Spectra and morphological structures in the inner coma of comet C/2020 R4 (ATLAS)



Federico Manzini (Stazione Astr. di Sozzago), Paolo Ochner (Uni-PD), Virginio Oldani (Staz. Astr. di Sozzago), Luigi R. Bedin (OAPD-INAF)


We followed comet C/2020 R4 (ATLAS) for three nights from April 22, 2021, with a resolution of 270 km/pixel with the 0.4 m Cassegrain at the Sozzago Astronomical Station (Italy - MPC A12) and with the 1.22m Galileo telescope at the Asiago Astrophysical Observatory-University of Padua (Italy - MPC 098).

Following an enhancement processing of the faint details and of the brightness gradients of the inner coma, our CCD images of April 22.9 UT showed a prominent morphological structure, attributable to at least one large active area located on the cometary nucleus. The comet was at the minimum distance from Earth of its path through the inner solar system, at an angle of -33 ° between the Earth and its orbital plane.

The structure looks like a jet/fan in PA 100° with respect to the optocenter of the comet. The analysis of the images taken in Bj, Vj, Rj (preliminarily calibrated) clearly shows a reddening of the nucleus. An isophote visualization of the original images also highlights a clear asymmetry of the area east of the nucleus, corresponding to the jet described above (see images). This may suggest a strong activity modulated by nucleus rotation.

Spectra obtained on April 23 and 24, 2021 with the Galileo telescope show the typical cometary emissions with evidence of NH at 3360 Å, CN at 3880 Å, OI at 6300 and 6364 Å and the Swan bands due to C2 and C3. The average of the fluxes of CN and OI in the calibrated spectra provides the following values:

CN        2021.04.23.95 UT        2.25*10-12 erg cm-2 s-1 Å-1

CN        2021.04.24.95 UT        5.16*10-12 erg cm-2 s-1 Å-1

OI        2021.04.23.95 UT        0.97*10-13 erg cm-2 s-1 Å-1

OI        2021.04.24.95 UT        2.60*10-13 erg cm-2 s-1 Å-1

The flux appears to have doubled between the two days, after only 24 hours, perhaps connected with the rotation of the nucleus.


Figures and extra material at https://web.oapd.inaf.it/bedin/files/PAPERs_eMATERIALs/ATel/C2020R4/