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1.
Dwarf-planet (1) Ceres is one of the two targets, along with (4) Vesta, that will be studied by the NASA Dawn spacecraft via imaging, visible and near-infrared spectroscopy, and gamma-ray and neutron spectroscopy. While Ceres’ visible and near-infrared disk-integrated spectra have been well characterized, little has been done about quantifying spectral variations over the surface. Any spectral variation would give us insights on the geographical variation of the composition and/or the surface age. The only work so far was that of Rivkin and Volquardsen ([2010], Icarus 206, 327) who reported rotationally-resolved spectroscopic (disk-integrated) observations in the 2.2–4.0 μm range; their observations showed evidence for a relatively uniform surface.Here, we report disk-resolved observations of Ceres with SINFONI (ESO VLT) in the 1.17–1.32 μm and 1.45–2.35 μm wavelength ranges. The observations were made under excellent seeing conditions (0.6″), allowing us to reach a spatial resolution of ～75 km on Ceres’ surface. We do not find any spectral variation above a 3% level, suggesting a homogeneous surface at our spatial resolution. Slight variations (about 2%) of the spectral slope are detected, geographically correlated with the albedo markings reported from the analysis of the HST and Keck disk-resolved images of Ceres (Li et al. [2006], Icarus 182, 143; Carry et al. [2008], Astron. Astrophys. 478, 235). Given the lack of constraints on the surface composition of Ceres, however, we cannot assert the causes of these variations.  相似文献   

2.
The aim of this work is to understand the absence of objects along the orbits of Mimas and Enceladus in contrast to their presence at the orbits of neighbouring Tethys and Dione from the point of view of dynamical stability. Large scale numerical simulations of 360 test particles within the coorbital regions of these four saturnian satellites were carried out for 4×105 yr or 1.6×108 revolutions of the innermost moon Mimas. The tidal forcing of the satellites' orbits was not taken into account in these simulations. We have quantitatively reproduced the Mimas-Tethys 4:2 and Enceladus-Dione 2:1 mean motion resonances in the system and devised a scheme by which the parameter space of the coorbital resonance is sampled uniformly by our test particles. We observe that 6 out of the 36 integrated horseshoe particles of Enceladus escaped the coorbital region. All 54 tadpole particles remained stable. The main cause of instability for Enceladus coorbitals appears to be the overlap between the coorbital resonance and the 2:1 mean motion resonance between the particle and Dione. This leads particles with starting semimajor axes near the horseshoe-tadpole separatrix to be ejected from the resonance, as proposed by Morais [Morais, M.H.M., 2000. The effect of secular perturbations and mean motion resonances on trojan dynamics. Ph.D. thesis, Univ. of London], over timescales of ∼8×107 revolutions of Enceladus. For Mimas we observe a larger number of coorbital escapes overall, both of tadpole (7/54) and horseshoe (29/36) librators. An analysis of the observed dynamical evolution suggests a two-stage process at work: The semimajor axis of particles with starting conditions near the horseshoe-tadpole separatrix undergoes a slow random walk over timescales of 105 yr through a mechanism similar to that at Enceladus but involving the 4:2 inclination resonance with Tethys. These particles are eventually injected into a region of short-term (?104 yr) instability just inside the nominal boundary of stable, symmetric horseshoe motion. The presence of the 4:2 eccentricity triplet at that location is the most likely culprit for the instability. In both the cases of Mimas and Enceladus small-amplitude tadpoles remain stable until the end of the integration. The existence of fast escapers at Mimas provides a dynamical avenue for the short-term survival of impact ejecta in horseshoe orbits within Mimas' coorbital region.  相似文献   

3.
We perform numerical simulations to investigate potential Earth-like planets in the GJ 876 planetary system. We show that the secular resonances ν1 and ν2 (resulting respectively from the inner and outer giant planets) can excite the eccentricities of Earth-like planets with orbits 0.21 AU ≤ a < 0.50 AU and cause them to be ejected out of the system in a short time. However, in the dynamical sense, Earth-like planets potentially exist in the region 0.50 AU ≤ a ≤ 1.00 AU, in stable low-eccentricity orbits which may last up to 105 yr.  相似文献   

