1. 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

2. 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 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 Entire Hill sphere

3. Motivation for a new Satellite Search
Pluto’s Hill sphere is big
rH = 6 x 106 km (4.6’ from earth)
Charon located at ≈ 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 < rH (pro.) r < 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

4. 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”

5. 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

6. 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

7. 2005 May 15
5 arcsec scale bar
Artifacts from overlapping star trails

8. 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

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

10. 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.

11. 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

12. 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

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

14. Size Limits of Satellites