Techniques d'observation spectroscopique d'astéroïdes
Techniques d'observation spectroscopique d'astéroïdes Techniques d'observation spectroscopique d'astéroïdes
6 M. Birlan et al. tel-00785991, version 1 - 7 Feb 2013 Natural Log Reflectance 0.3 0.2 0.1 0.0 -0.1 -0.2 -0.3 -0.4 -0.5 -0.6 1333 Cevenola LCP Ol 0.5 1.0 1.5 2.0 2.5 wavelength (µm) Figure 7. MGM modeling of VNIR data of (1333) Cevenola. The presence of both olivine (Ol) and pyroxene (LCP) minerals are necessary for this fit. The individual absorption bands are draw in black for LCP and gray for the olivine. (Ol), orthopyroxene (OPx) and clinopyroxene (CPx) 8 sets that were identified contain 44, 33 and 55 spectra respectively. The olivine set includes synthetic spectra that span the Fa 0Fo 100 - Fa 100Fo 0 domain in 5-10 mol% increments (Dyar et al. 2009) while the members of orthopyroxene set sample the range En 0Fs 100 - En 100Fs 0 (Klima et al. 2007). The clinopyroxene spectra represents minerals with different Wo, En, Fs content. The wide variety of minerals included in the three above sets make them suitable for generating synthetic mixtures spectra. The high resolution, high S/N RELAB spectra were fitted using a cubic B-spline function with 25 fitting coefficients. For the asteroids spectra, in order to avoid the overfitting, the number of coefficients was reduced to 12. In both cases it was confirmed by visual inspection that the interpolating functions approximate well the overall shape of the spectra. For each possible combination of olivine, ortho- and clinopyroxene, synthetic spectra were generated from b- splined end-members spectra using a linear mixture by varying the end-member’s concentration in 0.5% increment (step). Each of the obtained spectra were compared to spline smoothed spectra of (854) Frostia, (1333) Cevenola, and (3623) Chaplin. The χ 2 calculations were performed following the formula: χ 2 = 1 N ∑ w N w i=1 (R i − f(w i)) 2 f(w i) where N w is the number of R i reflectance values at w i wavelength, and f(w i) the reflectance value of the geometric mixture obtained from laboratory spectra (i.e. additive contribution of individual component). The concentration for each of the components will span the range between 0 and 100%, and the sum of the mixture 8 structures of orthorhombic and monoclinic crystals of pyroxenes (1) is 100%. The best mixture identified by the above χ 2 -test is further refined in 0.1% concentrations step this time with fixed end-members. The χ 2 minimization will allow the derivation of a map of possible/plausible models. This method allows the plot of 2D mineralogical charts, an interesting tool for visualize the best mineralogical solutions. Two dimensional charts derived from the χ 2 fit are presented in Figure 8, Figure 9, and Figure 10 for (854) Frostia (1333) Cenevola, and (3623) Chaplin respectively. These charts plot olivine on the X axis, and OPx/(CPx+OPx) on the Y axis. The color code indicates the concordance of the model to the observational data, the blue color representing the best fit. The white regions of the 2D charts are an indicator of the limit where the χ 2 minimization fails. The best fit mixture for (854) Frostia was obtained using (Ol, OPx, CPx) = (0, c1dl14, c1dl07) with following ratios (0%, 33.5%, 66.5%), while for the asteroid (1333) Cevenola the best fit was obtained using (Ol, OPx, CPx) = (cgpo84, c1dl01, c1dl12) with ratios (25%, 67.5%, 7.5%). In the case of (3623) Chaplin the closest analogue was the mixture (0, c1dl01, c1dl08) with ratios (0%, 96.5%, 3.5%). The inferred mineralogical solutions explain well the asteroids spectra. However, giving the inherent complications of curve matching procedure as the χ 2 -test Gaffey et al. (2002) a family of mineralogical solutions could fit our spectroscopic data. 5 DISCUSSION The binarity of (854) Frostia is supported by photometric data (Behrend et al. 2006). Unfortunately, no physical ephemerides of Frostia are not known to have a precise timing of