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Phase Curves of Themis Family Asteroids from NEOWISE Photometry

Presentation #212.04 in the session MBAs: Physical Characteristics (iPosters).

Published onOct 20, 2022
Phase Curves of Themis Family Asteroids from NEOWISE Photometry

Asteroid phase curves show how an asteroid’s brightness varies with changing phase angle, the angle between the Sun, the asteroid, and the observer. The shape of an asteroid phase curve is linked to the object’s surface properties, including its surface composition. There is a known relationship between the shape of a visible light phase curve and an asteroid’s taxonomic type, which itself is representative of surface composition. Since asteroid surface components are most commonly identified via the presence of spectral absorption features at known wavelengths, making the connection between phase curve shape and the presence of such absorption features would prove to be an extremely useful tool for broadly characterizing asteroids across a wide range of spectral types. In particular, the Themis dynamical family, a C-complex family in the outer main belt consisting of over 4000 asteroids, is one group of asteroids that warrants further study as several of its members have had water ice detected either on their surfaces or sublimating from their surfaces. This suggests potentially many more uncharacterized objects that are harboring water ice in the inner solar system. By using observations from the NEOWISE mission’s W1 filter, which covers the 3 μm region where water ice has a notable absorption feature, we construct phase curves at these wavelengths to help gain a better understanding of how the presence (or lack thereof) of water ice is impacting the shape of the W1 phase curve. Here, we present W1 phase curves of several hundred Themis family members as well as phase curves of non-C-complex asteroids for comparison. We estimate phase slopes in the linear regime of these phase curves and compare the derived phase slopes with the presence of a water ice spectral feature (when 3μm spectra are available). Additionally, we investigate potential trends between phase slope and geometric albedo, W1 albedo, and spectral type. This serves as a demonstration of the techniques used to construct phase curves from NEOWISE photometry as well as a basis for similar phase curve work to be applied to the remainder of the Themis family and to other asteroid spectral types.

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