The accelerated Kepler problem

The accelerated Kepler problem (AKP) is obtained by adding a constant acceleration to the classical two-body Kepler problem. This setting models the dynamics of a jet-sustaining accretion disk and its content of forming planets as the disk loses linear momentum through the asymmetric jet-counterjet system it powers. The dynamics of the accelerated Kepler problem is analyzed using physical as well as parabolic coordinates. The latter naturally separate the problem’s Hamiltonian into two unidimensional Hamiltonians. In particular, we identify the origin of the secular resonance in the AKP and determine analytically the radius of stability boundary of initially circular orbits that are of particular interest to the problem of radial migration in binary systems as well as to the truncation of accretion disks through stellar jet acceleration.     

The stellar-disk electric (short) circuit: observational predictions for a YSO jet flow

We discuss the star-disk electric circuit for a young stellar object (YSO) and calculate the expected torques on the star and the disk. We obtain the same disk magnetic field and star-disk torques as given by standard magnetohydrodynamic (MHD) analysis. We show how a short circuit in the star-disk electric circuit may produce a magnetically-driven jet flow from the inner edge of a disk surrounding a young star. An unsteady bipolar jet flow is produced that flows perpendicular to the disk plane. Jet speeds of order hundreds of kilometers per second are possible, while the outflow mass loss rate is proportional to the mass accretion rate and is a function of the disk inner radius relative to the disk co-rotation radius.     

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

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.     

Absolute Magnitudes of Be Stars Based on Hipparcos Parallaxes

The absolute visual magnitudes of 457 Be stars are determined from Hipparcos parallaxes, subsequently the mean absolute visual magnitudes of Be stars for luminosity classes III, IV and V are obtained. The new Mv calibration is compared with existing calibrations. It is found that Be stars are generally brighter than B stars of the corresponding spectral types, and that there is no direct correlation between absolute magnitudes and the stellar rotational velocity, these results are in agreement with some earlier results. A new and interesting result is that there is no direct correlation between near infrared excess and absolute visual magnitudes for Be stars. Moreover, possible biases, such as the Malmquist bias and the Lutz–Kelker bias, are also discussed.     

新四面体理论与磁极倒转

本文以最新修正了的新四面体理论模型来解释地球磁极倒转现象及其规律性。文章简要回顾了依附于收缩说的地球四面体理论的兴衰,接着简述了应归属于脉动说范畴的新四面体理论研究的最新进展——地球多级驻波(式)脉动演化模型。新模型可概括为:地球的形态在演化过程中呈“准球体→负准四面体→准球体→正准四面体→准球体→…”的驻波脉动;在呈每种形体的大阶段中,又包含着次级、更次级的驻波脉动。然后论述了地磁极性倒转的规律性(即多变→固定[正向]→多变→固定[反向] →多变→…)以及新模型对其所做的解释(即固定正向和固定反向阶段分别为负和正准四面体阶段,多变阶段为准球体阶段)。     

Differential rotation of some HK-Project stars and the butterfly diagrams

We analyze the long-term variability of the chromospheric radiation of 20 stars monitored in the course of the HK-Project at the Mount Wilson Observatory. We apply the modified wavelet algorithm for this set of gapped time series. Besides the mean rotational periods for all these stars, we find reliable changes of the rotational periods from year to year for a few stars. Epochs of slower rotation occur when the activity level of the star is high, and the relationship repeats again during the next maximum of an activity cycle. Such an effect is traced in two stars with activity cycles that are not perfectly regular (but labeled “Good” under the classification in [Baliunas, S.L., Donahue, R.A., Soon, W.H., Horne, J.H., Frazer, J., Woodard-Eklund, L., Bradford, M., Rao, L.M., Wilson, O.C., Zhang, Q. et al., 1995. ApJ 438, 269.]) but the two stars have mean activity levels exceed that of the Sun. The averaged rotational period of HD 115404 is 18.5 days but sometimes the period increases up to 21.5 days. The sign of the differential rotation is the same as the Sun’s, and the value ΔΩ/Ω=-0.14. For the star HD 149661, this ratio is −0.074. Characteristic changes of rotational periods occur over around three years when the amplitude of the rotational modulation is large. These changes can be transformed into latitude-time butterfly diagrams with minimal a priori assumptions. We compare these results with those for the Sun as a star and conclude that epochs when surface inhomogeneities rotate slower are synchronous with the reversal of the global magnetic dipole.     

Stellar and extragalactic radiation at the Earth’s surface

Reviving a calculation made by Eddington in the 1920s, and using the most recent and comprehensive databases available on stars and galaxies, including more than 2,500,000 stars and around 20,000 galaxies we have computed their total radiation received at the Earth just outside its atmosphere. This radiation density, if thermalized, would be equivalent to a temperature of 4.212 K. The comparability of this temperature to that of the cosmic microwave background (2.723 K) may either be a pure coincidence or may hold a key to some as yet unknown, aspect of the universe.     

恒星内部电荷屏蔽对电子俘获反应的影响

本文分析了在恒星环境下，电荷强屏蔽对原子核电子俘获反应的影响，分析中忽略了电荷屏蔽对核跃迁矩阵元的影响．作为例子，分别讨论了电荷屏蔽对核素33S、30P和57Fe的电子俘获率的影响．结果表明：在较低温度和较高密度下，由于电荷屏蔽，其电子俘获率相对于无屏蔽情形有明显降低，这可能对恒星晚期演化和超新星爆发理论带来明显的影响．     

Secular contraction in extreme helium stars and the future of V4334 Sgr

Speculative connections have been made between Sakurai's Object andother hydrogen-deficient stars, principally the RCB stars and [WC]central stars of planetary nebulae. RCBs have also been postulated as the precursors of extreme helium stars (EHes). The question arises to whether Sakurai's Object will evolve down the [WC]-PG1159 evolution track, or the RCB-EHe-HesdO track. From a number of IUE observations, we have measured thesecular contraction rates and pulsation masses of several EHes. These are inconsistent with the predicted contraction rates for helium-shell burning giants produced by a final helium-shell flash (the [WC]-PG1159 track). Although there may be some similarities between Sakurai's Object and some RCBs, if the RCB-EHe conection is valid then these must be superficial rather than structural.     

Regular and chaotic orbits in a self-consistent triaxial stellar system with slow figure rotation

We created a self-consistent triaxial stellar system through the cold disipationless collapse of 100,000 particles whose evolution was followed with a multipolar code. The resulting system rotates slowly even though its total angular momentum is zero, i.e., it offers an example of figure rotation. The potential of the system was subsequently approximated with interpolating formulae yielding a smooth potential stationary in the rotating frame. The Lyapunov exponents could then be computed for a randomly selected sample of 3,472 of the bodies that make up the system, allowing the recognition of regular and partially and fully chaotic orbits. The regular orbits were Fourier analyzed and classified using their locations on the frequency map. A comparison with a similar non-rotating model showed that the fraction of chaotic orbits is slightly but significantly enhanced in the rotating model; alternatively, there are no significant differences between the corresponding fractions neither of partially and fully chaotic orbits nor of long axis tubes, short axis tubes, boxes and boxlets among the regular orbits. This is a reasonable result because the rotation causes a breaking of the symmetry that may increase chaotic effects, but the rotation velocity is probably too small to produce any other significant differences. The increase in the fraction of chaotic orbits in the rotating system seems to be due mainly to the effect of the Coriolis force, rather than the centrifugal force, in good agreement with the results of other investigations.