Work Package 1

A major unknown at the onset of star formation is how a molecular cloud with ∼10^5 solar masses of gas forms compact cores of order 1 solar mass out of which individual stars are born. Understanding this is the key to knowing why stars form at all, with what efficiency, and in which hierarchy, including clusters and binaries. Cloud fragmentation must be involved, and both simulations (e.g., Bate, Bonnell, & Bromm 2003; Jappsen et al. 2005) and observations (e.g., Motte, Andre, & Neri 1998) show small-scale, dense structures forming out of turbulent gas in supersonic motion. Two fundamentally different theoretical frameworks have been proposed for how star formation proceeds. It is tempting to infer that the turbulent clumpy structure leads directly to the stellar IMF and, indeed, sub-millimetre continuum mapping shows a very similar distribution of core masses. However, interactions between protostars lead to competition for the surrounding gas which may also shape the IMF (Zinnecker, McCaughrean, & Wilking 1993). Limited resolution in the simulations to date has made it difficult to implement all relevant physical processes exactly, and some basic forces have even been omitted, including magnetic fields, which may be energetically important (Greaves, Holland, & Ward-Thompson 2001). Similarly, observations have also lacked precision, with little data on the internal dynamics of cores or their magnetic support. Revealing the destiny of individual cores (i.e., are they transient features, stable entities, or prone to collapse) is essential if if there is to be any hope of determining whether or not these map directly onto the future stellar population.

constellation will combine multi-wavelength cloud mapping, including wide-field surveys, inter- ferometry, and polarimetry, with dynamical simulations of molecular clouds, to explore three questions:

— How are gravitationally-bound, dense cores formed within molecular clouds?

— How is core collapse initiated and how is the subsequent mass accretion regulated?

— What determines the mass spectrum of the resulting protostars?

Task 1A—From molecular clouds to dense cores

To compare real and simulated clouds, wide-field mapping of molecular cloud cores and their environments will be obtained at radio, millimetre, sub-millimetre, and far-IR wavelengths, concentrating on their kinematics, magnetic fields, and internal structure. Complementary optical and near-IR surveys will show where young stars are located in relation to parental cloud material. Simultaneously, existing large-scale numerical calculations will be comprehensively extended to make detailed predictions of the structure and kinematics, with different prescriptions for large-scale motions, turbulence, and magnetic fields. The models will pursue resolving structures down to the scale of individual stellar systems, while correctly handling the dynamics up to cloud-wide scales.

Task 1B—Gravitational collapse; from cores to stars

The next step is to assess the forces acting on these cores and how the collapse proceeds towards higher densities. The core mass function will be constructed for a variety of molecular clouds and sub-divided into different classes of object, i.e., pre-stellar, protostellar, and circumstellar material (Andre, Ward-Thompson, & Barsony 2000). The numbers in each class yields timescales for each evolutionary phase for direct comparison with the theoretical calculations. Moving down to the scale of individual cores, the morphology and dynamics will be determined from high-resolution interferometric millimetre imaging and polarimetry, for direct comparison with predictions. For example, in ambipolar diffusion models, magnetic support gradually leaks out of cores making them long-lived and with a well-defined magnetic axis (e.g., Galli, Walmsley, & Goncalves 2002), while cores undergoing competitive accretion should be amorphous but with significant rotation.

Task 1C—The formation of stars in cores

Once gravitational collapse is initiated in a core, star formation is almost inevitable. However, the resulting stellar properties depend on further dynamical evolution: splitting and mergers of cores, ejection in multiple stellar systems (Bate, Bonnell, & Bromm 2002), and the mass-control processes of accretion and outflow (Greaves, Holland, & Pound 2003). These phenomena will be studied through high spatial resolution near- and mid-IR imaging (NACO/VLT, VISIR/VLT, MICHELLE/Gemini, Spitzer) and small-scale clustering properties compared to those in simulations. The multiplicity and separations of objects will probe the relevance of binary splitting and competitive accretion as protostars accumulate their final masses. In addition, models of ejections from stellar groups predict that this terminates accretion and sets the final masses of expelled stars: proper motion measurements can be used to trace the characteristic imprint that such ejections should leave on the resulting protostellar velocity distribution.