possible mutual phenomena of this system. Nevertheless there is little chance for a geometry allowing mutual phenomena at the time of our observations. The spectrum likely represents this object globally, thus being a first attempt in the characterization of the asteroid’s mineralogy. The spectrum of (854) Frostia (Fig 1) reveals large and deep absorption bands around 1 and 2 microns. This spectrum, similar to the asteroid (4) Vesta, allows the classification of (854) Frostia in the V type taxonomic class. Based on spectroscopic behavior and dynamical consideration from the main-belt through the resonances 3:1 and ν 6 resonances (Binzel & Xu 1993), Vesta and the vestoids are supposed to be at the origin of Howardite-Eucrite-Diogenite meteorites. The structure of HED meteorites is very close to mafic materials. Thus, the parent body of HED meteorites are supposed to have experienced volcanism and metamorphism in the process of formation during the early solar system. The parent body of Vesta and vestoids underwent accretion, total melting, fractionation, and differentiation during the first few million of years of solar system formation (Keil 2002). (854) Frostia was not included in the family of (4) Vesta by Zappala et al. (1995). The location of (854) Frostia inside the Main-Belt is very similar to that of Vesta family in semimajor axis and inclination and may justify its membership to the same clan. Frostia’s eccentricity of 0.17 is slightly over the greater boundary (of 0.12) of Vesta family. This case is not particular while other V-type asteroids were already c○ 0000 RAS, MNRAS 000, 000–000
Spectral properties of (854), (1333), and (3623) 7 Figure 8. Two dimension chart of concentration for the asteroid (854) Frostia using olivine, orthopyroxene, and clinopyroxene from RELAB is presented in the right side of the figure. The best fit mixture for (854) Frostia using (Ol, OPx, CPx) = (0, c1dl14, c1dl07) with ratios (0%, 33.5%, 66.5%) is presented in the left side in the figure. tel-00785991, version 1 - 7 Feb 2013 Figure 9. Two dimension chart of concentration for the asteroid (1333) Cevenola using olivine, orthopyroxene, and clinopyroxene from RELAB is presented in the right side of the figure. The best mineralogical model for (1333) Cevenola using (Ol, OPx, CPx) = (cgpo84, c1dl01, c1dl12) and the ratios (25%, 67.5%, 7.5%) is presented in the left side of the figure. Figure 10. Two dimension chart of concentration for the asteroid (3623) Chaplin using olivine, orthopyroxene, and clinopyroxene from RELAB. The best mineralogical mixture using (Ol, OPx, CPx) = (0, c1dl01, c1dl08) with ratios (0%, 96.5%, 3.5%) is presented in the left of the figure. c○ 0000 RAS, MNRAS 000, 000–000
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Spectral properties of (854), (1333), and (3623) 7<br />
Figure 8. Two dimension chart of concentration for the asteroid (854) Frostia using olivine, orthopyroxene, and clinopyroxene from<br />
RELAB is presented in the right side of the figure. The best fit mixture for (854) Frostia using (Ol, OPx, CPx) = (0, c1dl14, c1dl07)<br />
with ratios (0%, 33.5%, 66.5%) is presented in the left side in the figure.<br />
tel-00785991, version 1 - 7 Feb 2013<br />
Figure 9. Two dimension chart of concentration for the asteroid (1333) Cevenola using olivine, orthopyroxene, and clinopyroxene from<br />
RELAB is presented in the right side of the figure. The best mineralogical model for (1333) Cevenola using (Ol, OPx, CPx) = (cgpo84,<br />
c1dl01, c1dl12) and the ratios (25%, 67.5%, 7.5%) is presented in the left side of the figure.<br />
Figure 10. Two dimension chart of concentration for the asteroid (3623) Chaplin using olivine, orthopyroxene, and clinopyroxene from<br />
RELAB. The best mineralogical mixture using (Ol, OPx, CPx) = (0, c1dl01, c1dl08) with ratios (0%, 96.5%, 3.5%) is presented in the<br />
left of the figure.<br />
c○ 0000 RAS, MNRAS 000, 000–000
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