ARAVIND K

Comets can be considered to be the time-capsules of the early Solar system since they spend a larger part of their life away from the Sun. Studying various aspects of these minor bodies can help us gain insight into the conditions that prevailed during the formation of the Solar System since they are made up of pristine materials that comprised the proto-solar nebula. In this aspect, comparing comets from our Solar system with interstellar ones can shed light on the difference/similarity in the materials present in different proto-stellar systems. McGlynn & Chapman(1989), explored the possibility of predicting the number of interstellar comets that can be detected passing through the inner Solar system. Sen & Rana(1993), refining the concepts used by McGlynn & Chapman, developed an acceptable theory predicting the number of interstellar comets to be detected as only 1 in 200 years. This also helped in explaining the non-detection of interstellar comets. It was only recently, (August 30th 2019) that the first interstellar comet was discovered. Gennadiy Borisov discovered this comet with his 0.65m telescope. This in-bound comet was later named 2I/Borisov. The objective of our work was to investigate the properties of the interstellar comet so as to compare their properties with those of Solar system comets. Figure 1 depicts the low-resolution optical spectrum of a typical solar system comet, containing emissions from the radicals like CN, C2, C3, NH2 etc produced by the sublimation, photoionisation and photodissociation of various parent molecules present in comet’s nucleus as the comet comes closer to the Sun. Our secondary objective was to use imaging mode to study the colours of the dust present in the comet’s coma and compare its resemblance with the observed dust colours of various solar system comets. Figure 2 and 3 depicts the two observatories used for observation (IAO and MIRO). The comet was observed using spectroscopic technique at two different epochs. After systematic reduction, the optical spectrum of the comet showed emissions from CN, C2 and C3 radicals (Figure 4) similar to what is observed in Solar system comets. All the three emissions were detected on both the epochs of observation (Figure 5). Upon computation of the production rate (molecules/s), an indicator of activity and amount of molecules in the comet, it was found that the comet was depleted in carbon chain molecules, as per the classification criterion defined by A'Hearn (1995). From imaging study, it was seen that the colours of the dust in the coma of 2I/Borisov were redder than solar and similar to the colours of long-period comets of the solar system and 1I/`Oumuamua, the first interstellar object to be detected. Using these results, we conclude that the gaseous composition of the interstellar comet resembles that of the solar system comets with a depletion in carbon chain molecules. Similar carbon chain molecule depletion is also observed among Solar system comets with a larger fraction of these comets belonging to the Jupiter Family Comets (JFC). This similarity can imply that the formation scenario which occurred in 2I/Borisov’s parent star system maybe similar to that of our solar system. Based on the observation of variation in the ratio of the production rate of various molecular emissions, as the comet approached the Sun, it was realized that the comet nucleus maybe heterogenous or the nature of volatility of the pristine materials in 2I/Borisov is different from what is observed in solar system comets. I thank the staff at IAO-Hanle, CREST-Bangalore and MIRO-Mt. Abu and also my colleagues at both PRL and IIA who supported me on the proposal for telescope time.