We consider the instability of the cometary plasma tail which is composed of a neutral sheet,two lobes of the ion tail and solar wind.The plasma is assumed to be highly conductive and incompressible.The unstable state yields a magnetic field which is perpendicular to the tail axis.Our result is consistent with findings about plasma from the International Cometary Explorer(ICE).
The Damocloids are a group of unusual asteroids that recently added a new member: 2010 EJ104. The dynamical evolution of the Damocloids may reveal a connection from the Main Belt to the Kuiper Belt and beyond the scattered disk. According to our simulations, two regions may be considered as possible origins of the Damocloids: the scattered disk, or a part of the Oort cloud, which will be perturbed to a transient region located between 700 AU and 1000 AU. Based on their potential origin, the Damocloids can be classified into two types, depending on their semi-major axes, and about 65.5% of the Damocloids are classified into type Ⅰ which mainly originate from the Oort cloud. Whether the Damocloids are inactive nuclei of the Halley Family of Comets may depend on their origin.
Su WangHai-Bin ZhaoJiang-Hui JiSheng JinYan XiaHao LuMin WangJin-Sheng Yao
The CCD photometric observation of comet 103P/Hartley 2 with broadband optical filters was carried out at Xuyi Station of Purple Mountain Observatory, Chinese Academy of Sciences by use of Near Earth Object Telescope (NEOT). We monitored the comet from September 30 to November 4, 2010 and investigated the variation of activities. The Afρ parameter derived from our r broadband data varies from 46.0 to 126.0 cm and increases with time. The dust production rate in terms of Afρ was measured and a tentative calculation of the dust (or gas) loss-rate of nucleus was executed under the assumption that the nucleus of comet 103P/Hartley 2 is spherical. The r-band image and the surface brightness profile taken on September 30, 2010 was measured and the gradient of surface brightness profile of 103P/Hartley 2 showed m < 1.5.
The Purple Mountain Observatory of the Chinese Academy of Sciences and the Department of Astronomy, Nanjing University, cooperating with the Albert Einstein Institute in Germany, put forward a model of the planar co-orbital circular restricted three-body problem.
The formation of the present configuration of three hot super-Earths in the planet system HD 40307 is a challenge to dynamical astronomers.With the two successive period ratios both near and slightly larger than 2,the system may have evolved from pairwise2:1 mean motion resonances(MMRs).In this paper,we investigate the evolutions of the period ratios of the three planets after the primordial gas disk was depleted.Three routines are found to probably result in the current configuration under tidal dissipation with the center star,they are:(i)through apsidal alignment only;(ii)out of pairwise 2:1 MMRs,then through apsidal alignment;(iii)out of the 4:2:1 Laplace Resonance(LR),then through apsidal alignment.All the three scenarios require the initial eccentricities of planets～0.15,which implies a planetary scattering history during and after the gas disk was depleted.All the three routines will go through the apsidal alignment phase,and enter a state with near-zero eccentricities finally.We also find some special characteristics for each routine.If the system went through pairwise 2:1 MMRs at the beginning,the MMR of the outer two planets would be broken first to reach the current state.As for routine(iii),the planets would be out of the LR at the place where some high-order resonances are located.At the high-order resonances 17:8 or 32:15 of the planets c and d,the system will possibly enter the current state as the final equilibrium.
Research about asteroids has recently attracted more and more attention, especially focusing on their physical structures, such as their spin axis, rotation period and shape. The long distance between observers on Earth and asteroids makes it impossible to directly calculate the shape and other parameters of asteroids, with the exception of Near Earth Asteroids and others that have passed by some spacecrafts. Photometric measurements are still generally the main way to obtain research data on asteroids, i.e. the lightcurves recording the brightness and positions of asteroids. Supposing that the shape of the asteroid is a triaxial ellipsoid with a stable spin, a new method is presented in this article to reconstruct the shape models of asteroids from the lightcurves, together with other physical parameters. By applying a special curvature function, the method calculates the brightness integration on a unit sphere and Lebedev quadrature is employed for the discretization. Finally, the method searches for the optimal solution by the Levenberg-Marquardt algorithm to minimize the residual of the brightness. By adopting this method, not only can related physical parameters of asteroids be obtained at a reasonable accuracy, but also a simple shape model of an ellipsoid can be generated for reconstructing a more sophisticated shape model.
We present measurements of the optical broadband atmospheric extinction coefficients and the night sky brightness at the Xuyi Observation Station of Purple Mountain Observatory. The measurements are based on CCD imaging data taken in the Sloan Digital Sky Survey's g, r and i bands with the Xuyi 1.04/1.20 m Schmidt Telescope for the Xuyi Schmidt Telescope Photometric Survey of the Galactic Anti-center (XSTPS-GAC), the photometric part of the Digital Sky Survey of the Galactic Anti-center (DSS-GAC). The data were collected during more than 140 winter nights from 2009 to 2011. We find that the atmospheric extinction coefficients for the g, r and i bands are 0.69, 0.55 and 0.38 mag/airmass, respectively, based on observations taken on several photometric nights. The night sky brightness determined from images with good quality has median values of 21.7, 20.8 and 20.0 mag arcsec-2 and reaches 22.1, 21.2 and 20.4 mag arcsec-2 under the best observing conditions for the g, r and i bands, respectively. The relatively large extinction coefficients compared with other good astronomical observing sites are mainly due to the relatively low elevation (i.e. 180 m) and high humidity at the station.
