the commercial graphics processing units (GPU) NVIDIA Quadro FX1400 and GeForce 8800GTX, and compare the results with GRAPE-6Af special purpose hardware. The force evaluation of the N-body problem was implemented in Cg using the GPU directly to speed-up the calculations. The integration of the equations of motions were, running on the host computer, implemented in C using the 4th order predictor–corrector Hermite integrator with block time steps. We find that for a large number of particles (N  10⁴) modern graphics processing units offer an attractive low cost alternative to GRAPE special purpose hardware. A modern GPU continues to give a relatively flat scaling with the number of particles, comparable to that of the GRAPE. The GRAPE is designed to reach double precision, whereas the GPU is intrinsically single-precision. For relatively large time steps, the total energy of the N-body system was conserved better than to one in 10⁶ on the GPU, which is impressive given the single-precision nature of the GPU. For the same time steps, the GRAPE gave somewhat more accurate results, by about an order of magnitude. However, smaller time steps allowed more energy accuracy on the grape, around 10⁻¹¹, whereas for the GPU machine precision saturates around 10⁻⁶ For N  10⁶ the GeForce 8800GTX was about 20 times faster than the host computer. Though still about a factor of a few slower than GRAPE, modern GPUs outperform GRAPE in their low cost, long mean time between failure and the much larger onboard memory; the GRAPE-6Af holds at most 256k particles whereas the GeForce 8800GTX can hold 9 million particles in memory.  相似文献   

Orbit of Comet C/1845 D1 (de Vico) and the utility of Bulirsch-Stoer integration     

Richard L. Branham Jr. 《Planetary and Space Science》2010,58(10):1307-1313

Comet C/1845 D1 (de Vico) is one of a large number of comets with parabolic orbits. Given that there are sufficient observations of the comet, 291 in right ascension and 269 in declination, it proves possible to calculate a better orbit. The calculations are based on a 12th order predictor-corrector method. The comet's orbit is hyperbolic and, from calculated mean errors, statistically different from a parabola. Bulirsch-Stoer integration, although more flexible and of comparable precision, proves considerably more laborious.  相似文献   

On the peculiarities in the rotational frequency evolution of isolated neutron stars     

Anton Biryukov  Gregory Beskin  Sergey Karpov 《Astrophysics and Space Science》2007,308(1-4):551-555

The measurements of pulsar frequency second derivatives have shown that they are 10²−10⁶ times larger than expected for standard pulsar spin-down law, and are even negative for about half of pulsars. We explain these paradoxical results on the basis of the statistical analysis of the rotational parameters ν, and of the subset of 295 pulsars taken mostly from the ATNF database. We have found a strong correlation between and for both and , as well as between ν and . We interpret these dependencies as evolutionary ones due to being nearly proportional to the pulsars’ age. The derived statistical relations as well as “anomalous” values of are well described by assuming the long-time variations of the spin-down rate. The pulsar frequency evolution, therefore, consists of secular change of ν _ev(t), and according to the power law with n≈5, the irregularities, observed within a timespan as a timing noise, and the variations on the timescale larger than that—several decades. This work has been supported by the Russian Foundation for Basic Research (grant No 04-02-17555), Russian Academy of Sciences (program “Evolution of Stars and Galaxies”), and by the Russian Science Support Foundation. The authors would also like to thank the anonymous referee for valuable comments.  相似文献   

徐州市地质灾害防灾减灾方法研究     

陈万利 《江苏地质》2014,38(1):165-168

以徐州市地质灾害详细调查资料为依据,总结了徐州市地质灾害的主要类型,归纳分析了徐州市地质灾害形成的地质环境条件、诱发因素、发育现状及造成的损失。根据近年来省内外尤其是徐州市地质灾害调查评价和邳州石膏矿区地面塌陷现状及预防措施的建议,结合徐州市地质灾害隐患的具体特点,提出了加强地质灾害防治的有关制度建设、宣传培训工作、科学部署地质灾害防治、监测工程、强化建设项目的地质灾害危险性评估等地质灾害防灾减灾措施。  相似文献   