4.
Metamorphic CK carbonaceous chondrites display matrix textures that are best explained by a transient thermal event with temperatures in the 550–950 K range and durations in the order of days to years, longer than what is commonly admitted for shock events but shorter than what is required for nuclide decay. We propose that radiative heating of small carbonaceous meteoroids with perihelia close to the Sun could account for the petrological features observed in CK chondrites. Numerical thermal modeling, using favorable known NEOs orbital parameters (perihelion distances between 0.07 and 0.15 AU) and physical properties of CV and CK chondrites (albedo in the range 0.01–0.1, 25% porosity, thermal diffusivity of 0.5–1.5 W m?1 K?1), shows that radiative heating can heat carbonaceous meteoroids in the meter size range to core temperatures up to 1050 K, consistent with the metamorphic temperatures estimated for CK chondrites. Sizes of known CV and CK chondrites indicate that all these objects were small meteoroids (radii from a few cm to 2.5 m) prior to their atmospheric entry. Simulations of dynamic orbits for NEO objects suggest that there are numerous such bodies with suitable orbits and properties, even if they are only a small percentage of all NEOs. Radiative heating would be a secondary process (superimposed on parent-body processes) affecting meteoroids formed by the disruption of an initially homogeneous CV3-type parent body. Different petrologic types can be accounted for depending on the sizes and heliocentric distances of the objects in such a swarm.  相似文献   

5.
Based on the latest release of the SGP4/SDP4 (Simplified General Perturbation Version 4/ Simplified Deep-space Perturbation Version 4) model, in this paper we have designed an orbit determination program. Through calculations for the 1120 objects with various types and orbital elements selected from the space objects database, we have obtained the accuracies of the orbit determination prediction dealt with various types of space objects by the SGP4/SDP4 model. The results show that the accuracies of the near-earth objects are in the order of magnitude of 100 meters; the averages of the orbit determination accuracies of the semi-synchronous and geosynchronous orbits are, respectively, 0.7 and 1.9 km. The orbit determination accuracies of the elliptical orbit objects are related to their eccentricities. Except for few elliptical orbit objects with e > 0.8, the orbit determination errors of the vast majority of the elliptical orbit objects are all less than 10 km. By using the SGP4/SDP4 model to make 3 days predictions for near-earth objects, 30 days for semi-synchronous orbit objects, 15 days for geosynchronous orbit objects and 1 day for elliptical orbit objects, the errors of prediction generally don’t exceed 40 km.  相似文献   

6.
Phase angle and temperature are two important parameters that affect the photometric and spectral behavior of planetary surfaces in telescopic and spacecraft data. We have derived photometric and spectral phase functions for the Asteroid 4 Vesta, the first target of the Dawn mission, using ground-based telescopes operating at visible and near-infrared wavelengths (0.4–2.5 μm). Photometric lightcurve observations of Vesta were conducted on 15 nights at a phase angle range of 3.8–25.7° using duplicates of the seven narrowband Dawn Framing Camera filters (0.4–1.0 μm). Rotationally resolved visible (0.4–0.7 μm) and near-IR spectral observations (0.7–2.5 μm) were obtained on four nights over a similar phase angle range. Our Vesta photometric observations suggest the phase slope is between 0.019 and 0.029 mag/deg. The G parameter ranges from 0.22 to 0.37 consistent with previous results (e.g., Lagerkvist, C.-I., Magnusson, P., Williams, I.P., Buontempo, M.E., Argyle, R.W., Morrison, L.V. [1992]. Astron. Astrophys. Suppl. Ser. 94, 43–71; Piironen, J., Magnusson, P., Lagerkvist, C.-I., Williams, I.P., Buontempo, M.E., Morrison, L.V. [1997]. Astron. Astrophys. Suppl. Ser. 121, 489–497; Hasegawa, S. et al. [2009]. Lunar Planet. Sci. 40. ID 1503) within the uncertainty. We found that in the phase angle range of 0° < α ? 25° for every 10° increase in phase angle Vesta’s visible slope (0.5–0.7 μm) increases 20%, Band I and Band II depths increase 2.35% and 1.5% respectively, and the BAR value increase 0.30. Phase angle spectral measurements of the eucrite Moama in the lab show a decrease in Band I and Band II depths and BAR from the lowest phase angle 13° to 30°, followed by possible small increases up to 90°, and then a dramatic drop between 90° and 120° phase angle. Temperature-induced spectral effects shift the Band I and II centers of the pyroxene bands to longer wavelengths with increasing temperature. We have derived new correction equations using a temperature series (80–400 K) of HED meteorite spectra that will enable interpretation of telescopic and spacecraft spectral data using laboratory calibrations at room temperature (300 K).  相似文献   

