Mass segregation in young stellar clusters     

Ian A. Bonnell  & Melvyn B. Davies 《Monthly notices of the Royal Astronomical Society》1998,295(3):691-698

We investigate the evolutionary effect of dynamical mass segregation in young stellar clusters. Dynamical mass segregation acts on a time-scale of order the relaxation time of a cluster. Although some degree of mass segregation occurs earlier, the position of massive stars in rich young clusters generally reflects the cluster's initial conditions. In particular, the positions of the massive stars in the Trapezium cluster in Orion cannot be due to dynamical mass segregation, but indicate that they formed in, or near, the centre of the cluster. Implications of this for cluster formation and for the formation of high-mass stars are discussed.  相似文献   

Spectroscopic observations of the eclipsing polar MN Hya (RX J0929–24)     

Gavin Ramsay  & Peter J. Wheatley 《Monthly notices of the Royal Astronomical Society》1998,301(1):95-100

We present low–medium resolution optical spectroscopy of the eclipsing AM Her system MN Hya (RX J0929–24). We determine the magnetic field strength at the primary accretion region of the white dwarf to be 42 MG from the spacing of cyclotron features visible during π ∼ 0.4–0.7. From spectra taken during the eclipse we find that the secondary has an M3–4 spectral type. Combined with the eclipse photometry of Sekiguchi, Nakada &38; Bassett and an estimate of the interstellar extinction we find a distance of ∼300–700 pc. We find unusual line variations at π ∼ 0.9: Hα is seen in absorption and emission. This is at the same point in the orbital phase at which a prominent absorption dip is seen in soft X-rays.  相似文献   

High-redshift galaxies, their active nuclei and central black holes     

Martin G. Haehnelt  Priyamvada Natarajan  & Martin J. Rees 《Monthly notices of the Royal Astronomical Society》1998,300(3):817-827

We demonstrate that the luminosity function of the recently detected population of actively star-forming galaxies at redshift z  = 3 and the B -band luminosity function of quasi-stellar objects (QSOs) at the same redshift can both be matched with the mass function of dark matter haloes predicted by standard variants of hierarchical cosmogonies for lifetimes of optically bright QSOs anywhere in the range 10⁶ to 10⁸ yr. There is a strong correlation between the lifetime and the required degree of non-linearity in the relation between black hole and halo mass. We suggest that the mass of supermassive black holes may be limited by the back-reaction of the emitted energy on the accretion flow in a self-gravitating disc. This would imply a relation of black hole to halo mass of the form M _bh ∝  v ⁵_halo ∝  M ^5/3_halo and a typical duration of the optically bright QSO phase of a few times 10⁷ yr. The high integrated mass density of black holes inferred from recent black hole mass estimates in nearby galaxies may indicate that the overall efficiency of supermassive black holes for producing blue light is smaller than previously assumed. We discuss three possible accretion modes with low optical emission efficiency: (i) accretion at far above the Eddington rate, (ii) accretion obscured by dust, and (iii) accretion below the critical rate leading to an advection-dominated accretion flow lasting for a Hubble time. We further argue that accretion with low optical efficiency might be closely related to the origin of the hard X-ray background and that the ionizing background might be progressively dominated by stars rather than QSOs at higher redshift.  相似文献   

An 'outside-in' outburst of Aql X–1     

T. Shahbaz  R. M. Bandyopadhyay  P. A. Charles  R. M. Wagner  P. Muhli  P. Hakala  J. Casares  & J. Greenhill 《Monthly notices of the Royal Astronomical Society》1998,300(4):1035-1040

We present optical spectroscopy and optical and infrared photometry of the neutron star soft X-ray transient Aql X–1 during its X-ray outburst of 1997 August. By modelling the X-ray, optical and IR light curves, we find a 3-d delay between the IR and X-ray rise times, analogous to the UV–optical delay seen in dwarf novae outbursts and black hole X-ray transients. We interpret this delay as the signature of an 'outside-in' outburst, in which a thermal instability in the outer disc propagates inward. This outburst is the first of this type definitively identified in a neutron star X-ray transient.  相似文献   

The expulsion and retention of dust grains by galactic discs     

J. I. Davies  P. Alton  S. Bianchi  & M. Trewhella 《Monthly notices of the Royal Astronomical Society》1998,300(4):1006-1014

