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1.
J.P Emery  R.H Brown 《Icarus》2003,164(1):104-121
We present new near-infrared spectra of 20 Trojan asteroids. The spectra were recorded at the NASA Infrared Telescope Facility (IRTF) using the recently commissioned medium-resolution spectrograph SpeX and at the Multiple Mirror Telescope (MMT) using the instrument FSPEC. Spectra of all of these objects were measured in K-band (1.95-2.5 μm), 8 of these in L-band (2.8-4.0 μm), and 14 in the range 0.8-2.5 μm. These observations nearly double the number of published 0.8-2.5 μm spectra of Trojan asteroids and provide the first systematic study of the L-band region for these distant asteroids. The data show that the red spectral slope measured in the near-IR extends through the L-band, though it is not as steep here as at shorter wavelengths. A significant diversity is apparent in the near-IR spectral slopes of this sampling of objects. Most of the spectra do not contain any definitive absorption features characteristic of surface composition (e.g., H2O, organics, silicates) as seen on main-belt asteroids and several Centaur and Kuiper Belt objects. A few objects may display spectral activity, and the reliability of these possible features is discussed. While these spectra are generally compatible with silicate surfaces to explain the spectral slope mixed with some fraction of low albedo material, there is no absolute indication of silicates. The spectral slope could also be explained by the presence of hydrocarbons, but the lack of absorption features, especially in L-band where very strong fundamental absorptions from these molecules appear, constrains the character and abundance of these materials at the surface.  相似文献   

2.
We have used the ESO Schmidt telescope during the apparitions 1996, 1997 and 1998 to study the preceding Lagrangian cloud of Jupiter, L4. In our observed field of view, about 700 square degrees, during 1996 we found 399 moving objects that we classified as Trojans. From this we conclude that down to absolute magnitude H ≤ 13.0 there are ≈ 1100 Trojans in the L4 cloud. Proper elements of 696 numbered and multiopposition Trojans were calculated and one of these in the L5 cloud were found to be a non Trojan. A discussion of the Trojan proper elements is also presented.  相似文献   

3.
We present a number of spectra of Near-Earth Objects taken in the period 1998-2003 with two different instruments (CGS4 and UIST) on the UKIRT telescope. Since observations with CGS4 require multiple spectral fragments to be observed sequentially and then spliced together we assess the reliability of this technique using comparisons between multiple observations of the same object, between observations of the same object with both instruments and with independent spectra of common objects. We conclude that while problems in the spectral splicing can occur, they are usually intuitively obvious and that overall our dataset is sound. The objects for which we present new spectral data are: 1627 Ivar, 4179 Toutatis, 5381 Sekhmet, (5587) 1990 SB, 6489 Golevka, (11405) 1999 CV3, (14402) 1991 DB, 25143 Itokawa, (25330) 1999 KV4, (52760) 1998 ML14, (66391) 1999 KW4, and (101955) 1999 RQ36. Our results, together with albedo data from the literature, suggest carbonaceous compositions for 25330 and 101955. The available data for 14402 suggest it may belong to the relatively rare M class. Our analysis suggests an S or Sq classification for 52760 and a V classification for 5381 Sekhmet. For all remaining objects the UKIRT data are consistent with published spectral classifications. We find that only 3 of the 12 objects are not S/Q/V-class, which is roughly consistent with the results of Binzel et al. [Binzel, R.P., Rivkin, A.S., Stuart, J.S., Harris, A.W., Bus, S.J., Burbine, T.H., 2004. Icarus 170, 259-294]. Four spectra of Toutatis taken over a range of solar phase angles between 0.7°-81° and at intervals of several weeks are indistinguishable within the uncertainties and therefore do not reveal any evidence for phase reddening or surface variegation.  相似文献   

