Transcript AAS_Jan_2008_Schneide - University of Arizona

Coronagraphic Polarimetry of HST-Resolved
Circumstellar T Tauri and Debris Disks
Glenn Schneider (The University of Arizona, Dean C. Hines (Space Science Institute)
1
2
and the HST/GO 10852 & 10847 Teams
Abstract
Optically Thin Debris Disk Scattering
The formation of planetary systems is intimately linked to the dust population in circumstellar disks,
thus understanding dust grain evolution is essential to advancing our understanding of how planets
form. While it is well established that stars form in ISM-like protostellar environments, the
connection to now observable light-scattering circumstellar disks and the processes of planet
formation is still very uncertain. Mid-IR spectral studies suggest that disk grains are growing in the
environments of young stellar objects during the putative planet-formation epoch. Structures
revealed in well resolved images of older circumstellar debris disks suggest gravitational influences
on the disks from putative co-orbital bodies of planetary mass. To further elucidate the dust and
systemic properties in potentially planet-forming systems, we have undertaken two symbiotic HST
imaging programs that exploit the recently commissioned capabilities of coronagraphic polarimetry
with the Near Infrared Camera and Multi-Object Spectrometer (NICMOS), probing dust structures in
T Tauri circumstellar disks during the early epochs of planet formation, and debris disks around older
stars. We present the first observational results from these two programs in light of earlier
commission observations of TW Hya, focusing on the scattered light disks around the T Tauri star
GM Aur and the debris disk associated with HR 32297, along with optical (ACS) coronagraphic
polarimetry of the unusual dust structure around HD 61005.
Support for this work was provided by NASA through grant numbers GO-9768. 10847 and 10852
from the Space Telescope Science Institute, which is operated by Association of Universities for
Research in Astronomy Incorporated, under NASA contract NAS5-26555NASA.
Optically Thick T-Tauri Disk Scattering
Figure 2: HST/NICMOS coronagraphic polarimetry of the debris disk associated with HD 32297,
originally imaged with NICMOS in total light by Schneider, Silverstone & Hines 2005, ApJ, 629, L117.
Polarizations ~20% are measured along the mid-plane of this nearly edge-on disk.
HD 61005 - ACS 0.6m Coronagraphic Polarimetry
Total Intensity (I)
Polarization (P)
0%
Position Angle ()
Figure 1: Coronagraphic polarimetry of GM Aur comparing polarimetry results without (top) and with (bottom)
matched PSF-subtraction. Without subtraction of an unpolarized PSF-star, the polarization fraction of the GM Aur disk
is diluted by residual instrumentally diffracted and scattered light from the coronagraphicaly occulted (unpolarized)
central star. The efficacy of the technique is born out by the essentially identical position angle and polarized intensity
images in the right two columns. PSF subtraction enables decoupling the polarization fraction and total intensity (2 mm
flux density) with accurate measurements of both (overcoming one of the primary difficulties encountered in groundbased AO polarimetry). These observations are from GO 10852 (PI: Schneider).
1Glenn
Schneider (PI: University of Arizona), Dean C. Hines (Space Science Institute), Angela S. Cotera (SETI Institute), Francois Menard
(Universite de Grenoble), Christophe Pinte (Universite de Grenoble), Karl Stapelfeldt (JPL), Barbara A. Whitney (Space Science Institute)
C. Hines (PI: Space Science Institute), Glenn Schneider (University of Arizona), Jean-Charles Augereau (Universite de Grenoble, Dana
E. Backman (NASA Ames), Angela S. Cotera (SETI Institute), James R. Graham (UC Berkeley), Paul Kalas (UC Berkeley), Francois Menard
(Universite de Grenoble), Stanimir A. Metchev (UCLA), Deborah Padgett (SSC), Dan E. Potter (University of Arizona), Christophe Pinte
(Universite de Grenoble), Murray Silverstone (Eureka), Karl Stapelfeldt (JPL), Barbara A. Whitney (Space Science Institute)
2Dean
0˚
100%
Polarized Intensity (P*I)
180˚
Figure 3: Optical coronagraphic imaging polarimetry of the debris disk associated ~100 Myr old star
HD 61005, which is a G9V star at D = 34.6 pc. These optical images were obtained with the Advanced
Camera for Surveys (ACS) aboard HST. This extraordinary system was fisrt imaged in total light with
NICMOS (Hines et al. 2007, ApJ, 671, L165). The swept morphology appears to be caused by a collision
between the debris system and an enhanced density cloud within the interstellar medium. The polarized
intensity image reveals a ridge of highly polarized (~30%) light along the putative leading egde of the
disk.