Disks, toroids and the formation of massive stars

1. Disks, toroids and the formationof massive stars Riccardo Cesaroni O-B star  >103 LO >8 MO high-mass • Observations: where do (massive) stars form? • Theory: how do (massive) stars form? • Search for disks in high-mass (proto)stars • 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: >400 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. 3.6cm Testi et al. Cesaroni et al.

8. High-mass star forming region 0.5 pc

9. Clump UCHII HMC Core

10. Clump UCHII HMC

11. Clump HMC

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

13. nR-pwith p=1.5-2.5 no break at HMC •  unstable density profile • Mclump > Mvirial  clumps unstable • Vclump = VHMC  HMCs at rest wrt clumps • TR-q with q=0.4-0.5  clumps centrally heated • Clumps might be collapsing • HMCs are density peaks in clumps • HMCs are T peaks “enlightened’’ by embedded stars

14. HMC Clump nH2 R-2.6 Fontani et al. (2002)

15. nR-pwith p=1.5-2.5 no break at HMC •  unstable density profile • Mclump > Mvirial clumps unstable • Vclump = VHMC  HMCs at rest wrt clumps • TR-q with q=0.4-0.5  clumps centrally heated • Clumps might be collapsing • HMCs are density peaks in clumps • HMCs are T peaks “enlightened’’ by embedded stars

16. Fontani et al. (2002) sample of 12 Clumps

17. nH2R-pwith p=1.5-2.5 no break at HMC •  unstable density profile • Mclump > Mvirial clumps unstable • Vclump = VHMC  HMCs at rest wrt clumps • TR-qwith q=0.4-0.5 clumps centrally heated • Clumps might be collapsing • HMCs are density peaks in clumps • HMCs are T peaks “enlightened’’ by embedded stars • HMCs are deeply related to clumps

18. nH2R-pwith p=1.5-2.5 no break at HMC •  unstable density profile • Mclump > Mvirial clumps unstable • Vclump = VHMC  HMCs at rest wrt clumps • TR-qwith q=0.4-0.5 clumps centrally heated • Clumps might be collapsing • HMCs are density peaks in clumps • HMCs are T peaks “enlightened’’by embedded stars • HMCs are deeply related to clumps

19. How do massive stars form?

20. 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

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

23. PROBLEM: High-mass stars “switch on” still accreting  radiation pressure stops accretion (Kahn 1976)  stars > 8 MOcannot form!? SOLUTIONS Yorke (2003): Kdust<Kcrit M*/L* • “Increase’’ M*: large accretion rates • “Reduce’’ L*: non-spherical accretion • Reduce Kdust: large grains (coalescence of lower mass stars)

24. Possible models • Large accretion rates: competitive accretion (Bonnell et al. 2004); turbulent cores (McKee & Tan 2002) • Non-spherical accretion: disk+outflowfocus ram pressure and dilute radiation pressure (Yorke & Sonnhalter 2002; Krumholz et al. 2003) • Coalescence: many low-mass stars merge into one massive star (Bonnell et al. 2004)

25. Disk + outflow may be the solution (Yorke & Sonnhalter, Kruhmolz et al.): Outflow  channels stellar photons   lowers radiation pressure Disk  focuses accretion   boosts ram pressure Detection of accretion disks would support O-B star formation by accretion, otherwise other mechanisms are needed

26. Disks in young (proto)stars Disks seem natural outcome of star formation: collapse+angular momentum conservation  flattening+rotation speed up  disk • Disks detected in low- & intermediate-mass (< 8 MO) pre-main-sequence stars (Simon et al. 2000; Natta et al. 2000) • Disks of a few AU found in young ZAMS B stars (Bik & Thi 2004) • Disks disappear rapidly in intermediate-mass (2-8 MO), pre-main-sequence stars (Fuente et al. 2003)

27. The search for disks in massive YSOs Disks are likely associated with 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., …) • disks should be widespread! BUT… Where and what to search for…?

28. disk? Where to search for? 0.5 pc

29. disk outflow outflow What to search for? Theorist’s definition: Disk = long-lived, flat, rotating structure in centrifugal equilibrium Observer’s definition: Disk = elongated structure with velocity gradient perpendicular to outflow axis core

30. Which tracer…?

32. 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:

33. Examples of rotating toroids:

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

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

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

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

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

39. Mdyn= 19 MO Mdyn= 55 MO Furuya et al. (2002) Beltran et al. (2004,2005) Moscadelli et al. (2007)

40. First result: • velocity gradient perpendicular to bipolar outflow rotatingtoroid • conservation of angular momentum from 2” (15000 AU) to 0.5” (4000 AU)  possible formation of circumstellardisk?

41. absorption HC HII hypercompact HII +dust O9.5 (20 MO) + 130 MO outflow axis

42. Beltran et al. (2006, Nature) outflow axis

43. Second result: • Red-shifted absorption in molecular line towards HII region infall towards star  accretion onto star?

44. Hypercompact HII region Moscadelli et al. (2007) Beltran et al. (2007) 7mm free-free & H2O masers 500 AU

45. Hypercompact HII region Moscadelli et al. (2007) Beltran et al. (2007) 7mm free-free & H2O masers 30 km/s

46. Third result: • H2O masers along HII region border have proper motions away from star expansion of shell HII region with tHII = 500 AU/50 km/s = 50 yr !!! note that this is distance independent  hyperyoung HII region

47. Final scenario: • G24 A1 is a massive toroid, rotating about a bipolar outflow and infalling towards an O star with very young expanding HII region a 20 MO star has been formed through accretion (now finished…?)

48. Example of rotating disk:

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

50. 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: