DDA 33rd Meeting, Mt. Hood, OR, April 2002
Session 14. Satellites in the Outer Solar System
Wednesday, April 24, 2002, 10:20am-12:00noon

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[14.01] Saturn's Irregular Satellites

P. D. Nicholson, J. A. Burns (Cornell), B. J. Gladman, J-M. Petit (Observatoire de la Cote d'Azur), M. Holman (Harvard-CfA), J. J. Kavelaars (McMaster Univ.)

After discovering the first irregular satellites of Uranus in 1997 (Gladman, etal (1998) Nature, 392, 897), our team has undertaken a systematic program to search the Hill spheres of the outer planets for additional small satellites. In 2000/2001 we carried out a survey of the environs of Saturn comparable to our deep 1999 surveys at Uranus and Neptune (Gladman, etal (2000) Icarus, 147, 320). Because Saturn's Hill sphere subtends a diameter of almost 6\circ, such a survey was impracticable before the advent of large-format CCD array cameras. After a preliminary survey in August 2000, we carried out our main survey with the 3.6~m CFHT telescope and the CFH12k array camera in September 2000. This survey covered a roughly circular region with an outer radius of ~.0\circ to a limiting magnitude of mR=24. These two surveys netted a total of 15 candidate irregular satellites, of which 11 were subsequently recovered on several observing runs between October 2000 and January 2001. A 12th satellite was identified in November 2000, just outside our original survey area. These discoveries were reported by Gladman, etal (2001) Nature, 412, 163.

In August--November 2001 we recovered all 12 candidate satellites within a few arcminutes of their predicted positions, leading to confirmation of their status as bona fide saturnian satellites and the calculation of improved orbital elements. A striking result of this survey is that 11 of the 12 satellites fall into just three different inclination groups: three at i=34\circ, four at i=46\circ, and four at i=165-175\circ. The latter group includes Phoebe, the only previously-known saturnian irregular. The final object, S/2000 S8, has an inclination of 153\circ. It has been suggested that these groups may represent the later break-up of three originally-captured planetesimals, although the semimajor axes and eccentricities are much less tightly clustered than are the inclinations. The tightest overall groupings consist of S5/S6 and S4/S10/S11. We will summarize the orbital characteristics of the satellites, as well as discuss arguments for and against a break-up origin. A companion paper by Cuk etal will address the unusual secular dynamics exhibited by several of these objects. This work is supported by NASA's Planetary Astronomy program.

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Bulletin of the American Astronomical Society, 34, #3
© 2002. The American Astronomical Society.