Activity of comet C/2017 K2 (PanSTARRS)

Dynamically new comets differ from other representatives of the cometary population as they are making their first passage through the inner Solar System. These objects contain a larger amount of pristine material than short-period comets and returning comets. They tend to have higher activity levels, more consistent pre- and post-perihelion behavior, and shallower brightening [1, 2]. Among these objects, comet C/2017 K2 (PanSTARRS) is one of the most remarkable examples of distant cometary activity ever observed. The comet’s orbit was hyperbolic, and it passed perihelion on 19 December, 2022, at 1.79 au from the Sun [3, 4]. The comet was discovered active at a heliocentric distance of about 16 au and was later identified in archival images obtained at a larger distance of about 23.7 au [4, 5, 6], indicating sustained activity far beyond the typical water-ice sublimation zone. Such activity is thought to be driven by sublimation of hypervolatile species, particularly CO and CO₂, or by crystallization of amorphous water ice [5, 7]. It was also suggested that there can be heterogeneous, clumpy volatile sources on the comet’s nucleus [8].

We present results of a comprehensive study of the activity and dust properties of comet C/2017 K2 (PanSTARRS) based on quasi-simultaneous photometric, spectroscopic, and polarimetric observations obtained on 1 and 5 June and 2 July 2022 with the 6-m Big Telescope Alt-azimuth (BTA) of the Special Astrophysical Observatory at heliocentric distances of about 3.0 - 2.7 au, together with long-term photometric monitoring carried out between March and July 2021 at heliocentric distances of about 6 - 7 au using the 0.61-m and 1.3-m telescopes of Skalnaté Pleso Observatory. The BTA observations were performed using narrowband cometary continuum and C₂ filters, as well as Sloan g and r filters, allowing us to investigate the contribution of gaseous emissions to broadband photometry and their effect on the cometary color properties. In contrast, the Skalnaté Pleso observations were obtained at significantly larger heliocentric distances using broadband B, V, and R filters, where the recorded signal is expected to be dominated mainly by sunlight scattered by dust particles. Previous spectroscopic studies showed that gaseous emissions in C/2017 K2 (PanSTARRS) were weak or absent at large heliocentric distances [8], suggesting that the cometary activity during the early stages of our monitoring campaign was predominantly dust-driven.

Our results show a gradual increase in cometary activity as the comet approached perihelion without evidence of major outbursts, although moderate color variations were detected during the monitoring campaign. The dust production, expressed by the Afρ parameter, reached about 8200 cm at a heliocentric distance of 6.017 au, placing C/2017 K2 (PanSTARRS) among the most dust-productive distant long-period comets. To constrain physical properties of dust, we employ combined light-scattering and dust dynamical models to analyze the observed polarimetric and color maps. In addition, we use existing data such as JWST/MIRI [9] to further constrain the dust model and its evolution with the heliocentric distance.

References

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