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 Tombaugh’s search in 1930. Kuiper’s in 1950. Kuiper failed to find Charon, even though it was a maximum elongation and V=17.5. Did get anomalously large value for Pluto’s diameter. Stern et al. (1991) used non standard filter bandpass Kitt peak and McDonald observatory Whole stability radius. Stern et al (1994) used archival HST images Nicholson & Gladman Palomar 5m in June 1999. Entire Hill sphere

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 Hill radius scales as semi –major axis Szebehely stability radius is 1/3 hill radius Analytical work by Hamilton & Krivov unstable for r > 0.53rH Numerical simulations by Nesvorny showed unsable for r > 0.4 rH

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 Proposal was originally turned down. Only selected when STIS died Original proposal used ACS/WFC Followup used ACS/HRC Stability radius is 185” FOV is 202”

Flatfielded ACS/WFC Data PSFs include statistical noise Important to add PSFs to data while searching for satellites otherwise data not treated the same way

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 PSFs include statistical noise Important to add PSFs to data while searching for satellites otherwise data not treated the same way

2005 May 15 5 arcsec scale bar Artifacts from overlapping star trails

2005 May 15 with PSFs PSFs 11 o’clock ~13” 8 o’clock ~10” 8 o’clock very near edge 5 o’clock near edge

A note on detection efficiency mV = 26.8 90% detection limit mV = 27.4 50% detection limit HRC 3”-5” 50% detection limits are often quoted in the literature Advantages: common, well-defined Disadvantages: Usefulness?

2006 Feb 16 ACS/HRC HRC data Nix & Hydra in upper right Clear gradient in scattered light from Pluto & Charon. Near Pluto, detection efficiency will clearly be a function of radial distance.

ACS/HRC with mV = 25 PSFs 1 arcsec rings All PSFs are V mag 25 If this were actual spatial distribution of PSFs then would be

ACS/HRC with random PSFs 40 PSFs total 20 between 1”-3” & 20 between 3”-5” PSFs span range in magnitude 24.5 – 28.5 Repeat 10 times with different PSF locations

Detection Limit vs. Radial Distance Dotted line is 2x Charon semi-major axis

Size Limits of Satellites