Distribution of the near-earth objects

This paper analyzes the distribution of the orbits of near-Earth minor bodies from the data on more than 7500 objects. The distribution of large near-Earth objects (NEOs) with absolute magnitudes of H < 18 is generally consistent with the earlier predictions (Bottke et al., 2002; Stuart, 2003), although we have revealed a previously undetected maximum in the distribution of perihelion distances q near q = 0.5 AU. The study of the orbital distribution for the entire sample of all detected objects has found new significant features. In particular, the distribution of perihelion longitudes seriously deviates from a homogeneous pattern; its variations are roughly 40% of its mean value. These deviations cannot be stochastic, which is confirmed by the Kolmogorov-Smirnov test with a more than 0.9999 probability. These features can be explained by the dynamic behavior of the minor bodies related to secular resonances with Jupiter. For the objects with H < 18, the variations in the perihelion longitude distribution are not so apparent. By extrapolating the orbital characteristics of the NEOs with H < 18, we have obtained longitudinal, latitudinal, and radial distributions of potentially hazardous objects in a heliocentric ecliptic coordinate frame. The differences in the orbital distributions of objects of different size appear not to be a consequence of observational selection, but could indicate different sources of the NEOs.     

Applying machine learning to catalogue matching in astrophysics

We present the results of applying automated machine learning techniques to the problem of matching different object catalogues in astrophysics. In this study, we take two partially matched catalogues where one of the two catalogues has a large positional uncertainty. The two catalogues we used here were taken from the H  i Parkes All Sky Survey (HIPASS) and SuperCOSMOS optical survey. Previous work had matched 44 per cent (1887 objects) of HIPASS to the SuperCOSMOS catalogue.
A supervised learning algorithm was then applied to construct a model of the matched portion of our catalogue. Validation of the model shows that we achieved a good classification performance (99.12 per cent correct).
Applying this model to the unmatched portion of the catalogue found 1209 new matches. This increases the catalogue size from 1887 matched objects to 3096. The combination of these procedures yields a catalogue that is 72 per cent matched.     

A high galactic latitude survey of far-ultraviolet excess objects

Optical spectra have been obtained for a selection of objects included in a catalog of far ultraviolet bright, high galactic latitude objects detected with a balloon-borne survey telescope. The observed objects provide a sample of subdwarf O and B stars, white dwarfs, and binary systems including a hot subluminous member. Model atmospheres analysis of the subdwarf sample is used to determine the temperature, gravity, and helium to hydrogen ratio of the individual objects. A smooth distribution of objects is found on the gravity versus temperature diagram near the theoretical location of the extended horizontal branch. A break between the helium rich and helium poor objects occurs at 40,000 K. Suspected binary objects were found and analyzed to determine the temperature and gravity of the hot subluminous member in each system. The number of subdwarf stars contained in binaries is determined to be from 65% to 100%. The number versus ultraviolet magnitude distribution of the subdwarf B sample is modeled to obtain a midplane density of 3.3 10(-6) pc-3 and a population scale height of 240 pc. The proportion of white dwarfs that experience the subdwarf phase of evolution is found to be 0.94%.     

Analysis of the spatial distribution of gamma-ray bursts in their host galaxies

We compare the radial distributions of known localized gamma-ray bursts (GRBs) relative to the centers of their host galaxies with the distributions of known objects in nearby galaxies (supernovae of various types, X-ray binaries), the hypothetical dark-matter profiles, and the distribution of luminous matter in galaxies in the model of an exponential disk. By comparing the moments of empirical distributions, we show that the radial distribution of GRBs in galaxies differs significantly from that of other sources. We suggest a new statistical method for comparing empirical samples that is based on estimating the number of objects within a given radius. The exponential disk profile was found to be in best agreement with the radial distribution of GRBs. The distribution of GRBs relative to the centers of their host galaxies also agrees with the dark matter profile at certain model parameters.     

The unification of RBLs with XBLs

Many astronomers have discussed the property of BL Lacertae objects, including the variation of spectrum, the correlation of multi-wave bands and the property of polarization, which could give good information for studying intrinsic correlation of components and position of BL Lacertae objects. In the paper, we investigated the properties of RBLs and XBLs and RBLs/XBLs (these objects can also be found by radio survey as by X-ray survey). Firstly, we collected the light curve of 28 BL Lacertae objects, and gained their short timescales by Structure Function (SF). Secondly, we analyzed the distribution of the short timescales of some BL Lacertae objects, including 28 objects’ timescales calculated by SF, as well as the distribution of the redshift and the black hole mass and the flux densities of the multi-wavebands (radio, near-infrared, optical and Gamma-ray). Based on statistical analysis, the result of the paper support the unification model of RBLs and XBLs.     

