近地小行星探测和危险评估

<正>1994年7月,苏梅克-列维9号彗星的21个碎片以约60 km/s的速度撞击木星,这是人类第1次直接观测到的大规模天体相撞.这次事件表明地球面临着天体碰撞的现实威胁.1995年在纽约联合国大厦召开的预防近地天体撞击地球的国际科学会议上,紫金山天文台提出了中国近地天体探测计划(CNEOS).1998年,CNEOS计划正式立项启动.从选址建站、望远镜和CCD系统研制,到投入试观测,实现海量数据处理和碰撞危险评估,经历了7 yr多时间.本论文综述CNEOS观测系统的概     

冥族小天体与海王星特洛伊的近密交会

海王星外天体中的冥族小天体与海王星成2:3的平运动轨道共振,且具有较大的轨道偏心率,因此它们能与海王星特洛伊的轨道发生重叠,导致近密交会和碰撞,从而深刻地影响两者的动力学演化。利用数值模拟的方法,有效地获得了这两群小天体间近密交会的信息,讨论了可能影响两者近密交会频率的因素,包括小天体质量、轨道倾角和轨道偏心率等。在合理近似条件下,建立了估算两群小天体近密交会和碰撞次数的理论公式。结合已有的数值模拟结果,以及对冥族小天体观测数据的分析,对实际情况下冥族小天体群与典型特洛伊小天体之间的近密交会和碰撞次数进行了估算,证明近密交会较为频繁地发生,而碰撞则极其罕见,并且各尺寸范围的小天体对近密交会和碰撞次数的贡献各有不同。这一套分析和估算的方法可以直接应用在其他类似小天体间交会过程的估算上。     

Kuiper带天体的轨道分布特性

1992年9月,夏威夷大学的D.Jewitt和加利福尼亚大学的J.Lun发现了海王星外绕太阳运行的第一个小天体1992QB1[1],开创了人类对于海王星外天体的实际观测的研究.近10年的接连不断发现,已经证实了海王星轨道外面存在着一个由大量的环绕太阳运动的小天体组成的环带[2].由于G.P.Kuiper曾在1951年的文章中提出过在冥王星的外边可能存在小天体的问题,因此人们一般把这个环带称为Kuiper带,你这些天体为“KuiperBelt　Objects”(KBOs),或从逻辑上称它们为“Trans-NeptunianObjects”(TNOs)[3]     

ROSAT选射电源的性质：Ⅰ.样本

ＲＯＳＡＴ卫星的全天巡天观测到了一大批射电星系和各种活动星核,为我们研究这些活动性剧烈的河外天体的辐射机制、多波段辐射相关性以及各类星系核之间的演化联系提供了重要数据。通过将ＲＯＳＡＴ全天巡天观测与ＰＭＮ射电（５ＧＨｚ）巡天观测进行交叉证认,我们得到了６４２颗天射电源。对这些对应体随后进行的光学证认构建了一个星系和活动星系核的大样本。其中３１１个对应体（占５２％）被过去的光学观测证实为河外天体（包     

彗木两次碰撞过程分析

Ｓｈｏｅｍａｋｅｒ－Ｌｅｖｙ９号彗星与木星碰撞，是人类有史以来第一次有预报的天体大碰撞。有关观测的初步结果，很快通过新闻报道和电子网络传遍了全世界。本文通过对其１４号、１１号核撞击木星的大量报道的综合分析，对彗木碰撞的物理过程、彗木碰撞时刻的确定等重要问题进行了探讨。这对于分析和改进对这类碰撞的预报，确定撞击时刻和撞击点位置，以及今后研究撞击黑斑的变化，都有一定的意义。     

银河系自转曲线的测定

银河系自转曲线研究有着重要的天体物理意义.自转曲线可以利用多种星族Ⅰ示踪天体来加以测定,如经典造父变星、行星状星云、碳星、疏散星团、OB型星,以及中性氢巡天等.相关研究表明,在太阳圈之外,银河系自转曲线大致保持为平坦状,甚至略有抬高,从而为大质量暗晕的存在提供了有力的观测证据.     

Blazars天体的多波段的理论研究

在收集大量资料的基础上,较为全面的综述了Ｂｌａｚａｒｓ天体的基本观测特征,给出了目前较为完整的ＢＬＬａｃ天体总表。对该类天体的相关理论,多波段相关性,光变周期分析,Ｈｕｂｂｌｅ关系,ＢＬＬａｃ天体的偏振度的变化作了一定研究。并介绍了我们用ＣＣＤ测光和光谱对３Ｃ３４５作准同时性观测的结果,我们讨论了３Ｃ３４５的历史光变曲线和光变周期分析,研究结果表明：３Ｃ３４５具有（１０±０．８）ａ（年）的光变周期,其中心有一个１０６Ｍ⊙的黑洞。Ｊｕｒｋｅｖｉｃｈ方法是一种Ｂｌａｚａｒｓ天体的光变周期分析研究的好方法     

宇宙能被挤压到多小？

宇宙能被挤压到多小？之所以提出这个问题是因为宇宙在膨胀并假设所有天体来源于同一地点，一次原始火球的大爆炸。如果认为宇宙中的一切物质都是由夸克和电子组成的，根据观测到的各层次天体的分布情形，我们就可以用考虑数量级的方法求出这一问题的答案。已观测到的宇宙...     

