Molecularcloud formation

The first stage of the star-formation process is the formation of molecular clouds as a necessary physical condition for star formation. We know that molecular clouds are where stars are currently forming in our Galaxy but their formation mechanism and their lifetimes are still being debated (Blitz & Williams 1999). Molecular-cloud lifetimes have been debated for decades, but what is clear is that, since there are few clouds in the nearby Galaxy that are not undergoing star formation, the timescale for an individual cloud to commence forming stars is short, comparable to its dynamical timescale. Mechanisms have been suggested to form molecular clouds from gravitational instabilities, thermal instabilities and the shock-driven molecular-cloud formation as detailed below. What is clear is that dense physical

Figure 39.1. The formation of molecular clouds as the gas passes through a spiral shock (Dobbs et al. 2006). Note the spurs and feathering that appear as the dense clumps are sheared away upon leaving the spiral arm.

conditions are necessary in order to get a sufficient collision rate of H atoms onto dust grains.

One way of getting the necessary physical conditions for H2 formation is through the dense shocks which occur either in turbulent clouds (Glover & Mac Low 2006) or when gas passes through a spiral arm and shocks (Dobbs et al. 2006); see Figure 39.1. In either case, high-Mach-number shocks induce the formation of high-density regions, which can then form molecular gas rapidly. The gas need be cold, possibly due to cooling in the shock itself, in order to attain the high densities required.

Spiral shocks can also explain the large-scale distribution of molecular clouds in spiral arms. Structures in the spiral arms arise due to the shocks that tend to gather material together on converging orbits. Thus, structures grow in time through multiple spiral-arm passages. These structures present in the spiral arms are also found to form the spurs and feathering in the inter-arm region as they are sheared by the divergent orbits when leaving the spiral arms (Dobbs & Bonnell 2006a). In this model, molecular clouds are limited to spiral arms since it is only there that the gas is sufficiently dense to form molecules. These clouds need not be self-gravitating because their formation is independent of self-gravity.

Figure 39.2. The evolution of cold interstellar gas through a spiral arm is shown relative to the spiral potential of the galaxy (upper left). The minimum of the spiral potential is shown as black and the overall galactic potential is not shown for clarity. The three other panels, arranged clockwise, show close-ups of the gas as it is compressed in the shock and sub-regions become self-gravitating. Gravitational collapse leading to star formation occurs within 2 x 106 yr of the gas reaching molecular-cloud densities. The cloud produces stars inefficiently because the gas is not globally bound.

Figure 39.2. The evolution of cold interstellar gas through a spiral arm is shown relative to the spiral potential of the galaxy (upper left). The minimum of the spiral potential is shown as black and the overall galactic potential is not shown for clarity. The three other panels, arranged clockwise, show close-ups of the gas as it is compressed in the shock and sub-regions become self-gravitating. Gravitational collapse leading to star formation occurs within 2 x 106 yr of the gas reaching molecular-cloud densities. The cloud produces stars inefficiently because the gas is not globally bound.

Spiral shocks can also trigger the occurrence of star formation since the shock can produce locally bound regions that undergo collapse to form stars (Roberts 1969; Pringle et al. 2001; Bonnell et al. 2006). The evolution, over 34 million years, of 106Mq of gas passing through the spiral potential is shown in Figure 39.2, from Bonnell et al. (2006). The initially clumpy, low-density gas (p ^ 0.01Mq pc-3) is compressed by the spiral shock as it leaves the minimum of the potential. The shockforms some very-dense (>103Mq pc-3) regions, which become gravitatinally bound and thus collapse to form regions of star formation, with masses of typical stellar clusters, namely (102-104)Mq. Star formation occurs within 2 x 106 yr of the attainment of molecular-cloud densities. The total spiral-arm passage lasts for ^ 2 x 107yr. The gas remains globally unbound throughout the simulation and re-expands in the post-shock region. Star formation in unbound clouds produces low star-formation efficiencies of the order of 10% or less even in the absence of any form of stellar feedback (Clark & Bonnell 2004; Clark et al. 2005). In addition, the passage of clumpy gas through a spiral (or other) shock induces a supersonic velocity dispersion that follows the observed t>disp a R-05 relation found in molecular clouds (Bonnell et al. 2006; Dobbs & Bonnell 2006b). Thus, a dynamical onset of star formation is able to produce the observable structure and kinematics in molecular clouds.

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