| Résumé | The correlation between giant molecular clouds (GMCs) and high-mass star formation (HMSF) is now well established (McKee and Ostriker 2007; Tan et al. 2014; Dobbs et al. 2014 and references therein). From the nearest cluster of high-mass stars and GMC, the Orion Nebula Cluster (390 pc; Sandstrom et al. 2007) to molecular clouds in external galaxies (e.g. Wilson et al. 2000; Sakamoto et al. 2007), the presence of OB stars and young clusters is clearly associated with molecular gas (Lada and Lada 2003). Unlike low-mass star formation, which can occur in relatively quiescent and even isolated clouds, the nature of HMSF requires a clustered, dense environment. This presents particular problems in attempting to understand HMSF as the nascent stars remain deeply embedded in their natal clouds for a few million years, and generally HMSF occurs at large distances from the Sun. The relatively short lifetime of high-mass stars (typically a few 106 years or less) means that they do not have the time to migrate significantly from their birthplace, so HMSF regions are often found adjacent to HII regions and photo-dissociation regions (PDRs), which make observations of the forming stars challenging at short wavelengths. At longer wavelengths (far-IR to millimeter), the optical nebula is not visible and the dust and gas associated with star formation can be studied directly. Furthermore, the formation of high-mass stars can have devastating effects on the surrounding molecular gas, such that the cloud may not survive to host subsequent epochs of star formation. For example, it has been established that clusters older than 5 Myr are not typically associated with molecular clouds, implying that molecular clouds do not survive the onset of star formation by more than 5–10 Myr (Leisawitz et al. 1989). Recent estimates of molecular cloud lifetimes in the LMC and M51 are ~25 Myr (Kawamura et al. 2009) and ~100 Myr (Koda et al. 2009), consistent with the earliest estimates of 10 8 yr (Scoville et al. 1979) based on the detection of GMCs in the inter-arm regions of the Milky Way. |
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