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Register a new userA model of the 3-{mu}m hydration band with Exponentially Modified Gaussian (EMG) profiles: application to hydrated chondrites and asteroids
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We present here a new method to model the shape of the 3-{mu}m absorption band in the reflectance spectra of meteorites and small bodies. The band is decomposed into several OH/H2O components using Exponentially Modified Gaussian (EMG) profiles, as well as possible organic components using Gaussian profiles when present. We compare this model to polynomial and multiple Gaussian profile fits and show that the EMGs model returns the best rendering of the shape of the band, with significantly lower residuals. We also propose as an example an algorithm to estimate the error on the band parameters using a bootstrap method. We then present an application of the model to two spectral analyses of smectites subjected to different H2O vapor pressures, and present the variations of the components with decreasing humidity. This example emphasizes the ability of this model to coherently retrieve weak bands that are hidden within much stronger ones.
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Asteroids belonging to the Ch spectral taxonomic class are defined by the presence of an absorption near 0.7 {mu}m, which is interpreted as due to Fe-bearing phyllosilicates. Phyllosilicates also cause strong absorptions in the 3-{mu}m region, as do other hydrated and hydroxylated minerals and H2O ice. Over the past decade, spectral observations have revealed different 3-{mu}m band shapes the asteroid population. Although a formal taxonomy is yet to be fully established, the Pallas-type spectral group is most consistent with the presence of phyllosilicates. If Ch class and Pallas type are both indicative of phyllosilicates, then all Ch-class asteroids should also be Pallas-type. In order to test this hypothesis, we obtained 42 observations of 36 Ch-class asteroids in the 2- to 4-{mu}m spectral region. We found that 88% of the spectra have 3-{mu}m band shapes most consistent with the Pallas-type group. This is the first asteroid class for which such a strong correlation has been found. Because the Ch class is defined by the presence of an absorption near 0.7 {mu}m, this demonstrates that the 0.7-{mu}m band serves not only as a proxy for the presence of a band in the 3-{mu}m region, but specifically for the presence of Pallas-type bands. There is some evidence for a correlation between band depth at 2.95 {mu}m and absolute magnitude and/or albedo. However, we find only weak correlations between 2.95-{mu}m band depth and semi-major axis. The connection between band depths in the 0.7- and 3-{mu}m regions is complex and in need of further investigation.
Here we report a comparison between reflectance spectroscopy of meteorites under asteroidal environment (high vacuum and temperature) and Main Belt and Near Earth Asteroids spectra. Focusing on the OH absorption feature around 3{mu}m, we show that the asteroidal environment induces a reduction of depth and width of the band, as well as a shift of the reflectance minimum. We then decompose the OH feature into several components with a new model using Exponentially Modified Gaussians. Unlike previous studies, we confirme the link between these components, the aqueous alteration history and the amount of water molecules inside of the sample, using the shape of this spectral feature only. We then apply this deconvolution model to asteroids spectra which were obtained with a space-borne telescope and two space probes, and find a strong similarity with the components detected on meteorites, and among asteroids from a same type. Based on the conclusions drawn from our meteorites experiment, we suggest to use the 3-{mu}m band as a tracer of the alteration history of the small bodies. Using the 3-{mu}m band only, we show that Ryugu has been heavily altered by water, which is consistent with its parent body being covered with water ice, then went through a high temperature sequence, over 400{deg}C. We also point out that the 3-{mu}m band of Bennu shows signs of its newly discovered surface activity.
We present spectra of the 3.3 $mu$m and 11.2 $mu$m PAH features of a large number of (extra-) galactic sources, obtained with ISO-SWS. Clear variations are present in the profiles of these features. The sources are classified independently based on the 3.3 and 11.2 $mu$m feature profiles and peak positions. Correlations between these classes and those based on the 6--9 $mu$m features (Peeters et al. 2002) are found. Also, these classifications depend on the type of object. The observed pronounced contrast in the spectral variations for the CH modes (3.3 and 11.2 $mu$m bands) versus the CC modes (6.2, 7.7 and 8.6 $mu$m bands) is striking : the peak wavelengths of the features attributed to CC modes vary by $sim$15--80 cm$^{-1}$, while for the CH modes the variations are $sim$4--6.5 cm$^{-1}$. We summarize existing laboratory data and theoretical calculations of PAH molecules and complexes. In contrast to the 6.2 and 7.7 $mu$m components which are attributed to PAH cations, the 3.3 $mu$m feature appears to originate in neutral and/or negatively charged PAHs. We attribute the variations in peak position and profile of these features to the composition of the PAH family. The variations in FWHM of the 3.3 $mu$m feature remains an enigma while those of the 11.2 $mu$m can be explained by anharmonicity and molecular structure. The possible origin of the observed contrast in profile variations between the CH modes and the CC modes is highlighted.
We propose a novel exponentially-modified Gaussian (EMG) mixture residual model. The EMG mixture is well suited to model residuals that are contaminated by a distribution with positive support. This is in contrast to commonly used robust residual models, like the Huber loss or $ell_1$, which assume a symmetric contaminating distribution and are otherwise asymptotically biased. We propose an expectation-maximization algorithm to optimize an arbitrary model with respect to the EMG mixture. We apply the approach to linear regression and probabilistic matrix factorization (PMF). We compare against other residual models, including quantile regression. Our numerical experiments demonstrate the strengths of the EMG mixture on both tasks. The PMF model arises from considering spectroscopic data. In particular, we demonstrate the effectiveness of PMF in conjunction with the EMG mixture model on synthetic data and two real-world applications: X-ray diffraction and Raman spectroscopy. We show how our approach is effective in inferring background signals and systematic errors in data arising from these experimental settings, dramatically outperforming existing approaches and revealing the datas physically meaningful components.
We present $V$-band photometry of the 20,000 brightest asteroids using data from the All-Sky Automated Survey for Supernovae (ASAS-SN) between 2012 and 2018. We were able to apply the convex inversion method to more than 5,000 asteroids with more than 60 good measurements in order to derive their sidereal rotation periods, spin axis orientations, and shape models. We derive unique spin state and shape solutions for 760 asteroids, including 163 new determinations. This corresponds to a success rate of about 15%, which is significantly higher than the success rate previously achieved using photometry from surveys. We derive the first sidereal rotation periods for additional 69 asteroids. We find good agreement in spin periods and pole orientations for objects with prior solutions. We obtain a statistical sample of asteroid physical properties that is sufficient for the detection of several previously known trends, such as the underrepresentation of slow rotators in current databases, and the anisotropic distribution of spin orientations driven by the nongravitational forces. We also investigate the dependence of spin orientations on the rotation period. Since 2018, ASAS-SN has been observing the sky with higher cadence and deeper limiting magnitude, which will lead to many more new solutions in just a few years.
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