deletions | additions       diff --git a/section_Introduction_Massive_stars_strongly__.tex b/section_Introduction_Massive_stars_strongly__.tex  index 8e1a4c4..6a10989 100644  --- a/section_Introduction_Massive_stars_strongly__.tex  +++ b/section_Introduction_Massive_stars_strongly__.tex 
...
\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. \citet{Watson_2008, 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. \citet{Deharveng_2010} also interpreted these as classical HII regions.   \citet{Watson_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 \cite{Elmegreen_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 \cite{1994A&A...289..559L} (Lefloch \& Lazareff, 1994), \cite{1994A&A...289..559L},  for example, described ambient material as an already existent clump, but whose contraction is aided by the external radiation of the hot star. The selection of bubbles with an overabundance of YSOs along the bubble-ISM boundary are a potentially excellent set of sources to study the mechanisms of triggered star-formation. The method of identifying YSOs through photometry, however, is limited. Robitaille \& Whitney (2006) showed that YSO age is degenerate with the observer's inclination angle. Briefly, an early-stage YSO and a late-stage YSO seen edge on, so the accretion or debris disk is observed as thick and blocking the inner regions, can appear similar, even in the IR. Thus, we require other diagnostics of the YSOs along the bubble edge to determine the youngest, and most likely to have been triggered, YSOs.  
...