7.
We present a new set of CCD photometric observations for the short period eclipsing binary 1SWASP J1743 (= V1067 Her). We have determined the available times of light minima and two new linear and quadratic ephemerides have been obtained. The photometric solutions for the system have been performed using Wilson and Devinney Code. The 3D and fill out configuration revealed that V1067 Her is an over contact W UMa binary with relatively low fill-out factor of about 16%.We investigated the period variation for the system. It showed a strong evidence of period changes by using the (O-C) residual diagram method and we have concluded long-term orbital period decrease rate dP/dt= −3.0 × 10−7 d/yr, corresponding to a time scale 8.6 × 105 yr. Such period decrease in the A-type W UMa systems is usually interpreted to be due to mass transfer from the more to the less massive component.  相似文献   

8.
P. Abolmasov 《New Astronomy》2011,16(7):421-429
Using the archives of the American Association of Variable Stars Observers and our own data, we analyse the long-term variability of several well-studied luminous blue variables (LBVs) aiming on a general picture of stochastic variability of these objects. The power density spectra of all the selected objects may be generally described by a single power law contaminated by observational noise at higher frequencies. The slopes of the power-law component are close to p = 2 (where PDS ∝ f?p, and f is frequency) for strongly variable flaring objects like AG Car and significantly smaller (p ～ 1.3) for P Cyg where brightness variation amplitude is ? 1m and dominated by slow low-amplitude variability. The slope holds for about two orders of magnitude in the frequency domain, though peaks and curvatures are present at f ? 10?2–10?3 d?1. We show that pseudo-photosphere approach to variability may explain the power-law shape of the variability spectrum at higher frequencies. However, the observed spectra are actually rather “red” than “brown”: flux variations are correlated up to tens of years that is much longer than the characteristic refreshment time scales of the pseudo-photosphere. We propose that several stochastic noise components produce the power spectra of LBVs.  相似文献   

9.
NASA’s Dawn spacecraft observations of Asteroid (4) Vesta reveal a surface with the highest albedo and color variation of any asteroid we have observed so far. Terrains rich in low albedo dark material (DM) have been identified using Dawn Framing Camera (FC) 0.75 μm filter images in several geologic settings: associated with impact craters (in the ejecta blanket material and/or on the crater walls and rims); as flow-like deposits or rays commonly associated with topographic highs; and as dark spots (likely secondary impacts) nearby impact craters. This DM could be a relic of ancient volcanic activity or exogenic in origin. We report that the majority of the spectra of DM are similar to carbonaceous chondrite meteorites mixed with materials indigenous to Vesta. Using high-resolution seven color images we compared DM color properties (albedo, band depth) with laboratory measurements of possible analog materials. Band depth and albedo of DM are identical to those of carbonaceous chondrite xenolith-rich howardite Mt. Pratt (PRA) 04401. Laboratory mixtures of Murchison CM2 carbonaceous chondrite and basaltic eucrite Millbillillie also show band depth and albedo affinity to DM. Modeling of carbonaceous chondrite abundance in DM (1–6 vol.%) is consistent with howardite meteorites. We find no evidence for large-scale volcanism (exposed dikes/pyroclastic falls) as the source of DM. Our modeling efforts using impact crater scaling laws and numerical models of ejecta reaccretion suggest the delivery and emplacement of this DM on Vesta during the formation of the ～400 km Veneneia basin by a low-velocity (<2 km/s) carbonaceous impactor. This discovery is important because it strengthens the long-held idea that primitive bodies are the source of carbon and probably volatiles in the early Solar System.  相似文献   

