Protostellar collapse

Stars form in dense cores within giant molecular clouds (see Fig. 1.4, Alves et al. 2001). About 1 % of their mass is in dust grains, produced in the final phases of stellar evolution. Molecular clouds are complex entities with extreme density variations, whose nature and scales are defined by turbulence. These transient environments provide dynamic reservoirs that thoroughly mix dust grains of diverse origins and composition before the violent star-formation process passes them on to young stars and planets. Remnants of this primitive dust from the Solar System formation exist as presolar grains in primitive chondritic meteorites and IDPs. 

Infrared absorption spectroscopy of interstellar clouds shows that the interstellar dust population varies with the line of sight, yet it maintains a similar character. In particular, submicron-sized amorphous silicate grains are the dominant component in every direction. The absence of crystalline grains is likely the result of rapid amorphization by the interstellar radiation field. 

The extent to which the dust from the ISM survives planet formation intimately depends on the details of the core collapse and the formation of the accretion disk. 

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The key uncertainty in relating the astronomical observations to the Solar System constraints is at the zero point. However, the different constraints seem to line up well if CAIs formed less than 1 Myr after the protostellar collapse. If so, chondrules would have formed within 3 Myr, consistent with the presence of fine dust in many astronomical analogs. The presence of millimeter- and centimeter-sized objects at a few million years after CAI formation is also broadly consistent with the astronomical constraints, as is the timing for planetesimal collisions indicated by the freed planetary debris. This phase is likely to have started by 3-5 Myr after CAI formation and would have lasted for tens to hundreds of million years, until the final planetary architecture was reached. 

Boss A. P. and Hartmann L. W. (2001) Protostellar collapse in a rotating, self-gravitating sheet. Astrophys. J. 562, 842-851. 

Commerfon B, Hennebelle P, Audit E, Chabrier G, Teyssier R (2010) Protostellar collapse radiative and magnetic feedbacks on small-scale fragmentation. Astron Astrophys 510 L3... 

The continued addition of matter increases the density and temperature of the core until H2 begins to dissociate. The dissociation consumes heat, which holds temperature approximately constant, i.e. the heat capacity becomes very high and y - 1. The stability condition y >4/3 becomes violated and a new collapse of the core ensues. The core collapses until all H2 is dissociated and the H finally becomes ionized. The temperature then increases again with further contraction and the second core is formed that approaches stellar density. The second collapse phase is short and lasts for a solar-type star of the order of 103 years. By this event a protostellar embryo is born, which continues to grow in mass by collecting the remaining material from its environment. 

In the case of nonmagnetic collapse of a spherical cloud (Yorke and Bodenheimer, 1999), the protostar that forms is orbited by a protostellar disk with a similar mass. When angular momentum is transported outward by assumed gravitational torques, and therefore mass is transported inward onto the protostar, the amount of mass remaining in the disk is still so large that most of this matter must eventually be accreted by the protostar through other processes. Hence, the disk at this phase must still be considered a protostellar disk, not a relatively late phase, protoplanetary disk where any objects which form have some hope of... 

Rapid mass and angular momentum transport. Models of the growth of nonaxisymmetry during the collapse and formation of protostellar disks show that large-scale bars and spirals can form with the potential to transfer most of the disk angular momentum outward on timescales as short as 1,000 yr to 0.1 Myr (Boss, 1989), sufficiently fast to allow protostellar disks to transport the most of their mass inward onto the protostar and thereby evolve into protoplanetary disks. 

Weidenschilling S. J. and Ruzmaikina T. V. (1994) Coagulation of grains in static and collapsing protostellar clouds. Astrophys. J. 430, 713-726. 

Ceccarelli C, Hollenbach DJ, Tielens AGGM (1996) Far-infrared line emission from collapsing protostellar envelopes. Astrophys J 471 400-426. doi 10.1086/177978... 

Furthermore, HNC (alongside HCN) is a commonly used tracer of dense gas in molecular clouds, as referenced in this paper. Aside from the potential to use HNC to investigate gravitational collapse as the means of star formation, HNC abundance (relative to the abundance of other nitrogenous molecules) can be used to determine the evolutionary stage of protostellar cores. This is demonstrated in the aforementioned paper by Tennekes et al. In the same paper, the authors also elaborate on the HNC/HCN abundance ratio as a means of determining the temperature of the environment. 

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