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The split long-period comet C/2019 Y4 (ATLAS)

Russian version

    As the decay of the nucleus into fragments is not common in comets, it is observed very rarely. About 40 comets have been observed after their splitting in the last 150 years. The partial fragmentation of the cometary nucleus or its complete disintegration allows us to study the process regarding nucleus mass loss, internal structure, and composition of the nucleus. There are two main types of comet splitting: comets, the nuclei of which disintegrate into several large fragments (secondary nuclei), and comets, the nuclei of which disintegrate into small fragments, and the object itself "disappears" catastrophically, as happened with the long-period comet C/2019 Y4 (ATLAS).
    Spectroscopic and photometric observations of the comet C/2019 Y4 (ATLAS) were carried out at the 6-m telescope BTA of the SAO RAS using the SCORPIO-2 focal reducer. The comet was observed during the disintegration of its nucleus on April 14 and 16, 2020, when it was at heliocentric distances of 1.21 and 1.17 au, 47 and 45 days before its perihelion, respectively.
    Based on photometric studies, four fragments belonging to the coma were detected in both observational runs. They were visible both in the continuum and in emission lines, which indicates the formation of four "mini-comets" as a result of the split of the main nucleus. Spectral observations showed that the disintegration of the nucleus led to a significant increase in the proportion of gas in the cometary coma. The distribution of the gas component was asymmetric for solar-antisolar directions. A change in the color of the dust along the cometary coma was found: the color changed from red, near the main component of the nucleus, to very blue, at the periphery of the cometary coma. A significant difference in the coma color behavior is associated with the asymmetry of the contribution of the gas component (the presence of several nuclear fragments in the coma) and with the evolution of the scattering properties of particles with a cometocentric distance.
    The correlation between the findings on the composition of the gaseous coma in 2019Y4 and those emerging from the study of its dusty coma is worth noting. The CN and C2 molecules in a cometary coma could appear as a result of the disintegration of parental molecules, and also to evaporate from submicron and/or micron-sized organic particles in the cometary jets. The high abundance of C2 and CN molecules in a coma observed in the spectral and photometric data, and the absence of the negative polarization is an indicator of the predomination of highly absorbing particles, whose refractive index corresponds to organic materials.
Fig.1. Spectral observations of comet C/ 2019 Y4 (ATLAS) on April 14 (top) and April 16, 2020 (below). The contribution of the gas component to the total flux (in percent) in each pixel of the slit image is shown in the form of a color diagram. The spectrum of the comet (a) and the normalized transmission curves of the чу, RC, and SED500 filters (b) are superimposed on the diagram.
Fig.2. Direct images of comet C / 2019 Y4 (ATLAS) obtained with narrow-band filters: blue BC continuum (a), red RC continuum (b), the filter focused at the C2 band system, panel (d) shows the pure C2 intensity map after the dust-component subtraction. 4 fragments of the nucleus (A, B, C, D) are marked.

O.Ivanova, I.Luk'yanyk, D.Tomko, A.Moiseev. Photometry and long-slit spectroscopy of the split comet C/2019 Y4 (ATLAS). / // Monthly Notices of the Royal Astronomical Society. - 2021. - V. 507(4). - P. 5376-89.

Contact - Ivanova O.,, DSc, Senior Researcher in Department of Interplanetary Matter in Astronomical Institute of Slovak Academy of Science