The physics of star formation

Abstract

Our current understanding of the physical processes of star formation is reviewed, with emphasis on processes occurring in molecular clouds like those observed nearby. The dense cores of these clouds are predicted to undergo gravitational collapse characterized by the runaway growth of a central density peak that evolves toward a singularity. As long as collapse can occur, rotation and magnetic fields do not change this qualitative behavior. The result is that a very small embryonic star or protostar forms and grows by accretion at a rate that is initially high but declines with time as the surrounding envelope is depleted. Rotation causes some of the remaining matter to form a disk around the protostar, but accretion from protostellar disks is not well understood and may be variable. Most, and possibly all, stars form in binary or multiple systems in which gravitational interactions can play a role in redistributing angular momentum and driving episodes of disk accretion. Variable accretion may account for some peculiarities of young stars such as flareups and jet production, and protostellar interactions in forming systems of stars will also have important implications for planet formation. The most massive stars form in the densest environments by processes that are not yet well understood but may include violent interactions and mergers. The formation of the most massive stars may have similarities to the formation and growth of massive black holes in very dense environments. ‡ In his first letter to Bentley, as quoted by Jeans (1929), Newton said "It seems to me, that if the matter of our sun and planets, and all the matter of the universe, were evenly scattered throughout all the heavens, and every particle had an innate gravity towards all the rest, and the whole space throughout which this matter was scattered, was finite, the matter on the outside of this space would by its gravity tend towards all the matter on the inside, and by consequence fall down into the middle of the whole space, and there compose one great spherical mass. But if the matter were evenly disposed throughout an infinite space, it could never convene into one mass; but some of it would convene into one mass and some into another, so as to make an infinite number of great masses, scattered great distances from one to another throughout all that infinite space. And thus might the sun and fixed stars be formed, supposing the matter were of a lucid nature."

The JCMT Gould Belt Survey: properties of star-forming filaments in Orion A North

“…The observed respective Class durations are 10 4 -10 5 and 10 6 yr (Larson 2003) allowing for much greater movement of the more evolved protostars from their parent filament. Previous radial velocity studies of protostars suggest a relative gas to source velocity of ∼1 km s −1 (Covey et al 2006).…”

CN Zeeman observations of the NGC 2264-C protocluster

“…the review by Larson 2003). However, the conditions under which these dense cores form, collapse, and fragment remain a matter of debate.…”

The JCMT Spectral Legacy Survey: physical structure of the molecular envelope of the high-mass protostar AFGL2591

“…Molecular signatures from star-forming regions are an important probe in studies of star formation. While the energy budget is reasonably well understood for protostars with masses 8 M (e.g., Larson 2003), the problem is more complex for high-mass stars ( 8 M ). Not only do they require more material and involve (orders of magnitude) more energy, high-mass stars also form in more deeply embedded natal clouds and, generally, in more clustered environments.…”