10.
From a comparison between the different observations of Martian methane existing today, including the new TES methane maps (Fonti and Marzo, 2010), we show that all sets of data are globally consistent with each other, and that a well definite seasonal cycle of methane has been at work for at least 10 yr. With a simple model of the balance between the loss fluxes of H and O, using up-to-date values of the escape fluxes, we show that the long-standing enigma of the imbalance between H and O escape fluxes may be solved by assuming that the missing sink of oxygen is the oxidation of methane. If no H2 is released together with CH4, a good agreement is found between the present CH4 flux and the value imposed by the balance between H and O escape fluxes, an average over the last ≈103 yr. If H2 is released together with CH4, as expected if CH4 originates in serpentinization, the average level of CH4 during the last 103 yr should have been at least ten times lower than the present one. The lack of present H2 release could suggest a long-term storage of methane in the subsurface under the form of clathrates, whereas H2 has been lost to the atmosphere shortly after being produced. We suggest that the thin layer of CO2 ice covering the permanent southern polar cap could result from the release of methane since the end of the last obliquity transition (time scale: 1 Myr), at an average rate of 0.1 Mt yr?1, consistent with the values derived from: (i) the present observations of methane (time scale: 10 yr), (ii) the estimate from the observed imbalance between the H and O escape fluxes (time scale: 1 kyr). If so, the present release of methane from subsurface clathrates would have acted at a similar rate since at least 3 Myr.  相似文献   

11.
Canada’s Near-Earth Object Surveillance Satellite (NEOSSat), set to launch in early 2012, will search for and track Near-Earth Objects (NEOs), tuning its search to best detect objects with a < 1.0 AU. In order to construct an optimal pointing strategy for NEOSSat, we needed more detailed information in the a < 1.0 AU region than the best current model (Bottke, W.F., Morbidelli, A., Jedicke, R., Petit, J.M., Levison, H.F., Michel, P., Metcalfe, T.S. [2002]. Icarus 156, 399–433) provides. We present here the NEOSSat-1.0 NEO orbital distribution model with larger statistics that permit finer resolution and less uncertainty, especially in the a < 1.0 AU region. We find that Amors = 30.1 ± 0.8%, Apollos = 63.3 ± 0.4%, Atens = 5.0 ± 0.3%, Atiras (0.718 < Q < 0.983 AU) = 1.38 ± 0.04%, and Vatiras (0.307 < Q < 0.718 AU) = 0.22 ± 0.03% of the steady-state NEO population. Vatiras are a previously undiscussed NEO population clearly defined in our integrations, whose orbits lie completely interior to that of Venus. Our integrations also uncovered the unexpected production of retrograde orbits from main-belt asteroid sources; this retrograde NEA population makes up ?0.1% of the steady-state NEO population. The relative NEO impact rate onto Mercury, Venus, and Earth, as well as the normalized distribution of impact speeds, was calculated from the NEOSSat-1.0 orbital model under the assumption of a steady-state. The new model predicts a slightly higher Mercury impact flux.  相似文献   

12.
We suggest that planets, brown dwarfs, and even low mass stars can be formed by fragmentation of protoplanetary disks around very massive stars (M ? 100 M⊙). We discuss how fragmentation conditions make the formation of very massive planetary systems around very massive stars favorable. Such planetary systems are likely to be composed of brown dwarfs and low mass stars of ～0.1–0.3 M⊙, at orbital separations of ～ few × 100–104 AU. In particular, scaling from solar-like stars suggests that hundreds of Mercury-like planets might orbit very massive stars at ～103 AU where conditions might favor liquid water. Such fragmentation objects can be excellent targets for the James Webb Space Telescope and other large telescopes working in the IR bands. We predict that deep observations of very massive stars would reveal these fragmentation objects, orbiting in the same orbital plane in cases where there are more than one object.  相似文献   

13.
We present a multicolor photometry for the eclipsing binary WY Hydrae, observed on four nights of 2008 December. From our new observations and Carr’s data, the photometric solutions were deduced by using the updated W–D program. The results show that WY Hya is a detached binary with a mass ratio of q = 0.970(±0.005).By analyzing the O–C curve, it is found that there exists either a continuous period increase or a cyclic variation. From Eq. (2), the orbital period of WY Hya secularly increases at a rate of dP/dt = +3.56(±0.37) × 10?7 days/yr, which may be interpreted by some mass transfer for the near-contact configuration or tidal dissipation. From Eq. (3), the period and semi-amplitude of the periodic oscillation are P3 = 95.4(±4.2) yr and A = 0d.0087(±0d.0003), respectively. This may be likely attributed by light-time effect via the presence of the assumed third body. Assumed in the coplanar orbit with the binary, the mass of the third body should be M3 = 0.18 M⊙. If the unseen additional companion exists, it will extract angular momentum from the binary system. Finally, WY Hya with high fill-out factors (i.e., f1,2 > 80%), may evolve into a semi-detached configuration.  相似文献   

