HST/STIS Imaging of the HR 4796A Debris Ring Glenn Schneider Steward Observatory University of Arizona A.J. Weinberger (UCLA) E.E. Becklin (UCLA) B.A. Smith (U. Hawaii) D.C. Hines (UofA) M.D Silverstone (UofA) E. Malumuth (GSFC) NICMOS IDT & EONS Teams

HR 4796A Young (~8±3 Myr; Stauffer 1995) “Vega-like” A0V M.S. star. Member of TW Hya Association (Webb et al, 1999), at a distance of 67±3pc (Hipparchos). Large amount of circumstellar dust inferred from IRAS excess, Ldisk/L* = 5 x 10-3 (Jura 1991), TWICE that of b Pictoris. Tdust = 110K suggested lack of material @ r < 40AU, and grains > 3mm @ 40AU < r < 20 AU to be bound (Jura 1995). Koerner et al. (1998) and Jayawardhana et al. (1998) independently imaged the disk in the mid-IR. An inner depleted region was evident in 20.8mm image, and reproduced with a model suggesting: Koerner et al. (1998) i =72°± ; rin = 55AU; rout = 80AU; PA=28°±6°. 6° 9°

The Dusty Disk/Planet Connection? Current theories of disk/planet evolution suggest a presumed epoch of planet-building via the formation and agglomerative growth of embryonic bodies, and the subsequent accretion of gaseous atmospheres onto hot giant planets, is attendant with a significant decline in the gas-to-dust ratios in the remnant protostellar environments. In this critical phase of newly formed (or forming) extra-solar planetary systems, posited from a few megayears to a few tens of megayears, the circumstellar environments become dominated by a second-generation population of dust containing larger grains arising from the collisional erosion of planetesimals.

Direct (Scattered Light) Imaging of Dusty Debris Observing scattered light from circumstellar debris has been observationally challenging because of the very high Star:Disk contrast ratios in such systems. 1984 - B.A. Smith & R.J. Terrile 6" radius coronagraphic mask, Las Campanas (discovery image) Until very recently the large, and nearly edge-on disk around b Pictoris remained the only such disk imaged. b Pictoris Need space, or ground-based/AO*, coronagraphic systems. * but still very challenging. Resolved imaging spatial distribution of dust/debris. Asymmetries (radial & azimuthal): • May implicate low-mass perturbers (planets) from: Rings, Central Holes, Gaps, Clumps, Arcs, Arclets • Help Elucidate the scattering & physical properties of the grains.

HST Provides... Very high stability and repeatability of Point Spread Function. Strehl ratios 95—98% depending upon imaging mode. Near-IR (NICMOS) and Optical (STIS) coronagraphy with rejection factors of a few to ~ 10 within a few arcseconds of an occulted star. Sub-arcsecond spatial resolution (~ 50—160mas at 0.5—1.6mm). (E.g., see poster [77.17] in this special session).

NICMOS Observations of the HR 4796A Circumstellar Debris Ring GEOMETRY PA = 26.8°±0.6° i = 73.1°±1.2° a = 1.05”±0.02” E N 20 40 60 80 100 Arc Seconds (Y) -1.0 -0.5 0.0 0.5 1.0  1.5  1.0  0.5 -1.5 Arc Seconds (X) F160W F110W mJy/pixel MORPHOLOGY r = 70AU width < 14AU “abrupt” truncation “clear” @ r < 50 AU FLUX DENSITY 12.8±1.0mJy @ 1.1mm 12.5±2.0mJy @ 1.6mm H(F160W) = 12.35± J(F110W) = 12.92±0.08 0.16 0.19 Tdust ~ Ldisk/L* 1.4±0.2x10-3 @ 1.1mm 2.4±0.5x10-3 @ 1.6mm NIR scattered flux in good agreement with visible absorption & mid-IR re-radiation.

NICMOS Observations of the HR 4796A Circumstellar Debris Ring 20 40 60 80 100 Arc Seconds (Y) -1.0 -0.5 0.0 0.5 1.0  1.5  1.0  0.5 -1.5 Arc Seconds (X) F160W F110W mJy/pixel Anisotropies NE ansa ~ 15% brighter than SW ansa. Suggestion of preferential (forward) scattering to SE. Implications Possible dynamical confinement of particles by one or more unseen bodies. Mean particle size > few mm. debris origin, not I.S. dust.

HR 4796A Circumstellar Debris Ring NICMOS Additional processing recovered ring flux closer in and suggested somewhat higher inclination (~76°). “Clumpiness” due to residuals in PSF subtraction, not attributed to structure of ring.

HR 4796A Circumstellar Debris Ring STIS

OBSERVATIONS

STIS Observations of the HR 4796A Circumstellar Debris Ring Orient #1 HR 4796A

STIS Observations of the HR 4796A Circumstellar Debris Ring Orient #1 HR 4796A HR 4748 (PSF)

STIS Observations of the HR 4796A Circumstellar Debris Ring Orient #1 Orient #2 HR 4796A DOrient = 16° HR 4748 (PSF)

STIS Observations of the HR 4796A Circumstellar Debris Ring Orient #1 Orient #2 HR 4796A DOrient = 16° HR 4748 (PSF)

+ _ STIS Observations of the HR 4796A Circumstellar Debris Ring Orient #1 Orient #2 + _ HR 4796A HR 4748 (PSF)

Brightening & Tendrils STIS Observations of the HR 4796A Circumstellar Debris Ring Orient #1 Orient #2 Edge Brightening & Tendrils From Decentering Defocus & Zonal Artifacts From “Breathing” PSF Subtractions

