To appear in the XVth Special Courses of the National Observatory of Rio de Janeiro.
astro-ph/1101.5172

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STAR FORMATION IN MOLECULAR CLOUDS

Mark R. Krumholz


Department of Astronomy & Astrophysics, University of California, Santa Cruz


Abstract: Star formation is one of the least understood processes in cosmic evolution. It is difficult to formulate a general theory for star formation in part because of the wide range of physical processes involved. The interstellar gas out of which stars form is a supersonically turbulent plasma governed by magnetohydrodynamics. This is hard enough by itself, since we do not understand even subsonic hydrodynamic turbulence very well, let alone supersonic non-ideal MHD turbulence. However, the behavior of star-forming clouds in the ISM is also obviously influenced by gravity, which adds complexity, and by both continuum and line radiative processes. Finally, the behavior of star-forming clouds is influenced by a wide variety of chemical processes, including formation and destruction of molecules and dust grains (which changes the thermodynamic behavior of the gas) and changes in ionization state (which alter how strongly the gas couples to magnetic fields). As a result of these complexities, there is nothing like a generally agreed-upon theory of star formation, as there is for stellar structure. Instead, we are forced to take a much more phenomenological approach. These notes provide an introduction to our current thinking about how star formation works.


Table of Contents

FORWARD

OBSERVING STAR-FORMING CLOUDS
Observational Techniques
The Problem of H2
Observing the Dust
Molecular lines
Two-Level Atoms
Molecular Cloud Properties from Molecular Lines
Density Inference
Velocity and temperature inference
Mass inference

Physical processes in molecular clouds
Heating and cooling proceses
Heating by cosmic rays
CO cooling
Flows in Molecular Clouds
Equations of Motion
Dimensionless Numbers
The Virial Theorem

MOLECULAR CLOUD COLLAPSE
Stability Conditions
Thermal Pressure: the Bonnor-Ebert Mass
Magnetic Support: the Magnetic Critical Mass
Collapsing Cores
Spherical collapse
Rotation Collapse and the Angular Momentum Problem
Magnetized Collapse and the Magnetic Flux Problem

TWO PROBLEMS: THE STAR FORMATION RATE AND THE INITIAL MASS FUNCTION
The Star Formation Rate
The Observational Problem: Slow Star Formation
Potential Solutions
The Initial Mass Function
The Observed IMF
The IMF in the Gas Phase?
A Possible Model: Turbulent Fragmentation and Radiation-Suppressed Fragmentation

REFERENCES

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