短脉冲激光在实验室天体物理方面的研究进展

随着啁啾脉冲放大技术(Chirped Pulse Amplification,CPA)的飞速发展,激光功率密度实现了飞跃式的提升,利用短脉冲激光开展实验室天体物理研究的条件日趋成熟.短脉冲激光与靶相互作用可以产生相对论粒子(正负电子、质子、中子等)和高能电磁辐射(X射线、γ射线),这些粒子和辐射的产生过程与天体中的某些...     

Type Ia supernovae: An explosion in the regime of a convergent delayed detonation wave

The model of a presupernova’s carbon-oxygen (C-O) core with an initial mass of 1.33 M _⊙, an initial carbon abundance X _C ⁽⁰⁾ =0.27, and a mean rate of increase in mass of 5 × 10^?7 M _⊙ yr^?1 through accretion in a binary system evolved from the central density and temperature ρ_c=10⁹ g cm^?3 and T _c=2.05 × 10⁸K, respectively, by forming a convective core and its subsequent expansion to an explosive fuel ignition at the center. The evolution and explosion equations included only the carbon burning reaction ¹²C+¹²C with energy release corresponding to the complete conversion of carbon and oxygen (at the same rate as that of carbon) into ⁵⁶Ni. The ratio of mixing length to convection-zone size α_c was chosen as the parameter. Although the model assumptions were crude, we obtained an acceptable (for the theory of supernovae) pattern of explosion with a strong dependence of its duration on α_c. In our calculations with sufficiently large values of this parameter, α_c=4.0 × 10^?3 and 3.0×10^?3, fuel burned in the regime of prompt detonation. In the range 2.0×10^?3≥α_c≥3.0×10^?4, there was initially a deflagration with the generation of model pulsations whose amplitude gradually increased. Eventually, the detonation regime of burning arose, which was triggered from the model surface layers (with m ? 1.33 M _⊙) and propagated deep into the model up to the deflagration front. The generation of model pulsations and the formation of a detonation front are described in detail for α_c=1.0 × 10^?3.     

Supersonic steady accretion of a rotating cold “Iron Gas” onto a neutron star after gravitational collapse

We analytically generalize the well-known solution of steady supersonic spherically symmetric gas accretion onto a star (Bondi 1952) for an iron atmosphere with completely degenerate electrons with an arbitrary degree of relativity. This solution is used for typical physical conditions in the vicinity of protoneutron stars produced by gravitational collapse with masses M ₀=(1.4?1.8)M _⊙ and over a wide range of nonzero “iron gas” densities at infinity, ρ_∞=(10⁴?5×10⁶)g cm^?3. Under these conditions, we determine all accretion parameters, including the accretion rate, whose value is ～(10?50)M _⊙s^?1 at M ₀=1.8M _⊙ (it is a factor of 1.7 lower for M ₀=1.4M _⊙, because the accretion rate is exactly ∝M ₀ ² ). We take into account the effect of accreting-gas rotation in a quasi-one-dimensional approximation, which has generally proved to be marginal with respect to the accretion rate.     

The hydrogen emission spectrum of a shock wave in a stellar atmosphere

Based on a self-consistent solution of the equations of gas dynamics, kinetics of hydrogen atomic level populations, and radiative transfer, we analyze the structure of a shock wave that propagates in a partially ionized hydrogen gas. We consider the radiative transfer at the frequencies of spectral lines by taking into account the effects of a moving medium in the observer's frame of reference. The flux in Balmer lines is shown to be formed behind the shock discontinuity at the initial hydrogen recombination stage. The Doppler shift of the emission-line profile is approximately one and a half times smaller than the gas flow velocity in the Balmer emission region, because the radiation field of the shock wave is anisotropic. At Mach numbers M₁?10 and unperturbed gas densities σ₁=10^?10 g cm^?3, the Doppler shift is approximately one third of the shock velocity U₁. The FWHM of the emission-line profile δ _? is related to the shock velocity by δ _? ≈ k _? U₁, where k _? = 1, 0.6, and 0.65 for the Hα, Hβ, and Hγ lines, respectively.     

