High-Resolution Optical and Near-Infrared Images of the FS Tauri Circumbinary Disk

Abstract

We present an H-band image of FS Tauri, a 0.''2-separated classical T Tauri binary system, taken with the Coronagraphic Imager with Adaptive Optics (CIAO) on the Subaru Telescope. This new image, combined with Hubble Space Telescope/Advanced Camera for Surveys (HST/ACS) F606W-band polarimetric images, shows that the binary has complicated circumbinary features, including a circumbinary disk, western and eastern arm-like structures, and two cavities at the northeast and southwest. The circumbinary disk is 630 AU in radius and the southeast side of the disk is bright in the H-band. The brightness ratio (southeast/northwest) is 1.6±0.4. A single Rayleigh-like scattering model indicates that the disk is inclined by 30° to 40° and that the southeast side corresponds to the near side along our line of sight. The H-band surface brightness of the southeast side decreases as r^-1.9±0.1 from 15.2 mag arcsec^-2 to 16.8 mag arcsec^-2. The outer portion of the disk is possibly more flared than its inner portion. The weak centro-symmetric polarization pattern and redder F606W - H color (4.2 ± 0.2 mag) of the southeast side are probably caused by multiple-scattering events from dust grains associated with the binary. The F606W-band image shows the bright northwest side of the disk in contrast with the H-band image. The F606W - H color of the northwest side is between 1.7 mag and 3.0 mag. We consider that Haro 6-5 B (FS Tauri B), 20'' away, produces neutral scattered light from the northwest side. This idea is supported by the polarization pattern of the northwest side, which is centro-symmetric with respect to Haro 6-5 B. The arms appear to encompass the western and eastern cavities, suggesting that the arms + cavity systems are created by a bipolar outflow from the binary. However, the direction of this outflow is inconsistent with that of outflows inferred from the circumbinary disk model. These differences may arise from a misalignment between the circumbinary disk and the circumstellar disks. Another mechanism forming the arms + cavity systems is considered to be the inhomogeneous density distribution of materials in the circumbinary disk.