Coronagraphic Imaging with
HST and STIS
C. Grady, C. Proffitt, E. Malumuth,
T. Gull, B. Woodgate, C. Bowers,
S. Heap, R. Kimble, D. Lindler, P. Plait
The STIS Wedge Mask
• Stars to be observed
coronagraphically are
placed behind one of
two crossed wedges or
a bar.
• For convenience, a
number of “apertures”
have been defined by
STScI.
Comparison of Direct and
Coronagraphic Imaging
Direct Light Imaging With STIS
ghost
• Deep, direct light images obtained with HST/STIS suffer
from saturation, & the presence of window ghost(s).
• Image field of view is 20” on a side.
Coronagraphic Imaging
•
•
The coronagraphic image can be exposed ~400 times longer than the
direct light image before reaching the CCD full-well. The wings of
the stellar PSF are reduced by a factor of 3-5 within 5” of the star.
The diffraction spikes are the highest power features in the image.
Raw STIS Coronagraphic Images
• Suitable for sources
where the nebulosity
is comparable in
brightness to the
wings of the PSF.
• Higher S/N (deeper
exposures) than PSFsubtracted WFPC/2
direct imagery.
Color Effects with HST/STIS’s
coronagraphic imaging capability
• STIS is a white-light
coronagraph sensitive to
0.2-1.0 mm.
• The large bandpass
washes out the Airy rings,
makes the mode extremely
sensitive to color
differences between the
science and PSF star.
• Color effects are
conspicuous for
D(B-V)>0.08
Stability of the PSF
• STIS is on the sun-lit
side of the HST
spacecraft.
• We see systematic
variations in PSF
subtraction residuals
with orbital phase.
Detectability Limit V=5.2, back
to back images
• The limiting noise pedestal, due principally to read noise,
is shown for V=5.2 with the difference of successive 84s
images of HR 4413.
Detectability Limit V=5.2, noncontemporaneous images.
•
Larger breathing residuals occur when the science star and the
calibration star were obtained under different focus conditions, and
limit the ability to detect faint nebulosity within 3” of the star. Here
data for HD 141653 (V=5.2) from 1999 6/29 and 2000 3/29 are
differenced.
Detectability Limit V=5.2, with
filtering of the PSF data.
Subtracting a median of 3 orbits-worth of observations from a
Single observation pushes the contrast to better than 108 at 2”
From the star. Beyond 4” the net image is read noise limited.
Coronagraphic Imaging
• The HST/STIS coronagraphic imaging data fall
into 3 groups:
– Objects with nebulosity visible in direct light imaging,
which benefit from deeper exposures
– Objects with distant nebulosity (HH objects) visible in
the raw coronagraphic data, and where disks become
visible upon PSF subtraction.
– Objects where the nebulosity is not detectable in direct
light images or in the raw coronagraphic data, and
becomes detectable only upon PSF subtraction.
Large (800-1300 AU)
Circumstellar Envelopes
• AB Aur: Nebulosity beyond 2” is apparent in the raw
coronagraphic image.
• coronagraphic imagery enable discovery of spiral structure
continuing to within 0.5” of the star (70 AU).
Large (800-1300 AU)
Circumstellar Envelopes
•
•
Coronagraphic imagery of SU Aur revealed that the “cometary”
nebulosity seen in direct light images is not the disk, but is the
illuminated margin of the molecular cloud. The field of view is 25” on
a side.
STIS also produced the first detection of what appears to be a jet
extending into the cloud.
Optically bright disks
• Bright, face-on disks are comparatively easy to identify in
STIS data as an azimuthally-symmetric signal present
when the diffraction spikes have been nulled. Here TW
Hya (Weinberger et al. 2002) is shown. The field of view
is 12” on a side.
High Contrast Images of Jets
• The jet of CW Tau was previously known (Gomez
de Castro 1993, Hirth et al. 1994,Dougados et al.
2000). Note the structure within the jet.
DL Tau: Bipolar Jet and Disk
•
•
The outer portion of the jet was detectable after PSF subtraction in a F606W
WFPC/2 image, but the full jet and counterjet are revealed in the STIS
coronagraphic image.
The disk has not previously been optically imaged, but is coincident with the
mm disk (Simon et al. 2000). The NW side of the disk is 1 magnitude brighter
than the SE side, indicating that the NW side is the forward scattering side,
and the grains appear to be similar to zodiacal dust.
Nebulosity Requiring both the
coronagraph and PSF subtraction
• Nebulosity which is bright only within 2” of the star may
not be detectable in the raw coronagraphic imagery for
stars like HD 100546 (Grady et al. 2001). Following PSF
subtraction, both a 515 AU (5”) disk and 1000 AU (~10”)
envelope are visible.
HD 163296: A disk and jet
associated with a 4-6 Myr old
Herbig Ae star.
• The outer HH knots associated with HD 163296 were
discovered in the raw coronagraphic imagery.
• Detection of the disk required PSF subtraction, and this is
one of the optically faintest of our disk detections (Grady
et al. 2000).
Alternate Techniques for High Contrast
Imaging
Placing the star off the edge
of the active detector area,
results in a similar PSF intensity,
But can reduce the impact of read
Noise due to the longer integration
For each individual exposure.
Circumstellar Disks are Diverse:
• Both geometrically flat (Sµr-3 to r-4) and highly
flared disks (r-2) are present in the same age range,
as well as a large range in surface brightness for a
given angular distance from the star.
• Large (800-1000 AU, or 8-10”) envelopes can
persist around single stars for at least 10 Myr,
overlapping with the era of planet formation.
• Both azimuthally and, more interestingly, nonazimuthally symmetric structure is seen beginning
in some systems at 2 Myr. This may be the earliest
remotely detectable signature of planets.
The STIS Coronagraph in an Era
of Multiple HST Coronagraphs
• Large field of view may be useful in the analysis
of some complex scenes.
• STIS offers the narrowest occulter diameters
available in the optical, down to r=0.6”
• Broad bandpass enables deep observations in one
orbit and can efficiently identify sources
benefiting from narrower band imaging by ACS.
• Efficiently detect both reflection nebulosity and
emission line features.
Small Debris Disks: HR 4796A
•
•
Small debris disks (r~1”) present the most challenging coronagraphic
imaging attempted by HST, since they lie in the angular region most
affected by breathing.
HR 4796A’s disk is marginally detectable in the raw coronagraphic
image, which accounts for its detectability against a color mis-match
with the PSF star and breathing effects. The field of view is 4” on a
side.
Debris Disks
Raw coronagraphic
image
• HD 141569 (after Mouillet et al. 2000). The disk is a factor
of 25 below the PSF near the wedge, and is a factor of 6
fainter than the PSF at 2”.