4.
Many asteroids with a semimajor axis close to that of Mars have been discovered in the last several years. Potentially some of these could be in 1:1 resonance with Mars, much as are the classic Trojan asteroids with Jupiter, and its lesser-known horseshoe companions with Earth. In the 1990s, two Trojan companions of Mars, 5261 Eureka and 1998 VF31, were discovered, librating about the L5 Lagrange point, 60° behind Mars in its orbit. Although several other potential Mars Trojans have been identified, our orbital calculations show only one other known asteroid, 1999 UJ7, to be a Trojan, associated with the L4 Lagrange point, 60° ahead of Mars in its orbit. We further find that asteroid 36017 (1999 ND43) is a horseshoe librator, alternating with periods of Trojan motion. This asteroid makes repeated close approaches to Earth and has a chaotic orbit whose behavior can be confidently predicted for less than 3000 years. We identify two objects, 2001 HW15 and 2000 TG2, within the resonant region capable of undergoing what we designate “circulation transition”, in which objects can pass between circulation outside the orbit of Mars and circulation inside it, or vice versa. The eccentricity of the orbit of Mars appears to play an important role in circulation transition and in horseshoe motion. Based on the orbits and on spectroscopic data, the Trojan asteroids of Mars may be primordial bodies, while some co-orbital bodies may be in a temporary state of motion.  相似文献   

5.
J.P. Emery  R.H. Brown 《Icarus》2004,170(1):131-152
We present the results of spectral modeling of 17 Trojan asteroids. The surface composition of this group of objects (located just beyond the main belt, trapped in Jupiter's stable Lagrange points) remains uncertain due to an absence of diagnostic absorption features in their spectra. We quantitatively analyze spectra of these objects covering the range 0.3-4.0 μm using the formulation for scattering in a particulate medium developed by Hapke. Since the widest spectral range possible is desired to provide the most robust results, recently measured near-IR spectra are combined with previously published visible and near-IR data. These composite spectra are converted to and modeled in terms of geometric albedo to provide the additional constraint of the absolute brightness of the asteroids. It is important that this modeling is performed for a large number of objects, and results are derived based on trends among best-fit models. Under this rigorous examination, we find that it is unlikely that the red spectral slope is a result of organics on the surfaces, due mainly to the lack of absorptions in the L-band. Instead, anhydrous silicates adequately describe the spectral characteristics of this group of objects. A significant fraction of carbonaceous material is also likely present, but is not responsible for the red spectral slope in these models. Also, using these models, we estimate that these surfaces contain at most a few wt% of H2O ice and no more than 10-30 wt% of hydrated silicates.  相似文献   

6.
C.L Dandy  A Fitzsimmons 《Icarus》2003,163(2):363-373
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.  相似文献   

7.
All the Trojan asteroids orbit about the Sun at roughly the same heliocentric distance as Jupiter. Differences in the observed visible reflection spectra range from neutral to red, with no ultra-red objects found so far. Given that the Trojan asteroids are collisionally evolved, a certain degree of variability is expected. Additionally, cosmic radiation and sublimation are important factors in modifying icy surfaces even at those large heliocentric distances. We search for correlations between physical and dynamical properties, we explore relationships between the following four quantities; the normalised visible reflectivity indexes (S′), the absolute magnitudes, the observed albedos and the orbital stability of the Trojans. We present here visible spectroscopic spectra of 25 Trojans. This new data increase by a factor of about 5 the size of the sample of visible spectra of Jupiter Trojans on unstable orbits. The observations were carried out at the ESO-NTT telescope (3.5 m) at La Silla, Chile, the ING-WHT (4.2 m) and NOT (2.5 m) at Roque de los Muchachos observatory, La Palma, Spain. We have found a correlation between the size distribution and the orbital stability. The absolute-magnitude distribution of the Trojans in stable orbits is found to be bimodal, while the one of the unstable orbits is unimodal, with a slope similar to that of the small stable Trojans. This supports the hypothesis that the unstable objects are mainly byproducts of physical collisions. The values of S′ of both the stable and the unstable Trojans are uniformly distributed over a wide range, from 0%/1000 Å to about 15%/1000 Å. The values for the stable Trojans tend to be slightly redder than the unstable ones, but no significant statistical difference is found.  相似文献   

8.
New 30-channel narrowband photometry from 0.32 to 1.05 μm of the retrograde Jovian satellites J9 (to 0.7 μm) and J8 and the trailing Trojan asteroids 617, 884, 1172, and 1173 is presented. The data confirm previous measurements of J8, 617, 884, and 1172 at λ < 0.8 μm, but the extension into the infrared shows that the normalized spectral reflectance of these objects rises steadily from ～0.8 at 0.4 μm to ～ 1.4 at 1.05 μm, suggesting they are too bright in the near infrared to be C-type asteroids. The C classification of 1173 is confirmed. J9 is markedly redder than J8 at visible wavelengths. The results indicate a greater taxonomic contrast between these distant objects and main-belt asteroids than previously thought.  相似文献   

