Big+Questions+in+Star+Formation

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1187609477 //**To avoid clobbering this page with additional discussion I have started a page with Assorted SF Answers**//

0. What is the nature of supersonic MHD turbulence in weakly ionized plasmas?
> -Why does Lighthill's (1955) hypothesis, which implies media type="custom" key="164909" Kolmogorov scaling, work so well? > -How do the density distributions depend on the macroscopic turbulence properties?

1. What maintains the turbulence in molecular clouds?
> -Is this turbulence almost universal? > -What leads to deviations from universal behavior, such as in regions of high-mass star formation? > -How does the level of turbulence determine the structure of molecular clouds? > -Are internal sources of energy shared throughout a cloud?

2. What is the role of magnetic fields in star formation?
> -How do magnetic fields affect the rate of star formation? > -What is the role of magnetic fields in mass and angular momentum transfer in circumstellar disks, both due to the MRI and protostellar winds? > -Why do stars have such small magnetic fluxes?

3. What sets the rate of star formation?
> -Is the rate different in galactic bulges and in elliptical galaxies from that in disks? (This would imply an evolution with redshift.) > //**comment by Elmegreen (still in NY):** I think the evidence indicates that all SF is in disks or pieces of disks: that bulges and ellipticals are mixed up disks, globular clusters form in disks, etc., so we only have to consider SF in the disk environment (ApJ 658 763)// > -how much does the mean SFE vary with cloud mass, and why is the SFE dispersion so large? > //**comment by Elmegreen:** the total SFE pretty consistently increases with local density and that is simply a result of hierarchical structure. Since mass varies inversely with density by Larson-laws, the SFE decreases with increasing cloud mass.// > -how long do GMCs live in different environments? > //**comment by Elmegreen:** The cores where stars form seem to evolve from start to finish in only a few instantaneous crossing times (at low density this crossing time is long in absolute terms and at high density it is short). GMC envelopes can evolve much more slowly than cores even relative to the crossing time because the envelopes seem to be magnetically subcritical. However, most envelopes are forced to evolve more quickly than this as a result of compression and disruption from SF in the cores (astroph/07072252)//

4. What determines the IMF?
> -What determines the characteristic stellar mass? > //**comment by Elmegreen:**// I have not seen any theory for this which get the characteristic stellar mass as constant as it seems to be: considering the large variations in both gas and dust temperatures in different environments. The characteristic mass cannot just be the result of a characteristic density for certain physical processes. > -Does the IMF of stars today vary with physical conditions? > -In particular, brown dwarfs may be exponentially sensitive to initial conditions (e.g., the theory of Padoan and Nordlund) > //**comment by Elmegreen:** the physical condition that makes the most sense is density: high density environments (e.g., dense cluster cores) somewhat correlate with more massive stars (flatter IMFs).//

5. How do massive stars form?
> -How do they overcome radiation pressure? > -What are the initial conditions that lead to massive star formation? > -Why are massive stars always formed as part of clusters? > //**comment by Elmegreen:** there seems to be good evidence that a few percent of O stars do not form in clusters, and this fraction is what you'd get from the cluster mass function if you go down to a 100 Msun cluster; i.e., the O star as a 100 Msun cluster (de Wit et al. 2005; Parker & Goodwin 2007)// > -What is the nature of disks and outflows in massive protostars?

6. How do star clusters form?
> -What determines the initial cluster mass function? > //**comment by Elmegreen:** only the hierarchical structure of clouds is needed to explain the ICMF (hierarchical structure comes from turbulence + self-gravity).// > -Is the distribution of stellar masses within a cluster established prior to cluster formation, or after the cluster has begun forming stars? > //**comment by Elmegreen: t**he IMF appears to be independent of the cluster environment: e.g., it is the same in Scaled OB associations (unbound) as in super starclusters having the same total mass -- a point made by Deidre Hunter long ago.//

7. How do cores collapse and disks accrete at various stages?
> -what is the resolution of the luminosity problem in low mass star formation?(are early infall rates higher than current predictions?) > -are purely hydrodynamic (non-self-gravitating) processes important to disk accretion? > -how does planet formation affect the evolution of disks and the accretion process?

OTHER QUESTIONS
> ===A. How do stars form in the Galactic Center?=== > ===B. How do binaries form?===