1. 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

2. 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 r < 40AU, and grains > 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°

3. 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.

4. 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.
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.

5. 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).

6. 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
r < 50 AU
FLUX DENSITY
1.1mm
1.6mm
H(F160W) = 12.35±
J(F110W) = 12.92±0.08
Tdust ~ Ldisk/L*
1.1mm
1.6mm
NIR scattered flux in good agreement with visible
absorption & mid-IR re-radiation.

7. NICMOS Observations of the HR 4796A Circumstellar Debris Ring20 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.

8. 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.

9. HR 4796A Circumstellar Debris Ring
STIS

10. OBSERVATIONS

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

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

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

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

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

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

17. 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

18. 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

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

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

21. COMBINATION & POST-PROCESSING

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

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

24. 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

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

26. < 2% Flux & No Wedge/Spikes
STIS Observations of the HR 4796A Circumstellar Debris Ring N E
< 2% Flux & No Wedge/Spikes

27. 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

28. 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

29. 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

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

31. RESULTS (thus far) from this image

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

33. HR 4796A Circumstellar Debris Ring - WIDTH
WIDTH AT NE ANSA
FWHM: 12.9±0.7AU % 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 = 0.126”

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

35. “FACE-ON” PROJECTION - With Flux Conservation

37. Spatially Resolved Relative PHOTOMETRY of the Ring

38. N-Sigma Brightness Ratio (Percent)

39. PHOTOMETRIC ERROR ESTIMATION

40. N-Sigma Brightness Ratio (Percent)

41. Aperture Photometry
STIS(leff 0.58 mm): F(ring[unobscured])/F(star) = ± (7.3%)
NICMOS (leff 1.10 mm): F(ring[unobscured])/F(star) = ± (14.3%)
NICMOS (leff 1.60 mm): F(ring[unobscured])/F(star) = ± (20.8%)

42. Wavelength Dependent Scattering Efficiency (Color)

43. 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).

44. 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.