1996A&A...311..858B

We present a detailed study of outflow activity in a sample of 45 low-luminosity embedded young stellar objects (YSOs). We use maps in the J=2-1 line of ¹²CO to characterize this activity for YSOs that are still sufficiently embedded to show molecular outflows. Our CO outflow survey benefits from coordinated millimeter continuum measurements of circumstellar masses which allow us to estimate the evolutionary states of the central driving sources. Our sample comprises 36 near-IR (Class I) protostars and 9 far-IR/submm (Class 0) protostars, and should be representative of the ``self-embedded'' phase of (low-mass) protostellar evolution characterizing young stars still surrounded by significant circumstellar envelopes. We find that virtually all the objects in our sample have detectable CO outflow activity. We make homogeneous estimates of the outflow momentum flux deposited in the close environment of the driving sources in order to assess the dynamical properties of the underlying driving winds/jets. As is well-known, a tight correlation between outflow energetics and driving source luminosity is found for Class I sources. However, Class 0 sources lie a factor of ∼10 above this correlation, suggesting they have qualitatively different (e.g., more powerful) CO outflows. In addition, we find that the outflow momentum flux correlates well with the circumstellar envelope mass of the exciting source for both Class I and Class 0 sources. We show that this new correlation is independent of the F_CO-L_bol correlation and most likely results from a more or less continuous decrease of outflow power with time during the accretion phase. For a young star of final mass ∼0.6M_☉, the outflow momentum flux is typically F_CO∼10^–4M_☉.km/s/yr at the early Class 0 stage and F_CO∼2x10^–6M_☉.km/s/yr at the late Class I stage. We suggest that this decrease of outflow energetics reflects a corresponding decay in the mass accretion/infall rate.