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

Abstract

We present an in-depth near-IR 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 2.12 μm and [FeII] 1.64 μm and spectroscopic observations in the range 1-2.5 μm. From [FeII] images we detected spots of ionized gas in ∼74% of the outflows which in some cases indicate the presence of embedded HH-like objects. H2 line ratios have been used to estimate the visual extinction and average temperature of the molecular gas. AV values range from ∼2 to ∼15 mag; average temperatures range between ∼2000 and ∼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 (H2, 2.12 μm and [Fe II], 1.64 μm) with the parameters derived from the spectroscopic analysis, we are able to measure the total luminosity of the H2 and [Fe II] shocked regions (LH2 and L[Fe II]) in each flow. H2 is the major NIR coolant with an average LH2/L[Fe II] ratio of ∼102. We find that ∼83% of the sources have a LH2/Lbol ratio ∼0.04, irrespective of the Class of the driving source, while a smaller group of sources (mostly Class I) have LH2/Lbol 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. © ESO 2006.