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

We present a NIR analysis of a sample of H2 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 H2 2.12um and FeII 1.64um and spectroscopic observations in the range 1-2.5um. H2 line ratios have been used to estimate the visual extinction and average temperature of the molecular gas. Av values range from 2 to 15mag; average temperatures range between 2000 and 4000K. In several knots a stratification of temperatures is found with maximum values up to 5000K. Such a stratification is more commonly observed in those knots which also show FeII emission, while a thermalized gas at a single temperature is generally found in knots emitting only in molecular lines. Combining narrow band imaging with the parameters derived from the spectral analysis, we are able to measure the total luminosity of the H2 and FeII shocked regions in each flow. H2 is the major IR coolant with an average L(H2)/L(FeII) ratio of 10^{-2}. 83% of the sources have a L(H2)/L(bol) ratio of 0.04, irrespective of the Class of the driving source, while a smaller group of sources have L(H2)/L(bol) an order of magnitude smaller.Such a separation reveals the non-homogeneous behaviour of ClassI, where sources with very different outflow activity can be found. This is consistent with other studies showing that among ClassI one can find objects with different accretion properties and it demonstrates that the H2 power in the jet can be a powerful tool to identify the most active sources of this class.