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

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 submillimeter-sized 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, C , is related to the heliocentric distance by a power law, , with heliocentric index s = 1.14 ± 0.05. This dependence is significantly weaker than the r H −2 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 10 3 kg s −1 , while loss rates in gas may be much smaller, depending on the unknown dust to gas ratio. Consequently, the ratio of the nongravitational acceleration to the local solar gravity, α ′, may, depending on the nucleus size, attain values of ∼10 −7 ≲ α ′ ≲ 10 −5 , comparable to values found in short-period comets at much smaller distances. Nongravitational 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.