2002ApJ...572..227S

We present an analysis of velocity-resolved near-IR molecular hydrogen observations of a variety of protostellar outflows with very different energetics, degrees of collimation, and morphologies. Observations in the 2.12 µm line of H₂ were obtained using an IR Fabry-Pérot interferometer with a spectral resolution of 23 km.s^–1. The integrated flux-velocity diagrams for each outflow show a flat spectrum for low velocities followed by a decreasing power law dF/dv∝v^γ, with γ between -1.8 and -2.6, for velocities higher than a clearly defined break velocity at 2-17 km.s^–1. Contrary to shock model predictions, it is shown that the H₂ intensity is constant with velocity. We argue that the flux-velocity relation can then be interpreted as a mass-velocity relation, in striking similarity to the power-law mass spectra observed in CO outflows. By comparing H₂ and CO mass-velocity spectra, it is shown that there is a velocity regime in which both molecules coexist and produce similar γ-values. Evolution effects in outflows appear as a correlation between outflow length and γ; as outflows age, the spectra becomes steeper. Our results support a common physical origin for both CO and H₂ emission and a strong association between the molecular outflows traced in each molecule.