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”.