1. First Constraints on Rings in the Pluto System A.J. Steffl and S.A. Stern Southwest Research Institute With thanks to H.A. Weaver, M.J. Mutchler, M.W. Buie, W.J. Merline, J.R. Spencer, E.F. Young, and L.A. Young

2. Motivation Objects in the Kuiper Belt are impacted by debris. Characteristic ejecta velocity is 1-10% impactor velocity or 10-100 m/s (Durda & Stern, 2000). Escape velocities of Nix and Hydra are 30-90 m/s. Stern et al. (2006) predicted highly time-variable rings with an characteristic optical depth of  =5x10 -6. Use existing HST ACS images of the Pluto system to constrain the present-day optical depth of rings.

3.  Observations designed to detect Nix and Hydra ( ~ 9 mag fainter)  Pluto and Charon are saturated  Light from Pluto and Charon completely dominates the sky background near the orbits of Nix and Hydra  Lots of complex spatial structure (extended PSF, halos, diffraction spikes) in 2-D distribution of flux from Pluto and Charon Pluto System on February 15, 2006 from HST/ACS F606W N E

4.  Fit tilted plane to circular region around each pixel  Exclude circular aperture at center from fit  Subtract fit value from central pixel  Technique similar to that used by Showalter & Lissauer (2006) to find faint rings of Uranus.  No evidence for rings seen in filtered image The Pluto System Through a High-Pass Filter N E

5. Pluto System on February 15, 2006 from HST/ACS F606W  Rings should be in Pluto- Charon orbital plane  Divide plane into circular annuli 1,500 km wide (3 pixels)  Assume limiting case where all flux is due to ring backscatter  Calculate avg. I/F in each annulus  Exclude regions around Pluto, Charon, Nix, Hydra  Convert I/F to optical depth: I/F = p  N E

6. 3  Limit on Ring Optical Depth  1.3x10 -5

7. Upper limits of  =1.3x10 -5 used to derive # of particles New Horizons would hit: –Unimodal ring of 1  m particles: 2x10 8 (not a problem) –Unimodal ring of 100  m particles: 2x10 4 (ouch!) Without tighter constraints, New Horizons should cross ring plane inside 42,000 km from barycenter. –Rings unstable inside this distance (Nagy et al. 2006) –Current trajectory well inside this safe zone Implications for New Horizons

8. Estimate of Ring Particle Lifetime In steady state: T p =M R /(dM R /dt) M R =8/3  r p  p  R dR dM R /dt=2  M sat /T SS For r sat =50 km,  sat =2 g/cm 3,  =10 - 4, r p = 0.5  m,  p =1 g/cm 3 Ring particle lifetime constraint< 900 yr

9. Conclusions No rings detected at Pluto 3  upper limit optical depth  =1.3x10 -5 Without tighter constraints, New Horizons should cross ring plane where rings are not stable (inside 42,000 km) Ring particle lifetime <900 years Steffl & Stern, AJ, submitted astro-ph/0608036 Image credit: NASA/ESA/STScI