Bipolar molecular outflows from high-mass protostars

Abstract

We report observations of the bipolar molecular outflows associated with the luminous (∼2 × 104 L⊙) far-IR sources IRAS 21519+5613 and IRAS 22506+5944, as well the dust and molecular gas condensations on which these outflows appear to be centered. The observations were made in 12CO, 13CO, C18O, and continuum at 3 mm with the BIMA array and in 12CO and 13CO with the NRAO 12 m telescope to recover extended emission filtered out by the interferometric array. We find that the outflow associated with each IRAS source shows a clear bipolar morphology in 12CO, with properties (i.e., total mass of order 10-100 M⊙, mass-outflow rate ≥10-3 M⊙, dynamical timescale 10 4-105 yr, and energetics) comparable with those of other massive outflows associated with luminous young stellar objects. Each outflow appears to be centered on a dust and gas condensation with a mass of 200-300 M⊙, likely marking the location of the driving source. The outflow lobes of both sources are fully resolved along their major but not minor axes, and they have collimation factors that may be comparable with young low-mass stars. The mass-velocity diagrams of both outflows change in slope at a velocity of ∼10 km s-1, suggesting that the high-velocity component (HVC) may drive the low-velocity component (LVC). Although the HVC of IRAS 21519+5613 shows evidence for deceleration, no such signature is seen in the HVC of IRAS 22506+5944. Neither HVC has a momentum supply rate sufficient to drive their corresponding LVCs, although it is possible that the HVC is more highly excited and hence its thrust underestimated. Like for other molecular outflows the primary driving agent cannot be ionized gas, leaving atomic gas as the other remaining candidate. Neither IRAS 21519+5613 nor IRAS 22506+5944 exhibits detectable free-free emission, which together with the observed properties of their molecular outflows and surrounding condensations make them credible candidates for high-mass protostars. The mass-accretion rate required to produce their observed IRAS luminosity is ≳10-4 M ⊙ yr-1, which is more than sufficient to quench the development of an UC-H II region. On the other hand, the individual IRAS sources may be associated with a group of stars whose dominant member is a main-sequence star that is responsible for the observed outflow. Such a star would be required to have a spectral type of ∼B2 (luminosity of ∼3000 L⊙) or later to not excite a detectable UC-H II region; the time-averaged mass-accretion rate needed to produce this star is then 10 -3 to 10-4 M⊙ yr-1. Thus, regardless of the evolutionary stage of the outflow driving source, the inferred mass-accretion rate is much higher than that allowed by simple inside-out collapse models but can be accommodated by recently proposed variants.