Triggering the Formation of Massive Clusters

There are at least 2 distinct mechanisms for the formation of young massive clusters (YMC), all of which require galactic-scale processes. One operates in harrassed fragile galaxies, in the dense cores of low mass galaxies, at the ends of spiral arms, or in galactic tidal shocks where transient and peculiar high pressures make massive clouds at high densities. The result of this process is usually only one or two YMC without the usual morphologies of local star formation, i.e., without hierarchical structure and a continuous power law distribution of cluster masses up to the largest mass. The other operates in the more usual way: continuously for long periods of time in large parts of the interstellar medium where the ambient pressure is already high as a result of the deep potential well from background stars and other gas. This second process makes clusters in a hierarchical fashion with size-of-sample effects, and tends to occur in nuclear rings, merger remnants, and even the ambient ISM of normal galaxies if the star formation rate is high enough to sample out to the YMC range. In Formation and Evolution of Massive Young Star Clusters, Cancun, Mexico, November 17-21, 2003, eds. Henny Lamers, Linda Smith and Antonella Nota, Astronomical Society of the Pacific (PASP Conference series), in press 1. Triggering Low Mass Clusters Local clusters often show the signatures of high-pressure triggering: they form in the heads of pressure-swept, cometary clouds that are adjacent to older massive stars, or they form in compressed layers or shells between expanding HII regions or wind bubbles and the surrounding gas. Examples of the former include Orion (Bally et al. 1987; Lada, et al. 1991; Reipurth, Rodriguez & Chini 1999), the Eagle nebula (Hester et al. 1996), the rho Oph core (de Geus 1992), and many places in the clouds surrounding 30 Dor in the LMC (Walborn et al. 2002). Examples of the latter include the Carina nebula (Brooks, et al. 1998, 2001), the Rosette nebula (Phelps & Lada 1997), and other regions near 30 Dor. A list of likely triggered regions is in Elmegreen (1998). Sometimes clusters form at the tips of elongated clouds with no obvious pressure source nearby. IC 5146 looks like this (Lada, Alves & Lada 1999): it is a long, filamentary cloud with most of the star formation near the eastern tip. Another example is in the Taurus region where most of the famous filaments have their star formation toward the east (Elmegreen 2002), often with short-lived molecules at these places, suggesting recent compression (Hartquist et al. 2001). In these places, the star formation is occurring at the most vulnerable places in the cloud where stray pressure bursts would have the greatest cross section