Physics – Plasma Physics
Scientific paper
Oct 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000aps..dppji2001f&link_type=abstract
American Physical Society, 42nd Annual Meeting of the APS Division of Plasma Physics combined with the 10th International Congre
Physics
Plasma Physics
Scientific paper
Photoionized Gaseous Nebulae are generated by hot stars located at the two ends of the stellar mass spectra (and near the extremes of their evolutionary tracks): HII regions are excited by recently formed massive stars, and Planetary Nebulae (PNe) are generated by low mass stars evolving toward the white-dwarf phase. These two types of nebulae display a variety of fascinating shapes, with forms delineated by dusty filaments and bright rims, and with a complex network of velocity fields, ionization fronts and shock waves. At large scales, integrated over time and space, these objects can have a strong impact in the evolution of gaseous galaxies. For instance, massive stars inject large amounts of radiative energy which photoionizes and disrupts the parental interstellar clouds, setting the efficiency of star formation at galactic scales. In contrast, evolved low mass stars do not have such a disrupting effect but they provide a generous gas mass return rate. Thus, PNe ejecta can maintain the gaseous component at the late stages of galaxy evolution, and are responsible for the enrichment of several heavy elements. HII regions form a class of relatively well studied objects, in which the hot photoionized plasma is outflowing from the parental clouds. After Stromgren and Kahn set the physical basis to their modeling in the mid-fifties, the details of the expansion have been studied with a variety of different analytical and numerical tools, and under different cloud conditions. In the case of PNe, there are a variety of different morphological classes. They have been cataloged as bipolar, elliptical, point-symmetric, irregular, spherical, and quadrupolar. Recently, a series of collimated, jet-like outflows have been also found. These nebulae are formed by the mass ejected at the final stages of low-mass star evolution, and the structuring is apparently created by at least two interacting winds. The details of PNe formation and evolution is now explored with this interacting stellar wind scenario, and photoionization and magnetized rotating winds can shape the expanding shocked shells. Stellar wind asymmetries and magnetic fields from rotating stars, along with the ionizing flux from the star, can create the wide range of observed PNe morphologies and magnetically collimated outflows (jets). This magnetic collimation is also operative in young stellar objects and can be responsible for the generation of protostellar jets. Perhaps the most important issue here is that magnetic collimation, and jet formation, becomes very efficient after the flow has been passed through a shock.
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