H2 active jets in the near IR as a probe of protostellar evolution

We present an in-depth near-IR analysis of a sample of H-2 outflows from young embedded sources to compare the physical properties and cooling mechanisms of the different flows. The sample comprises 23 outflows driven by Class 0 and I sources having low-intermediate luminosity. We have obtained narrow band images in H-2 2.12 mu m and [Fe II] 1.64 mu m and spectroscopic observations in the range 1-2.5 mu m. From [ Fe II] images we detected spots of ionized gas in similar to 74% of the outflows which in some cases indicate the presence of embedded HH-like objects. H-2 line ratios have been used to estimate the visual extinction and average temperature of the molecular gas. A(v) values range from similar to 2 to similar to 15 mag; average temperatures range between similar to 2000 and similar to 4000 K. In several knots, however, a stratification of temperatures is found with maximum values up to 5000 K. Such a stratification is more commonly observed in those knots which also show [ Fe II] emission, while a thermalized gas at a single temperature is generally found in knots emitting only in molecular lines. Combining narrow band imaging (H-2, 2.12 mu m and [Fe II], 1.64 mu m) with the parameters derived from the spectroscopic analysis, we are able to measure the total luminosity of the H-2 and [Fe II] shocked regions (L-H2 and L-[Fe II]) in each flow. H-2 is the major NIR coolant with an average L-H2 /L-[Fe II] ratio of similar to 10(2). We find that similar to 83% of the sources have a L-H2 /L-bol ratio similar to 0.04, irrespective of the Class of the driving source, while a smaller group of sources ( mostly Class I) have L-H2 / L-bol an order of magnitude smaller. Such a separation reveals the non-homogeneous behaviour of Class I, where sources with very different outflow activity can be found. This is consistent with other studies showing that among Class I one can find objects with different accretion properties, and it demonstrates that the H-2 power in the jet can be a powerful tool to identify the most active sources among the objects of this class.