9.
Up to now, 17 Neptune Trojan asteroids have been detected with their orbits being well determined by continuous observations. This paper analyzes systematically their orbital dynamics. Our results show that except for two temporary members with relatively short lifespans on Trojan orbits, the vast majority of Neptune Trojans located within their orbital uncertainties may survive in the solar system age. The escaping probability of Neptune Trojans, through slow diffusion in the orbital element space in 4.5 billion years, is estimated to be ～50%. The asteroid 2012 UW177 classified as a Centaur asteroid by the IAU Minor Planet Center currently is in fact a Neptune Trojan. Numerical simulations indicate that it is librating on the tadpole-shaped orbit around the Neptune's L4 point. It was captured into the current orbit approximately 0.23 million years ago, and will stay there for at least another 1.3 million years in the future. Its high inclination of i ≈ 54° not only makes it the most inclined Neptune Trojan, but also makes it exhibit the complicated and interesting co-orbital transitions between the leading and trailing Trojans via the quasi-satellite orbit phase.  相似文献   

10.
Anita L Cochran  Faith Vilas 《Icarus》2004,167(2):360-368
We present spectral observations of Minor Planet 4 Vesta, of five V-type asteroids which are physically near Vesta, and of two V-type NEAs. We use these spectra to determine the presence or absence of a weak feature at 506.5 nm which is indicative of the presence of spin-forbidden Fe2+ in sixfold coordination. As with our earlier observations [Cochran and Vilas, Icarus 134 (1998) 207-212], we find this feature at all observed rotational phases of Vesta and again see the trend that spectra at longitudes between 240° and 360° have a smaller 506.5 nm feature equivalent width than spectra obtained at other longitudes. Additionally, we searched for this feature in V-class main-belt and NEA asteroids and positively detected the feature in main-belt Asteroid 2579 Spartacus and possibly in 3376 Armandhammer. The other objects lacked the feature. Our results are compared with previous observations of this feature by Vilas et al. [Icarus 147 (2000) 119-128]. The spatial distribution of the bodies as a function of the presence of this feature was investigated. We discuss the implication of the presence of this feature and the depth of the 0.9 μm pyroxene band for the scenario that pieces of Vesta were transported, via the 3:1 and ν6 resonances, to the NEAs, and thence to inclusion in our meteorite collections as HED meteorites.  相似文献   

11.
H.J. Reitsema 《Icarus》1981,48(1):140-142
The 1980 observations of the Saturn system have revealed objects at both the preceding (L4) and following (L5) triangular libration points of Tethys (S4). The observations indicate a small (～2°) libration amplitude for the L4 body while the data on the L5 object are insufficient to define its libration amplitude.  相似文献   

12.
We use two independent methods to reduce the data of the surveys made with RATAN-600 radio telescope at 7.6 cm in 1988–1999 at the declination of the SS433 source. We also reprocess the data of the “Cold” survey (1980–1981). The resulting RCR (RATAN COLD REFINED) catalog contains the right ascensions and fluxes of objects identified with those of the NVSS catalog in the right-ascension interval 7 h ≤ R.A. < 17 h . We obtain the spectra of the radio sources and determine their spectral indices at 3.94 and 0.5 GHz. The spectra are based on the data from all known catalogs available from the CATS, Vizier, and NED databases, and the flux estimates inferred from the maps of the VLSS and GB6 surveys. For 245 of the 550 objects of the RCR catalog the fluxes are known at two frequencies only: 3.94 GHz (RCR) and 1.4 GHz (NVSS). These are mostly sources with fluxes smaller than 30mJy. About 65% of these sources have flat or inverse spectra (α > −0.5). We analyze the reliability of the results obtained for the entire list of objects and construct the histograms of the spectral indices and fluxes of the sources. Our main conclusion is that all 10–15 mJy objects found in the considered right-ascension interval were already included in the decimeter-wave catalogs.  相似文献   

