Cometary Activity Begins at Kuiper Belt Distances: Evidence from C/2017 K2

TL;DR

This study reveals that comet C/2017 K2 begins activity at large distances from the Sun, with dust production starting around 35 AU, challenging previous assumptions about comet activation thresholds.

Contribution

It provides the first detailed analysis of comet activity initiation at Kuiper Belt distances, supported by observational data and Monte Carlo modeling.

Findings

Dust production starts at approximately 35 AU from the Sun.

Mass loss rates in dust at 10 AU are around 1000 kg/s.

Non-gravitational acceleration could be significant, affecting orbit predictions.

Abstract

We study the development of activity in the incoming long-period comet C/2017 K2 over the heliocentric distance range 9 < r_H < 16 AU. The comet continues to be characterized by a coma of sub-millimeter and larger particles ejected at low velocity. In a fixed co-moving volume around the nucleus we find that the scattering cross-section of the coma is related to the heliocentric distance by a power law with heliocentric index $s = 1.14\pm0.05$. This dependence is significantly weaker than the inverse square variation of the insolation as a result of two effects. These are, first, the heliocentric dependence of the dust velocity and, second, a lag effect due to very slow-moving particles ejected long before the observations were taken. A Monte Carlo model of the photometry shows that dust production beginning at r_H ~ 35 AU is needed to match the measured heliocentric index, with only a slight dependence on the particle size distribution. Mass loss rates in dust at 10 AU are of order 1000 kg/s, while loss rates in gas may be much smaller, depending on the unknown dust to gas ratio. Consequently, the ratio of the non-gravitational acceleration to the local solar gravity may, depending on the nucleus size, attain values comparable to values found in short-period comets at much smaller distances. Non-gravitational acceleration in C/2017 K2 and similarly distant comets, while presently unmeasured, may limit the accuracy with which we can infer the properties of the Oort cloud from the orbits of long-period comets.