Fragments of comet 205P/Giacobini in 2021

ATel #16104; Quanzhi Ye (U. Maryland), Michael S. P. Kelley (U. Maryland), on behalf of the Zwicky Transient Facility Collaboration
on 29 Jun 2023; 03:23 UT
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Credential Certification: Kumar Venkataramani (kumarv@caltech.edu)

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Comet 205P/Giacobini was first discovered in 1896, then lost for over a century before being recovered again in 2008 (IAUC 8975 ). Two fragments were subsequently reported within a few days of the recovery (IAUC 8978 , 8980). The comet returned again in 2015 but neither of the two fragments were reportedly observed.

The 2021 return of 205P/Giacobini was first observed by the Pan-STARRS survey on 2021 July 31 UTC (heliocentric distance rh=2.24 au, geocentric distance Delta=1.90 au; cf. MPC 160519). Through the ZChecker program (Kelley et al. 2019, ASPC, 523, 471), the Zwicky Transient Facility (ZTF; Bellm et al. 2019, PASP, 131, a8002) has been monitoring the 205P's ephemeris position since the beginning of 2021. The comet was visible in nightly ZTF stacked images from 2021 July 7 UTC (rh=2.38 au, Delta=1.78 au, r=20.8±0.3 mag), and remained visible until 2021 November 14 UTC (rh=1.65 au, Delta=2.24 au, r=17.75±0.03 mag) when the comet moved behind the Sun. (The photometry quoted above were measured with a 4” radius aperture, equivalent to 5200 to 6500 km at the comet’s distance, and were calibrated to the PS1 catalog, cf. Tonry et al. 2012, ApJ, 750, 99.)

Starting on 2021 October 15, a secondary component became visible, drifting away from the primary at a rate of 0.4”/day towards PA=280 deg, translating to a projected velocity of 7.4 m/s with respect to the primary (Figure). Both components were also brightening rapidly, with activity roughly increasing with rh^-9. Neither component is visible in the next ZTF nightly stack taken on 2022 August 9 UTC when the comet reemerged from the solar avoidance zone (rh=2.49 au, Delta=2.99 au, limiting r=20.4 mag). Using the pre-perihelion brightness as a guide, an apparent brightness of r=22.5 mag would have been expected.

It is presently unclear if the secondary component results from a new fragmentation or from the 2008 fragmentation. Calculations by Zdenek Sekanina (IAUC 8987 ) showed that the 2008 fragments could be long-lived, with fragment C appearing to be released as far back as 1998. At this time, we cannot rule out the possibility that our observed secondary is the return of one of the fragments observed in 2008.

This work is based on observations obtained with the Samuel Oschin Telescope 48-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under Grant No. AST-2034437 and a collaboration including Caltech, IPAC, the Weizmann Institute of Science, the Oskar Klein Center at Stockholm University, the University of Maryland, Deutsches Elektronen-Synchrotron and Humboldt University, the TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee, Trinity College Dublin, Lawrence Livermore National Laboratories, and IN2P3, France. Operations are conducted by COO, IPAC, and UW.