A high-resolution radio survey of Class I protostars

Abstract

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 formation region were observed with the VLA at 0.25-arcsec resolution. All seven sources drive CO outflows and display Herbig-Haro flows in the optical or near-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, three sources were not detected to a 3σ limit of 0.10 mJy/beam, which indicates that significant cm continuum emission is not a universal feature of Class I systems with outflow activity. Subsequent observations of HH30, a more evolved Class II system, found no emission to a 3σ limit of 0.03 mJy/beam. After comparison with near infrared data, we suggest that the discriminating feature of the detected systems is a relatively high ionization fraction in the stellar wind. Temporal variability of the outflow may also play a role: only recently ejected knots may have sufficiently dense plasma to be optically thick to free-free emission, and hence produce detectable flux. The one relatively bright source, IRAS 04016+2610 (L1489 IRS), is clearly resolved on a 0.4-arcsec scale at 2 and 3.5 cm. Additional imaging with MERLIN did not detect this source with a 0.04-arcsec beam, indicating that the radio emission is generated in a region with a radius of ≈25 au, which is broadly similar to the radius of the bipolar cavities inferred from models of NIR data. Interpretation of this system is complicated by the existence of a quadrupolar outflow, i.e. two bipolar outflows along roughly perpendicular axes, which we originally detected through polarimetric imaging. We present an NIR H2 image in which a bow shock in the secondary outflow is clearly seen. This complicated structure may have been caused by a gravitational interaction between two protostars.