Very Large Array Observations of High‐Velocity Hiin L1551

Abstract

We report Very Large Array (VLA) observations of the 21 cm continuum and H I line emission of the neutral wind in the prototypical molecular outflow source L1551. The intermediate-velocity gas observed with the VLA has a velocity with respect to the ambient cloud in the range 10-50 km s-1. Although the blueshifted flow appears weaker than the red one, the maps are indicative of a bipolar morphology confined inside the large-scale CO bipolar lobes. This morphology strongly suggests that the atomic wind is driving the bipolar CO outflow. We fit to the H I line profiles, at four different positions along the redshifted flow axis, a model of a decelerating conical wind that entrains ambient cloud material in a mixing layer at the walls of the cone. This model has a velocity at the axis v0 = 200 km s-1, a mass-loss rate * 9 × 10-7 M☉ yr-1, and a momentum rate 2 × 10-4 M☉ yr-1 km s-1. This is sufficient to drive the observed molecular flow, provided an age ≥8 × 104 yr. We find that the continuum emission spectrum of IRS 5, on scales of 10'', is similar to those of H II regions. This can be explained if most of the emission comes from material ionized by UV radiation from shock fronts due to the interaction of the stellar wind against the ambient cloud. The momentum rate required to produce the extended centimeter emission agrees well with the wind obtained by fitting the line profiles. Finally, the location of a background extragalactic continuum source in the red lobe allows us to probe the physical conditions of the neutral wind. We find that the most probable H I spin temperature in the wind is in the range 25-50 K, in agreement with predictions of the thermal structure of such winds.