So far, only two interstellar objects have been observed within our Solar System. While the first one, 1I/'Oumuamua, had asteroidal characteristics, the second one, 2I/Borisov, showed clear evidence ...of cometary activity. We performed polarimetric observations of comet 2I/Borisov using the European Southern Observatory Very Large Telescope to derive the physical characteristics of its coma dust particles. Here we show that the polarization of 2I/Borisov is higher than what is typically measured for Solar System comets. This feature distinguishes 2I/Borisov from dynamically evolved objects such as Jupiter-family and all short- and long-period comets in our Solar System. The only object with similar polarimetric properties as 2I/Borisov is comet C/1995 O1 (Hale-Bopp), an object that is believed to have approached the Sun only once before its apparition in 1997. Unlike Hale-Bopp and many other comets, though, comet 2I/Borisov shows a polarimetrically homogeneous coma, suggesting that it is an even more pristine object.
Context.
The
Gaia
mission of the European Space Agency is measuring reflectance spectra of a number to the order of 10
5
small Solar System objects. A first sample will be published in the
Gaia
Data ...Release scheduled for 2021.
Aims.
The aim of our work was to test the procedure developed to obtain taxonomic classifications for asteroids based only on
Gaia
spectroscopic data.
Methods.
We used asteroid spectra obtained using the DOLORES (Device Optimised for the LOw RESolution) instrument, a low-resolution spectrograph and camera installed at the Nasmyth B focus of the Telescopio Nazionale
Galileo
. Because these spectra have a higher spectral resolution than that typical of the
Gaia
spectra, we resampled them to more closely match the expected
Gaia
spectral resolution. We then developed a cloning algorithm to build a database of asteroid spectra belonging to a variety of taxonomic classes, starting from a set of 33 prototypes chosen from the 50 asteroids in our observing campaign. We used them to generate a simulated population of 10 000 representative asteroid spectra and employed them as the input to the algorithm for taxonomic classification developed to analyze
Gaia
asteroid spectra.
Results.
Using the simulated population of 10 000 representative asteroid spectra in the algorithm to be used to produce the
Gaia
asteroid taxonomy at the end of the mission, we found 12 distinct taxonomic classes. Two of them, with 53% of the sample, are dominant. At the other extreme are three classes each with <1% of the sample, and these consist of the previously known rare classes A, D/Ld, and V; 99.1% of the simulated population fall into a single class.
Conclusions.
We demonstrated the robustness of our algorithm for taxonomic classification by using a sample of simulated asteroid spectra fully representative of what is expected to be in the
Gaia
spectroscopic data catalogue for asteroids. Increasingly larger data sets will become available as soon as they are published in the future
Gaia
data releases, with the next one coming in 2021. This will be exploited to develop a correspondingly improved taxonomy, likely with minor tweaks to the algorithm described here, as suggested by the results of this preliminary analysis.
ABSTRACT Asteroid (3200) Phaethon is a Near-Earth Apollo asteroid with an unusual orbit that brings it closer to the Sun than any other known asteroid. Its last close approach to the Earth was in ...2017 mid-December and the next one will be on 2026 October. Previous rotationally time-resolved spectroscopy of Phaethon showed that its spectral slope is slightly bluish, in agreement with its B/F taxonomic classification, but at some rotational phases, it changes to slightly reddish. Motivated by this result, we performed time-resolved imaging polarimetry of Phaethon during its recent close approach to the Earth. Phaethon has a spin period of 3.604 h, and we found a variation of the linear polarization with rotation. This seems to be a rare case in which such variation is unambiguously found, also a consequence of its fairly large amplitude. Combining this new information with the brightness and colour variation as well as previously reported results from Arecibo radar observations, we conclude that there is no variation of the mineralogy across the surface of Phaeton. However, the observed change in the linear polarization may be related to differences in the thickness of the surface regolith in different areas or local topographic features.
We present a new calibration of the geometric albedo versus linear polarization relation for the asteroids. We use the classical relation logpv=C1logh+C2, where pV is the geometric albedo and h is ...the slope of the phase–polarization curve. We have obtained new values for the C1 and C2 coefficients and their nominal uncertainties, by means of dedicated polarimetric observations of a number of asteroids for which the albedo is supposed to be known with good accuracy Shevchenko and Tedesco. Icarus 2006;184:211–220. The new calibration proposed in this paper represents the state of the art based on currently available data. However, the uncertainties on the derived calibration coefficients are still not negligible, and we suggest that alternative forms of the albedo–polarization relation should be explored in the future, possibly based on a bigger data set of polarimetric measurements.
► A classical relation exists between asteroid geometric albedo and polarization. ► A long-debated problem is the calibration of this relation. ► We present new asteroid polarimetric observations and derive a new calibration.