Dynamical relaxation and massive extrasolar planets

Following the suggestion of Black that some massive extrasolar planets may be associated with the tail of the distribution of stellar companions, we investigate a scenario in which 5 N 100 planetary mass objects are assumed to form rapidly through a fragmentation process occuring in a disc or protostellar envelope on a scale of 100 au. These are assumed to have formed rapidly enough through gravitational instability or fragmentation that their orbits can undergo dynamical relaxation on a time-scale of ∼100 orbits.
Under a wide range of initial conditions and assumptions, the relaxation process ends with either (i) one potential 'hot Jupiter' plus up to two 'external' companions, i.e. planets orbiting near the outer edge of the initial distribution; (ii) one or two 'external' planets or even none at all; (iii) one planet on an orbit with a semi-major axis of 10 to 100 times smaller than the outer boundary radius of the inital distribution together with an 'external' companion. Most of the other objects are ejected and could contribute to a population of free-floating planets. Apart from the potential 'hot Jupiters', all the bound objects are on orbits with high eccentricity, and also with a range of inclination with respect to the stellar equatorial plane. We found that, apart from the close orbiters, the probability of ending up with a planet orbiting at a given distance from the central star increases with the distance. This is because of the tendency of the relaxation process to lead to collisions with the central star. The scenario we envision here does not impose any upper limit on the mass of the planets. We discuss the application of these results to some of the more massive extrasolar planets.     

Optical colors of 56 near-Earth objects: trends with size and orbit

We present the results of BVRIZ photometry of 56 near-Earth objects (NEOs) obtained with the 1-m Jacobus Kapteyn telescope on La Palma during 2000 and 2001. Our sample includes many NEOs with particularly deep 1-μm pyroxene/olivine absorption bands, similar to Q-type asteroids. We also classify three NEOs with particularly blue colors. No D-type asteroids were found, placing an upper limit of ∼2% on the fraction of the NEO population originating in the outer main belt or the Trojan clouds. The ratio of dark to bright objects in our sample was found to be 0.40, significantly higher than current theoretical predictions. As well as classifying the NEOs, we have investigated color trends with size and orbit. We see a general trend for larger silicate objects to have shallower absorption bands but find no significant difference in the distribution of taxonomic classes at small and large sizes. Our data clearly show that different taxonomic classes tend to occupy different regions of (a, e) space. By comparing our data with current model predictions for NEO dynamical evolution we see that Q-, R-, and V-type NEOs tend to have orbits associated with “fast track” delivery from the main belt, whereas S-type NEOs tend to have orbits associated with “slow track” delivery. This outcome would be expected if space weathering occurs on time scales of >10⁶ years.     

The fraction of Compton-thick sources in an INTEGRAL complete AGN sample

We study the N _H distribution in a complete sample of 88 active galactic nuclei (AGN) selected in the 20–40 keV band from INTEGRAL /Imager on Board the Integral Satellite (IBIS) observations. We find that the fraction of absorbed  ( N _H≥ 10²² cm²)  sources is 43 per cent while the Compton thick AGN comprise 7 per cent of the sample. While these estimates are fully compatible with previous soft gamma-ray surveys, they would appear to be in contrast with results reported by Risaliti, Maiolino & Salvati using an optically selected sample. This apparent difference can be explained as being due to a selection bias caused by the reduction in high energy flux in Compton thick objects rendering them invisible at our sensitivity limit. Taking this into account, we estimate that the fraction of highly absorbed sources is actually in close agreement with the optically selected sample. Furthermore, we show that the measured fraction of absorbed sources in our sample decreases from 80 to ∼20–30 per cent as a function of redshift with all Compton thick AGN having   z ≤ 0.015  . If we limit our analysis to this distance and compare only the type 2 objects in our sample with the Risaliti et al. objects below this redshift value, we find a perfect match to their N _H distribution. We conclude that in the low-redshift bin we are seeing almost the entire AGN population, from unabsorbed to at least mildly Compton thick objects, while in the total sample we lose the heavily absorbed 'counterparts' of distant and therefore dim sources with little or no absorption. Taking therefore this low z bin as the only one able to provide the 'true' distribution of absorption in types 1 and 2 AGN, we estimate the fraction of Compton thick objects to be ≥24 per cent.     

