High-Mass Star Formation: Observations, Theory, and Solutions

1. High-mass star formation Riccardo Cesaroni INAF - Osservatorio Astrofisico di Arcetri O-B star  >103 LO >8 MO high-mass • Observations: where do (massive) stars form? • Theory: how do (massive) stars form? • Radiation pressure problem:possiblesolutions • The role of disks in high-mass star formation • Results: disks in B stars, toroids in O stars • Implications: different formation scenarios for B and O stars?

2. High-mass star forming regions: Observational problems • IMF  high-mass stars are rare • large distance: >300 pc, typically a few kpc • formation in clusters  confusion • rapidevolution: tacc=20 MO /10-3MOyr-1=2104yr • parental environment profoundly altered • Advantage: • very luminous (cont. & line) and rich (molecules)!

3. Where do massive stars form?

4. G9.62+0.19 NIR J+H+K 10 pc

5. 2 pc

6. G9.62+0.19 350 micron 0.5 pc Hunter et al. (2000)

7. Testi et al. Cesaroni et al.

8. Typical high-mass star forming region 0.5 pc

9. How do massive stars form?

10. Low-mass vs High-mass Shu et al. (1987): star formation from inside-out collapse onto protostar Two relevant timescales: accretion  tacc = M*/(dM/dt) contraction  tKH = GM*/R*L* • Lowmass (< 8 MO): tacc < tKH “birthline’’ • Highmass (> 8 MO): tacc > tKH  accretion onZAMS

11. PROBLEM High-mass stars “switch on” still accreting  radiation pressure stops accretion (Kahn 1976)  stars > 8 MOcannot form!??

12. Solution 1: Coalescence Many low-mass stars merge into one massive star (Bonnell et al. 2004) Pro: Massive stars do not form in isolation Contra: Required >106 stars pc-3>>104 stars pc-3 as observed e.g. in Orion

13. Solution 2: Large accretion rates Competitive accretion (Bonnell et al. 2004) Turbulent cores (McKee & Tan 2002) Pro: Outflow mass loss rates >10 times than in low-mass stars Contra: Infall difficult to measure (vfree-fall = a few km s-1 over 1000 AU i.e. <1” at several kpc)

14. Solution 3: Non-spherical accretion Formation of disk+outflow (Yorke & Sonnhalter 2002; Krumholz et al. 2003): Outflow channels stellar photons   lowers radiation pressure Disk  focuses accretion  boosts ram pressure Pro: Collapse + angular momentum conservation  spinup and flattening  rotating disk Contra:rotation looks “similar’’ to expansion

15. Bipolar outflow Plane of the sky

16.  The detectionof disk-outflow systems would supportO-B star formation by non-spherical accretion, otherwise other mechanisms are needed

17. The search for disks in massive YSOs Disks are likely associated with bipolar outflows. Outflow detection rate = 40-90% in massive YSOs (luminous IRAS sources, UC HIIs, H2O masers,…) (Osterloh et al., Beuther et al., Zhang et al., …)  also disks must be widespread!

18. CO(2-1) outflow & 1mm continuum Beuther et al. (2002) Single-dish (12’’ beam)

19. 05358+3543 Beuther et al. (2003) interferometer (4’’ beam)

20. Where to search for disks? Hot molecular cores with outflows • What to search for? Velocity gradient perpendicular to outflow

21. disk outflow outflow 0.5 pc

22. Toroids M > 100 MO R ~ 10000 AU L > 105 LO O stars (dM/dt)star > 10-3 MO/yr trot~ 105 yr tacc~ M/(dM/dt)star~ 104 yr tacc << trot non-equilibrium, circum-cluster structures Disks M < 10 MO R ~ 1000 AU L ~ 104 LO B stars (dM/dt)star ~ 10-4 MO/yr trot~ 104 yr tacc~ M/(dM/dt)star~ 105 yr tacc >> trot equilibrium, circumstellar structures Results of disk searchTwo types of objects found:

