A Submillimetre Study of Massive Star Formation Within the W51 Complex and Infrared Dark Clouds
Parsons, Harriet Alice Louise
Despite its importance the fundamental question of how massive stars form remains unanswered, with improvements to both models and observations having crucial roles to play. To quote Bate et al. (2003) computational models of star formation are limited because “conditions in molecular clouds are not suﬃciently well understood to be able to select a representative sample of cloud cores for the initial conditions”. It is this notion that motivates the study of the environments within Giant Molecular Clouds (GMCs) and Infrared Dark Clouds (IRDCs), known sites of massive star formation, at the clump and core level. By studying large populations of these objects, it is possible to make conclusions based on global properties. With this in mind I study the dense molecular clumps within one of the most massive GMCs in the Galaxy: the W51 GMC. New observations of the W51 GMC in the 12 CO, 13 CO and C18 O (3 – 2) transitions using the HARP instrument on the JCMT are presented. With the help of the clump ﬁnding algorithm CLUMPFIND a total of 1575 dense clumps are identiﬁed of which 1130 are associated with the W51 GMC, yielding a dense mass reservoir of 1.5 × 105 M contained within these clumps. Of these clumps only 1% by number are found to be super-critical, yielding a super-critical clump formation eﬃciency of 0.5%, below current SFE estimates of the region. This indicates star formation within the W51 GMC will diminish over time although evidence from the ﬁrst search for molecular outﬂows presents the W51 GMC in an active light with a lower limit of 14 outﬂows. The distribution of the outﬂows within the region searched found them concentrated towards the W51A region. Having much smaller sizes and masses, obtaining global properties of clumps and cores within IRDCs required studying a large sample of these objects. To do this pre-existing data from the SCUBA Legacy Catalogue was utilised to study IRDCs within a catalogues based on 8 µm data. This data identiﬁes 154 IRDC cores that are detected at 850 µm and 51 cores that were not. This work suggests that cores not detected at 850 µm are low mass, low column density and low temperature cores that are below the sensitivity limit of SCUBA at 850 µm. Utilising observations at 24 µm from the Spitzer space telescope, allows for an investigation of current star formation by looking for warm embedded objects within the cores. This work reveals 69% of the IRDC cores have 24 µm embedded objects. IRDC cores without associated 24 µm emission (“starless” IRDC cores) may have yet to form stars, or may contain low mass YSOs below the detection limit. If it is assumed that cores without 24 µm embedded sources are at an earlier evolutionary stage to cores with embedded objects a statistical lifetime for the quiescent phase of a few 103 – 104 years is derived.