1. The B-V colors and photometric variability of Nix and Hydra, Pluto’s two small satellites Max Mutchler (STScI) S. Alan Stern (SwRI) Hal Weaver (JHU/APL) Andrew Steffl (SwRI) DPS Meeting in Pasadena 11 October 2006

2. Overview HST obs: 4 epochs in V-band, different detectors/strategies, B filter for last, pre- covery Show images: data quality, photometric issues (spikes, PSF halos, artifacts) Describe photometry: apertures, corrections, etc. Photometric results: neutral, no variability Publications, future work, NH!

3. Hubble ACS observations of Nix and Hydra 15 May 2005: WFC, discovery!, V-band 18 May 2005: WFC, preliminary orbits, V-band 15 Feb 2006: HRC, confirmation, V-band 2 March 2006: HRC, B and V-band (B-V colors) Pre-discovery HRC detections in 2002-2003 (Buie et al.) Merge this with actual images?

4. North-up images from each epoch 6.4 x 6.4 arcseconds (compact system) Short exposures: Pluto-Charon unsaturated Long exposures: Nix and Hydra detected ACS / WFC: longer exposures (fishing), gap ACS / HRC: less saturation of Pluto-Charon Nix & Hydra not optimally placed in every frame (background stars, diffraction spikes, bright Pluto-Charon PSFs): only use ideal frames Images provide sense of data quality, photometry hazards…

5. 15 May 2005, ACS / WFC V-band, median 4 long exposures

7. 15 Feb 2006, ACS / HRC V-band, drizzle 4 long exposures

8. 2 March 2006, ACS / HRC V-band, drizzle 3 long exposures

9. 2 March 2006, ACS / HRC B-band, drizzle 3 long exposures

10. Pre-discovery HRC observations Hubble program by Buie & Young, 2002-2003 Primarily designed to map surface features (albedo) of Pluto and Charon New moons marginally detected Good orbit determination Photometry implied Nix reddish (B-V=+0.91), like Pluto (B-V=+0.00) and Hydra neutral (like Charon) (Buie et al. ???)

11. Photometry ACS filters: F606W “broad V”, F435W “Johnson B” Short and long exposures (moons 5000x fainter than Pluto-Charon) Hazards: diffraction spikes, PSF halos, artifacts… Measurements on 3 of 4 dates for each satellite Aperture phot? Aperture used? Aperture corrections, CTE effect (WFC edge! PSFs help fill traps) Adjust for changing heliocentric/geocentric distances, and solar phase angle (assume Charon’s phase law) Instrumental magnitudes (Sirianni et al. 2005) Uncertainty: background, PSFs, CTE? Hydra (P1) variation: 1.00 : 0.78 : 0.73 Nix (P2) variation: 1.00 : 0.79 : 0.86 (put in table?) Limiting magnitudes (Steffl): V=26.8 (90% confidence level); diamiter limits 10-40 km

12. IAU Circular 8686 (at left) Stern et al., AJ (submitted), astro-ph/0605014 Both have neutral (grey) reflectivities, like Charon Similar origins (collision event) for all 3 moons No strong photometric variability Both towards larger end of allowable sizes (over ~100 km)? Lower albedos than Charon? No other satellites of similar size, to limiting magnitude V=26.8, 90% confidence (Steffl et al. AJ) Photometric results for Nix & Hydra

13. Hubble ACS photometry Date Nix (S/2005 P2) Hydra (S/2005 P1) 15.05 May 2005V = 23.41 +/- 0.15V = 22.96 +/- 0.15 18.14 May 2005 V = 23.41 +/- 0.15V = 22.96 +/- 0.15 15.66 Feb 2006V = 23.7 +/- 0.2V = 23.26 +/- 0.15 2.75 March 2006 V= 00.0 +/- 0.0 B-V = +0.654 +/- 0.065 V= 00.0 +/- 0.0 B-V = +0.653 +/- 0.026 Charon B-V=0.710 (from Buie et al., 1997, Icarus, 125, 233)

14. Team is planning more elaborate observations in July 2015…

15. Publications IAU Circulars 8625, 8676, 8686 Weaver et al. 2006, Nature Stern et al., 2006, Nature Steffl et al., 2006, ApJ Stern et al., 2006, AJ, submitted

16. Hubble ACS astrometry (Pluto-centric, center of light) Date Nix (S/2005 P2) Hydra (S/2005 P1) 15.05 May 2005 d = 2.09 +/- 0.035 arcsec PA = 326.9 +/- 0.5 deg d = 1.85 +/- 0.035 arcsec PA = 264.2 +/- 0.5 deg 18.14 May 2005 d = 2.22 +/- 0.035 arcsec PA = 355.5 +/- 0.5 deg d = 2.36 +/- 0.035 arcsec PA = 305.8 +/- 0.5 deg 15.66 Feb 2006 d = 2.03 +/- 0.010 arcsec PA = 355.3 +/- 0.3 deg d = 2.86 +/- 0.010 arcsec PA = 343.0 +/- 0.3 deg 2.75 March 2006 d = 1.43 +/- 0.015 arcsec PA = 213.7 +/- 0.5 deg d = 2.69 +/- 0.015 arcsec PA = 138.2 +/- 0.5 deg * Confirmed orbital solution from Buie et al., 2005

17. The B-V colors and photometric variability of Nix and Hydra, Pluto’s two small satellites Max Mutchler, Alan Stern, Hal Weaver, Andrew Steffl Pluto’s two small satellites, Nix and Hydra, were observed on four dates (15.1 and 18.1 May 2005, 15.7 February 2006, and 2.8 March 2006) using the Hubble Space Telescope’s (HST) Advanced Camera for Surveys (ACS). V-band magnitudes were obtained for all dates, and B- band magnitudes were additionally measured on the last date. We find that both satellites have essentially neutral (grey) reflectivities, like Charon. We also find that neither satellite exhibited strong photometric variation, which might suggest that Nix and Hydra are toward the large end of their allowable size range, and therefore may have lower albedos than Charon. Support for this work was provided by NASA through Grants GO-10427 and GO-10774 from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., and the New Horizons Pluto-Kuiper Belt mission.

18. Division for Planetary Sciences of the American Astronomical Society http://www.aas.org/~dps/dps.html