ACS Multiband Coronagraphic Imaging of the Debris Disk around ß Pictoris

David Golimowski

STScI/NRDD 19 Oct 2005

Collaboration

Observations part of HST/ACS GTO debris disk coronagraphic imaging program

PI: H. C. Ford (JHU)

Deputy PI: G. D. Illingworth (UCSC/Lick Obs.)

GTO Debris Disk Team:

Mark Clampin (NASA/GSFC)David Ardila (IPAC/SSC)John Krist (JPL)David Golimowski (JHU)

Motivation Only two multiband scattered-light imaging

studies of ß Pic’s disk have been published: Paresce & Burrows (1987) Lecavelier des Etangs et al. (1993)

Ground-based measurements consistent with neutral colors and amin >> 1 µm, but errors are large (15-30%)

Better colors may better constrain grain properties

HST SM-3B: ACS/HRC equipped with high-contrast Lyot coronagraph with broadband filter set

ACS GTO observations of ß Pic HRC coronagraph

images taken in Cycle 12

0".9 radius occulting spot

Short & long exps. in F435W (B), F606W (Broad V), F814W (Broad I)

1 orbit/filter X two orients

Similar images of Pic (A7 IV) taken for PSF reference

PSF-subtracted images

PSF-deconvolved images

Ratio of images before & after PSF deconvolution

Morphology and Brightness

Inner “warp” is clearly resolved as secondary disk inclined by ~5º to outer disk.

Supports notion that giant planet has perturbed planetesimals from main disk into a coplanar orbit.

The main disk (r < 120 AU) has FWHM ~ 11-13 AU, i.e., ~50% narrower than previously measured.

PSF deconvolution is crucial for accurate modeling of the disk.

The radial surface-brightness profiles of the main and secondary disks are not alike.

Single power-law fit of secondary disk suggests:• No depletion of dust down to 80 AU• Smaller/larger ice-sublimation zone than main disk

Uncertainties are 5-10 times smaller than those of previous studies.

Midplane Colors

Grain characteristics

We compared the midplane colors to simulated colors of grain distributions with various compositions, porosities, and minimum grain sizes.

r < 120 AU: Colors matched by compact or moderately porous (P < 75%), non-icy grains of astrosil/graphite with amin ~ 0.15-0.2 µm

10X smaller than ground-based estimates Consistent with cometary activity within 120 AU

r > 120 AU: Icy grains with amin increasing from 0.15 µm at 120 AU to perhaps ~2 µm at 250 AU.

Legacy of ACS ß Pic Study

Our multiband ACS images are the finest optical images of the inner (30-250 AU) region of ß Pic’s disk obtained to date.

These images will not be superseded by TPF and other proposed exoplanet-imaging missions due to tiny FOV. Comparable IR images are expected from JWST if current imaging specs are maintained.

Thus, our results will be a standard reference for comparative and theoretical studies of debris disks for at least a decade.