BL Lac天体某些性质研究的新进展

对ＢＬＬａｃ天体观测性质及相关理论,多波段相关性、光变、偏振度、射电结构与超光速运动、发射线、宁静问题及γ辐射等作一定的综述和探讨。报导我们在这个领域的某些研究的新进展。     

大尺度赫比格-哈罗天体的光学研究进展

近期大视场CCD成像观测表明，由年轻星激发的部分赫比格－哈罗（HH）天体的尺度范围超过1pc，比过去认识的HH天体的尺度大了一个量级。目前的观测已经发现pc尺度的HH天体有数十个，表明HH天体达到pc尺度是很普遍的，这将深刻地改变人们对年轻星与其周围环境作用的认识。综述评述了pc尺度HH天体的观测性质，讨论了巨HH天体对恒星形成的重大意义及其对周围星际介质的物理和化学影响。     

A Statistical Test of Correlations and Periodicities in the Geological Records

Matsumoto & Kubotani argued that there is a positive and statistically significant correlation between cratering and mass extinction. This argument is critically examined by adopting a method of Ertel used by Matsumoto & Kubotani but by applying it more directly to the extinction and cratering records. It is shown that on the null-hypothesis of random distribution of crater ages, the observed correlation has a probability of occurrence of 13%. However, when large craters are excluded whose ages agree with the times of peaks of extinction rate of marine fauna, one obtains a negative correlation. This result strongly indicates that mass extinction are not due to accumulation of impacts but due to isolated gigantic impacts. Further, correlations between cratering and flood basalts and between mass extinction and flood basalts originally recognized by Stothers are further investigated. It is shown that for the former, large craters corresponds to 7 of 11 major flood basalts. Periodicity analyses are carried out for the geomagnetic reversal events and extinction rate peaks recently given by Sepkoski for the interval up to 540 M yr BP. The former exhibits a significant periodicity at P = 15 M yr, but the latter shows no significant period. It seems premature to regard the periodicity in geological records established. This revised version was published online in July 2006 with corrections to the Cover Date.     

Are cratering and probably related geological records periodic?

The controversial topic of periodicity in geological records in relation to astronomical modeling is reviewed. Although impact cratering, frequency distribution of geo-magnetic reversals, and mass extinction of fauna yield periods when certain tests are applied, none of them can be regarded significant in the sense of mathematical statistics. The first two records yield periods of 30 Myr, while the faunal-extinction record yields a period of ~ 26 – 27 Myr. It seems that although catastrophes in the form of large impacts trigger mass extinctions, certain geophysical or geological conditions need be satisfied for mass extinctions to be realized. One should not expect to find an indisputable periodicity in cratering record because random impacts by asteroids are dominant. Thus, the earth-crossing cometary flux modulated by the galactic tidal force appears consistent with the weak detected periodicity.     

Are cratering and probably related geological records periodic?

The controversial topic of periodicity in geological records in relation to astronomical modeling is reviewed. Although impact cratering, frequency distribution of geo-magnetic reversals, and mass extinction of fauna yield periods when certain tests are applied, none of them can be regarded significant in the sense of mathematical statistics. The first two records yield periods of 30 Myr, while the faunal-extinction record yields a period of ~ 26 – 27 Myr. It seems that although catastrophes in the form of large impacts trigger mass extinctions, certain geophysical or geological conditions need be satisfied for mass extinctions to be realized. One should not expect to find an indisputable periodicity in cratering record because random impacts by asteroids are dominant. Thus, the earth-crossing cometary flux modulated by the galactic tidal force appears consistent with the weak detected periodicity.     

Organic-chemical clues to the theory of impacts as a cause of mass extinctions

The reasons for the mass extinctions, which occur from time to time in Earth's history-as, e.g., the dinosaur extinction at the Cretaceous/Tertiary boundary 65 myr ago - are still not satisfactorily cleared up. A possible reason might be the impact of one or several comets of several kilometers in diameter. In this paper the astrophysical background of this hypothesis and organic-chemical processes during an impact will be discussed.Quantitative estimations are given, which show that the amount of organic substances brought to the Earth may be of the same order of magnitude as the normal biological production of organic material.Investigations are proposed to examine the organic-chemical composition of profiles of the Cretaceous/Tertiary boundary and other boundaries, at which mass extinction had occurred, in order to find anomalies as consequences of impacts.     

