ABSTRACT
Potentially hazardous asteroids (PHAs) are objects that can have close encounters with Earth and are significantly large to cause significant regional damage in the event of impact. The ...mitigation strategy requires to understand their physical properties. We aim to investigate the physical nature of PHAs, using data obtained within the framework of the Visible NEOs Observations Survey (ViNOS). We analysed and characterized a sample comprised of 14 of these objects. To conduct this study, we obtained visible spectra of 14 PHAs in the 0.5–0.9-μm region using the 2.5 m Nordic Optical Telescope, located at the El Roque De Los Muchachos Observatory in La Palma (Spain). The resulting spectra were combined with their corresponding near-infrared counterparts, available in the literature from the Small Main-Belt Asteroid Spectroscopic Survey (SMASS). We performed a taxonomical classification, computed several diagnostic spectral parameters (slopes, band centres, and band area ratios), and provide completely new mineralogical information for 10 of these objects. We also compared the data with laboratory spectra of meteorites from the RELAB data base. Among the studied sample of PHAs, approximately 90 per cent of the objects (13 out of 14) were classified as silicaceous (S-types and subclasses). Only one object, 489 486, was classified as carbonaceous. Five of the studied PHAs did not have previous taxonomic classifications. The comparisons of the silicaceous PHAs with meteoritic spectra yielded, in all cases, ordinary chondrites as the best match for meteoritic analogues. The computed mineralogy of all of our targets is also consistent with this results.
Aims. Our aim is to obtain more information about the physical nature of B-type asteroids and extend previous work by studying their physical properties as derived from fitting an asteroid thermal ...model to their NASA’s Wide-field Infrared Survey Explorer (WISE) data. We also examine the Pallas collisional family, a B-type family with a moderately high albedo in contrast to the large majority of B-types. Methods. We applied a combination of the near-Earth asteroid thermal model and a model of the reflected sunlight to WISE asteroid data in order to derive up to four parameters: effective diameter (D), the so-called infrared beaming parameter (η), ratio of infrared to visible albedo (Rp = pIR/pV), and visible geometric albedo (pV). Results. We obtained the effective diameter, geometric visible albedo, infrared-to-visible albedo ratio, and beaming parameter for ≳ 100 B-types asteroids and plotted the value distributions of pV, Rp, and η (p̅V = 0.07 ± 0.03, R̅p = 1.0 ± 0.2, and η̅ = 1.0 ± 0.1). By combining the IR and visible albedos with 2.5 μm reflectances from the literature we obtained the ratio of reflectances at 3.4 and 2.5 μm, from which we found statistically significant indications that the presence of a 3-μm absorption band related to water may be commonplace among the B-types. Finally, the Pallas collisional family members studied (~50 objects) present moderately high values of pV, p̅V = 0.14 ± 0.05, which is significantly higher than the average albedo of B-types. In addition, this family presents the lowest and most homogeneously distributed Rp-values of our whole sample, which shows that this group is clearly different from the other B-types, probably because its members are fragments likely originating from the same region of (2) Pallas, a particularly high-albedo B-type asteroid.
Context. The Centaurs are a transitional population of minor bodies of the solar system and the evolutionary link between the trans-Neptunian objects and the short period comets. The surface ...properties of these objects are very peculiar, because currently available data suggest that their visual surface colors divide the population into two distinctive groups, those with reddish slopes of the visual reflection spectra and those with neutral spectra. Moreover, some of them are known to posses comas produced by cometary activity. Aims. We aim to investigate possible links between the orbital dynamical history and the surface physical properties of the bodies of this population. Methods. By means of numerical integrations of the equations of motion we calculated the orbital evolution of three groups of Centaurs: the Red group, the Gray group, and the Active group. We looked for statistical differences in the timescales spent by the objects of each group at heliocentric distances below certain values that are associated with locations where certain particular physical processes occur at the surfaces. Results. We find remarkable differences when we compare the fraction of objects that penetrate below typical heliocentric distances for each group. Conclusions. Our results suggest that the observed bimodality in the distribution of surface colors of the Centaurs is caused by the different thermal reprocessing on the surface of bodies of the Red group on one side and the Active and Gray groups on the other. Centaurs of the Gray group likely had cometary activity, therefore their color distribution is similar to that of comet nuclei.
