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V-Type Mineralogical Calibration Equations for a Variety of Astrophysical Observing Scenarios

Presentation #101.06 in the session “Main Belt Asteroids 1: Taxonomy and Composition”.

Published onOct 26, 2020
V-Type Mineralogical Calibration Equations for a Variety of Astrophysical Observing Scenarios

Vesta and Vesta-like asteroids have been convincingly linked, through near-infrared (NIR; 0.7–2.5 µm) spectral analysis, to a clan of basaltic achondritic meteorites - howardites, eucrites, and diogenites (HEDs). NIR reflectance spectra of V-type asteroids and HED meteorites have two absorption features centered near 1 µm (Band I) and 2 µm (Band 2) caused primarily by Fe2+ and Ca2+ cations in pyroxene. Previous studies have shown a correlation between the mol% Fs and Wo with the central wavelengths of Band I and Band II, hereafter called Band I Center (B1C) and Band II Center (B2C). This dependency on mineral composition allows for the generation of calibration equations that link B1C and B2C to mol% Fs and Wo, which can be applied to estimate the mineralogy of V-type asteroids from their NIR spectra. The ability to accurately determine B1C and B2C wavelength positions is dependent on the signal-to-noise (S/N) of the spectrum. The typical S/N of asteroid spectra is S/N ~ 50, noticeably lower than the S/N of meteorite spectra, which is about ten times higher. Sanchez et al. (2020) artificially reduced the S/N of the ordinary chondrite meteorite spectra to explore mineralogical determinations using S-type asteroid spectra of comparable S/N. Sanchez et al. (2020) also extended the applicability of S-type calibration equations to incomplete spectral data sets lacking visible data or having unreliable/incomplete data between 2.4–2.5 µm.

Here we performed a similar analysis on HEDs and V-type asteroids using a reduction in S/N for HED meteorite spectra. In addition, we used six versions of each spectra, covering wavelength ranges from five different terminal wavelengths (~0.75 and 0.8 for Band I; 2.4, 2.45, and 2.5 µm for Band II), mimicking incomplete spectral data sets. Like Sanchez et al. (2020), we found that the decreased S/N calibration equations yielded a lower R2, and an increased rms error, compared with their high S/N calibrations. We also created calibration equations using the minimum of Band I as an alternative to the center of Band I, and explored the efficacy of those equations. When applied to V-type asteroids, we found that the derived mineralogy from Band I is often very different from that of Band II, and concluded that averaging the results from both bands gives mineralogical results for the V-type asteroids most consistent with those of the HEDs.

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