Star Formation in OB Associations and Clusters

128 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2007.Star formation can be characterized at two levels: at microscopic level by the initial mass function (IMF), and at macroscopic level by space, time, and intensity. To understand how star formation proceeds across a region, all four parameters need to be examined. We used the resolved massive stars and young stellar objects (YSOs) in N 44 to study star formation at both levels with extended coverage in space and time. Our study of massive stars showed that the present-day mass functions (PDMFs) of four star formation regions do not have obvious variations and are shallower than those of field regions. The PDMFs of field stars indicates that star formation varies from location to location; therefore, assumptions made for converting PDMFs to IMFs have to be critically examined. Using Spitzer mid-IR observations and complementary multi-wavelength data, we confirmed 59 YSOs in N 44. Most of the YSOs are found in molecular clouds and concentrated toward three molecular peaks. The central molecular peak has the highest concentration of YSOs and molecular material. The peak is centered on the superbubble rim and the YSOs are distributed along it, suggesting that the current star formation is most likely triggered by the expansion of the superbubble. We used the resolved cluster content of luminous giant H II regions (GHRs) in M101 to study star formation in a starburst environment at macroscopic level. Cluster types are different in the three GHRs with distinct morphologies. NGC 5461 is dominated by a very luminous core, and has been suggested to host a super-star cluster (SSC). Our observations show that it contains three R136-class clusters on a bright stellar background in a small region. This tight group of clusters may dynamically evolve into an SSC in the future, and may appear unresolved and be identified as an SSC at large distances. In contrast, NGC 5462 consists of loosely distributed H II regions and clusters without a prominent core. NGC 5462 has the steepest cluster luminosity function and the most loosely distributed interstellar gas, qualitatively consistent with the hypothesis that massive clusters are formed in high-pressure interstellar environments.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD