High-Resolution Near-Infrared Observations of the Circumstellar Disk System in the Bok Globule CB 26

We report on results of near-infrared and optical observations of the millimeter disk embedded in the Bok globule CB 26. The near-infrared images show a bipolar reflection nebula with a central extinction lane that coincides with the millimeter disk. Imaging polarimetry of this object yielded a polarization pattern that is typical of a young stellar object (YSO) surrounded by a large circumstellar disk and an envelope, seen almost edge-on. The strong linear polarization in the bipolar lobes is caused by single scattering at dust grains and reveals the location of the illuminating source that coincides with the center of the millimeter disk. The spectral energy distribution of the YSO embedded in CB 26 resembles that of a Class I source with a luminosity of 0.5 L☉. Using the pre-main-sequence evolutionary tracks and the stellar mass inferred from the rotation curve of the disk, we derive an age of the system of ≤106 yr. Hα and [S II] narrowband imaging as well as optical spectroscopy revealed a Herbig-Haro object 615 northwest of CB 26 YSO 1, perfectly aligned with the symmetry axis of the bipolar nebula. This Herbig-Haro object (HH 494) indicates ongoing accretion and outflow activity in CB 26 YSO 1. Its excitation characteristics show that the Herbig-Haro flow is propagating into a low-density environment. We suggest that CB 26 YSO 1 represents the transition stage between embedded protostellar accretion disks and more evolved protoplanetary disks around T Tauri stars in an undisturbed environment.