14.
《New Astronomy》2007,12(2):146-160
We point out that although conventional stars are primarily fed by burning of nuclear fuel at their cores, in a strict sense, the process of release of stored gravitational energy, known as, Kelvin–Helmholtz (KH) process is either also operational albeit at an arbitrary slow rate, or lying in wait to take over at the disruption of the nuclear channel. In fact, the latter mode of energy release is the true feature of any self-gravity bound object including stars. We also highlight the almost forgotten fact that Eddington was the first physicist to introduce special relativity into the problem and correctly insist that, actually, total energy stored in a star is not the mere Newtonian energy but the total mass energy (E = Mc2). Accordingly, Eddington defined an “Einstein time scale” of Evolution where the maximum age of the Sun turned out to be tE ≈ 1.4 × 1013 yr. This concept has a fundamental importance though we know now that Sun in its present form cannot survive for more than 10 billion years. We extend this concept by introducing general relativity and show that the minimum value of depletion of total mass–energy is tE = ∞ not only for Sun but for and sufficiently massive or dense object. We propose that this time scale be known in the name of “Einstein–Eddington”. We also point out that, recently, it has been shown that as massive stars undergo continued collapse to become a Black Hole, first they become extremely relativistic radiation pressure supported stars. And the life time of such relativistic radiation pressure supported compact stars is indeed dictated by this Einstein–Eddington time scale whose concept is formally developed here. Since this observed time scale of this radiation pressure supported quasistatic state turns out to be infinite, such objects are called eternally collapsing objects (ECO). Further since ECOs are expected to have strong intrinsic magnetic field, they are also known as “Magnetospheric ECO” or MECO.  相似文献   

15.
We consider a network of telescopes capable of scanning all the observable sky each night and targeting Near-Earth objects (NEOs) in the size range of the Tunguska-like asteroids, from 160 m down to 10 m. We measure the performance of this telescope network in terms of the time needed to discover at least 50% of the impactors in the considered population with a warning time large enough to undertake proper mitigation actions. The warning times are described by a trimodal distribution and the telescope network has a 50% probability of discovering an impactor of the Tunguska class with at least one week of advance already in the first 10 yr of operations of the survey. These results suggest that the studied survey would be a significant addition to the current NEO discovery efforts.  相似文献   

16.
The 1:1 mean motion resonance may be referred to as the lowest order mean motion resonance in restricted or planetary three-body problems. The five well-known libration points of the circular restricted three-body problem are five equilibriums of the 1:1 resonance. Coorbital motion may take different shapes of trajectory. In case of small orbital eccentricities and inclinations, tadpole-shape and horseshoe-shape orbits are well-known. Other 1:1 libration modes different from the elementary ones can exist at moderate or large eccentricities and inclinations. Coorbital objects are not rare in our solar system, for example the Trojans asteroids and the coorbital satellite systems of Saturn. Recently, dozens of coorbital bodies have been identified among the near-Earth asteroids. These coorbital asteroids are believed to transit recurrently between different 1:1 libration modes mainly due to orbital precessions, planetary perturbations, and other possible effects. The Hamiltonian system and the Hill’s three-body problem are two effective approaches to study coorbital motions. To apply the perturbation theory to the Hamiltonian system, standard procedures involve the development of the disturbing function, averaging and normalization, theory of ideal resonance model, secular perturbation theory, etc. Global dynamics of coorbital motion can be revealed by the Hamiltonian approach with a suitable expansion. The Hill’s problem is particularly suitable for the studies on the relative motion of two coorbital bodies during their close encounter. The Hill’s equation derived from the circular restricted three-body problem is well known. However, the general Hill’s problem whose equation of motion takes exactly the same form applies to the non-restricted case where the mass of each body is non-negligible, namely the planetary case. The Hill’s problem can be transformed into a “canonical shape” so that the averaging principle can be applied to construct a secular perturbation theory. Besides the two analytical theories, numerical methods may be consulted, for example the approach of periodic orbit, the surface of section, and the computation of invariant manifolds carried by equilibriums or periodic orbits.  相似文献   