We have constructed a numerical model of a galaxy that consists of a stellar, gas and dust disc imbedded within a dark halo. We have used this model to assess the radiation, gravitational and viscous forces on dust grains and to trace their motion through the interstellar medium over a period of 10⁹ yr. We conclude that the disc opacity is a crucial factor in understanding the motion of the grains. Large grains (≈0.1 μm) with low disc opacity will lead to dust expulsion from the stellar disc, while high opacity leads to dust retention. Reasonable disc opacities lead to the recycling of the larger grains from the outer to the inner regions of the galaxy. The larger grains travel at higher velocities than small grains (0.01−0.001 μm), and so the smaller grains remain relatively close to their formation sites. Dust can 'leak' out over the entire surface of the disc because of the imbalance of radiation and gravitational forces. The dust is dynamically coupled to the gas and so although the gas lags behind the dust it is carried along with it. This explains the close correlation between the far-infrared emission from dust and the gas column density. We use a simple analytical model to show how the dust mass of a galaxy may evolve with time and how a significant fraction (90 per cent) of the total dust mass produced may have been expelled into the intergalactic medium.  相似文献   

Numerical simulations of protostellar encounters — II. Coplanar disc–disc encounters     

S. J. Watkins  A. S. Bhattal  H. M. J. Boffin  N. Francis  & A. P. Whitworth 《Monthly notices of the Royal Astronomical Society》1998,300(4):1205-1213

It is expected that an average protostar will undergo at least one impulsive interaction with a neighbouring protostar whilst a large fraction of its mass is still in a massive, extended disc. Such interactions must have a significant impact upon the evolution of the protostars and their discs.   We have carried out a series of simulations of coplanar encounters between two stars, each possessing a massive circumstellar disc, using an SPH code that models gravitational, hydrodynamic and viscous forces. We find that during a coplanar encounter, disc material is swept up into a shock layer between the two interacting stars, and the layer then fragments to produce new protostellar condensations. The truncated remains of the discs may subsequently fragment; and the outer regions of the discs may be thrown off to form circumbinary disc-like structures around the stars. Thus coplanar disc–disc encounters lead efficiently to the formation of multiple star systems and small- N clusters, including substellar objects.  相似文献   

Line emission from accretion discs around black holes: the analytic approach     

Andrej ?ade?  Claudio Fanton  Massimo Calvani 《New Astronomy》1998,3(8):528-654

The broad X-ray iron line observed in many active galactic nuclei spectra is thought to originate from the accretion disc surrounding the putative supermassive black hole. We show here how to perform the analytical integration of the geodesic equations that describe the photon trajectories in the general case of a rotating black hole (Kerr metric), in order to write a fast and efficient numerical code for modelling emission line profiles from accretion discs.  相似文献   

地勘单位转变经济增长方式若干问题的思考     

古斌 《河南国土资源》1998,(4)

转变地勘单位的经济增长方式,提高各生产要素的使用效率,进而提高经济效益,推动地勘单位的经济持续、健康、全面的发展是当前急需解决的首要问题。本文在分析了地勘单位经济增长方式现状的基础上,指出了优化认识、优化资金配置、优化资本运行机制、优化内部管理、优化经营规模、优化人才成长环境等六个方面是转变地勘单位经济增长方式可选择的主要思路和途径。  相似文献   

Socioeconomic Correlates of Increasing Crime Rates in Smaller Communities     

William V. Ackerman 《The Professional geographer》1998,50(3):372-387

This research analyzes changes in crime rates by city size and determines the extent to which these changes can be explained by socioeconomic variables. More particularly it addresses rates of change in mean crime rates for violent and property crime between 1976–1984 and 1985–1994 for all U. S. cities, then compares results to Ohio cities. It provides a detailed analysis of changing crime rates in 111 Ohio cities with populations between 10,000 and 99,999 inhabitants and attempts to account for crime differentials between these cities employing linear regression and factor analysis. Results indicate that crime is significantly related to poverty and its associated conditions and processes.  相似文献   

Arc／Info—Grid模块及其水文模型应用   总被引：5，自引：0，他引：5  

张山山 《四川测绘》1998,21(1):10-13

利用地理信息系统可以建立流域的数字高程模型,并与grid模块的水文表面分析工具结合,形成直观的流域环境三维显示,再通过计算、统计与分析,即可获取精确的水文因子数据,为进一步水文定量研究及应用提出了一种高效的方法。  相似文献