A high-resolution radio survey of Class I protostars

We report the results of a survey of low-mass Class I protostars in the cm continuum. In the initial survey, seven sources in the Taurus star formatio n region were observed with the VLA at 0.25-arcsec resolution. All seven so urces drive CO outflows and display Herbig-Haro flows in the optical or nea r-infrared (NIR) wavebands. Four out of seven sources were detected, two of which are new discoveries in systems of very low luminosity, one being the lowest luminosity system detected to date in the cm continuum. Notably, th ree sources were not detected to a 3 sigma limit of 0.10 mJy/beam, which in dicates that significant cm continuum emission is not a universal feature o f Class I systems with outflow activity. Subsequent observations of HH30, a more evolved Class II system, found no emission to a 3 sigma limit of 0.03 mJy/beam. After comparison with near infrared data, we suggest that the di scriminating feature of the detected systems is a relatively high ionizatio n fraction in the stellar wind. Temporal variability of the outflow may als o play a role: only recently ejected knots may have sufficiently dense plas ma to be optically thick to free-free emission, and hence produce detectabl e flux. The one relatively bright source, IRAS 04016+2610 (L1489 IRS), is c learly resolved on a 0.4-arcsec scale at 2 and 3.5 cm. Additional imaging w ith MERLIN did not detect this source with a 0.04-arcsec beam, indicating t hat the radio emission is generated in a region with a radius of approximat e to 25 au, which is broadly similar to the radius of the bipolar cavities inferred from models of NIR data. Interpretation of this system is complica ted by the existence of a quadrupolar outflow i.e. two bipolar outflows alo ng roughly perpendicular axes, which we originally detected through polarim etric imaging. We present an NIR H-2 image in which a bow shock in the seco ndary outflow is clearly seen. This complicated structure may have been cau sed by a gravitational interaction between two protostars.