Collisional velocities and rates in resonant planetesimal belts

We consider a belt of small bodies (planetesimals, asteroids, dust particles) around a star, captured in one of the external or 1:1 mean-motion resonances with a massive perturber (protoplanet, planet). The objects in the belt collide with each other. Combining methods of celestial mechanics and statistical physics, we calculate mean collisional velocities and mean collisional rates, averaged over the belt. The results are compared to collisional velocities and rates in a similar, but non-resonant belt, as predicted by the particle-in-a-box method. It is found that the effect of the resonant lock on the velocities is rather small, while on the rates more substantial. At low to moderate eccentricities and libration amplitudes of tens of degrees, which are typical of many astrophysical applications, the collisional rates between objects in an external resonance are by about a factor of two higher than those in a similar belt of objects not locked in a resonance. For Trojans under the same conditions, the collisional rates may be enhanced by up to an order of magnitude. The collisional rates increase with the decreasing libration amplitude of the resonant argument, depend on the eccentricity distribution of objects, and vary from one resonance to another. Our results imply, in particular, shorter collisional lifetimes of resonant Kuiper belt objects in the solar system and higher efficiency of dust production by resonant planetesimals in debris disks around other stars.     

Kuiper Belt structure as a reflection of the migration process of a planet

One of the main particular features of the structure of the Kuiper Belt is that it contains clusters of objects of small orbital eccentricity and inclination (“cold population”). In order to solve the problem of the origin of the objects, we considered the process of the gravitational interaction of a comparatively small-mass planet with a planetesimal disk. We found that one particular property of the process is that the planet changes its direction of migration. The interaction with the planet results in the transportation of a considerable portion of planetesimals from the inner zone out to the Kuiper Belt. After such a transition of the objects, the planet returns to the inner regions of the planetesimal disk. Numerical simulations show that the reversible migration of a planet of a mass similar to that of the Earth can explain the main properties of the Kuiper Belt’s cold population orbit distribution.     

近地小行星发现数目与发现场景的统计分析

近地小行星是一类可能对地球安全造成潜在威胁的太阳系小天体, 目前绝大部分的近地小行星是由地基望远镜发现的, 且数目仍在不断增加. 为了对我国未来开展近地小行星发现监测提供参考和借鉴, 利用国际小行星中心公开的数据库对所有近地小行星首次发现时刻的观测资料开展了多维度统计分析. 发现望远镜探测能力的限制会对近地小行星的发现造成选择效应, 导致不同轨道类型近地小行星发现的相对比例逐年变化且与直径有关. 另外, 结合数值模拟获得的轨道数据, 对近地小行星首次发现时的观测场景进行了还原, 获得了发现时刻近地小行星位置在不同天球坐标系的分布, 分析了其分布特征与季节、测站纬度和小行星直径的依赖关系. 最后, 通过分析数据定量考察了太阳、月球和银道面对近地小行星发现的影响, 发现地基望远镜一般难以发现来自太阳方向90$^\circ$范围内直径140m以下的近地小行星, 并且随着小行星直径的减小该限制范围也将变大; 月光污染对近地小行星发现的影响也非常显著, 望月前后几天的观测限制可导致约29%的目标无法被发现, 而且分析表明农历上半月发现的目标一般比下半月发现的更难以被跟踪观测; 银道面特别是银心方向会对近地小行星发现产生影响, 使得黄道面附近存在与季节相关的观测``盲区''.     

Non-power law behavior in fragmentation cascades

Collisions resulting in fragmentation are important in shaping the mass spectrum of minor bodies in the asteroid belt, the Kuiper Belt, and debris disks. Models of fragmentation cascades typically find that in steady-state, the solution for the particle mass distribution is a power law in the mass. However, previous studies have typically assumed that the mass of the largest fragment produced in a collision with just enough energy to shatter the target and disperse half its mass to infinity is directly proportional to the target mass. We show that if this assumption is not satisfied, then the power law solution for the steady-state particle mass distribution is modified by a multiplicative factor, which is a slowly varying function of the mass. We derive analytic solutions for this correction factor and confirm our results numerically. We find that this correction factor proves important when extrapolating over many orders of magnitude in mass, such as when inferring the number of large objects in a system based on infrared observations. In the course of our work, we have also discovered an unrelated type of non-power law behavior: waves can persist in the mass distribution of objects even in the absence of upper or lower cutoffs to the mass distribution or breaks in the strength law.     

