First%20Constraints%20on%20Rings%20in%20the%20Pluto%20System

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
Pluto System on February 15, 2006 from HST/ACS
F606W

• 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

4
The Pluto System Through a High-Pass Filter

• 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

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 ?

6
3? Limit on Ring Optical Depth
??1.3x10-5
7
Implications for New Horizons

• Upper limits of ?1.3x10-5 used to derive of
  particles New Horizons would hit
• Unimodal ring of 1 ?m particles 2x108 (not a
  problem)
• Unimodal ring of 100 ?m particles 2x104 (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

8
Estimate of Ring Particle Lifetime

• In steady state TpMR/(dMR/dt)
• MR8/3?rp?p??R dR
• dMR/dt2?Msat/TSS
• For rsat50 km, ?sat2 g/cm3, ?10-4,
• rp 0.5 ?m, ?p1 g/cm3
• Ring particle lifetime constraintlt 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 lt900 years
• Steffl Stern, AJ, submitted
• astro-ph/0608036

Image credit NASA/ESA/STScI