23. Example of rotating disk:

24. IRAS 20126+4104 Cesaroni et al. Hofner et al. Moscadelli et al. Keplerian rotation: M*=7 MO Moscadelli et al. (2005)

25. Example of rotating toroid:

26. Furuya et al. (2002) Beltran et al. (2004) Beltran et al. (2005)

27. Furuya et al. (2002) Beltran et al. (2004) Beltran et al. (2005)

28. UC HII +dust O9.5 (20 MO) + 130 MO Furuya et al. (2002) Beltran et al. (2004) Beltran et al. (2005)

29. Mdyn= 19 MO Mdyn= 55 MO = Mstar+Mgas CH3OH masers Beltran et al. (2004,2005) Goddi et al. (in prep.)

30. Are there disks in O stars? • In Lstar~ 104 LO (B stars) true disks found • In Lstar > 105 LO (O stars) no true disk (only toroids) found Why is it difficult to detect disks in O (proto)stars?

31. Caveats!!! • rarity of O stars  very distant • confusion with envelope • chemistry • confusion with outflow/infall • non-keplerian rotation • disk flaring • inclination angle Nevertheless… disk lifetime in O stars might be short!

32. photo-evaporation tidal destruction rotational period

33. Conclusion: How do OB stars form? • Disksfound in B (proto)starsstar formation by accretion as in low-mass stars • No disk found yet (only massive, rotating toroids) in O (proto)stars • observational bias (confusion, distance, rarity,…) • disks hidden inside toroids and/or destroyed by tidal interactions with stellar companions • disks do not exist;alternative formation scenarios for O stars needed: coalescence of lower mass stars, competitive accretion (see Bonnell, Bate et al.)

35. Beltran et al. (2006) absorption UC HII outflow axis

36. infall and rotation! (dM/dt)infall > (dM/dt)HIIquench but HII exists  infall in disk! Beltran et al. (2006) outflow axis

37. Goddi et al. in prep. H2O maser proper motions accretion is finished!?? ALMA needed

38. Disk tracers

39. Palla & Stahler (1990) tKH=tacc dM/dt=10-5 MO/yr tKH>tacc Zero-age main sequence Sun

40. Clumps and hot molecular cores • Rclump = 10 RHMC • Mclump= 10 MHMC • nclump = 0.01 nHMC

41. IRAS 20126+4104 Edris et al. (2005) Sridharan et al. (2005) NIR & OH masers disk

42. A possible scenario for high-mass SF • Unstable clump: tff=105 yr Clump nR-2 Mclump > Mvirial

43. A possible scenario for high-mass SF • Unstable clump: tff=105 yr • Inside-out collapse: dMaccr/dt=Mclump/tff=10-2 MO/yr infalling Clump nR-2 nR-3/2

44. A possible scenario for high-mass SF • Unstable clump: tff=105 yr • Inside-out collapse: dMaccr/dt=Mclump/tff=10-2 MO/yr • Rotation of core with rotation period=105 yr infalling Clump nR-2 nR-3/2 rotating Core

45. A possible scenario for high-mass SF • Unstable clump: tff=105 yr • Inside-out collapse: dMaccr/dt=Mclump/tff=10-2 MO/yr • Rotation of core with rotation period=105 yr • Fragmentation over Rcentrifugal=RHMC/5=0.01 pc infalling Clump nR-2 nR-3/2 rotating Core rotating disks

46. infalling Clump nR-2 nR-3/2 rotating HMC circumstellar disks A possible scenario for high-mass SF • Unstable clump: tff=105 yr • Inside-out collapse: dMaccr/dt=Mclump/tff=10-2 MO/yr • Rotation of core with rotation period=105 yr • Fragmentation over Rcentrifugal=RHMC/5=0.01 pc • Formation of HMC with 53 ∼100 stars • (dMaccr/dt)star= 10-2 MO/yr /100 = • = 10-4 MO/yr over tSF=tff=105 yr

47. G192.16-3.82 Shepherd & Kurtz (1999) 2.6mm cont. disk CO outflow

48. G192.16-3.82 Shepherd & Kurtz (1999) Shepherd et al. (2001) 3.6cm cont. & H2O masers

49. Simon et al. (2000): TTau stars Velocity maps (CO J=21)

50. Fuente et al. (2003): mm continuum in Herbig Ae/Be stars (age ~ 106 yr) Mdisk(B) << Mdisk(A)