Distance from VLA1 (arcsec)
Figure 7.20. Jet diameter versus distance, illustrating that the opening half-angle is <0.5° Adapted from Reipurth & Bally (2001).
thought to be shocks - both internal shocks and collisions with the ISM. The optical spectra of HH objects are thus, not surprisingly, dominated by the forbidden transitions that typically accompany collisional-shock excitation (Figure 7.21). These are typically much brighter than the hydrogen lines which are more dominant in radiatively excited environments. In the IR, it is common to see strong H2 emission lines, indicating that significant molecular material is also present in the outflow region. The IR spectra also reveal strong [Fe ii] emission, again indicating grain disruption typically associated with shock activity.
Massive protostars are another related set of objects, which are again an important phase for the formation of many stars. In particular, there is some belief that massivestar formation may follow a different process than that which applies for low-mass stars. The details of the accretion process of course have critical implications for the final stellar mass. The mass distribution of high-mass stars in turn has a dramatic impact on galaxy evolution (chemical enrichment, ISM kinetic energetics, etc.) and the production of compact objects (neutron stars and black holes).
Despite the importance of this subject, relatively little work has been done to date. This is in part due to the relative rarity of high-mass stars compared with their numerous low-mass counterparts, and also due to the faster evolutionary track they take - this phase is both numerically rarer and temporally shorter than low-mass star-formation. Furthermore, work in the IR is essential to penetrate the high extinction typically associated with these objects.
Fe II Fe II
Fe II Fe II
HH47A Bow Shock
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