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

¹ INAF - Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monte Porzio, Italy
² Università degli Studi di Roma “Tor Vergata” - Dipartimento di Fisica, via della Ricerca Scientifica 1, 00133 Roma, Italy e-mail: [caratti;giannini;nisini;dloren]@mporzio.astro.it

Received: 6 October 2005
Accepted: 11 November 2005

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 [] 1.64 μm and spectroscopic observations in the range 1-2.5 μm. From [] 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. A_v 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 [] 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 [], 1.64 μm) with the parameters derived from the spectroscopic analysis, we are able to measure the total luminosity of the H₂ and [] shocked regions ( and ) in each flow. H₂ is the major NIR coolant with an average / ratio of ~10². We find that ~83% of the sources have a /L_bol ratio ~0.04, irrespective of the Class of the driving source, while a smaller group of sources (mostly Class I) have /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 H₂ power in the jet can be a powerful tool to identify the most active sources among the objects of this class.