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 } Â 2.12 \mu m and [ Fe ii ] Â 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 \sim 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 \sim 2 to \sim 15 mag ; average temperatures range between \sim 2000 and \sim 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 _ { H _ { 2 } } and L _ { [ { Fe \textsc { ii } } ] } ) in each flow .
H _ { 2 } is the major NIR coolant with an average L _ { H _ { 2 } } / L _ { [ { Fe \textsc { ii } } ] } ratio of \sim 10 ^ { 2 } .
We find that \sim 83 % of the sources have a L _ { H _ { 2 } } / L _ { bol } ratio \sim 0.04 , irrespective of the Class of the driving source , while a smaller group of sources ( mostly Class I ) have L _ { H _ { 2 } } / 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 .