The Influence of the Orbital Evolution of Main Belt Asteroids on Their Spin Vectors

It was found that certain features in the observed spin vector distribution of main belt asteroids can be explained by the differences in the dynamical spin vector evolution between objects with high and low orbital inclinations. In particular, the deficiency of high-inclination objects whose spin vectors are close to the ecliptic plane can be accounted for.The present spin vector distribution of main belt asteroids is due to several factors connected with their collisional and dynamical evolution. In this paper, the influence of the orbital evolution on the spin axis of asteroids is examined in the case of 25 objects with typical main belt orbital evolution and 125 synthetic objects, during an integration over a time period of 1 Myr. This investigation produced the following general results:• The difference between maximum and minimum obliquity increases in an approximately linear fashion with increasing orbital inclination of the studied objects.• The inclination is the major factor influencing the magnitude of the obliquity variation. This variation is generally larger for asteroids with their initial spin vectors located close to the orbital plane.• In general, the regular obliquity differences are relatively insensitive to differences in the shape, composition, and spin rate of the asteroids.The result is compared with the properties of the observed spin vectors for 73 main belt asteroids and good agreement is found between the above results and the existing spin vector distribution.     

Spectroscopic investigation of near-Earth objects at Telescopio Nazionale Galileo

In this paper we present new results obtained from our spectroscopic survey of near-Earth objects (called SINEO). We show a set of 36 visible and near-infrared spectra, recorded with the 3.5-m Italian Telescopio Nazionale Galileo at La Palma (Canary Island). We discuss their taxonomic classification (resulting in 25 objects belonging to the S-complex, five to the C-complex and six to the X-complex), and their overall compositional linkage with the principal source of near-Earth objects, namely the Main Belt. Moreover, for some near-Earth objects we found good spectral fit among meteorites. In particular, we achieved an excellent fit for chondrites of different clans. Finally, we discuss the influences of space weathering among small S-type near-Earth objects.     

Planet X and the Extended Scattered Disk

The effects that a hypothetical trans-Plutonian planet would produce on theorbital distribution of the Classical Edgeworth-Kuiper-Belt, has beensurveyed for different physical and orbital parameters of the hypotheticalbody in Melita et al. (2003a). The best fits were obtained by a moderatelyeccentric and inclined Earth-sized object with a semimajor axis of ～ 70AU. However the history of some objects in the `Extended Scattered disk’still represent a puzzle. One possibility is that they can be `extracted’from the Scattered disk by the planetoid. In this work we confirm that such anhypothesis would not explain the present orbit of 2000 CR105, given theconditions for a gap as observed to be formed in the Classical EKB.     

Spectroscopic observations of Jupiter Trojans

We present the results of a campaign of spectroscopic observations of Jupiter Trojan asteroids. Thirty-four objects were observed during three runs in July and November 1998, and March 2002 using the Danish 1.54-m telescope at ESO. The covered spectral range was between 5000 and 9000 Å. Our observations include objects belonging both L₄ to L₅ clouds. According to analyses of previous investigations of Trojans, the spectra of different taxonomic classes can be separated on the basis of the slope of the reflectance spectrum. The large majority of the objects of our sample have been found to belong to the D taxonomic class, but we found also objects of P- and C-type. In two cases, we found also evidence of blueish spectral trends. Our data are important, since they allow us to substantially enlarge the whole data set of available Trojan spectra.     

Spectral classification of stars in the second Byurakan Sky Survey. II

The results of the spectra/classification of 310 stars and objects having a continuous spectrum, selected in the course of the Second Byurakan Spectral Sky Survey, are given. Slit spectra were used in the work, which were obtained from 1978 to 1994, mainly on the 6-m telescope of the Special Astrophysical Observatory, Russian Academy of Sciences; some of the spectra were obtained on the 2.6-m telescope of the Byurakan Astrophysical Observatory, National Academy of Sciences of Armenia, and the 4.5-m Multiple-Mirror Telescope (USA), with 5–15 Å resolution. The sample contains objects in the range of stellar magnitudes 11.0 ≤ m(pg) ≤ 19.5. We found 93 DA, 2 DO, 12 DB, and 1 DC white dwarfs, 83 sdB and 20 sdO subdwarfs, 34 NHB variables, 2 cataclysmic variables, 48 stars of late spectral types, 8 spectroscopic binary systems, and 7 objects having a continuous spectrum. Several recordings are given for each of these spectral types.     

