Limits on Pluto’s Small Companions

Andrew Steffl (SwRI) Max Mutchler (STScI)

With thanks to Marc Buie, Dan Durda, Bill Merline, John Spencer, Alan Stern, Dirk Terrell, Hal Weaver, Eliot Young, and Leslie Young

Historical Satellite Searches• Tombaugh (1960)

– No satellites detected (published no formal upper limits)• Kuiper (1961)

– mp= 19 for 0.3”- 2” from Pluto– mp = 22.4 for > 2” from Pluto

• Stern et al. (1991)– m(90%) = 20.6±0.5 for 6”-10”– m(90%) = 22.6 for > 10” from Pluto

• Stern et al. (1994)– mV(90%) = 21.7 for 1”-2” from Pluto– mV(90%) = 21.9 for 2”-10”

• Nicholson & Gladman (2006)– mR(50%) = 25.0±0.2 for > 4” from Pluto

Motivation for a new Satellite Search

• Pluto’s Hill sphere is big– rH = 6 x 106 km (4.6’ from earth)– Charon located at ≈ 0.003 rH

– However, orbits near the edge of Hill sphere are not stable over the age of the solar system

• Szebehely’s stability criterion r < ⅓ rH

• Hamilton & Krivov r < 0.53 rH (pro.) r < 0.69 rH (retro)• Nesvorný r < 0.4 rH (pro.) r < 0.7 rH (retro)

• Stern et al. (1994) limit corresponds to 85 km object

HST Campaign of 2005• 2 visits of 1 orbit

using ACS/WFC– 2005 May 15

and 2005 May 18

– 1 0.5s image– 4 475s images

• After discovery of Nix & Hydra obtained 2 additional visits with ACS/HRC– 2006 Feb 16– 2006 Mar 02

Flatfielded ACS/WFC Data

Data Analysis• 400 artificial PSFs with 25.5 ≥ mV ≥

29.5 placed randomly within WFC field

• Median combine images using “drizzle” software

• Visually identify objects in field• Compare found objects to list of

added PSFs– Nix and Hydra detected in this way

2005 May 15

2005 May 15 with PSFs

A note on detection efficiency

HRC 3”-5”

50% detection limits are often quoted in the literature

Advantages: common, well-defined Disadvantages: Usefulness?

mV = 26.8 90% detection limit

mV = 27.4 50% detection limit

2006 Feb 16 ACS/HRC

ACS/HRC with mV = 25 PSFs

ACS/HRC with random PSFs

Detection Limit vs. Radial Distance

Size Limits of Satellites90% Limit