Context. B-type asteroids constitute a peculiar spectral class within the C-complex. Previous work has pointed out the difference between the visible geometric albedos of B-types and those of the ...Pallas collisional family (PCF), whose few members with observed spectra are B-types (one exception out of eight objects). This has been interpreted as being due to compositional differences. However, the PCF members are typically smaller than the spectroscopically classified B-types, and the following possibilities have not been ruled out: the albedo differences might be related to a size-albedo dependence and/or to the generally larger errors of the WISE data and best-fitting values of the derived parameters expected for smaller objects. Aims. We compare albedos and beaming parameters of PCF members and B-types of similar sizes and re-examine our conclusion on the different composition of the PCF. Methods. By modelling their WISE/NEOWISE data, we derived sizes and albedos of all objects whose Sloan Digital Sky Survey reflectances are similar to the typical B-type reflectance spectra. In particular, we derived the so-called infrared beaming parameters (η), effective diameters (D), and corresponding visible geometric albedos (pV), and studied their value distributions. Results. We obtained the effective diameter and geometric visible albedo for ~600 B-type asteroids whose sizes range between 2 and 100 km, approximately half of which have fitted η-values that are inversely correlated to size. We found that the albedo distributions of the PCF is significantly different from that of other B-types in the same size range (2−20 km), and we rule out any size-albedo dependency or biases related to the lower quality of the pV-values of smaller objects. In addition, we also found differences between the η-value distribution of the PCF and the other similarly-sized B-types. Conclusions. The differences in the visible albedos of PCF members and the other B-types of similar sizes is a strong indication that their compositions are different. The trend between beaming parameter and size of the B-types (excluding the PCF) is consistent with the expectation that the surfaces of smaller asteroids have coarser rocks than do those of larger objects (D> 40 km), which are thought to be covered by a finer, more insulating layer of dust.
Context. Dynamically, 133P/Elst-Pizarro and 176P/LINEAR are main belt asteroids, likely members of the Themis collisional family, and unlikely of cometary origin. They have been observed with ...cometary-like tails, which may be produced by water-ice sublimation. They are part of a small group of objects called Main Belt Comets (MBCs, Hsieh & Jewitt 2006). Aims. We attempt to determine if these MBCs have spectral properties compatible with those of comet nuclei or with other Themis family asteroids. Methods. We present the visible spectrum of MBCs 133P and 176P, as well as three Themis family asteroids: (62) Erato, (379) Huenna and (383) Janina, obtained in 2007 using three telescopes at “El Roque de los Muchachos” Observatory, in La Palma, Spain, and the 8 m Kueyen (UT2) VLT telescope at Cerro Paranal, Chile. The spectra of the MBCs are compared with those of the Themis family asteroids, comets, likely “dormant” comets and asteroids with past cometary-like activity in the near-Earth (NEA) population. As 133P was observed active, we also look for the prominent CN emission around 0.38 μm typically observed in comets, to test if the activity is produced by the sublimation of volatiles. Results. The spectra of 133P and 176P resemble best those of B-type asteroid and are very similar to those of Themis family members and another activated asteroid in the near-Earth asteroid population, (3200) Phaethon. On the other hand, these spectra are significantly different from the spectrum of comet 162P/Siding-Spring and most of the observed cometary nuclei. CN gas emission is not detected in the spectrum of 133P. We determine an upper limit for the CN production rate Q(CN) = 1.3 × 1021 mol/s, three orders of magnitude lower than the Q(CN) of Jupiter family comets observed at similar heliocentric distances. Conclusions. The spectra of 133P/Elst-Pizarro and 176P/LINEAR confirm that they are likely members of the Themis family of asteroids, fragments that probably retained volatiles, and unlikely have a cometary origin in the trans-Neptunian belt or the Oort Cloud. They have similar surface properties to activated asteroids in the NEA population, which supports the hypothesis that these NEAs are scattered MBCs. The low Q(CN) of 133P means that, if water-ice sublimation is the activation mechanism, the gas production rate is very low and/or the parent molecules of CN present in the nuclei of normal comets are much less abundant in this MBC.
