Testing Massive Star Formation Theory in Orion

I compare theoretical models of massive star formation with observations of the Orion Hot Core, which harbors one of the closest massive protostars. Although this region is complicated, many of its features (size, luminosity, accretion disk, H II region, outflow) may be understood by starting with a simple model in which the star forms from a massive gas core that is a coherent entity in approximate pressure balance with its surroundings. The dominant contribution to the pressure is from turbulent motions and magnetic fields. The collapse can be perturbed by interactions with other stars in the forming cluster, which may induce sporadic enhancements of the accretion and outflow rate. 1. Theoretical Models of Massive Star Formation Stars much more massive than the Sun, although rare, are important for the energetics and metal production of galaxies. How do these stars assemble themselves from the interstellar medium (ISM)? Observationally it is clear that massive stars are born in the densest clumps of gas inside giant molecular clouds (GMCs) (e.g. Mueller et al. 2002), which undergo quite efficient (∼ 10 − 50%) transformation to star clusters. The new stellar mass is mostly in low-mass stars, and it appears that the majority of Galactic star formation occurs in this clustered mode (Lada & Lada 2003). This concentration of star formation in a relatively small part of the total Galactic molecular ISM, suggests that the creation of clumps may be triggered by processes external to GMCs. One possibility is an origin in local regions of pressure enhancement created in GMC collisions (Tan 2000). An alternative to triggering is the gradual condensation of clumps in regions of GMCs that become sufficiently self-shielded from the Galactic far UV background (McKee 1989). This question can be addressed by studying the infrared dark clouds (IDCs) (e.g. Egan et al. 1998), which are the likely precursors of star-forming clumps. The collisional model predicts IDCs are surrounded by coherent, supersonic (∼ 10km s) flows. Teyssier et al. (2002) report significant velocity structure towards all their IDCs, and in at least one case the gas is spatially connected across this velocity range. Since the angular momentum vectors of collisions in a thin shearing disk can be both parallel and anti-parallel to that of the host galaxy, the collisional model can account for the almost equal proportions of proand retrograde GMC rotations in M33 (Rosolowsky et al. 2003). The dependence of collision rate on shear leads to reduced star formation efficiency in galaxies with rising rotation curves — a general trend of the Hubble sequence. In this article I focus on the separate question of how clumps, once formed, transform a small part of themselves into massive stars. Clumps can be regarded as quasi-virialized structures: virial mass estimates are similar to estimates of the total gas and stellar mass (Plume et al. 1997); and their mor-