Ionospheric measurements of relative coronal brightness during the total solar eclipses of 11 August, 1999 and 9 July, 1945

Swept-frequency (1/10 MHz) ionosonde measurements were made at Helston, Cornwall (50°06N, 5°18W) during the total solar eclipse on August 11, 1999. Soundings were made every three minutes. We present a method for estimating the percentage of the ionising solar radiation which remains unobscured at any time during the eclipse by comparing the variation of the ionospheric E-layer with the behaviour of the layer during a control day. Application to the ionosonde date for 11 August, 1999, shows that the flux of solar ionising radiation fell to a minimum of 25±2% of the value before and after the eclipse. For comparison, the same technique was also applied to measurements made during the total solar eclipse of 9 July, 1945, at Sörmjöle (63°68N, 20°20E) and yielded a corresponding minimum of 16 ± 2%. Therefore the method can detect variations in the fraction of solar emissions that originate from the unobscured corona and chromosphere. We discuss the differences between these two eclipses in terms of the nature of the eclipse, short-term fluctuations, the sunspot cycle and the recently-discovered long-term change in the coronal magnetic field.     

天体物理环境中的硅酸盐尘粒

硅酸盐尘粒是宇宙尘埃的主要成分之一,它广泛存在于许多天体物理环境中,其特性随环境而变化。由于近年来观测数据的不断增加和红外光谱质量的逐步提高,宇宙空间中的硅酸盐尘粒正受到越来越多的关注．该文详细地介绍了在各种天体环境(星际空间、演化晚期恒星的星周尘埃包层、绕年轻恒星和主序星的星周尘埃盘、彗星的彗发和行星际空间)中的硅酸盐尘粒的观测特征,并分别对其物理和化学性质进行了综合比较．观测已经证实在星际尘埃演化的前身(演化晚期恒星的星周尘埃包层)和其遗迹(彗星)均有可观数量的结晶硅酸盐存在。但是至今还没有在其中间态(弥散星际介质)找到结晶硅酸盐存在的证据。这一尚未解决的难题突出了结晶态硅酸盐在天体物理研究中的重要意义。     

星系团的分层效应

对星系团各类分层效应的有关问题做了概要的评述,包括成员星系在位置空间和(或)速度空间中的形态分层、光度(质量)分层和元素丰度分层的表现形式和探测途径,分层效应可能的形成机制及其对星系和星系团的结构和演化的影响。     

Laboratory Exploration of Solar Energetic Phenomena

The solar atmosphere displays a wide variety of dynamic phenomena driven by the interaction of magnetic fields and plasma. In particular, plasma jets in the solar chromosphere and corona, coronal heating, solar flares and coronal mass ejections all point to the presence of magnetic phenomena such as reconnection, flux cancellation, the formation of magnetic islands, and plasmoids. While we can observe the signatures and gross features of such phenomena we cannot probe the essential physics driving them, given the spatial resolution of current instrumentation. Flexible and well-controlled laboratory experiments, scaled to solar parameters, open unique opportunities to reproduce the relevant unsteady phenomena under various simulated solar conditions. The ability to carefully control these parameters in the laboratory allows one to diagnose the dynamical processes which occur and to apply the knowledge gained to the understanding of similar processes on the Sun, in addition directing future solar observations and models. This talk introduces the solar phenomena and reviews the contributions made by laboratory experimentation.     

Simulating Poynting Flux Acceleration in the Laboratory with Colliding Laser Pulses

We review recent PIC simulation results which show that double-sided irradiation of a thin over-dense plasma slab with ultra-intense laser pulses from both sides can lead to sustained comoving Poynting flux acceleration of electrons to energies much higher than the conventional ponderomotive limit. The result is a robust power-law electron momentum spectrum similar to astrophysical sources. We discuss future ultra-intense laser experiments that may be used to simulate astrophysical particle acceleration.     

黑洞吸积理论及其天体物理学应用的近期发展(Ⅱ)

黑洞吸积理论是天体物理学的一个基础理论,是认识许多高能天体系统如活动星系核、黑洞X射线双星,以及伽马暴等的重要物理基础.该文评述近年来黑洞吸积理论尤其是径移主导吸积流模型(advection-dominated accretion flow)及其变种的主要发展,并介绍该理论在银河系中心、低光度活动星系核、黑洞X射线双星等方面的应用.共分为两篇,该文是第2篇,内容是关于黑洞热吸积流理论在低光度活动星系核以及黑洞X射线双星方面的应用.