A compact density condensation around L1551-IRS 5: 2.7mm continuum observations with 4" resolution

The dark cloud L1551 contains the best known examples of bipolar molecular outflow. Such outflows are assumed to be driven by winds from young stars embedded in a cloud but the mechanism for collimation of the outflows is still in doubt, though it has been much debated. Among the possibilities put forth to date are intrinsically anisotropic stellar winds, isotropic stellar winds collimated by interstellar toroidal shaped clouds on the order of 10(17) cm in size, or circumstellar disks of order 10(15) cm in size. Because the outflow in L1551 as revealed by the Very Large Array (VLA) cm continuum observations is collimated even at the arc second level it seems as though the stellar wind powering the outflow must either be initially anisotropic or be collimated by something very close to the star, such as a circumstellar disk. We have observed L1551 in the continuum at 2.7 mm with the OVRO millimeter-wave interferometer in the winter of 1983-4 and again, more extensively, in 1985-6. The resulting map shows for the first time direct evidence for a density condensation capable of collimating an initially isotropic flow from IRS 5. This map made from data taken in 1985-6 with projected baselines up to 100 m in length (37k lambda). It has been cleaned and reconstructed with a 4'' gaussian beam. It shows a nearly unresolved source with a suggestion of extension at the 3'' level (assuming a gaussian source shape). The integrated flux density in this map is 170 mJy. A composite spectrum of L1551 is presented which shows that the flux at 2.7 mm is due to the thermal radiation from dust that is also seen at shorter wavelengths. Very little of it can be due to the continuation of the nearly flat cm wavelength spectrum which is assumed to be from thermal bremsstrahlung radiation. The map and spectrum provide strong constraints on the size, temperature, and optical depth of the density condensation surrounding IRS 5.