A Comet Active Beyond the Crystallization Zone

Abstract

We present observations showing in-bound long-period comet C/2017 K2 (PANSTARRS) to be active at record heliocentric distance. Nucleus temperatures are too low (60 K to 70 K) either for water ice to sublimate or for amorphous ice to crystallize, requiring another source for the observed activity. Using the Hubble Space Telescope we find a sharply-bounded, circularly symmetric dust coma 10${}^5$ km in radius, with a total scattering cross section of $\sim$10${}^5$ km${}^2$. The coma has a logarithmic surface brightness gradient -1 over much of its surface, indicating sustained, steady-state dust production. A lack of clear evidence for the action of solar radiation pressure suggests that the dust particles are large, with a mean size $\gtrsim$ 0.1 mm. Using a coma convolution model, we find a limit to the apparent magnitude of the nucleus $V>$ 25.2 (absolute magnitude $H>$ 12.9). With assumed geometric albedo $p_V$ = 0.04, the limit to the nucleus circular equivalent radius is $<$ 9 km. Pre-discovery observations from 2013 show that the comet was also active at 23.7 AU heliocentric distance. While neither water ice sublimation nor exothermic crystallization can account for the observed distant activity, the measured properties are consistent with activity driven by sublimating supervolatile ices such as CO${}_2$, CO, O${}_2$ and N${}_2$. Survival of supervolatiles at the nucleus surface is likely a result of the comet's recent arrival from the frigid Oort cloud.

20 pages, 3 figures, 2 tables, published on Astrophysical Journal Letters, 847:L19 (5pp), 2017 October 1

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Cited by in corpus (12)

• Thermophysical evolution of planetesimals in the Primordial Disk
• Pre-discovery Activity of New Interstellar Comet 2I/Borisov Beyond 5 AU
• CO-Driven Activity in Comet C/2017 K2 (PANSTARRS)
• Cometary Activity Begins at Kuiper Belt Distances: Evidence from C/2017 K2
• Prediscovery Observations and Orbit of Comet C/2017 K2 (PANSTARRS)
• C/2010 U3 (Boattini): A Bizarre Comet Active at Record Heliocentric Distance
• Discovery of Carbon Monoxide in Distant Comet C/2017 K2 (PANSTARRS)
• ALMA Observations of the HD~110058 debris disk
• Thermal processing of Jupiter Family Comets during their chaotic orbital evolution
• A LOOK at Outbursts of Comet C/2014 UN${}_{271}$ (Bernardinelli-Bernstein) Near 20 au
• Hubble Space Telescope Detection of the Nucleus of Comet C/2014 UN${}_{271}$ (Bernardinelli-Bernstein)
• The Discus Comet: C/2014 B1 (Schwartz)