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 Pasadena11 October 2006

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!

Hubble ACS Hubble ACS observations observations of Nix and Hydraof 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?

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…

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

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

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

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

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

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. ???)

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

• 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

Hubble ACS photometry

DateNix

(S/2005 P2)

Hydra

(S/2005 P1)

15.05 May 2005 V = 23.41 +/- 0.15 V = 22.96 +/- 0.15

18.14 May 2005V = 23.41 +/- 0.15 V = 22.96 +/- 0.15

15.66 Feb 2006 V = 23.7 +/- 0.2 V = 23.26 +/- 0.15

2.75 March 2006 V= 00.0 +/- 0.0B-V = +0.654 +/- 0.065

V= 00.0 +/- 0.0B-V = +0.653 +/- 0.026

Charon B-V=0.710 (from Buie et al., 1997, Icarus, 125, 233)

Team is planning more elaborate observations inJuly 2015…

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

Hubble ACS astrometry(Pluto-centric, center of light)

DateNix

(S/2005 P2)

Hydra

(S/2005 P1)

15.05 May 2005d = 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 2005d = 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 2006d = 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 2006d = 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

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.

Division for Planetary Sciences of the

American Astronomical Society

http://www.aas.org/~dps/dps.html