Comets change rapidly when approaching the Sun, volatile substances of the nuclei of comets begin to evaporate they have gas and dust atmosphere coma, and “grow” tails. But to predict how it will behave each new comet trapped in the inner regions of the Solar system, is almost impossible, each “experiment” is unique, and in addition, carried out in conditions that are unattainable in terrestrial laboratories. It is therefore important to examine each comet with all available astrophysical methods — this will allow to better understand the origin and evolution of matter in the Solar system.
Especially scientists interested in the decay of cometary nuclei. It doesn’t happen with every comet, but it is relatively common. Over the last 150 years there are more than 40 disintegrated comets. Partial fragmentation of the nucleus or its complete disintegration allow to study the internal structure of the nucleus.
Disintegrating comets are divided into two groups: the comet, the core of which disintegrate into several large fragments (secondary nucleus), and comet nuclei which break up into small pieces, and the object “disappears” catastrophically quickly. The analysis of the dynamics of fragmentation of the fragments indicates that the decay of the nucleus of a comet can occur at large heliocentric distances (more than 50 astronomical units) for long-period and dynamically new comets, and at any time — for short-period comets. Astronomer Zdenek Sekanina (Zdeněk Sekanina) in 1997 described two possible mechanisms of disintegration of cometary nuclei: 1) separation of the surface layer (cortex) of a rapidly rotating core and 2) the collapse of a rapidly rotating core in which tensile strength at break and density was quite low.
It is accepted to consider, that the nuclei of comets — aggregates several cometesimals that occurred in the protoplanetary disk, closer to the periphery. There was an intensive cooling of substances and dust particles condensed different the ice. It is therefore likely that the decay of nuclei into fragments should occur at the borders between cometesimals. The physical and chemical differences between individual cometesimals can you explain the difference in activity of the fragments. The heterogeneity of the physical structure of the kernel can be combined with the heterogeneity of the chemical composition.
Long-period comet C/2019 Y4 (ATLAS) was discovered on 28 December 2019. Since she quickly gained brightness, and in the period from early February to late March its apparent magnitude was doubled. It was expected that the comet will be very bright and like the character of the activity in comet C/1995 O1 (Hale-Bopp). However, in early April, the comet began to disintegrate, losing brightness. The core was broken not into dust, but several large fragments, so we had the opportunity to observe the collapse and to examine the characteristics of individual fragments.
We managed to observe the comet on then six-meter telescope BTA of the Special astrophysical Observatory of RAS with a multimode device SCORPIO-2 in two modes — the spectral and photometric.
Photometry is the precise measurement of brightness is one of the most effective methods for the study of comets. This method allows, for example, to evaluate the rate of formation of gas and dust and changing them as you get closer to the Sun, to measure the period of rotation core size. Images of the comet obtained with BTA, confirmed that the nucleus of the comet ATLAS was divided into several fragments in the head of the comet and along the tail.
The study of the spectra of comets allow to investigate properties of the three components of their atmospheres − neutral gas, dust particles and ionized gas (plasma). A preliminary analysis of the spectral data for the ATLAS of the comet showed that the comet has a very rich emission spectrum, including the line emission of various molecules and ions and neutral oxygen.
Spectra help to see not all the substances of the comet. Many of them disintegrate under the action of solar radiation, but we register the decay products is a subsidiary of the molecules and can build models of reactions to understand what are the parent molecules of the comet’s nucleus could produce. It is the differences in the relative contents of the parent molecules in different comets may be the most interesting. Cometary ICES reflect the molecular composition protocolecho nebula in the field of large planets, where they formed, and the temperature of the environment, which has led to their condensation.
But spectra allow us to find the chemical and physical differences of individual fragments of the core. For example, differences in the relative contents subsidiary of the molecules CN, C2 and C3 in different fragments, the differences in spectral reflectance of dust will allow you to judge the internal chemical heterogeneity, as the parent of the comet’s nucleus and its fragments.