Cometary Science Newsletter

September 2020
Michael S. P. Kelley (

Refereed Articles

Abstracts of articles in press or recently published. Limited to 3000 characters.

Observations of Disintegrating Long-period Comet C/2019 Y4 (ATLAS): A Sibling of C/1844 Y1 (Great Comet)

  • Hui, Man-To 1
  • Ye, Quan-Zhi 2
  1. Institute for Astronomy, University of Hawaii at Manoa
  2. Department of Astronomy, University of Maryland

We present a study of C/2019 Y4 (ATLAS) using Sloan gri observations from 2020 mid-January to early April. During this time span, the comet brightened with a growth in the effective cross-section of (2.0 ± 0.1)× 102 m2 s−1 from the beginning to ~70 days preperihelion in late 2020 March, followed by a brightness fade and the comet gradually losing the central condensation. Meanwhile, the comet became progressively bluer, and was even bluer than the Sun (g − r ≈ 0.2) when the brightness peaked, likely due to activation of subterranean fresh volatiles exposed to sunlight. With the tailward bias-corrected astrometry we found an enormous radial nongravitational parameter, A1 = (+2.25 ± 0.13) × 10-7 au day−2 in the heliocentric motion of the comet. Taking all of these findings into consideration, we conclude that the comet has disintegrated since 2020 mid-March. By no means was the split new to the comet, as we quantified that the comet had undergone another split event around last perihelion ~5 kyr ago, during which its sibling C/1844 Y1 (Great Comet) was produced, with the in-plane component of the separation velocity ≳1 m s−1. We constrained that the nucleus of C/2019 Y4 before disintegration was ≳60 m in radius, and has been protractedly ejecting dust grains of ~10–40 μm (assuming a dust bulk density of 0.5 g cm−3) with an ejection speed of ~30 m s−1 in early 2020 March and increased to ~80 m s−1 toward the end of the month for grains of ~10 μm.

The Astronomical Journal (Published)

DOI: 10.3847/1538-3881/ab9d81 arXiv: 2004.10990

Physical Characterization of Interstellar Comet 2I/2019 Q4 (Borisov)

  • Hui, Man-To 1
  • Ye, Quan-Zhi 2
  • Föhring, Dora 1
  • Hung, Denise 1
  • Tholen, David J. 1
  1. Institute for Astronomy, University of Hawai'i
  2. Department of Astronomy, University of Maryland

We present a study of interstellar comet 2I/2019 Q4 (Borisov) using both preperihelion and postperihelion observations spanning late September 2019 through late January 2020. The intrinsic brightness of the comet was observed to continuously decline throughout the timespan, likely due to the decreasing effective scattering cross-section as a result of volatile sublimation with a slope of −0.43 ± 0.02 km2 d−1. We witnessed no significant change in the slightly reddish color of the comet, with mean values of <g - r> = 0.68 ± 0.04, <r - i> = 0.23 ± 0.03, and the normalized reflectivity gradient across the g and i bands S'(g, i) = (10.6 ± 1.4)% per 103 Å, all unremarkable in the context of solar system comets. Using the available astrometric observations, we confidently detect the nongravitational acceleration of the comet following a shallow heliocentric distance dependence of rH-1 ± 1. Accordingly, we estimate that the nucleus is most likely ≲ 0.4 km in radius, and that a fraction of ≳0.2% of the total mass of the nucleus has been eroded due to the sublimation activity since the earliest observation of the comet in 2018 December by the time of perihelion. Our morphology simulation suggests that the dust ejection speed increased from ~4 m s−1 in 2019 September to ~7 m s−1 around perihelion for the optically dominant dust grains with β ~ 0.01, and that the observable dust grains are no smaller than micron size.

The Astronomical Journal (Published)

DOI: 10.3847/1538-3881/ab9df8 arXiv: 2003.14064

The Transient Jupiter Trojan-Like Orbit of P/2019 LD2 (ATLAS)

  • Hsieh, H. H. 1,2
  • Fitzsimmons, A. 3
  • Novaković, B. 4
  • Denneau, L. 5
  • Heinze, A. N. 5
  1. Planetary Science Institute, USA
  2. Academia Sinica, Taiwan
  3. Queen's University, Belfast, UK
  4. University of Belgrade, Serbia
  5. University of Hawaii, USA

Comet P/2019 LD2 has orbital elements currently resembling those of a Jupiter Trojan, and therefore superficially appears to represent a unique opportunity to study the volatile content and active behavior of a member of this population for the first time. However, numerical integrations show that it was previously a Centaur before reaching its current Jupiter Trojan-like orbit in 2018 July, and is expected to return to being a Centaur in 2028 February, before eventually becoming a Jupiter-family comet in 2063 February. The case of P/2019 LD2 highlights the need for mechanisms to quickly and reliably dynamically classify small solar system bodies discovered in current and upcoming wide-field surveys.

Icarus (In press)

DOI: 10.1016/j.icarus.2020.114019 arXiv: 2007.13945

Monitoring polarization in comet 46P/Wirtanen

  • Zheltobryukhov, M. 1
  • Zubko, E. 2
  • Chornaya, E. 3
  • and 9 co-authors
  1. Institute of Applied Astronomy of RAS, Russia
  2. Kyung Hee University, South Korea
  3. Far Eastern Federal University, Russia

We measure the degree of linear polarization of comet 46P/Wirtanen during two months, embracing the perihelion passage in December, 2018 with phase angles ranging from α = 18.1° to 46.4°. The polarimetric response P_Q obtained resembles what was previously found in comet C/1975 V1 (West). This suggests 46P/Wirtanen belongs to a group of comets with high maximum positive polarization. In the first half of February of 2019, we conducted BVRI photometry of 46P/Wirtanen and found either neutral or blue colour of its dust, which is in good accordance with measurements of C/1975 V1 (West). While aperture-averaged polarimetry of 46P/Wirtanen reveals a nearly zero polarization P_Q at the lowest phase angle α = 18.1°, simultaneous imaging polarimetry suggests that the negative polarization (P_Q < 0) arises in a region of within 5,000 km of the nucleus, where the negative polarization could be as strong as PQ = –(1.44 ± 0.15)%. This observation suggests the existence of the circumnucleus halo and that the coma is populated by at least two types of dust particles. One of those reveals a low positive polarization at side scattering and high negative polarization near backscattering. Both polarimetric features are simultaneously produced by weakly absorbing Mg-rich silicate particles. Another type of dust produces solely positive polarization that could be attributed to carbonaceous particles. This composition of 46P/Wirtanen coma appears to be similar with what was previously found in comet C/1975 V1 (West).

Monthly Notices of the Royal Astronomical Society (In press)

DOI: 10.1093/mnras/staa2469