Abstract
Abstract We report the detection of CH3OH emission in comet 46P/Wirtanen on UT 2018 December 8 and 9 using the Atacama Compact Array (ACA), part of the Atacama Large Millimeter/Submillimeter Array (ALMA). These interferometric measurements of CH3OH along with continuum emission from dust probed the inner coma (<2000 km from the nucleus) of 46P/Wirtanen approximately one week before its closest approach to Earth (Δ = 0.089–0.092 au), revealing rapidly varying and anisotropic CH3OH outgassing during five separate ACA executions between UT 23:57 December 7 and UT 04:55 December 9, with a clear progression in the spectral line profiles over a timescale of minutes. We present spectrally integrated flux maps, production rates, rotational temperatures, and spectral line profiles of CH3OH during each ACA execution. The variations in CH3OH outgassing are consistent with Wirtanen’s 9 hr nucleus rotational period derived from optical and millimeter wavelength measurements and thus are likely coupled to the changing illumination of active sites on the nucleus. The consistent blue offset of the line center indicates enhanced CH3OH sublimation from the sunward hemisphere of the comet, perhaps from icy grains. These results demonstrate the exceptional capabilities of the ACA for time-resolved measurements of comets such as 46P/Wirtanen.
Highlights
Comets, kilometer-sized bodies of ice and dust, were assembled in the solar nebula during the era of planet formation and subsequently scattered to the Kuiper disk or the Oort cloud
We transformed the images from astrometric coordinates to projected cometocentric distances, with the location of the peak continuum flux chosen as the origin, which was in good agreement with the comet’s predicted ephemeris position
Molecular line emission was modeled using a three-dimensional radiative transfer method based on the Line Modeling Engine (LIME; Brinch & Hogerheijde 2010) adapted for cometary atmospheres, including a full non-LTE treatment of coma gases, collisions with H2O and electrons, and pumping by solar radiation
Summary
Kilometer-sized bodies of ice and dust, were assembled in the solar nebula during the era of planet formation and subsequently scattered to the Kuiper disk or the Oort cloud. The 12 × 7 m antennas of the ACA provide short baselines between 9 and ∼50 m (corresponding to an angular resolution ranging from 5 45 to 29 0 at 230 GHz), resulting in greater sensitivity to extended flux than the main 12 m array (which was in configuration C43-4 at the time of our observations with an angular resolution ranging from 0 39 to 4 89 at 230 GHz) We note that these observations were most sensitive to the inner coma of 46P/Wirtanen and that the largest recoverable scale of the ACA is small compared to the size of 46P/Wirtanen’s extended coma (up to several arcminutes). ⎝ frest ⎠ dt where c is the speed of light (km s−1), frest is the rest frequency of a given transition (GHz), f is the frequency of the channel (GHz), and dΔ/dt is the topocentric velocity of the comet at the time of the observations (km s−1)
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