2009MNRAS.393..354P

Bipolar outflows appear to represent the ubiquitous products of post- and pre-main-sequence stellar evolution. However, although the outflows around young stellar objects (YSOs) have been observed over a broad range of wavelengths, from the ultraviolet through to the radio, there has been very little mapping of these sources at mid-infrared (MIR) wavelengths. The recent advent of the Spitzer Space Telescope (SST) now permits us to correct this deficiency, and we present maps, profiles and images of seven bipolar sources, six of which appear to be new to the literature. It is found that although the bipolars have differing morphologies, reflecting the differing phases of their evolution, they nevertheless have many properties in common. These include the evidence for central tori of neutral material, in some cases associated with appreciable levels of extinction. We also note evidence for exterior photodissociation regimes (PDRs) about the ionized flows, responsible for strong polycyclic aromatic hydrocarbon (PAH) band emission in the 5.8 and 8 µm bands. Such PDRs are also likely to be responsible for appreciable variations in flux ratios with position within the sources. Finally, it is noted that a large fraction of the sources are associated with stars in the earliest phases of their evolution - Class 0 and I stars in which infalling material is important, and dominates their spectral energy distributions in the MIR.

One of the sources (G035.2-00.7) has previously been observed at visual, near-infrared (NIR) and millimetric wavelengths, whence it has been proposed that the spatio-kinematic properties of the source are best explained in terms of multiple nuclear outflows, and/or precession of an ionized jet. Our present results suggest that the characteristics of this source are much simpler than has previously been inferred, and suggest that the outflow can be described in terms of a very low-collimation bipolar outflow.

Finally, we have also compared the present MIR results with NIR mapping from the Two Micron All Sky Survey (2MASS). These show that whilst the structures of the sources are comparable within these differing wavelength regimes, there are differences attributable to grain extinction and shock or fluorescently excited H₂.