Activity and composition of the hyperactive comet 46P/Wirtanen during its close approach in 2018

Abstract

Context. Hyperactive comets are a small group of comets whose activity is higher than expected. They seem to emit more water than would normally be expected given the size of their nucleus. Comet 46P/Wirtanen (hereafter, 46P) is among these objects of interest. Investigating its activity and composition evolution could provide clues about its origins and its formation region in the Solar nebulae. Aims. Given the exceptional close approach of comet 46P to the Earth in 2018, we aim to study the evolution of its activity and composition as a function of heliocentric distances before and after perihelion. Methods. We used both TRAPPIST telescopes to monitor the comet for almost a year with broad and narrow-band filters. We derived the production rates of five gaseous species (OH, NH, CN, C 3 , and C 2 ) using a Haser model as well as the A(θ)fρ dust proxy parameter. The comet was also observed with the two optical high-resolution spectrographs UVES and ESPRESSO, mounted on the 8-m ESO VLT, to measure the isotopic ratios of C and N, along with the oxygen forbidden-line ratios and the NH 2 ortho-to-para ratios.Results. Over nearly a year, we followed the rise and decline of the production rates of different species as well as the dust activity of 46P on both pre-and post-perihelion. Relative abundances with respect to CN and OH along the orbit of the comet show constant and symmetric abundance ratios and a typical coma composition. We determined the rotation period of the nucleus using high-cadence observations and long series of CN images on several nights. We obtained a value of (9.18±0.05) hours at perihelion. Using the high-resolution spectra of 46P coma, we derived C and N isotopic ratios of 100±20 and 150±30 as well as a green-to-red forbidden oxygen [OI] line ratio of 0.23±0.02. We measured a NH 2 ortho-to-para ratio of 3.31±0.03 and derived an ammonia ratio of 1.19±0.03, corresponding to a spin temperature of 27±1 K. Conclusions. Narrow-band observations show that comet 46P is a hyperactive comet for which 40% of its nucleus surface is active. It has a typical composition, similar to other normal comets; however, an asymmetric behavior with respect to perihelion has been seen in its activity, which is typical of seasonal effects. Photometric measurements show no evidence for a change in the rotation period of the nucleus during this apparition. High-resolution spectra show that 46P has typical NH 2 ortho-to-para, [OI] lines ratios, and C and N isotopic ratios.

The Evolution of Activity and Chemical Composition in Rosetta’s Comet Targets across Multiple Apparitions: Complications for CS2 as the CS Parent in Comet Nuclei

“…We also note that the NUV/visible Afρ ratio for 46P is consistent with that of 19P/Borelly as measured by Weaver et al (2003) and 67P pre-perihelion as measured here but is much lower than the 1997 values of 0.65-0.78, although those were measured at 1.31-2.45 au. When our NUV value is corrected for phase angle, it is still substantially smaller than A (0)fρ values derived from visible wavelength sources of Kelley et al (2021) and Moulane et al (2023), as well as our own MIRVIS-derived A(0)fρ. However, the 1997 NUV dust slope (22%-30%/100 nm) is similar to that inferred from our comparison of the NUV and R/R-band A(0)fρ of 17%/100 nm, which in turn is matched by our NUV dust slope of 16%/ 100 nm between 2600 and 3140 Å.…”

“…Since initial characterization as a "hyperactive" 14 comet in 1997-1998 based on water production rate, 46P has experienced a factor of two decrease in activity (Combi et al 2020b;Knight et al 2021;Moulane et al 2023) despite no changes to its perihelion distance since . Apparitions in 1992, 1997, 2008, 2013 have shown the evolution of 46P's activity from frequently outbursting in 2002 to a steeper activity slope leading into and out of perihelion (Combi et al 2020b).…”

“…The extensive results of the Rosetta mission have transformed our understanding of the short-term evolution of cometary activity and formation (Bieler et al 2015;Feldman et al 2016;Vincent et al 2016;Jones et al 2017, among others), but 67P was not the original target of the mission; it was chosen after the initial launch date was canceled owing to problems with the Ariane 5 launch vehicle (Schwehm 2003;Ferri 2006). 46P/Wirtanen (hereafter 46P), a hyperactive comet that exhibited a decreased water production in its most recent apparition (Combi et al 2020b;Knight et al 2021;Moulane et al 2023), was the primary target with the best trajectory for an escort mission. The change in targets was done irrespective of their activity, composition, or orbital history and to some degree requires the implicit Copernican assumption that when selected at random a dynamically favorable Jupiterfamily comet (JFC) for a mission is likely to have the mean compositional properties of the JFC population as a whole.…”

Dust Production Rates in Jupiter-family Comets: A Two Year Study with ATLAS Photometry

“…This is where there is a volatile-rich surface of the comet illuminated by the Sun preperihelion and then a different region of the comet which is less abundant in volatiles becomes exposed after perihelion, causing a decrease in activity. Example of this asymmetric activity behavior are shown in Moulane et al (2023) and Ehlert et al (2019) for 46P/ Wirtanen and 21P/Giacobini-Zinner, respectively, both reaching a maximum A(0°)fρ before perihelion.…”

Gas Sources from the Coma and Nucleus of Comet 46P/Wirtanen Observed Using ALMA

“…Comet 46P/Wirtanen is also hyperactive (Lis et al 2019;Moulane et al 2023), and the large H 2 O production rate for its nucleus size can, at least in part, be attributed to the presence of icy grains in the coma (Bonev et al 2021). Knight et al (2021) explained the different OH and CN morphologies in 46P as being partly due to OH production from an icy grain H 2 O source.…”