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Fig. 9

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Average spectrum of the HCN(3–2) line observed from 8.5 to 13.4 December in comet C/2021 A1, with simulated profiles. The model in blue assumes a production rate of 2.1 × 1025 molecules s−1 at 0.88 km s−1 on the sunward hemisphere and 1.1 × 1025 molecules s−1 at 0.58 km s−1 on the opposite hemisphere mostly facing the observer. The mean phase angle is ~140°, and the tilt of 40°(or 140°) with respect to the comet-observer line of sight is taken into account in this 3D simulation, as depicted in the upper right. The model in purple uses variable velocities in both hemispheres (see text), with production rates of 2.3 × 1025 molecules s−1 and 0.9 × 1025 molecules s−1, respectively. The vertical axis is main beam brightness temperature in K. The horizontal axis is the Doppler velocity in the rest frame of the comet.

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