35th Meeting of the AAS Division on Dynamical Astronomy, April 2004
Session 5 Planets
Oral, Thursday, April 22, 2004, 9:30am-12:45pm,

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[5.02] An Impact Formation of Pluto-Charon

R.M. Canup (Southwest Research Institute)

The angular momentum of Pluto-Charon, LPC, is estimated to be between about 5.6 and 7 x 1037 g-cm2/sec (e.g., Olkin et al. 2003), likely exceeding the critical angular momentum for rotational stability for a single body containing the same total mass. An impact-triggered formation is favored (e.g. Stern, McKinnon & Lunine 1997), although unlike the relatively well-constrained situation for the formation of the Earth's Moon, key properties of Pluto and Charon - their mass ratio, rock/ice fractions, and densities - are still somewhat uncertain. However, a primary challenge to the impact hypothesis is obtaining sufficient material in bound orbit: Charon contains ~ 10% of Pluto's mass, whereas the Moon has only ~ 1% of the Earth's mass.

Here I report results from a suite of smoothed particle hydrodynamics (SPH) simulations of potential Pluto-Charon forming impacts considering various impact angles, velocities, impactor sizes and pre-impact spin states. The most successful impacts involve an oblique, low velocity collision between two similarly sized objects. In the final stages of the collision, the rapidly rotating merged objects deform into a bar-like structure, angular momentum is redistributed as the cores of the objects merge and a 2-arm spiral structure emanates from the ends of the bar, leaving an ellipsoidal central planet and a circumplanetary disk. This evolution shares some commonalities with that seen in simulations of rapidly rotating protostellar clouds cores (e.g., Durisen et al. 1986; review by Tohline 2002).

The results here display a relationship between the angular momentum of the post-impact planet-disk system and the disk mass, such that impacts capable of producing a planet-disk system with a L ~ LPC also typically leave disks containing ~ 8 to 13% of the central planet's mass.

Support from the National Science Foundation's Planetary Astronomy program is gratefully acknowledged.

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Bulletin of the American Astronomical Society, 36 #2
© 2004. The American Astronomical Soceity.