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621.
Ben-Zhong Dai * Guang-Zhong Xie Ze-Jun Jiang Yunnan Observatory Chinese Academy of Science Kunming National Astronomical Observatories Chinese Academy of Science Beijing Yunnan Astrophysics Center Yunnan Univers 《中国天文和天体物理学报》2002,2(1):8-16
1 INTRODUCTIONBlazars, including BL Lacertae objects and Optical Violently Variable (OVV) quasars,belong to a class of active ga1actic nuclei (AGNs), the spectrum of which is dominated bynontherma1 radiation from re1ativistic electrons in jets pointing at us (e.g. BlandfOrd & Rees1978; Urry & Padovani 1995). The entire electromagnetic spectraI energy distributions (SEDs)usually reveal two broad components, the low energy part, peaking in the IR to soft X-rayrange, and the high en… 相似文献
622.
We present a light curve of C/1995 O1 (Hale-Bopp) compiledfrom more than 3000 visual observations of the comet made by members
of the The Astronomer Group world-wide. These observations cover the period from discovery through to the end of 1997. The
light curve shows that the rate of brightening of the comet varied widely at different times, with rapid rates of brightening
at high heliocentric distance pre-perhelion and a comparably rapid post-perihelion fade. There is no evidence that the comet
was suffering a large photometric outburst when first discovered, although a small outburst can be identified at perihelion.
At least five difficult brightening regimes can be identified in the light curve between discovery and perihelion. From 2.5
AU to perihelion the rate of brightening with decreasing heliocentric distance was typical for “fairly” new comets(n ∼ 3.5,
where “n” is the power law exponent of the heliocentric distance), although this was preceded by a period of very slow brightening
with n ∼ 1 from r ∼ 4.0 AU to r ∼ 2.8 AU and followed by an initially more rapid brightening which appears to be related to
the on-set of rapid water sublimation activity. We derive the light curve parameters at different stages of the comet's apparition.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
623.
Earth, Moon, and Planets - Comet Hale-Bopp has shown an extremely elevated level of dust activity during its apparition. We examine the dust activity of the comet, particularly the dust jets which... 相似文献
624.
本文对X射线选择BLLac天体(XBLs)的完备样品进行了较细致的研究,讨论了该样品各观测量的分布、最新发现的几例X射线和ROSAT选择的BLLac天体的Hubble关系及BLLac天体的红移起源问题. 相似文献
625.
E.I. Robson J.A. Stevens T. Jenness 《Monthly notices of the Royal Astronomical Society》2001,327(3):751-770
Calibrated data for 65 flat-spectrum extragalactic radio sources are presented at a wavelength of 850 μm, covering a three-year period from 1997 April. The data, obtained from the James Clerk Maxwell Telescope using the SCUBA camera in pointing mode, were analysed using an automated pipeline process based on the Observatory Reduction and Acquisition Control–Data Reduction ( orac–dr ) system. This paper describes the techniques used to analyse and calibrate the data, and presents the data base of results along with a representative sample of the better-sampled light curves. 相似文献
626.
We have worked out a ’statistical algorithm’ for obtaining the posterior probability density of the deceleration parameter
q0 from quasars where there is a luminosity indicator available. We point out that the role of the luminosity indicator is to
provide asecond estimate of individual luminosities after a first estimate has been obtained from measured brightness and redshift together
with an assumed q0. Discrimination of q0 is to be sought in the statistical properties of the set of differences between the two estimates (the residuals). We show
that the variance of the residuals and their correlation with redshifts (further refined to luminosity distances) are two
independent test-statistics for q0, whose known distributions then lead to the probability density sought.
We have applied the above algorithm to a sample of flat-spectrum radio quasars with measured CIV, MgII and Ly α lines. A combined
Baldwin’s relation was used for all 3 lines. Our result is that log q0 is normally distributed with a mean value of + 0.270± 0.135 (s.d.), or, q0
= + 1.86 ± 0.135 dex. This result, we believe, is the sharpest result so far published on q0. 相似文献
627.
628.
V. P. Arkhipova N. P. Ikonnikova R. I. Noskova G. V. Sokol S. Yu. Shugarov 《Astronomy Letters》2001,27(3):156-162
We present photoelectric and photographic observations of the supergiant HD 179821 with a large infrared excess, a candidate for protoplanetary objects. Over, ten years of our UBV observations, the star exhibited semiregular light variations with amplitudes $\Delta V = 0\mathop .\limits^m 10$ , $\Delta B = 0\mathop .\limits^m 15$ , and $\Delta U = 0\mathop .\limits^m 25$ , as well as systematic color and light variations. From 1990 until 1996, the yearly mean U-B and B-V color indices decreased by 0.25 and 0.15, respectively. After 1996, the motion of the star in the two-color (B-V)-(U-B) diagram upward and to the left slowed down. The color excess that we derived from our observations, by assuming that the star’s spectral type was F3 I in the 1990s, is E(B-V)=1.0. The photographic observations of HD 179821 from 1899 until 1989 show that its brightness m pg generally increased while significantly fluctuating. An analysis of the observational data suggests that HD 179821 is most likely a post-AGB star of intermediate or low mass. 相似文献
629.
