Star and Planet Formation. I. The Big Questions Based on P. Bodenheimer, “Principles of Star Formation” (2011)
1. What is the efficiency of star formation? Stars form in GMCs (giant molecular clouds) of mass >106 M Molecular clouds are not in thermal equilibrium, and should collapse on a freefall timescale (tff = √[3p/32Gr0]) Observed SFR ~ 1 Myr-1, not 500 (M/tff) What prevents/slows cloud collapse? Why is efficiency ~2%?
2. How/Why do clouds fragment? Stars typical form in clusters/associations of dozens to >104 stars. Most clusters/associations are gravitationally unbound, meaning <50% of gas is converted to stars. How do clusters form? What is the role of turbulence? How does the gas disperse?
3. Is Star Formation Triggered? What makes a GMC become unstable to collapse? Why are there chains of SFRs apparently lines up in chronological order (e.g. the Cen/Crux, Lupus, and Upper Sco associations)?
4. Do high- and low- mass stars form in the same way? High mass cores may collapse directly; low mass cores cannot in the presence of magnetic fields. Feedback from high mass stars may influence low mass cores
5. Why is there a universal Initial Mass Function (IMF)? dN/d log m = C m-a a = 1.35 (M > 0.5M) (Salpeter slope) a = 0.3 (0.5M>M > 0.08M) a = -0.7 (M < 0.08M) More complex formulations exist Why is the IMF continuous? Are there upper and lower limits to M?
6. Why do most stars seem to form in pairs or small groups? The multiplicity fraction is about 100% for O stars 50% for solar-like stars 25% for M dwarfs Some brown dwarfs are binary This must say something fundamental about the star formation process.
7. Can we see protostars? Opacities are very high Infall and rotation have similar observational signatures
8. What sets the size of circumstellar (protoplanetary) disks? Solar system disk >100 AU in radius Observed disks around young stars up to 1000AU What are the lifetimes of the dust and gas disks? Roles for conservation of angular momentum viscosity Planets (gaps in disk)
9. What drives bi-polar outflows from YSOs? Herbig-Haro (HH) objects T Tauri star winds Disk wind X-wind Disk channeling Role of the magnetic field
10. How do planets form in a circumstellar disk? Terrestrial planets Planetesimal formation Accumulation of planetesimals Gas giants Disk instabilities Disk migration
11. The role of Rotation and Magnetic Fields Rotation/magnetic fields add complexity to the models We observe rotation/magnetic fields in cloud cores and YSOs What is the role of ambi-polar diffusion in star formation? What determines the initial stellar rotation? How does rotation affect the evolution of low mass stars