Chapter 2: Protostellar collapse and star formation

1. Chapter 2: Protostellar collapse and star formation

2. One of 3 branches of proton-proton chain

3. CNO cycle:C, N O atoms act as catalysts

4. T-dependence of pp chain and CNO cycle

5. Hydrostatic equilibrium: negative feedback loop • If core T drops, • fusion rate drops, • core contracts • heats up • If core heats up, • fusion rate rises • core expands • cools down

6. Main sequence stars are modeled as concentric spherical shells in hydrostatic equilibrium Mass element dm Constant density  Inward force = outward force

7. The Main Sequence L = A sT4

9. Demographics of Stars • Observations of star clusters show that star formation makes many more low-mass stars than high-mass stars

10. Giant molecular clouds are the sites of star formation GMC: Length scale ~ 10-100 pc T = 10 – 20 K Mass ~ 105 – 106Msun Clumps: Length scale ~ 2-5 pc T = 10 – 20 K Mass ~ 103 – 104Msun Cores: Length scale ~ 0.1 pc T = 10 K Mass ~ 1 Msun

11. Clouds exhibit a clumpy structure

12. Star forming regions in Orion

14. What supports Cloud Cores from collapsing under their own gravity? • Thermal Energy (gas pressure) • Magnetic Fields • Rotation (angular momentum) • Turbulence

15. Gravity vs. gas pressure • Gravity can create stars only if it can overcome the forces supporting a cloud • Molecules in a cloud emit photons: • cause emission spectra • carry energy away • cloud cools • prevents pressure buildup

17. What happens when a cloud core collapses? Virial theorem: 2K + U = 0 If 2K > |U|, then • Force due to gas pressure dominates over gravity • Cloud is supported against collapse

18. Gravitational potential energy Kinetic energy where Assume a spherical cloud with constant density

19. where Using the equality and solving for M gives a special mass, MJ, called the Jeans Mass, after Sir James Jeans. In order for the cloud to collapse under its own gravity,

20. You can also define a Jeans length, RJ Jeans Criterion When the mass of the cloud contained within radius Rc exceeds the Jeans mass, the cloud will spontaneously collapse:

22. Figure from Jeff Hester & Steve Desch, ASU

23. Figure from Jeff Hester & Steve Desch, ASU

24. “protoplanetary disks”

26. HH Objects

28. Collapse slows before fusion begins: Protostar • Contraction --> higher density • --> even IR and radio photons can’t escape • --> Photons (=energy=heat) get trapped • --> core heats up (P ~ nT) • --> pressure increases • Protostars are still big --> luminous! • Gravitational potential energy --> light!

29. What supports Cloud Cores from collapsing under their own gravity? • Thermal Energy (gas pressure) • Magnetic Fields • Rotation (angular momentum) • Turbulence

30. Angular momentum problem • A protostellar core has to rid itself of 1000x Jsolar system • Core collapse produces a disk whose j increases with r • To redistribute (and/or lose) J takes >> orbital timescale • The disk is stable over ~106 years

31. Homework for Wednesday Sept. 14 • Problem 2-5 from book • One paragraph on a possible topic for your semester project (for topics, check out the author’s blog or astrobites; then find a peer-reviewed paper on the subject from NASA ADS) • Estimate how the angular momentum is currently distributed in the solar system (sun & planets). Compare to the angular momentum of a uniform spherical gas cloud with ‘typical’ properties for a collapsing molecular cloud core.

32. Protostellar evolution onto the main sequence

33. Protostellar evolution for Different Masses • Sun took ~ 30 million years from protostar to main sequence • Higher-mass stars evolve faster • Lower-mass stars evolve more slowly

35. 4000 K Hayashi Track Physical cause: at low T (< 4000 K), no mechanisms to transport energy out Such objects cannot maintain hydrostatic equilibrium They will rapidly contract and heat until closer to being in hydro. eq.

36. Mass accretes onto the star via an accretion disk (Krumholtz et al 2009) Necessary to build stars > 8 Msun

37. Phases of star formation

38. Spectral energy distribution http://feps.as.arizona.edu/outreach/sed.html

39. dust sublimes at ~2000 K p depends on grain properties, 0<p<2 Smaller grains = flatter T(R) =smaller p

40. Comparing disk observations to models:

42. Modeling SED’s with some simplifying assumptions: Dust grains are perfect blackbody emitters/absorbers Disk is optically thick Disk is geometrically thin Reality: Radiation absorption and emission depends on size, composition, shape, orientation (!) of grains (more so for optically thin disk) Optically thick = disk grains absorb only on the outside of disk, we only see emission from these grains Geometrically thick = disk self-gravity, etc

43. continuous disk that extends out from the surface of the star to 100 AU

44. same disk with an inner hole of 0.3 AU

45. A gap = cleared by a planet?

46. Class 0 Protostar: Earliest stage of collapse, no star visible, no disk visible Class I: bipolar outflow, jets ~100 km/s, still embedded in infalling material heated by star + disk Class II: “Classical T Tauri star” SED = star + disk, disk lifetime~ 106 yr Class III: PMS star w/ debris disk http://ssc.spitzer.caltech.edu/documents/compendium/galsci/

47. T Tauri : the prototype protostar

48. http://ssc.spitzer.caltech.edu/documents/compendium/galsci/

49. http://vinkovic.org/Projects/Protoplanetary/

50. http://vinkovic.org/Projects/Protoplanetary/