13.
We present moderate resolution CCD spectra and R photometry for seven KP2001 stars. We revised the spectral classification of the stars in the range λλ4000−8700? . Our photometric data confirmed the behavior of the light curves downloaded from the NSVS (Northern Sky Variability Survey) database. For KP2001-32, presented as a Mira-type variable in NSVS, we estimated absolute bolometric M b and K-band M K magnitudes as well as the distance using the period-luminosity relations. We also estimated the mass loss rate using the calibration relations between mass loss rate and K - [12] index. From the position in infrared color-color diagrams, we confirmed the photometric classification of KP2001-221 as a semiregular variable, based on the light curve of the NSVS database. For the N-type carbon star KP2001-77 we estimated distance and absolute magnitude M K using different calibration methods. For the remaining four objects we derived the absolute magnitudes and distances using our CCD spectra and published JHK S magnitudes. We discuss the nature of these objects on the basis of the obtained results.  相似文献   

14.
R. Gil-Hutton  J. Licandro 《Icarus》2010,206(2):729-734
In this paper we search for photometric data of asteroids in the outer region of the Hecuba gap in the Moving Object Catalogue of the Sloan Digital Sky Survey to find the spectrophotometric characteristics of small members of this group. We found that the correlation between size and spectral slope previously identified for Cybele asteroids is correct only for large objects (HV<12) but it is not supported by data obtained for the small ones. This result argues against the scenario suggesting that D-type objects are more fragile than P-types, favoring disruptive collisions of precursors of the first type and resulting in a larger fraction of the smaller body population being collisional fragments from a few large D-type precursors. A statistical comparison of the spectral slope histograms of Cybeles and Hildas showed that it is not possible to reject the hypothesis that both samples were obtained from the same population at a confident limit of 90%. This result could be indicative of certain homogeneity in the taxonomic distributions of the outer belt populations due to a similar original composition and/or a similar resurfacing processes of these distant bodies. Despite the intrinsic limitations of the five band photometry of the Sloan Digital Sky Survey, the analysis presented is based mainly in the detection of spectral slopes thus providing sufficient indication of the taxonomic type of these asteroids and making us confident about our conclusions.  相似文献   

15.
Using new laboratory spectra, we have calculated the real and imaginary parts of the index of refraction of amorphous and crystalline H2O-ice from 20 to 150 K in the frequency range 9000-3800 cm−1 (1.1-2.6 μm) at a spectral resolution of 1 cm−1. These optical constants can be used to create model spectra for comparison to spectra from Solar System objects. We also analyzed the differences between the amorphous and crystalline H2O-ice spectra, including weakening of bands and shifting of bands to shorter wavelength in amorphous H2O-ice spectra. We have also observed two spectrally distinct phases of amorphous H2O-ice.  相似文献   

16.
The Eurybates family is a compact core inside the Menelaus clan, located in the L4 swarm of Jupiter Trojans. Fornasier et al. (Fornasier, S., Dotto, E., Hainaut, O., Marzari, F., Boehnhardt, H., De Luise, F., Barucci, M.A. [2007]. Icarus 190, 622-642) found that this family exhibits a peculiar abundance of spectrally flat objects, similar to Chiron-like Centaurs and C-type main belt asteroids. On the basis of the visible spectra available in literature, Eurybates family’s members seemed to be good candidates for having on their surfaces water/water ice or aqueous altered materials.To improve our knowledge of the surface composition of this peculiar family, we carried out an observational campaign at the Telescopio Nazionale Galileo (TNG), obtaining near-infrared spectra of 7 members. Our data show a surprisingly absence of any spectral feature referable to the presence of water, ices or aqueous altered materials on the surface of the observed objects. Models of the surface composition are attempted, evidencing that amorphous carbon seems to dominate the surface composition of the observed bodies and some amount of silicates (olivine) could be present.  相似文献   

17.
In this review we provide the data needed to interpret remote spectroscopic studies of O2 molecules embedded in the icy surfaces of outer solar system bodies. O2 produced by radiolysis has been seen in the gas phase and as the so-called ‘solid O2’ trapped in the icy surfaces of Ganymede, Europa and Callisto. It may also have been indirectly observed on a number of objects by its radiolysis product, O3. These observations indicate the importance of O2 for understanding the chemical processes occurring on icy outer solar system surfaces. Therefore, the published absorption spectra of gaseous, liquid and solid O2 and of O2 embedded in H2O ice are reviewed in some detail. Particular emphasis has been placed on the presentation of transition probabilities for the various O2 spectral series so that their relative importances can be assessed when they are used for modelling the radiation chemistry occurring in such environments.  相似文献   

