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Methanol in Comet 46P/Wirtanen: Rapidly Varying and Anisotropic Outgassing Measured with ALMA

Presentation #108.03 in the session “Comet Comae: A Tail of Volatiles and Ice”.

Published onOct 26, 2020
Methanol in Comet 46P/Wirtanen: Rapidly Varying and Anisotropic Outgassing Measured with ALMA

The study of comets affords a unique window into the birth, infancy, and subsequent history of the solar system through study of their coma volatiles. Pure rotational transitions of methanol (CH3OH) have been observed in comets at millimeter wavelengths since the 1990s (e.g., Bockelée-Morvan et al. 1994, A&A, 287, 647) and are often used to probe coma rotational and kinetic temperature. More recently, the high spatial and spectral resolution provided by the Atacama Large Millimeter/Submillimeter Array (ALMA) has enabled the coma to be probed in unprecedented detail. These studies have revealed insights into the production mechanisms and thermal chemistry of CH3OH in comets, and time-resolved measurements have revealed rapid variations in gas and dust production (e.g., Cordiner et al. 2017, ApJ, 837, 166; Cordiner et al. 2017, ApJ, 838, 147). The historic 2018 perihelion passage of comet 46P/Wirtanen afforded an opportunity to couple the capabilities of ALMA with the most favorable apparition of a Jupiter-family comet in the previous decade. We targeted multiple CH3OH transitions in 46P/Wirtanen using five executions of the ALMA Atacama Compact Array (ACA) spread across UT 2018 December 8 and 9, with a spectral resolution of 0.15 km s-1 and spatial resolution (synthesized central beam) of 279 km × 502 km. Our time-resolved measurements revealed that CH3OH production in Wirtanen was anisotropic, and that the spectral line asymmetry varied on timescales of minutes. Variations in line asymmetry and CH3OH production rates were consistent with the reported nucleus rotation period of 9 hours. We will present spectrally integrated flux maps of CH3OH emission on each date, report CH3OH production rates and rotational temperatures, and place our results into the context of comets characterized to date.

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