The OSIRIS-REx spacecraft encountered the asteroid (101955) Bennu on December 3, 2018, and has since acquired extensive data from the payload of scientific instruments on board. In 2019, the ...OSIRIS-REx team selected primary and backup sample collection sites, called Nightingale and Osprey, respectively. On October 20, 2020, OSIRIS-REx successfully collected material from Nightingale. In this work, we apply an unsupervised machine learning classification through the K-Means algorithm to spectrophotometrically characterize the surface of Bennu, and in particular Nightingale and Osprey. We first analyze a global mosaic of Bennu, from which we find four clusters scattered across the surface, reduced to three when we normalize the images at 550 nm. The three spectral clusters are associated with boulders and show significant differences in spectral slope and UV value. We do not see evidence of latitudinal non-uniformity, which suggests that Bennu's surface is well-mixed. In our higher-resolution analysis of the primary and backup sample sites, we find three representative normalized clusters, confirming an inverse correlation between reflectance and spectral slope (the darkest areas being the reddest ones) and between b' normalized reflectance and slope. Nightingale and Osprey are redder than the global surface of Bennu by more than 1σ from average, consistent with previous findings, with Nightingale being the reddest (S′ = (−0.3 ± 1.0) × 10−3 percent per thousand angstroms). We see hints of a weak absorption band at 550 nm at the candidate sample sites and globally, which lends support to the proposed presence of magnetite on Bennu.
•We apply a machine learning classification to characterize the surface of Bennu, in particular Nightingale and Osprey.•We find three representative normalized clusters where the darkest areas being the reddest ones.•Nightingale and Osprey are redder than the global surface (more than 1σ from average), with Nightingale being the reddest.•We see hints of a weak absorption band at 550 nm, which lends support to the proposed presence of magnetite on Bennu.
ABSTRACT We present observations carried out using the 10.4 m Gran Telescopio Canarias and an interpretative model of the dust environment of activated asteroid 313 P/Gibbs. We discuss three ...different models relating to different values of the dust parameters, i.e., dust loss rate, maximum and minimum sizes of particles, power index of the size distribution, and emission pattern. The best model corresponds to an isotropic emission of particles which started on August 1. The sizes of grains were in the range of 0.1−2000 m, with velocities for 100 m particles between 0.4−1.9 m s−1, with a dust production rate in the range of 0.2−0.8 kg s−1. The dust tails' brightnesses and morphologies are best interpreted in terms of a model of sustained and low dust emission driven by water-ice sublimation, spanning since 2014 August 1, and triggered by a short impulsive event. This event produced an emission of small particles of about 0.1 m with velocities of ∼4 m s−1. From our model we deduce that the activity of this main-belt comet continued for at least four months since activation.
The active centaur 2020 MK4 C. de la Fuente Marcos; R. de la Fuente Marcos; Licandro, J ...