We have discovered that the Asteroid (234) Barbara exhibits very anomalous polarimetric properties. The phase–polarization curve of this asteroid is unique and is not matched by any other known ...atmosphereless body of our Solar System. Although a few preliminary conjectures can be made, for the moment the reasons of the peculiar polarimetric properties of this asteroid remain essentially unknown.
A sample of over 12,487 asteroid proper element triplets, computed by A. Milani and Z. Knežević (1994,
Icarus 107, 219-254), has been searched for statistically significant and robust families by ...both the hierarchical clustering (HCM) and the wavelet analysis (WAM) automated techniques. The current sample includes almost 8000 unnumbered objects with relatively well-determined orbits, which allow us to extend the previous family classifications to much smaller asteroid sizes. With both clustering methods about one-third of the whole asteroid population is found to belong to groupings of varying significance and robustness. Nearly all the families identified in previous searches by Ph. Bendjoya (1993,
Astron. Astrophys. Suppl. Ser. 102, 25-55) and V. Zappalà
et al. (1994,
Astron. J. 107, 772-801) are confirmed. Many more groupings are now recognized having a small number of members and/or a large percentage of unnumbered ones, particularly when the WAM technique is applied to asteroids located in the middle region of the main belt, between the 3:1 and the 5:2 Kirkwood gaps. Most of these new small families appear to derive from the (possibly recent) break-up of small parent asteroids, <50 km across. Some are very compact, suggesting ejection speeds not much in excess of the parent's escape velocity. Some fairly populous families can be readily interpreted as the outcome of giant cratering events, including those associated with (4) Vesta and (10) Hygiea. Others (e.g., Meliboea in the outer belt) are so depleted in small members that probably an old age is implied. Several populous families, and in particular those named "clans" (e.g., Eunomia, Vesta, Nysa), show a complex and size-dependent internal structure, suggesting a complex collisional history (i.e., multiple successive disruptive events). A problem with family searches applied to very numerous asteroid samples, such as the current one, is that neighboring, physically distinct groupings often partially overlap each other, and thus merge according to the clustering techniques.
Context. The Calern Asteroid Polarimetric Survey (CAPS), a collaboration between the INAF Astrophysical Observatory of Torino (Italy) and the Observatoire de la Côte d’Azur (Nice, France), has ...produced new asteroid polarimetric data for a number of years, and is one of the most important, currently active projects of asteroid polarimetry. Aims. The purpose of this paper is to make public the CAPS data collected thus far, to explain the adopted techniques of data reduction and computation of phase-polarisation curves for the measured objects, and explain, by means of some examples, the importance of the CAPS database. Methods. The pipeline of data reduction has been recently updated and made as automatic as possible, using numerical algorithms developed specifically for the purposes of CAPS. The derivation of phase-polarisation curves for the observed asteroids is done using established criteria and algorithms that have recently been slightly improved, and are also summarised in this paper. Results. The CAPS catalogue is a steadily growing source of information which can be exploited for different purposes, including, but not limited to, an updated calibration of the relations existing between different polarimetric parameters and the geometric albedo of the objects, and a study of classes of objects that can be most easily identified by means of their polarimetric properties. These subjects will be more specifically discussed in separate papers. Conclusions. Asteroid polarimetry data nicely complement the results of other more commonly used techniques, including visible and IR photometry and spectroscopy. CAPS contains a lot of much-desired information about physical properties, which can hardly be inferred by means of other techniques.
•Spectroscopic and polarimetric observation of Barbarian/L-type asteroids.•Hapke modeling of asteroid spectra including model of space-weathering.•CAIs were found to be the most probable cause of the ...large inversion angle of Barbarians.
Asteroids can be classified into several groups based on their spectral reflectance. Among these groups, the one belonging to the L-class in the taxonomic classification based on visible and near-infrared spectra exhibit several peculiar properties. First, their near-infrared spectrum is characterized by a strong absorption band interpreted as the diagnostic of a high content of the FeO bearing spinel mineral. This mineral is one of the main constituents of Calcium-Aluminum-rich Inclusions (CAI) the oldest mineral compounds found in the solar system. In polarimetry, they possess an uncommonly large value of the inversion angle incompatible with all known asteroid belonging to other taxonomical classes. Asteroids found to possess such a high inversion angle are commonly called Barbarians based on the first asteroid on which this property was first identified, (234) Barbara. In this paper we present the results of an extensive campaign of polarimetric and spectroscopic observations of L-class objects. We have derived phase-polarization curves for a sample of 7 Barbarians, finding a variety of inversion angles ranging between 25 and 30°. Spectral reflectance data exhibit variations in terms of spectral slope and absorption features in the near-infrared. We analyzed these data using a Hapke model to obtain some inferences about the relative abundance of CAI and other mineral compounds. By combining spectroscopic and polarimetric results, we find evidence that the polarimetric inversion angle is directly correlated with the presence of CAI, and the peculiar polarimetric properties of Barbarians are primarily a consequence of their anomalous composition.
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