18.
Mid-infrared spectra measured by Cassini's Composite InfraRed Spectrometer (CIRS) between July 2004 and January 2007 (Ls=293°-328°) have been used to determine stratospheric temperature and abundances of C2H2, C3H4, C4H2, HCN, and HC3N. Over 65,000 nadir spectra with spectral resolutions of 0.5 and 2.5 cm−1 were used to probe spatial and temporal composition variations in Titan's stratosphere. Cassini's 180° orbital transfer in mid-2006 allowed low emission angle observations of the north polar region for the first time in the mission and allowed us to probe the full latitude range. We present the first measurements of composition variations within the polar vortex, which display increasing abundances right up to 90° N. The lack of a homogeneous abundance-latitude variation within the vortex indicates limited horizontal mixing and suggests that subsidence is greatest at the vortex core. Contrary to numerical model predictions and tropospheric cloud observations, we do not see any evidence for a secondary circulation cell near the south pole, which suggests a single Hadley-type circulation in the stratosphere at this epoch. This difference can be reconciled if the secondary cell is restricted to altitudes below 100 km, where there is no sensitivity in our data. Temporal variations in composition were observed in the south, with volatile species becoming less abundant as the season progressed. The observed variations are compared to numerical model predictions and observations from Voyager.  相似文献   

19.
We have performed an ecliptic survey of the Kuiper belt, with an areal coverage of 8.9 square degrees to a 50% limiting magnitude of , and have detected 88 Kuiper belt objects, roughly half of which received follow-up 1–2 months after detection. Using this survey data alone, we have measured the luminosity function of the Kuiper belt, thus avoiding any biases that might come from the inclusion of other observations. We have found that the Cold population defined as having inclinations less than 5° has a luminosity function slope αCold = 0.82 ± 0.23, and is different from the Hot population, which has inclinations greater than 5° and a luminosity function slope αHot = 0.35 ± 0.21. As well, we have found that those objects closer than 38 AU have virtually the same luminosity function slope as the Hot population. This result, along with similar findings of past surveys demonstrates that the dynamically Cold Kuiper belt objects likely have a steep size distribution, and are unique from all of the excited populations which have much shallower distributions. This suggests that the dynamically excited population underwent a different accretion history and achieved a more evolved state of accretion than the Cold population. As well, we discuss the similarities of the Cold and Hot populations with the size distributions of other planetesimal populations. We find that while the Jupiter family comets and the scattered disk exhibit similar size distributions, a power-law extrapolation to small sizes for the scattered disk cannot account for the observed influx of comets. As well, we have found that the Jupiter Trojan and Hot populations cannot have originated from the same parent population, a result that is difficult to reconcile with scattering models similar to the NICE model. We conclude that the similarity between the size distributions of the Cold population and the Jupiter Trojan population is a striking coincidence.  相似文献   

20.
It has recently been shown that Jupiter Trojans may exhibit chaotic behavior, a fact that has put in question their presumed long term stability. Previous numerical results suggest a slow dispersion of the Trojan swarms, but the extent of the ‘effective’ stability region in orbital elements space is still an open problem. In this paper, we tackle this problem by means of extensive numerical integrations. First, a set of 3,200 fictitious objects and 667 numbered Trojans is integrated for 4 Myrs and their Lyapunov time, TL, is estimated. The ones following chaotic orbits are then integrated for 1 Gyr, or until they escape from the Trojan region. The results of these experiments are presented in the form of maps of TLand the escape time, TE, in the space of proper elements. An effective stability region for 1 Gyr is defined on these maps, in which chaotic orbits also exist. The distribution of the numbered Trojans follows closely the TE=1 Gyr level curve, with 86% of the bodies lying inside and 14% outside the stability region. This result is confirmed by a 4.5 Gyr integration of the 246 chaotic numbered Trojans, which showed that 17% of the numbered Trojans are unstable over the age of the solar system. We show that the size distributions of the stable and unstable populations are nearly identical. Thus, the existence of unstable bodies should not be the result of a size-dependent transport mechanism but, rather, the result of chaotic diffusion. Finally, in the large chaotic region that surrounds the stability zone, a statistical correlation between TLandTE is found.  相似文献   

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