A Detailed Study of Spitzer-IRAC Emission in Herbig-Haro Objects (II): Interaction Between Ejecta and Ambient Gas

TL;DR

This study analyzes Spitzer-IRAC data of Herbig-Haro objects to understand the distribution and nature of shocks and knots, revealing complex interactions and suggesting a 'shot gun' ejection mechanism that challenges existing jet models.

Contribution

It provides new insights into the physical conditions and structures in Herbig-Haro objects, proposing a novel explanation for knotty features involving episodic ejections.

Findings

Identification of warm/dense knots associated with shocks

Evidence of knots distributed across jet axes

Proposal of a 'shot gun' ejection mechanism

Abstract

We present a new analysis of the physical conditions in three Herbig-Haro complexes (HH 54, HH 212, and the L 1157 protostellar jet) using archival data from the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. As described in detail in Paper I, the emission observed using the 4.5-micron filter is enhanced in molecular shocks (T=1000-4000 K) at relatively high temperature or densities compared with that observed with the 8.0-micron filter. Using these data sets, we investigate different distributions of gas between high and low temperatures/densities. Our analysis reveals the presence of a number of warm/dense knots, most of which appear to be associated with working surfaces such as the head of bow shocks and cometary features, and reverse shocks in the ejecta. These are distributed not only along the jet axis, as expected, but also across it. While some knotty or fragmenting structures can be explained by instabilities in shocked flows, others can be more simply explained by the scenario that the mass ejection source acts as a "shot gun", periodically ejecting bullets of material along similar but not identical trajectories. Such an explanation challenges to some degree the present paradigm for jet flows associated with low-mass protostars. It also give clues to reconciling our understanding of the mass ejection mechanism in high and low mass protostars and evolved stars.