17.
《New Astronomy》2007,12(6):461-470
We present results of a study that combines UBVI photometry, MK spectral classification and proper motions in the area of the, up to now unknown, open cluster Ruprecht 58 at the Puppis region. Star counts from the 2MASS data catalog together with the analysis of CCD UBVI photometry demonstrate that it is a real open cluster with 9′ size approximately. The cluster is placed at a distance of 3.9 kpc and is about 250 Myr old with mean reddening E(B−V) = 0.33 mag. Proper motions confirm Ruprecht 58 is a real cluster with mean absolute proper motions μαcosδ = −2.77 ± 0.45 mas/yr and μδ = 4.54 ± 0.45 mas/yr in the magnitude range 13.5 < V < 14.5 and μαcosδ = −2.70 ± 0.32 mas/yr and μδ = 3.19 ± 0.32 mas/yr in the range 14.5 < V < 16.0. The computation of the cluster mass spectrum slope yielded x = 1.8 in the mass range from ≈1.4 to ≈4m⊙.  相似文献   

18.
We report the first results of quasi-simultaneous two passband optical monitoring of six quasi-stellar objects to search for micro-variability. We carried out photometric monitoring of these sources in an alternating sequence of R and V passbands, for five radio-quiet quasi-stellar objects (RQQSOs), 0748 + 291, 0824 + 098, 0832 + 251, 1101 + 319, 1225 + 317 and one radio-loud quasi-stellar object (RLQSO), 1410 + 429. No micro-variability was detected in any of the RQQSOs, but convincing micro-variability was detected in the RLQSO on two successive nights it was observed. Using the compiled data of optical micro-variability of RQQSOs till date, we got the duty cycle for micro-variability in RQQSOs is ～10%. The present investigation indicates that micro-variability is not a persistent property of RQQSOs but an occasional incident.  相似文献   

19.
We present the ensemble properties of 31 comets (27 resolved and 4 unresolved) observed by the Sloan Digital Sky Survey (SDSS). This sample of comets represents about 1 comet per 10 million SDSS photometric objects. Five-band (u, g, r, i, z) photometry is used to determine the comets’ colors, sizes, surface brightness profiles, and rates of dust production in terms of the Afρ formalism. We find that the cumulative luminosity function for the Jupiter Family Comets in our sample is well fit by a power law of the form N(<H) ∝ 10(0.49±0.05)H for H < 18, with evidence of a much shallower fit N(<H) ∝ 10(0.19±0.03)H for the faint (14.5 < H < 18) comets. The resolved comets show an extremely narrow distribution of colors (0.57 ± 0.05 in g ? r for example), which are statistically indistinguishable from that of the Jupiter Trojans. Further, there is no evidence of correlation between color and physical, dynamical, or observational parameters for the observed comets.  相似文献   

20.
The EGRET observations have confirmed and proposed the new isotropically distributed γ-ray background, but the known objects radiating γ-rays can not supply so much radiation. Meanwhile, EGRET also reveals a population of γ-ray sources with no radio counterparts which are isotropically distributed in the sky, indicating their possible cosmological origins. Wang et al.[13] proposed a new γ-ray radiation process driven by the radiation feedback of AGNs. The energy of the radiation peaks around 1 GeV—0.1 TeV with the typical luminosity of 1042—1043 ergs · s?1. This kind of radiation process in the radio quiet quasars make them the potential γ-ray radiation sources as well as the contributors to the γ-ray background. We consider two cases in which the seed photons in the inverse-Compton processes are from the accretion disks of quasars and cosmic microwave background (CMB), respectively. We find that the former contributes 78%—92% of the background radiation around 1 GeV, while the contribution from the latter is negligible. The radio quiet quasars are highly likely to become the objects which contribute the most energy to the γ-ray background around 1 GeV.  相似文献   

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