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تسجيل مستخدم جديدCometary Activity Begins at Kuiper Belt Distances: Evidence from C/2017 K2
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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 $s = 1.14pm0.05$. 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.
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Distant long-period comet C/2017 K2 has been outside the planetary region of the solar system for 3 Myr, negating the possibility that heat retained from the previous perihelion could be responsible for its activity. This inbound comet is also too co
ld for water ice to sublimate and too cold for amorphous water ice, if present, to crystallize. C/2017 K2 thus presents an ideal target in which to investigate the mechanisms responsible for activity in distant comets. We have used Hubble Space Telescope to study the comet in the pre-perihelion distance range 13.8 to 15.9 AU. The coma maintains a logarithmic surface brightness gradient $m = -1.010pm$0.004, consistent with steady-state mass loss. The absence of a radiation pressure swept tail indicates that the effective particle size is large (0.1 mm) and the mass loss rate is $sim$200 kg s$^{-1}$, remarkable for a comet still beyond the orbit of Saturn. Extrapolation of the photometry indicates that activity began in 2012.1, at 25.9$pm$0.9 AU, where the blackbody temperature is only 55 K. This large distance and low temperature suggest that cometary activity is driven by the sublimation of a super-volatile ice (e.g.~CO), presumably preserved by K2s long-term residence in the Oort cloud. The mass loss rate can be sustained by CO sublimation from an area $lesssim 2$ km$^2$, if located near the hot sub-solar point on the nucleus. However, while the drag force from sublimated CO is sufficient to lift millimeter sized particles against the gravity of the cometary nucleus, it is 10$^2$ to 10$^3$ times too small to eject these particles against inter-particle cohesion. Our observations thus require either a new understanding of the physics of inter-particle cohesion or the introduction of another mechanism to drive distant cometary mass loss. We suggest thermal fracture and electrostatic supercharging in this context.
(Abreviated) Comet C/2017 K2 PANSTARRS drew attention to its activity already at a time of its discovery in May 2017 when it was about 16 au from the Sun. This Oort spike comet will approach its perihelion in December 2022, and the question about its
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We present a study of comet C/2017 K2 (PANSTARRS) using prediscovery archival data taken from 2013 to 2017. Our measurements show that the comet has been marginally increasing in activity since at least 2013 May (heliocentric distance of $r_{mathrm{H
}} = 23.7$ AU pre-perihelion). We estimate the mass-loss rate during the period 2013--2017 as $overline{dot{M}} approx left(2.4 pm 1.1 right) times 10^{2}$ kg s$^{-1}$, which requires a minimum active surface area of $sim$10--10$^2$ km$^{2}$ for sublimation of supervolatiles such as CO and CO$_2$, by assuming a nominal cometary albedo $p_V = 0.04 pm 0.02$. The corresponding lower limit to the nucleus radius is a few kilometers. Our Monte Carlo dust simulations show that dust grains in the coma are $gtrsim0.5$ mm in radius, with ejection speeds from $sim$1--3 m s$^{-1}$, and have been emitted in a protracted manner since 2013, confirming estimates by Jewitt et al. (2017). The current heliocentric orbit is hyperbolic. Our N-body backward dynamical integration of the orbit suggests that the comet is most likely (with a probability of $sim$98%) from the Oort spike. The calculated median reciprocal of the semimajor axis 1 Myr ago was $a_{mathrm{med}}^{-1} = left( 3.61 pm 1.71 right) times 10^{-5}$ AU$^{-1}$ (in a reference system of the solar-system barycentre).
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