Observations suggest most stars originate in clusters embedded in giant molecular clouds [Lada, C.J., Lada, E.A., 2003. Annu. Rev. Astron. Astrophys. 41, 57-115]. Our Solar System likely spent 1-5 Myrs in such regions just after it formed. Thus the Oort Cloud (OC) possibly retains evidence of the Sun's early dynamical history and of the stellar and tidal influence of the cluster. Indeed, the newly found objects (90377) Sedna and 2000 CR105 may have been put on their present orbits by such processes [Morbidelli, A., Levison, H.F., 2004. Astron. J. 128, 2564-2576]. Results are presented here of numerical simulations of the orbital evolution of comets subject to the influence of the Sun, Jupiter and Saturn (with their current masses on orbits appropriate to the period before the Late Heavy Bombardment (LHB) [Tsiganis, K., Gomes, R., Morbidelli, A., Levison, H.F., 2005. Nature 435, 459-461]), passing stars and tidal force associated with the gas and stars of an embedded star cluster. The cluster was taken to be a Plummer model with 200-400 stars, with a range of initial central densities. The Sun's orbit was integrated in the cluster potential together with Jupiter and Saturn and the test particles. Stellar encounters were incorporated by directly integrating the effects of stars passing within a sphere centred on the Sun of radius equal to the Plummer radius for low-density clusters and half a Plummer radius for high-density clusters. The gravitational influence of the gas was modeled using the tidal force of the cluster potential. For a given solar orbit, the mean density, 〈ρ〉, was computed by orbit-averaging the density of material encountered. This parameter proved to be a good measure for predicting the properties of the OC. On average 2-18% of our initial sample of comets end up in the OC after 1-3 Myr. A comet is defined to be part of the OC if it is bound and has q>35 AU. Our models show that the median distance of an object in the OC scales approximately as 〈ρ〉−1/2 when . Our models easily produce objects on orbits like that of (90377) Sedna [Brown, M.E., Trujillo, C., Rabinowitz, D., 2004. Astrophys. J. 617, 645-649] within ∼1 Myr in cases where the mean density is or higher; one needs mean densities of order to create objects like 2000 CR105 by this mechanism, which are reasonable (see, e.g., Guthermuth, R.A., Megeath, S.T., Pipher, J.L., Williams, J.P., Allen, L.E., Myers, P.C., Raines, S.N., 2005. Astrophys. J. 632, 397-420). Thus the latter object may also be part of the OC. Close stellar passages can stir the primordial Kuiper Belt to sufficiently high eccentricities (e?0.05; Kenyon, S.J., Bromley, B.C., 2002. Astron. J. 123, 1757-1775) that collisions become destructive. From the simulations performed it is determined that there is a 50% or better chance to stir the primordial Kuiper Belt to eccentricities e?0.05 at 50 AU when . The orbit of the new object 2003 UB313 [Brown, M.E., Trujillo, C.A., Rabinowitz, D.L., 2005. Astrophys. J. 635, L97-L100] is only reproduced for mean cluster densities of the order of , but in the simulations it could not come to be on its current orbit by this mechanism without disrupting the formation of bodies in the primordial Kuiper Belt down to 20 AU. It is therefore improbable that the latter object is created by this mechanism. 相似文献
630.
新一代并行空间分析将面临空间大数据分析和实时空间分析服务的挑战。矢量空间计算作为GIS系统中的重要组成部分,在并行化算法设计中存在负载不均,并行扩展性差,IO性能低等技术瓶颈。本文首先从应用需求和技术发展的演变历史回顾了矢量空间分析算法发展过程;然后,从研究现状的角度详细阐述了并行矢量空间分析计算的研究成果,总结了并行空间分析算法的算法特征和技术瓶颈,对不同并行编程模型进行了对比,并提出了并行空间分析算法的研发流程;最后,从发展前景的角度预测了全空间信息系统中基于多粒度时空对象的空间数据模型和计算方法的发展趋势,提出了以内存计算等技术实现存算一体化的新型空间数据模型和分析方法的技术趋势。 相似文献