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Forbidden atomic oxygen lines far from the Sun for dynamically new comet C/2017 K2 (PanSTARRS)

Presentation #322.02 in the session Comets (Poster + Lightning Talk)

Published onOct 23, 2023
Forbidden atomic oxygen lines far from the Sun for dynamically new comet C/2017 K2 (PanSTARRS)

Comet is thought to be one of pristine objects in the solar system. The main components of cometary ice are H2O, CO2, and CO (Mumma & Charnley 2011). These chemical compositions restrict the chemical environment and evolution in the early solar system, especially formation temperatures. Since comets with eccentricity of e<1.0 have approached the Sun many times, it is thought that volatile species such as CO2 (sublimation temperature Tsub ~70 K) and CO (Tsub ~25 K) are selectively sublimated from the surface of the nucleus when the comet approaches the Sun. It is unclear whether the derived abundances of CO2 and CO with respect to H2O of these comets are nature or nurture. Hence, we focus on a comet firstly approaching the inner solar system from the Oort Cloud, so called a “Dynamically new comet” (DNC). The DNC is thought to be one of the most pristine icy small bodies because the DNC has undergone little thermal alternation due to solar heating since the formation in the solar nebula 4.6 Gyrs ago.

The abundance ratios of H2O:CO2:CO in comets has been determined from two different methods: (1) direct measurements by space infrared facilities (e.g., Ootsubo et al. 2012), and (2) indirect measurements using intensity ratio of three forbidden lines of the oxygen atoms, [O I], which are photo-dissociation products of these species in the cometary coma, using high-resolution optical spectroscopy (e.g., Shinnaka et al. 2020). In the method 2, the CO2/H2O abundance ratio has been estimated from the intensity ratio of the “green line” at 557.7 nm and the “red lines” at 630.0 and 636.4 nm of the [O I] lines using the empirical equation (Decock et al. 2013).

In order to derive the abundance ratios of of CO2 with respect to water of DNC C/2017 K2 (PanSTARRS), we performed high-resolution spectroscopy of the comet on UT 2022 April 8 using the HDS mounted on the Subaru Telescope, atop of the Mauna kea. We discuss the chemical composition of the comet based on our and previous studies regarding this research. This research was supported by JSPS grants (No. 20K14541).

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