Narrowband Observations of Comet 46P/Wirtanen During its Exceptional Apparition of 2018/19 II: Photometry, Jet Morphology, and Modeling Results

Abstract

We report on our extensive photometry and imaging of Comet 46P/Wirtanen during its 2018/19 apparition and use these data to constrain modeling of Wirtanen's activity. Narrowband photometry was obtained on nine epochs from 2018 October through 2019 March as well as 10 epochs during the 1991, 1997, and 2008 apparitions. The ensemble photometry reveals a typical composition and a secular decrease in activity since 1991. Production rates were roughly symmetric around perihelion for the carbon-bearing species (CN, C 3 , and C 2 ), but steeper for OH and NH outbound. Our imaging program emphasized CN, whose coma morphology and lightcurve yielded rotation periods reported in a companion paper (Farnham et al., PSJ, 2, 7). Here, we compare the gas and dust morphology on the 18 nights for which observations of additional species were obtained. The carbon-bearing species exhibited similar morphology that varied with rotation. OH and NH had broad, hemispheric brightness enhancements in the tailward direction that did not change significantly with rotation, which we attribute to their originating from a substantial icy grain component. We constructed a Monte Carlo model that replicates the shape, motion, and brightness distribution of the CN coma throughout the apparition with a single, self-consistent solution in principal axis rotation. Our model yields a pole having (R.A., Decl.) = 319 • , −5 • (pole obliquity of 70 • ) and two large sources (radii of 50 • and 40 • ) centered at near-equatorial latitudes and separated in longitude by ∼160 • . Applications of the model to explain observed behaviors are discussed.

Non-detection of water-ice grains in the coma of comet 46P/Wirtanen and implications for hyperactivity

“…The water production rate reported by Knight et al (2021) and derived from measurements of the OH 309 nm (0-0) band is in agreement with the H 16 2 O production rate of (7.7±1.5)×10 27 molecules s −1 quoted by Lis et al (2019) and based on observations of the 1 1,0 − 1 0,1 transition of H 18 2 O between 2018 December 14 and 20 UT. The water production rate was 10% higher on 2018 December 3 (8×10 27 molecules s −1 ; Knight et al 2021), which translates into an active fraction of 65%. This active fraction is similar to that of comet H2 during the Deep Impact observations (∼72%, Lis et al 2019) and it is at least 2 times higher than the typical active fraction for most comets (e.g., A'Hearn et al 1995).…”

“…In this paper, we test the hypothesis put forth by A'Hearn et al ( 2011), Protopapa et al (2014), andKelley et al (2013) that sublimating water ice in the coma in the form of small grains and/or large chunks is responsible for a comet hyperactivity. We point out that Knight et al (2021) and Bonev et al (2021) both provide indirect evidence for the presence of an extended source of water vapor in the coma of Wirtanen. However, no information on the properties of the ice that produces the water vapor, such as particle size and ice-to-dust ratio, has been retrieved based on their work.…”

“…Adopting the mean nucleus radius of 555±40 m estimated by Boehnhardt et al (2002), we translated the water production rate of Q H2O = 7.2 × 10 27 molecules s −1 reported by Knight et al (2021) on 2018 December 16 (Z = 3.19 × 10 21 molecules s −1 m −2 at R h =1.056 au), close in time to our spectroscopic measurements, into an active fraction of 58%. The water production rate reported by Knight et al (2021) and derived from measurements of the OH 309 nm (0-0) band is in agreement with the H 16 2 O production rate of (7.7±1.5)×10 27 molecules s −1 quoted by Lis et al (2019) and based on observations of the 1 1,0 − 1 0,1 transition of H 18 2 O between 2018 December 14 and 20 UT. The water production rate was 10% higher on 2018 December 3 (8×10 27 molecules s −1 ; Knight et al 2021), which translates into an active fraction of 65%.…”

“…Cometary activity is commonly expressed in terms of active fraction: the surface area of water ice needed to sustain the activity, generally referred to as "minimum active area" (Keller 1990;Combi et al 2011), divided by the surface area of the nucleus. For most comets, the active fraction is typically between a few percent to 30% (A'Hearn et al 1995, after correcting for the factor of 2 error noted by Knight et al 2021; see also Lis et al 2019). Hyperactive comets are unusual in that their gas production rates require an active fraction of ∼100% (A' Hearn et al 1995).…”