EVOLUTION OF OUTFLOW ACTIVITY AROUND LOW-MASS EMBEDDED YOUNG STELLAR OBJECTS

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 5 =2-1 line of (CO)-C-12 to characterize this activity for YSOs that are still sufficiently embedded to show molecular outflows. Our CO outflo w survey benefits from coordinated millimeter continuum measurements o f circumstellar masses which allow us to estimate the evolutionary sta tes of the central driving sources. Our sample comprises 36 near-IR (C lass I) protostars and 9 far-IR/submm (Class 0) protostars, and should be representative of the ''self-embedded'' phase of (low-mass) protos tellar evolution characterizing young stars still surrounded by signif icant 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 environ ment of the driving sources in order to assess the dynamical propertie s of the underlying driving winds/jets. As is well-known, a tight corr elation between outflow energetics and driving source luminosity is fo und for Class I sources. However, Class 0 sources lie a factor of simi lar to 10 above this correlation, suggesting they have qualitatively d ifferent (e.g., more powerful) CO outflows. In addition, we find that the outflow momentum flux correlates well with the circumstellar envel ope 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) co rrelation and most likely results from a more or less continuous decre ase of outflow power with time during the accretion phase. For a young star of final mass similar to 0.6 M., the outflow momentum flux is ty pically F-CO similar to 10(-4) M.km s(-1)yr(-1) at the early Class 0 s tage and F-CO similar to 2 x 10(-6) M.km s(-1)yr(-1) at the late Class I stage. We suggest that this decrease of outflow energetics reflects a corresponding decay in the mass accretion/infall rate.