1. Star and Planet Formation. I. The Big Questions
Based on P. Bodenheimer, “Principles of Star Formation” (2011)

2. 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 Myr-1, not 500 (M/tff)
What prevents/slows cloud collapse?
Why is efficiency ~2%?

3. 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?

4. 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)?

5. 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

6. 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?

7. 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.

8. 7. Can we see protostars? Opacities are very high
Infall and rotation have similar observational signatures

9. 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)

10. 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

11. 10. How do planets form in a circumstellar disk?
Terrestrial planets
Planetesimal formation
Accumulation of planetesimals
Gas giants
Disk instabilities
Disk migration

12. 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