Bayesian foreground analysis with CMB data     

H.K. Eriksen  C. Dickinson  C.R. Lawrence  C. Baccigalupi  A.J. Banday  K.M. Grski  F.K. Hansen  E. Pierpaoli  M.D. Seiffert 《New Astronomy Reviews》2006,50(11-12):861

The quality of CMB observations has improved dramatically in the last few years, and will continue to do so in the coming decade. Over a wide range of angular scales, the uncertainty due to instrumental noise is now small compared to the cosmic variance. One may claim with some justification that we have entered the era of precision CMB cosmology. However, some caution is still warranted: The errors due to residual foreground contamination in the CMB power spectrum and cosmological parameters remain largely unquantified, and the effect of these errors on important cosmological parameters such as the optical depth τ and spectral index n_s is not obvious. A major goal for current CMB analysis efforts must therefore be to develop methods that allow us to propagate such uncertainties from the raw data through to the final products. Here we review a recently proposed method that may be a first step towards that goal.  相似文献   

星系NGC5018的星族合成研究     

孔旭  张文浩  李成  程福臻  A.Weiss 《天文学报》2002,43(3):264-271

利用星团谱样本的星族合成方法,研究了邻近巨椭圆星系NGC5018中的星族成分和其内部的恒星形成历史,给出了星系中不同年龄和金属丰度星族的成分占有比.星族合成结果表明,NGC5018中不仅存在大量金属丰度低的年老恒星成分,而且较年轻的星族成分(T=5×108yr)对星系光度贡献也很重要.星系吞并和相互作用过程可能是触发这些较年轻星族形成的物理原因,椭圆星系内部的恒星形成历史可能是2次爆发或者多次爆发过程.这些结果可以很好地解释NGC5018颜色偏蓝、Mg2谱指数强度偏弱等观测特征.  相似文献   

Propagation in 3D spiral-arm cosmic-ray source distribution models and secondary particle production using Picard     

《Astroparticle Physics》2015

We study the impact of possible spiral-arm distributions of Galactic cosmic-ray sources on the flux of various cosmic-ray nuclei throughout our Galaxy. We investigate model cosmic-ray spectra at the nominal position of the sun and at different positions within the Galaxy. The modelling is performed using the recently introduced numerical cosmic ray propagation code Picard. Assuming non-axisymmetric cosmic-ray source distributions yields new insights on the behaviour of primary versus secondary nuclei.We find that primary cosmic rays are more strongly confined to the vicinity of the sources, while the distribution of secondary cosmic rays is much more homogeneous compared to the primaries. This leads to stronger spatial variation in secondary to primary ratios when compared to axisymmetric source distribution models. A good fit to the cosmic-ray data at Earth can be accomplished in different spiral-arm models, although leading to decisively different spatial distributions of the cosmic-ray flux. These lead to different cosmic ray anisotropies, where even reproducing the data becomes possible. Consequently, we advocate directions to seek best fit propagation parameters that take into account the higher complexity introduced by the spiral-arm structure on the cosmic-ray distribution. We specifically investigate whether the flux at Earth is representative for a large fraction of the Galaxy. The variance among possible spiral-arm models allows us to quantify the spatial variation of the cosmic-ray flux within the Galaxy in presence of non-axisymmetric source distributions.  相似文献   

巨分子云聚合形成N体模拟中粒子数的效应     

张同杰  宋国玄  杨志良  陈黎  张保洲  李楠  崔建华 《天文学报》2002,43(2):131-139

通过N体数值模拟以不同粒子数对星系中巨分子云的形成的影响进行了研究。结果表明：在聚合形成机制下，当对不同数目的分子云取相同的平均密度时，基本分子云的有效半径与其相应质量的立方根成正比，巨分子云的碎裂率与模拟基本分子云的数目无关。  相似文献