BUTTERFLY STAR IN TAURUS - STRUCTURES OF YOUNG STELLAR OBJECTS

We present high-resolution, near-infrared images of a sample of five l ow-mass young stellar objects in the Taurus-Auriga dark cloud, mostly observed with the 'shift and add' technique, together with deep polari metric images. These systems are at the 'Class I' phase of evolution, prior to the T Tauri phase. Monte Carlo models tailored to the individ ual sources support the view that the flux distribution of the circums tellar nebulosity arises from an envelope with a bipolar cavity, but s how that dust in outflows can also be prominent in the near-infrared. The source IRAS 04302+2247 is analysed in detail, since it displays a unique quadrupolar morphology, which is explained in terms of a system oriented precisely edge-on with a perpendicular outflow. The masses o f the envelopes derived from the models are in good agreement with est imates obtained from submillimetre thermal fluxes and estimates derive d from CS(J=2-1) emission. Constraints are placed on envelope and cavi ty structure, indicating higher density near the equatorial plane, smo oth, continuous density profiles and physically thin inner discs. Thre e of the five systems show evidence for two bipolar cavity/outflow sys tems, perhaps indicating that these are close binaries. The very high (up to 80 per cent) degrees of polarization observed in the envelopes show that the scattering is dominated by small, interstellar-type dust grains, and the morphologies and high polarizations of the cores show that the protostars themselves are usually obscured at this phase of evolution, even at 3.7 mu m.