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

References in corpus (9)

• The Active Centaurs
• Color Systematics of Comets and Related Bodies
• CO in distantly active comets
• What drives the dust activity of comet 67P/Churyumov-Gerasimenko?
• Ensemble Properties of Comets in the Sloan Digital Sky Survey
• Hubble Space Telescope Investigation of Main-Belt Comet 133P/Elst-Pizarro
• Acceleration of Individual, Decimetre-sized Aggregates in the Lower Coma of Comet 67P/Churyumov-Gerasimenko
• Activity of 50 Long-Period Comets Beyond 5.2 AU
• Frozen to death? -- Detection of comet Hale-Bopp at 30.7 AU

Cited by in corpus (23)

• Distant Comet C/2017 K2 and the Cohesion Bottleneck
• 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)
• Characterization of the Nucleus, Morphology and Activity of Interstellar Comet 2I/Borisov by Optical and Near-Infrared GROWTH, Apache Point, IRTF, ZTF and Keck Observations
• C/2010 U3 (Boattini): A Bizarre Comet Active at Record Heliocentric Distance
• Discovery of Carbon Monoxide in Distant Comet C/2017 K2 (PANSTARRS)
• C/2014 UN271 (Bernardinelli-Bernstein): the nearly spherical cow of comets
• ALMA Observations of the HD~110058 debris disk
• Thermal processing of Jupiter Family Comets during their chaotic orbital evolution
• Cometary Activity Beyond The Planets
• OSSOS: XV. No active Centaurs in the Outer Solar System Origins Survey
• 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)
• Coma environment of comet C/2017 K2 around the water ice sublimation boundary observed with VLT/MUSE
• Is the Cometary Nucleus Extraction Technique Reliable?
• The active centaur 2020 MK4
• Evolution of pits at the surface of 67P/Churyumov-Gerasimenko
• Variations in Volatile-Driven Activity of Comet C/2017 K2 (PanSTARRS) Revealed by Long-Term Multi-Wavelength Observations
• Comet C/2017 K2 (PANSTARRS): dynamically old or new?