•New polarimetric observations of 15 main belt asteroids are presented.•The mean polarimetric parameters of the main asteroid composition classes are determined.•Polarimetry gives valuable ...information for refining asteroid taxonomy.•The polarimetric classification of 283 main belt asteroids is given.
We present new results of polarimetric observations of 15 main belt asteroids of different composition. By merging new and published data we determined polarimetric parameters characterizing individual asteroids and mean values of the same parameters characterizing different taxonomic classes. The majority of asteroids show polarimetric phase curves close to the average curve of the corresponding class. We show that using polarimetric data it is possible to refine asteroid taxonomy and derive a polarimetric classification for 283 main belt asteroids. Polarimetric observations of asteroid (21) Lutetia are found to exhibit possible variations of the position angle of the polarization plane over the surface.
Context. Studying the coma environment of comet 67P/Churyumov-Gerasimenko (67P) is one of the primary scientific goals of the VIRTIS experiment on the ESA Rosetta mission. Aims. The distribution and ...variability of water vapour and carbon dioxide in the comet’s coma are needed to estimate their production rate, abundances in the nucleus, and the spatial distribution of the active regions. Methods. Infrared emission lines from vibrational bands of water and carbon dioxide at 2.67 and 4.27 μm, respectively, were observed by the VIRTIS-M imaging channel and mapped from close to the nucleus up to ~10 km altitude with a resolution of ~40 m/px. A dataset consisting of 74 observations in the 1−5 μm spectral range acquired from 8 to 14 April 2015 when 67P was at a heliocentric distance of 1.9 AU is analysed in this work. A statistical correlation between the gas distribution and the surface’s active regions was performed. Results. The maximum H2O emission is observed within 3 km from the nucleus and is mainly concentrated above two active regions, Aten-Babi and Seth-Hapi, while the CO2 distribution appears more uniform with significant emissions coming from both the “head” and southern latitude regions. In the equatorial region, the column densities of both species decrease with altitude, although CO2 decreases more rapidly than H2O. The calculated CO2/H2O column density ratios above Aten-Babi and Seth-Hapi are 2.4 ± 0.6% and 3.0 ± 0.7%, respectively. A value equal to 3.9 ± 1.0% is observed at equatorial latitudes in the region encompassing Imothep. Conclusions. VIRTIS-M has mapped the distribution of water vapour and carbon dioxide around the nucleus of 67P with unprecedented spatial resolution. The different water and carbon dioxide outgassing above the surface, seen in the VIRTIS-M data, might be indicative of a different thermal history of the northern and southern hemispheres of 67P.
Before Rosetta, the space missions Giotto and Stardust shaped our view on cometary dust, supported by plentiful data from Earth based observations and interplanetary dust particles collected in the ...Earth’s atmosphere. The Rosetta mission at comet 67P/Churyumov-Gerasimenko was equipped with a multitude of instruments designed to study cometary dust. While an abundant amount of data was presented in several individual papers, many focused on a dedicated measurement or topic. Different instruments, methods, and data sources provide different measurement parameters and potentially introduce different biases. This can be an advantage if the complementary aspect of such a complex data set can be exploited. However, it also poses a challenge in the comparison of results in the first place. The aim of this work therefore is to summarize dust results from Rosetta and before. We establish a simple classification as a common framework for intercomparison. This classification is based on the dust particle structure, porosity, and strength and also on its size. Depending on the instrumentation, these are not direct measurement parameters, but we chose them because they were the most reliable for deriving our model. The proposed classification has proved helpful in the Rosetta dust community, and we offer it here also for a broader context. In this manner, we hope to better identify synergies between different instruments and methods in the future.
