New polarimetric and spectroscopic evidence of anomalous enrichment in spinel-bearing calcium-aluminium-rich inclusions among L-type asteroids

Abstract

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.

Experimental phase function and degree of linear polarization of cometary dust analogues

“…As shown in Section 5.2, for the size range of the samples presented in this work, the position of the inversion angle depends on sample composition. This is also found by Devogèle et al (2018) in their analysis of Barbarian asteroids. Other observed polarimetric evidence (P min versus inversion angle plot) suggests that these objects should have a very fine surface regolith.…”

“…A few objects exhibit an inversion angle at lower phase angles (15 • − 17 • ). Only a few rare objects, the so-called Barbarians, show high values of inversion angles, up to 28 • (Cellino et al 2006;Devogèle et al 2018). As shown in Section 5.2, for the size range of the samples presented in this work, the position of the inversion angle depends on sample composition.…”

“…While the Mg-rich forsterite and enstatite present a flat wavelength dependence in the visible, the Fe content of the meteorite samples is expected to produce an effect on the refractive index at different visible wavelengths. This fact has been recently taken into account by Devogèle et al (2018) to interpret the observed wavelength dependence of the inversion angle of polarization of asteroid (234) Barbara as a consequence of the imaginary refractive index, in turn a consequence of the presence of nano-iron phase particles.…”

Near-Earth asteroids spectroscopic survey atIsaac NewtonTelescope

“…This taxonomic class exhibits variations in terms of slope and spectral features mostly in the NIR region. These are interpreted as diagnostic of minerals with a high FeO content (Devogèle et al 2018). However, based only on the optical region the classification is not uniquely determined as other classes such as S, K, and A can not be totally excluded.…”

Compositional distributions and evolutionary processes for the near-Earth object population: Results from the MIT-Hawaii Near-Earth Object Spectroscopic Survey (MITHNEOS)

“…We address each in turn. The L-types have been linked to bodies rich in calcium aluminum inclusions (CAIs; Sunshine et al, 2008;Devogèle et al, 2018). The particularly strong ν6 signature, even greater than the overall population (Fig.…”