A compositional interpretation of trans-neptunian objects taxonomies

Trans-neptunian objects (TNOs) are a population of small objects orbiting the Sun beyond Neptune. Because of their distance they are difficult to observe spectroscopically, but a large body of photometric observations is available and growing. TNOs are important tracers of the evolution of the outer Solar System and key when testing current dynamical evolution theories. Previous statistical studies of the colors of TNOs have yielded useful but limited results regarding the chemical history and evolution of these bodies.With the aim at obtaining compositional information on the small and distant TNOs we introduce a statistical cluster analysis (labelled albedo) based on colors and published albedos of TNOs. We compare it to a previous taxonomy, to illustrate the significance of including the albedo information when determining the composition of the objects. When the albedo contribution is removed from the data, the new taxonomy (now labelled classical) is in general agreement with the published ones, supporting the applicability of our approach. Making use of modeled reflectance spectra of a variety of plausible mixtures found on the surface of TNOs, we extract the average surface composition of each taxon, for both the classical and the albedo taxonomy, in a statistically consistent fashion.Differently from previous and classical, the albedo taxonomy establishes a direct link between the colors and albedos of the objects and their surface composition, allowing, for the first time, a quick assessment of the chemical history of TNOs. In fact, under closer examination the taxa show trends in composition that might be evolutionary in nature. If a simple ‘snow lines’ model is adopted, we can infer that albedo taxa relate the current objects’ locations to their original ones, prior to the migration of the outer planets. We regard the large population that characterizes the darkest classes spread at a variety of semi-major axis distances as one of the intriguing results of this work.     

Computing the effects of YORP on the spin rate distribution of the NEO population

The overall change of NEO spin rate due to planetary encounters and YORP is evaluated by using a Monte Carlo model. A large sample of test objects mimicking a source population is evolved over a timescale comparable with the Solar System age until they reach a steady state spin distribution that should reproduce the current NEO distribution. The spin change due to YORP is computed for each body according to a simplified model based on Scheeres [Scheeres, D.J., 2007a. Icarus 188, 430-450].The steady state cumulative distribution of NEO spin rates obtained from our simulation nicely reproduces the observed one, once our results are biased to match the diameter distribution of the sample of objects included in the observational database. The excellent agreement strongly suggests that YORP is responsible for the concentration of spin at low rotation rates. In fact, in the absence of YORP the steady state population significantly deviates from the observed one. The spin evolution due to YORP is also so rapid for NEOs that the initial rotation rate distribution of any source population is quickly relaxed to that of the observed population. This has profound consequences for the study of NEO origin since we cannot trace the sources of NEOs from their rotation rate only.     

Testing the circumstellar disc hypothesis: a search for H2 outflow signatures from massive young stellar objects with linearly distributed methanol masers

The results of a survey searching for outflows using near-infrared imaging are presented. Targets were chosen from a compiled list of massive young stellar objects associated with methanol masers in linear distributions. Presently, it is a widely held belief that these methanol masers are found in (and delineate) circumstellar accretion discs around massive stars. If this scenario is correct, one way to test the disc hypothesis is to search for outflows perpendicular to the methanol maser distributions. The main objective of the survey was to obtain wide-field near-infrared images of the sites of linearly distributed methanol masers using a narrow-band 2.12-μm filter. This filter is centred on the  H₂ v = 1–0 S(1)  line; a shock diagnostic that has been shown to successfully trace CO outflows from young stellar objects. 28 sources in total were imaged of which 18 sources display H₂ emission. Of these, only two sources showed emission found to be dominantly perpendicular to the methanol maser distribution. Surprisingly, the H₂ emission in these fields is not distributed randomly, but instead the majority of sources are found to have H₂ emission dominantly parallel to their distribution of methanol masers. These results seriously question the hypothesis that methanol masers exist in circumstellar discs. The possibility that linearly distributed methanol masers are instead directly associated with outflows is discussed.