Abstract
Simultaneously to the ESA Rosetta mission, a world-wide ground-based campaign provided measurements of the large scale activity of comet 67P/Churyumov–Gerasimenko through measurement of ...optically active gas species and imaging of the overall dust coma. We present more than 2 yr of observations performed with the FORS2 low-resolution spectrograph at the VLT, TRAPPIST and ACAM at the WHT. We focus on the evolution of the CN production as a tracer of the comet activity. We find that it is asymmetric with respect to perihelion and different from that of the dust. The CN emission is detected for the first time at 1.34 au pre-perihelion and production rates then increase steeply to peak about 2 weeks after perihelion at (1.00 ± 0.10) × 1025 molecules s−1, while the post-perihelion decrease is more shallow. The evolution of the comet activity is strongly influenced by seasonal effects with enhanced CN production when the Southern hemisphere is illuminated.
On 2015 July 18, near perihelion at a heliocentric distance of 1.28 au, the Visible InfraRed Thermal Imaging Spectrometer (VIRTIS-M) on board the Rosetta spacecraft had the opportunity of observing ...dust activity in the inner coma with a view of the night side (shadowed side) of comet 67P/Churyumov-Gerasimenko. At the time of the measurements we present here, we observe a dust plume that originates on the far side of the nucleus. We are able to identify the approximate location of its source at the boundary between the Hapi and Anuket regions, and we find that it has been in darkness for some hours before the observation. Assuming that this time span is equal to the conductive time scale, we obtain a thermal inertia in the range 25–36 W K−1 m−2 s−1/2. These thermal inertia values can be used to verify with a 3D finite-element method (FEM) numerical code whether the surface and subsurface temperatures agree with the values found in the literature. We explored three different configurations: (1) a layer of water ice mixed with dust beneath a dust mantle of 5 mm with thermal inertia of 36 J m−2 K−1 s−0.5; (2) the same structure, but with thermal inertia of 100 J m−2 K−1 s−0.5; (3) an ice-dust mixture that is directly exposed. Of these three configurations, the first seems to be the most reasonable, both for the low thermal inertia and for the agreement with the surface and subsurface temperatures that have been found for the comet 67P/Churyumov-Gerasimenko. The spectral properties of the plume show that the visible dust color ranged from 16 ± 4.8%/100 nm to 13 ± 2.6%/100 nm, indicating that this plume has no detectable color gradient. The morphology of the plume can be classified as a narrow jet that has an estimated total ejected mass of between 6 and 19 tons when we assume size distribution indices between −2.5 and −3.
Context.The population of known large trans-neptunian objects (TNOs) is growing very fast and the knowledge of their physical properties is a key issue to understand the origin and evolution of the ...Solar System. Aims.In this paper we studied the surface composition of the recently discovered TNO 2005 FY9, one of the largest known TNOs (~0.7 times the diameter of Pluto, i.e. 1600 km, if the albedo is similar, or 3100–1550 km in diameter assuming an albedo range 0.2 < pV < 0.8). Methods.We report visible and near infrared spectra covering the 0.35–2.5 μm spectral range, obtained with the 4.2 m William Herschel Telescope and the Italian 3.58 m Telescopio Nazionale Galileo at “El Roque de los Muchachos” Observatory (La Palma, Spain). Results.The spectrum of this large TNO is similar to that of Pluto, with an infrared region dominated by very prominent absorptions bands formed in solid CH4. At wavelengths shorter than 0.6 μm, the spectrum is almost featureless and red. The red color most likely indicates the presence of complex organics, as has been hypothesized for Pluto and many other TNOs. The icy-CH4 bands in this new giant TNO are significantly stronger than those of Pluto, implying that methane could be even more abundant on its surface. The existence of a volatile such as methane on the surface of 2005 FY9, likely accompanied by N2 and CO ices, coupled with its large size, make this Pluto-like TNO an excellent candidate to have an atmosphere comparable to Pluto's. Conclusions.
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.
