Collapse and fragmentation of rotating, prolate clouds

We present the results of collapse calculations for uniformly rotating, prolate clouds performed using the numerical method: smoothed particle hydrodynamics (SPH). The clouds considered are isothermal, prolate spheroids with different axial ratios ( a/b ), and with different values of β, the ratio of the rotational to gravitational energy. Small density perturbations are added to the clouds, and different initial perturbation spectra are studied. All of the clouds considered are strongly unstable to gravitational contraction, and so collapse to form a spindle configuration. Such a linear structure is unstable to fragmentation, so that the clouds break up into a number of subcondensations. The long-term evolution of the system is then determined by the angular momentum possessed by these fragments.
It is found that a number of the calculations performed result in the formation of orbitally stable binary systems, composed of two rotationally supported discs in orbit about their common centre of mass. Tidal interactions during closest approach, close three-body interactions and the continued accretion of material with high specific angular momentum are all found to increase the orbital separation during these calculations, ensuring that the systems do not merge at later times. The calculations are therefore relevant to the problem of binary star formation, though the systems produced tend to have large orbital separations and periods. One of the strong points of the models presented, however, is their ability to produce systems with a range of mass ratios and orbital eccentricities, without the explicit inclusion of biases in the initial conditions.     

Variability of long-period pulsating stars. II. Additional parameters for classifying stars

Results are presented from an analysis of observations in a sample of 53 long-period pulsating stars (44 of which are Mira-type and 9 are semiregular variables). A catalog of the characteristics of the individual pulsation cycles of the light curves is obtained for these stars. A correlation analysis of the characteristics of the individual pulsation cycles is carried out for each star. A number if diagrams relating the variability of the characteristics of the individual cycles to the average parameters of the variability of the stars and to their spectral classes. New criteria for classifying long-period variables are proposed on the basis of these diagrams. __________ Translated from Astrofizika, Vol. 49, No. 4, pp. 595–612 (November 2006).     

The unusual X-ray light curve of GRB 080307: the onset of the afterglow?

Swift -detected GRB 080307 showed an unusual smooth rise in its X-ray light curve around 100 s after the burst, at the start of which the emission briefly softened. This 'hump' has a longer duration than is normal for a flare at early times and does not demonstrate a typical flare profile. Using a two-component power-law-to-exponential model, the rising emission can be modelled as the onset of the afterglow, something which is very rarely seen in Swift -X-ray light curves. We cannot, however, rule out that the hump is a particularly slow early-time flare, or that it is caused by upscattered reverse shock electrons.     

E (B-V) determination from an UV-visual two-colour diagram: O and B stars in the catalogue of stellar ultraviolet fluxes

We present here, for the O and B type stars in the Catalogue of Stellar Ultraviolet Fluxes, an approach which does not require a precise knowledge of spectral type and luminosity class for derivingE(B-V) colour excesses. The method is based on the use of an UV-visual two-colour diagram; galactic variations in the interstellar extinction law are analyzed and fully taken into account. Our results have been compared with those derived by using the differences between observed and intrinsic colours for stars with known spectral classification. The very good agreement in a large number of cases (94 per cent) demonstrates that our approach permits the derivation of reliable colour excess values for early type stars even if only a rough spectral classification is available.     

Element spots in Ap and Hg‐Mn stars from current‐driven diffusion

The stars of the middle main sequence often have spot‐like chemical structures at their surfaces. We consider the diffusion process caused by electric currents that can lead to the formation of such chemical spots. Diffusion is considered using the partial momentum equations derived by the Chapman‐Enskog method. We argue that diffusion caused by electric currents can substantially change the surface chemistry of stars and form spotted chemical structures even in a relatively weak magnetic field. The considered mechanism can be responsible for a formation of element spots in Hg‐Mn and Ap‐stars. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

3XMM J185246.6+003317 as transient neutron star

We performed an X‐ray timing and spectral analysis of the variable source 3XMM J185246.6+003317 to investigate its physical nature. The data from all observations of 3XMMJ185246.6+003317 conducted by XMM‐Newton EPIC MOS1 and MOS2 with the same instrumental setup in 2004–2009 were reprocessed to form a homogenous data set of solar barycenter corrected photon arrival times and high S/N spectra of 3XMM J185246.6+003317. A Bayesian method for the search, detection, and estimation of the parameters of a periodic signal of unknown shape was employed, as developed by Gregory & Loredo (1992, 1993). The results show that 3XMM J185246.6+003317 is a transient neutron star with the genuine spin‐period of 23.11722 (23.11711–23.11727) s and its derivative of 5.3(0.3–5.5)×10^–11 s s^–1, implying a characteristic age of 7 (6–104) kyr, if the period derivative can be ascribed to the genuine spin‐down rate of the neutron star. The rotational‐phase averaged X‐ray spectra at the different brightness periods can be fitted with a highly absorbed blackbody model with different temperatures. The phase‐folded light curves in different energy bands with high S/N ratio show a double‐peaked profile; the variations depend on time and energy, indicating that radiation emerges from at least two emitting areas. The spectra at the phases corresponding to the maxima in the phase‐folded light curve show different spectral parameters of absorbed blackbody radiation, i.e. the hotter one has a smaller size. The source is detected only from September 2008 to April 2009 with persistently decreasing brightness, but not before, even though it was observed by XMM. Hence, it is a transient neutron star or a binary system hosting it. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

银河系铝元素丰度研究

恒星的Al元素丰度可以为探索星团和星系的化学演化提供重要线索.通过系统分析银河系薄盘、厚盘、核球、银晕以及M4、M5等球状星团中恒星的[Al/Fe]随恒星金属丰度[Fe/H]的变化趋势,得出银河系薄盘、厚盘和核球恒星的[Al/Fe]随着[Fe/H]的增加而缓慢下降,而球状星团M4和M5恒星的[Al/Fe]随[Fe/H]增加没有下降趋势,这暗示Ia超新星对M4和M5恒星元素丰度的贡献比较小.详细研究了银河系恒星[Al/Fe]与[Mg/Fe]、[Na/Fe]的相关性,结果表明银河系场星的[Al/Fe]与[Mg/Fe]正相关,但在球状星团M4和M5恒星中未见此相关性;银河系盘星及M4和M5等球状星团恒星的[Al/Fe]与[Na/Fe]都存在正相关.