We imaged a circumstellar nebula that lies symmetrically around the pre-main-sequence star T Tauri, using a 6″ diameter coronagraphic spot placed in front of an infrared array camera. The nebula is detected in polarized light our to ∼10″ (1400 AU) at both 1.25 μm and 2.2 μm, twice as far from T Tauri as the structure previously seen in the J = 1 → 0 rotational transition of carbon monoxide. The intensity of scattered light, measured as a function of distance from T Tauri, reveals an approximate r-2.5 dependence. We model the distribution of polarized intensity and deduce that the radial density gradient in the T Tauri nebula is approximately proportional to r-1.5. Between 3″ and 10″ from T Tauri the scattering optical depths, measured along lines of sight through the observed regions of the nebula, range from <0.05 to ∼0.21 at J band and from ∼0.05 to ∼0.14 at K band. The integrated dust mass, in an annulus between 3″ and ∼10″, is ∼3 × 10-4 M⊙. By analyzing the polarized light distribution in terms of the optical depth to the star along the line of sight and the optical depth to the star in the plane of the sky, we can study the three-dimensional shape of the nebula. Although the projection of the nebula in the plane of the sky appears circularly symmetric, analysis indicates that the nebula is not spherically symmetric. We deduce that we are probably viewing, from nearly pole-on, a small, preplanetary accretion disk surrounded by a larger structure that may be partly a large disk and partly an infalling envelope. This orientation agrees with that deduced previously from interferometric observations of the J = 1 → 0 transition of CO. Because the spectrum of T Tauri indicates that it is surrounded by a disk that is optically thick out to wavelengths ≥ 100 μm, our detection of scattered light in the near-infrared at distances as far as 10″ from the star suggests that the larger nebula is either a warped or flared disk or a toroid of large solid angle, as seen by the star. Alternatively, the disk may be more flattened if it is centered on the infrared companion and illuminated from above by T Tauri. The presence of an infalling envelope that is actively depositing heat in the outer disk may explain the shallow radial temperature gradient predicted from models of flat spectrum sources like T Tauri. We also obtained polarized, coronagraphic images of the vicinities of DG Tauri and HL Tauri. DG Tauri behaves like a point source. This implies that any substantial nebula surrounding this star is limited to the region r < 3″ or is not warped or flared. The HL Tauri images reveal an elliptical, 18″ × 12″ reflection nebula. The long axis of the nebula is oriented at PA ∼ 145°; hence, these results reinforce previous assertions that we are viewing a bipolar nebula surrounding a nearly edge-on disk. Most and possibly all of the scattered light seen in the 2.2 μm image of HL Tauri probably emerges from the bipolar nebula rather than from the disk itself. Polarization observations of the protostellar source CRL 2591, obtained primarily for calibration purposes, reveal density gradients and optical depth profiles that support the expanding bubble model for its outflow.
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science
- Circumstellar matter
- Radio lines: molecular: circumstellar
- Stars: individual (T Tauri)
- Stars: pre-main-sequence