ABSTRACT
Based on HST and MUSE data, we probe the stellar and gas properties (i.e. kinematics, stellar mass, star formation rate) of the radio-loud brightest cluster galaxy (BCG) located at the ...centre of the X-ray-luminous cool-core cluster Abell 2667 (z = 0.2343). The bi-dimensional modelling of the BCG surface brightness profile reveals the presence of a complex system of substructures extending all around the galaxy. Clumps of different size and shape plunged into a more diffuse component constitute these substructures, whose intense ‘blue’ optical colour hints at the presence of a young stellar population. Our results depict the BCG as a massive (M⋆ ≃ 1.38 × 1011 M⊙) dispersion-supported spheroid (Δv⋆ ≤ 150 km s−1, σ0 ∼ 216 km s−1) hosting an active supermassive black hole (MSMBH ≃ 3.8 × 109 M⊙) whose optical features are typical of low-ionization nuclear emission line regions. Although the velocity pattern of the stars in the BCG is irregular, the stellar kinematics in the regions of the clumps show a positive velocity of ∼100 km s−1, similarly to the gas component. An analysis of the mechanism giving rise to the observed lines in the clumps through empirical diagnostic diagrams points out that the emission is composite, suggesting contribution from both star formation and an active galactic nucleus. We conclude our analysis describing how scenarios of both chaotic cold accretion and merging with a gas-rich disc galaxy can efficaciously explain the phenomena the BCG is undergoing.
We present Chandra point-source catalogs for the Extended Chandra Deep Field-South (E-CDF-S) survey. The E-CDF-S consists of four contiguous 250 ks Chandra observations covering an approximately ...square region of total solid angle -0.3 deg super(2), which flank the existing -1 Ms Chandra Deep Field-South (CDF-S). The survey reaches sensitivity limits of -1.1 x 10 super(-16) and -6.7 x 10 super(-16) ergs cm super(-2) s super(-1) for the 0.5-2.0 and 2-8 keV bands, respectively. We detect 762 distinct X-ray point sources within the E-CDF-S exposure; 589 of these sources are new (i.e., not previously detected in the -1 Ms CDF-S). This brings the total number of X-ray point sources detected in the E-CDF-S region to 915 (via the E-CDF-S and -1 Ms CDF-S observations). Source positions are determined using matched-filter and centroiding techniques; the median positional uncertainty is -0.35. The basic X-ray and optical properties of these sources indicate a variety of source types, although absorbed active galactic nuclei (AGNs) seem to dominate. In addition to our main Chandra catalog, we constructed a supplementary source catalog containing 33 lower significance X-ray point sources that have bright optical counterparts (R < 23). These sources generally have X-ray-to-optical flux ratios expected for normal and starburst galaxies, which lack a strong AGN component. We present basic number-count results for our main Chandra catalog and find good agreement with the -1 Ms CDF-S for sources with 0.5-2.0 and 2-8 keV fluxes greater than 3 x 10 super(-16) and 1 x 10 super(-15) ergs cm super(-2) s super(-1), respectively. Furthermore, three extended sources are detected in the 0.5-2.0 keV band, which are found to be likely associated with galaxy groups or poor clusters at z - 0.1-0.7; these have typical rest-frame 0.5-2.0 keV luminosities of (1-5) x 10 super(42) ergs s super(-1).
Results of the first polarimetric observations of Centaurs (5145) Pholus and (10199) Chariklo, and new observations of (2060) Chiron are presented together with the estimates of their absolute ...magnitudes. Observations were carried out at the 8
m ESO Very Large Telescope in 2007–2008. They revealed noticeable negative polarization in the phase-angle range 0.5–4.4° with a minimum varying from −1% to −2.1% in the
R band. All three objects show diverse polarization phase-angle behaviour, each distinctly different from that of transneptunian objects. We found evidence of surface heterogeneity for Chariklo while Chiron and Pholus appear to have rather homogeneous surfaces. Polarization phase behaviours of Chiron and Pholus are significantly different from any other Solar System bodies studied so far. A shift of negative polarization minima toward small phase angles seems to be a characteristic feature of polarization properties of Centaurs. Presence of a small amount of water frost on a dark surface is considered as one of the possible ways to explain these properties.
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