Christer Watson edited section_Introduction_Massive_stars_strongly__.tex  almost 9 years ago

Commit id: 329fbd9fbe0a6eef024cc972704360bc67ff1fbe

deletions | additions      

       

\section{Introduction}  Massive stars strongly influence their surrounding environment. The two most important mechanisms before their post-main-sequence evolution are through bipolar outflows during formation and ionizing radiation during their main-sequence. This influence was observed in the form of bubble-shaped emission in the 8 $\mu$m band of the Spitzer-GLIMPSE survey of the Galactic Plane \cite{Benjamin_2003}. \citet{Churchwell_2006,Churchwell_2007} observed bubble-shaped 8 $\mu$m emission to be common throughout the Galactic plane. Watson et al. (2008, 2009) \citet{Watson_2008} \citet{Watson_2009}  found 24 $\mu$m and 20 cm emission centered within the 8 $\mu$m emission and interpreted the objects as caused by hot stars ionizing their surroundings, creating 20 cm free-free emission, and at larger distances exciting PAHs, creating 8 $\mu$m emission. Anderson et al. (2011) also interpreted these as classical HII regions. Watson et al. (2010) used 2MASS and GLIMPSE photometery to analyze the YSO population around 46 bubbles and found about a quarter showed an overabundance of YSOs near the boundary between the ionized interior and molecular exterior. These YSOs are interesting because they are candidates for being triggered by the expanding ionization and shock fronts created by the hot star. Star-formation triggered by previous generations of stars is known to occur (citation?) but the specific physical mechanism is still undetermined. The collect-and-collapse model (Elmegreen \& Lada, 1977) describes the ambient material swept up by the shock fronts as eventually becoming gravitationally unstable, resulting in collapse. Other mechanisms, however, have been proposed. Radiatively-driven implosion (Lefloch \& Lazareff, 1994), for example, described ambient material as an already existent clump, but whose contraction is aided by the external radiation of the hot star.