The “Shiva Hypothesis”: Impacts, mass extinctions, and the galaxy

The “Shiva Hypothesis”, in which recurrent, cyclical mass extinctions of life on Earth result from impacts of comets or asteroids, provides a possible unification of important processes in astrophysics, planetary geology, and the history of life. Collisions with Earth-crossing asteroids and comets ≥ a few km in diameter are calculated to produce widespread environmental disasters (dust clouds, wildfires), and occur with the proper frequency to account for the record of five major mass extinctions (from ≥ 10⁸ Mt TNT impacts) and ~ 20 minor mass extinctions (from 10⁷–10⁸ Mt impacts) recorded in the past 540 million years. Recent studies of a number of extinctions show evidence of severe environmental disturbances and mass mortality consistent with the expected after-effects (dust clouds, wildfires) of catastrophic impacts. At least six cases of features generally considered diagnostic of large impacts (e.g., large impact craters, layers with high platinum-group elements, shock-related minerals, and/or microtektites) are known at or close to extinction-event boundaries. Six additional cases of elevated iridium levels at or near extinction boundaries are of the amplitude that might be expected from collision of relatively low-Ir objects such as comets. The records of cratering and mass extinction show a correlation, and might be explained by a combination of periodic and stochastic impactors. The mass extinction record shows evidence for a periodic component of about 26 to 30 Myr, and an ~ 30 Myr periodic component has been detected in impact craters by some workers, with recent pulses of impacts in the last 2–3 million years, and at ~ 35, 65, and 95 million years ago. A cyclical astronomical pacemaker for such pulses of impacts may involve the motions of the Earth through the Milky Way Galaxy. As the Solar System revolves around the galactic center, it also oscillates up and down through the plane of the disk-shaped galaxy with a half-cycle ~ 30±3 Myr. This cycle should lead to quasi-periodic encounters with interstellar clouds, and periodic variations in the galactic tidal force with maxima at times of plane crossing. This “galactic carrousel” effect may provide a viable perturber of the Oort Cloud comets, producing periodic showers of comets in the inner Solar System. These impact pulses, along with stochastic impactors, may represent the major punctuations in earth history.     

The Galactic Theory of Mass Extinctions: an Update

Astronomical and geological evidence is consistent with the hypothesis that mass extinctions of life on Earth are related to impacts of comets whose flux is partly modulated by the dynamics of the Milky Way Galaxy. Geologic evidence for impact (ejecta and large impact craters) has been found at times of mass extinction events, and the record of large dated craters shows a significant correlation with extinctions. Statistical analyses suggest that mass extinction events exhibit a periodic component of about 30 Myr, and periodicities of 30 ± 0.5 Myr and 35 ± 2 Myr have been extracted from sets of well-dated large impact craters. These results suggest periodic or quasi-periodic showers of impactors, probably Oort Cloud comets, with an approximately 30 or 36 Myr cycle. The best explanation for these proposed quasi-periodic comet showers involves the Sun's vertical oscillation through the galactic disk, which may have a similar cycle time between crossings of the galactic plane. Further refinement of the model will depend on the identification and quantification of the dark matter component in the galactic disk, and discovery and accurate dating of additional impact craters. This revised version was published online in July 2006 with corrections to the Cover Date.     

Volcanogenic dark matter and mass extinctions

The passage of the Earth through dense clumps of dark matter, the presence of which is predicted by certain cosmologies, would produce large quantities of heat in the interior of this planet through the capture and subsequent annihilation of dark matter particles. This heat would lead to large-scale volcanism which could in turn have caused mass extinctions. The periodicity of such volcanic outbursts agrees with the frequency of paleontological mass extinctions as well as the observed periodicity in the occurrence of the largest flood basalt provinces on the globe.     

On the periodicity hypothesis of the ages of large impact craters

An analysis is made of the periodicity hypothesis of the ages of large craters, based on the compilation by Grieve with the addition of recently identified craters. A method earlier proposed by Broadbent is used to derive a period, and the significance of the derived period is tested by a Monte Carlo experiment. In accordance with the result of Stothers, the ages of large craters  ( D >30 km)  are shown to exhibit a period close to 37.5 Myr. Monte Carlo experiments show, however, that the derived period is far from being statistically significant. A subset of crater data earlier adopted by Napier for the purpose of similar investigation is also tested, and it is shown that they also exhibit a similar period at an almost identical level of confidence. A brief discussion is made of the relation between the derived period and that associated with faunal mass extinctions.     

Are periodicities in crater formations and mass

Periodicities in crater formation rate and mass-extinctions are reviewed. The former exhibits a period of 30 million yr, while the latter appear to have a periodicity at 26 myr. Results obtained earlier that small craters better satisfy the adopted criterion for statistical testing is shown due to the fact that there is a strong clustering of small craters in a recent past (<10 myr). On the basis of the dataset of craters compiled by Grieve, it is shown that there are several craters for which no mass extinctions correspond. The difference in the periods of the craters and of mass extinctions and the lack of mass extinctions that correspond to large craters appear to suggest that the two periodicities are not interrelated, and large impacts merely act as triggers for the mass-extinctions; the only exception being theK/T boundary.