Massive molecular outflows

With the aim of understanding the role of massive outflows in high-mass star formation, we mapped in the 12 CO $J=2-1$ transition 26 high-mass star-forming regions at very early stages of their evolution. At a spatial resolution of $11''$ bipolar molecular outflows were found in 21 of them. The other five sources show confusing morphology but strong line wings. This high detection rate of bipolar structure proves that outflows common in low-mass sources are also ubiquitous phenomena in the formation process of massive stars. The flows are large, very massive and energetic, and the data indicate stronger collimation than previously thought. The dynamical timescales of the flows correspond well to the free-fall timescales of the associated cores. Comparing with correlations known for low-mass flows, we find continuity up to the high-mass regime suggesting similar flow-formation scenarios for all masses and luminosities. Accretion rate estimates in the $10^4~L_{\odot}$ range are around $10^{-4}~M_{\odot}$ yr -1 , higher than required for low-mass star formation, but consistent with high-mass star formation scenarios. Additionally, we find a tight correlation between the outflow mass and the core mass over many orders of magnitude. The strong correlation between those two quantities implies that the product of the accretion efficiency $f_{\rm{acc}}=\dot{M}_{\rm{acc}}/(M_{\rm{core}}/t_{\rm{ff}})$ and $f_{\rm{r}}$ (the ratio between jet mass loss rate and accretion rate), which equals the ratio between jet and core mass ($f_{\rm{acc}} f_{\rm{r}}= M_{\rm{jet}}/M_{\rm{core}}$), is roughly constant for all core masses. This again indicates that the flow-formation processes are similar over a large range of masses. Additionally, we estimate median $f_{\rm{r}}$ and $f_{\rm{acc}}$ values of approximately 0.2 and 0.01, respectively, which is consistent with current jet-entrainment models. To summarize, the analysis of the bipolar outflow data strongly supports theories which explain massive star formation by scaled up, but otherwise similar physical processes – mainly accretion – to their low-mass counterparts.