A comprehensive study of comet 67P/Churyumov-Gerasimenko in the 2021/2022 apparition.

1. Introduction

Comet 67P/Churyumov˗Gerasimenko (67P/C˗G) is a well-studied comet of the Jupiter family. During the previous apparition, we extensively studied it in the post-perihelion period using the 6-m BTA telescope (Rosenbush et al. 2017; Ivanova et al. 2017). Now, in the recent 2021/22 apparition, we aimed to compare our findings with those of 2015/16. We conducted comprehensive pre- and post-perihelion observations with various telescopes, providing important insight into the composition and evolution of the comet.

 

2. Observations

Quasi-simultaneous photometric, spectroscopic, and polarimetric observations of comet 67P/C˗G were conducted at the 6-m BTA telescope (SAO) on October 6, 2021 with the g- and r-sdss filters and on February 6, 2022 with continuum BC (λ4450/62 Å) and RC (λ6839/96 Å) and emission CN (λ3870/58 Å) filters. In addition, aperture polarimetry of the comet was carried out at the 2.6-m of the CrAO and 2-m telescopes of the Peak Terskol Observatory. These observations cover a range of heliocentric distances from 1.248 au to 1.836 au, geocentric distances from 0.468 au to 0.950 au, and phase angles from 10.5° to 48.2°.

 

3. Results

Comet 67P/C˗G was active and showed an extensive coma with jets and a dust tail. On October 6, 2021, –31 days before passing perihelion, the sunward jet and long dust tail are detected in the images of the comet. The structure of the dust coma of the comet changed significantly during post-perihelion observations on February 6, 2022 (+96 days after perihelion) in comparison with pre-perihelion images. The enhanced images showed two jets which are located symmetrically relative to the rotation axis of the nucleus. However, the most prominent feature of these images is a bright linear strip-like structure extending in the direction of the Sun, which is similar to the neck-line structure. Using a geometric model, we found that two observed jets are formed from the same active area located at a latitude of φ = –58° ± 5° and the opening angle of the jet is 26° ± 8°. We also studied the unusual morphology of the CN coma in the comet and showed how it can be formed.

Strong CN and relatively weak  C₂, C₃, and NH₂  emissions are identified in the spectra of the comet in 2021 and 2022. The production rates of CN, C₃, and C₂ are in good agreement with data obtained by other authors. The production rate ratio log[Q(C2)/Q(CN)] is –0.2 ± 0.4 for 2021, and –0.6 ± 0.1 for 2022, respectively, which corresponds to the carbon-chain depleted class of comets. However, this conclusion has to be taken with caution, because, as Le Roy et al. (2015) showed, comet 67P/C˗G can be carbon-chain depleted in the summer hemisphere, whereas for the winter hemisphere, the comet would be carbon-chain normal. The dust production rate Afρ is smaller than 200 cm for both periods of observations.

The dust color (g–r) gradually changed from 0.8 mag within the innermost coma to about 0.2 mag in the outer coma for observations in 2021. In October 2021, the polarization in the near-nucleus area was about 11% at a phase angle of 47.9° and practically did not change over the coma. In February 2022, at a phase angle of 10.5°, the polarization varied between –1% in the near-nucleus area and –2.5% in the outer coma. In general, the polarization phase curve of comet 67P/C˗G resembles that of a highly polarized dust comet, however, it seems that higher polarization values are observed specifically in the 2021/22 apparition. Combination of our photometric and polarimetric data is used to model the physical characteristics of the dust in the coma.

 

Acknowledgments

OI is supported by the Slovak Academy of Sciences (grant Vega 2/0059/22), the Slovak Research and Development Agency under Contract no. APVV-19-0072. OI, CS and JM acknowledge the support from the International Space Science Institute (ISSI) through the team 472. 

 

References

Ivanova O.V. et.al. 2017. MNRAS 469, Issue 2. 386-395.

Le Roy L. et al. 2015. A&A 583, A1 

Rosenbush V.K. et al. 2017. MNRAS 469, 475-491.