Better Focus Match Better Position Match Better Position Match Better STIS Observations of the HR 4796A Circumstellar Debris Ring PSF Orient #1 PSF Orient #2 Better Focus Match Better Position Match Better Position Match Better Focus Match PSF Subtractions

2-rolls samples regions otherwise obscured by wedge & spikes STIS Observations of the HR 4796A Circumstellar Debris Ring PSF Orient #1 PSF Orient #2 2-rolls samples regions otherwise obscured by wedge & spikes N E PSF Subtractions

STIS Wedge and Diffraction Spike Masks Orient #1 Orient #2 N E Individually Unsampled or Corrupted Regions

STIS Observations of the HR 4796A Circumstellar Debris Ring PSF Orient #1 PSF Orient #2 N E Masked PSF Subtractions

COMBINATION & POST-PROCESSING

STIS Observations of the HR 4796A Circumstellar Debris Ring 4 3 N 2 1 E Combined (Weighted) Mask

Combined Image -20% to +100%Stretch STIS Observations of the HR 4796A Circumstellar Debris Ring N E But... Combined Image -20% to +100%Stretch

Global Background Nulling Not Combined Image -5% to +5%Stretch STIS Observations of the HR 4796A Circumstellar Debris Ring Empirically determined background correction... Global Background Nulling Not Perfectly Achieved Due To PSF Mis-Matches N E Combined Image -5% to +5%Stretch

STIS Observations of the HR 4796A Circumstellar Debris Ring < +2% Peak Flux

< 2% Flux & No Wedge/Spikes STIS Observations of the HR 4796A Circumstellar Debris Ring N E < 2% Flux & No Wedge/Spikes

Masked Background -5% to +5%Stretch STIS Observations of the HR 4796A Circumstellar Debris Ring Interpolate across regions of disk flux With variable length/width 2D-Gaussian kernel N E Masked Background -5% to +5%Stretch

Re-Masked Interpolated Background -5% to +5%Stretch STIS Observations of the HR 4796A Circumstellar Debris Ring N E Re-Masked Interpolated Background -5% to +5%Stretch

Interpolated Background Subtracted -5% to +5%Stretch STIS Observations of the HR 4796A Circumstellar Debris Ring N E Interpolated Background Subtracted -5% to +5%Stretch

STIS Observations of the HR 4796A Circumstellar Debris Ring Combined Image - Resampled

RESULTS (thus far) from this image

(Least-Squares Isophotal Ellipse Fit) HR 4796A RING GEOMETRY (Least-Squares Isophotal Ellipse Fit) Ansal Separation (Peaks) = 2.107” ± 0.0045” Major Axis of BFE = 2.114” ± 0.0055" P.A. of Major Axis (E of N) = 27.06° ± 0.18° Major:Minor Axial Length = (3.9658 ± 0.034): 1 Inclination of Pole to LOS = 75.73° ± 0.12° Photocentric Offset from BFE(Y) = -0.0159" ± 0.0048" Photocentric Offset from BFE(X) = +0.0031" ± 0.0028"

HR 4796A Circumstellar Debris Ring - WIDTH WIDTH AT NE ANSA FWHM: 12.9±0.7AU 9.6% Dring * Slightly asymmetric 1-e-1: 7.5±0.4AU 11.9% Rring Brightness (Normalized to NE Ansa) Measured = 0.197” PSF point source = 0.043” FWHM ring = 0.192” 1-e-1 = 0.126”

RING GEOMETRY - Least-Squares Isophotal Ellipse Fit Ansal Separation (Peaks) = 2.107” ± 0.0045” Major Axis of BFE = 2.114” ± 0.0055" P.A. of Major Axis (E of N) = 27.06° ± 0.18° Major:Minor Axial Length = (3.9658 ± 0.034): 1 Inclination of Pole to LOS = 75.73° ± 0.12° Photocentric Offset from BFE(Y) = -0.0159" ± 0.0048" Photocentric Offset from BFE(X) = +0.0031" ± 0.0028"

“FACE-ON” PROJECTION - With Flux Conservation

Spatially Resolved Relative PHOTOMETRY of the Ring

N-Sigma Brightness Ratio (Percent)

PHOTOMETRIC ERROR ESTIMATION

N-Sigma Brightness Ratio (Percent)

Aperture Photometry STIS(leff 0.58 mm): F(ring[unobscured])/F(star) = 0.00049 ± 0.000036 (7.3%) NICMOS (leff 1.10 mm): F(ring[unobscured])/F(star) = 0.00083 ± 0.00012 (14.3%) NICMOS (leff 1.60 mm): F(ring[unobscured])/F(star) = 0.00140 ± 0.00029 (20.8%)

Wavelength Dependent Scattering Efficiency (Color)

SUMMARY Ring geometry/astrometry defined by NICMOS improved by higher resolution STIS observations. Notably, i 2.6° larger than original (published) NICMOS solution. Spatially resolved photometry of ring with ±2% uncertainty at ansae (1”), and ±6—8% uncertainty at 0.6—0.5”. Characteristic width ~ 10% of 70AU radius ring. “Left/Right” brightness anisotropy or ~20% along at least 50° wide diametrically opposed arcs centered on ansae. “Front/Back” brightness anisotropy, roughly symmetric in both L/R “hemispheres”, increasing with longitudinal distance from ansae to 35% difference at 30° from ansae. Ring is uniformly RED from “V” to H with 1:1.7:2.9 spectral reflectance in CCD50(“V”):F110W(1.1mm):F160W(H).

SUMMARY We eagerly await the opportunity to continue exploiting these capabilities when both are re-awakened and fully restored to service. A unique venue for sub-arcsecond high contrast imaging in the near-IR and optical is afforded by HST/NICMOS & STIS.