ISSN:
1089-7674
Source:
AIP Digital Archive
Topics:
Physics
Notes:
The early evolution of hydrogen+ (H II) regions is controlled by the properties of the star-forming cloud cores. The observed density distributions in some young H II regions indicate that the power-law stratifications can be steeper than r−2. Ionization fronts can overrun these gradients and the ionized outflows are strongly accelerated along these steep density distributions. Thus, photoionized regions can either reach pressure equilibrium inside the inner parts of the high-pressure cores [with sizes and densities similar to those observed in ultra compact (UC) H II regions], or create bright H II regions with extended emission. The density inhomogeneities engulfed within the ionization fronts create corrugations in the front, which in turn drive instabilities in the ionization-shock (I-S) front. These instabilities grow on short time scales and lead to the fragmentation of the dense shells generated by the shock fronts. Thus, new clumps are continuously created from the fragmented shell, and the resulting finger-like structures can explain the existence of elephant trunks and cometary-like globules in most H II regions. In the case of planetary nebulae (PNe), wind asymmetries and magnetic fields from rotating stars, along with precession of the rotation axis, can create the wide range of observed PNe morphologies and collimated outflows (jets). Magnetic collimation and jet formation in PNe become very efficient after the flow has passed through the reverse shock of the PN. © 2001 American Institute of Physics.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.1351825
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