First Constraints on Rings in the Pluto

System

A.J. Steffl and S.A. SternSouthwest 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

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.

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 F606WN

E

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 FilterN

E

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

3 Limit on Ring Optical Depth

1.3x10-5

• 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

Implications for New Horizons

€

N p =τ

cosθ

σ NHσ p

Estimate of Ring Particle Lifetime

• In steady state: Tp=MR/(dMR/dt)

MR=8/3rpp R dR

dMR/dt=2Msat/TSS

For rsat=50 km, sat=2 g/cm3, =10-4,

rp= 0.5 m, p=1 g/cm3

Ring particle lifetime constraint< 900 yr

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