Astronomy & astrophysics,
05/2021, Letnik:
649
Journal Article
Recenzirano
Odprti dostop
Context. Centaurs go around the Sun between the orbits of Jupiter and Neptune. Only a fraction of the known centaurs have been found to display comet-like features. Comet 29P/Schwassmann-Wachmann 1 ...is the most remarkable active centaur. It orbits the Sun just beyond Jupiter in a nearly circular path. Only a handful of known objects follow similar trajectories. Aims. We present photometric observations of 2020 MK4, a recently found centaur with an orbit not too different from that of 29P, and we perform a preliminary exploration of its dynamical evolution. Methods. We analyzed broadband Cousins R and Sloan g′, r′, and i′ images of 2020 MK4 acquired with the Jacobus Kapteyn Telescope and the IAC80 telescope to search for cometary-like activity and to derive its surface colors and size. Its orbital evolution was studied using direct N-body simulations. Results. Centaur 2020 MK4 is neutral-gray in color and has a faint, compact cometary-like coma. The values of its color indexes, (g′− r′) = 0.42 ± 0.04 and (r′− i′) = 0.17 ± 0.04, are similar to the solar ones. A lower limit for the absolute magnitude of the nucleus is Hg = 11.30 ± 0.03 mag which, for an albedo in the range of 0.1–0.04, gives an upper limit for its size in the interval (23, 37) km. Its orbital evolution is very chaotic and 2020 MK4 may be ejected from the Solar System during the next 200 kyr. Comet 29P experienced relatively close flybys with 2020 MK4 in the past, sometimes when they were temporary Jovian satellites. Conclusions. Based on the analysis of visible CCD images of 2020 MK4, we confirm the presence of a coma of material around a central nucleus. Its surface colors place this centaur among the most extreme members of the gray group. Although the past, present, and future dynamical evolution of 2020 MK4 resembles that of 29P, more data are required to confirm or reject a possible connection between the two objects and perhaps others.
Context. The Arjuna asteroid belt is loosely defined as a diverse group of small asteroids that follow dynamically cold, Earth-like orbits. Most of them are not actively engaged in resonant, ...co-orbital behavior with Earth. Some of them experience temporary but recurrent horseshoe episodes. Objects in horseshoe paths tend to approach Earth at a low velocity, leading to captures as Earth’s temporary satellites or mini-moons. Four such objects have already been identified: 1991 VG, 2006 RH120, 2020 CD3, and 2022 NX1. Here, we focus on 2023 FY3, a recent finding, the trajectory of which might have a co-orbital status and perhaps lead to temporary captures. Aims. We want to determine the physical properties of 2023 FY3 and explore its dynamical evolution. Methods. We carried out an observational study of 2023 FY3 using the OSIRIS camera spectrograph at the 10.4 m Gran Telescopio Canarias, to derive its spectral class, and time-series photometry obtained with QHY411M cameras and two units of the Two-meter Twin Telescope to investigate its rotational state. N-body simulations were also performed to examine its possible resonant behavior. Results. The visible reflectance spectrum of 2023 FY3 is consistent with that of an S-type asteroid; its light curve gives a rotation period of 9.3±0.6 min, with an amplitude of 0.48±0.13 mag. We confirm that 2023 FY3 roams the edge of Earth’s co-orbital space. Conclusions. Arjuna 2023 FY3, an S-type asteroid and fast rotator, currently exhibits horseshoe-like resonant behavior and in the past experienced mini-moon engagements of the temporarily captured flyby type that may repeat in the future. The spectral type result further confirms that mini-moons are a diverse population in terms of surface composition.
We present observations and an interpretative model of the dust environment of the Main-Belt Comet P/2010 F5 (Gibbs). The narrow dust trails observed can be interpreted unequivocally as an impulsive ...event that took place around 2011 July 1 with an uncertainty of + or -10 days, and a duration of less than a day, possibly of the order of a few hours. The best Monte Carlo dust model fits to the observed trail brightness imply ejection velocities in the range 8-10 cm s super(-1) for particle sizes between 30 cm and 130 mu m. This weak dependence of velocity on size contrasts with that expected from ice sublimation and agrees with that found recently for (596) Scheila, a likely impacted asteroid. The particles seen in the trail are found to follow a power-law size distribution of index asymptotically =-3.7. Assuming that the slowest particles were ejected at the escape velocity of the nucleus, its size is constrained to about 200-300 m in diameter. The total ejected dust mass is > ~5 x 10 super(8) kg, which represents approximately 4%-20% of the nucleus mass.
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