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: possible solutions
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?
≡≫≪ 10M⊙  ′″∼

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
rapid evolution: tacc=20 MO /10-3MOyr-1=2 104yr
parental environment profoundly altered
Advantage:
very luminous (cont. & line) and rich (molecules)!

3. Where do massive stars form?

4. G
NIR J+H+K
10 pc

5. 2 pc

6. G
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 on ZAMS

11. PROBLEM
High-mass stars “switch on” still accreting
 radiation pressure stops accretion (Kahn 1976)
 stars > 8 MO cannot 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  spin up and flattening  rotating disk
Contra: rotation looks “similar’’ to expansion

15. Bipolar outflow
Plane of the sky

16.  The detection of disk-outflow systems would support O-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.
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. outflow
disk
0.5 pc

22. Results of disk search Two types of objects found:
Toroids
M > 100 MO
R ~ 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

23. Example of rotating disk:

24. Keplerian rotation: M*=7 MO
IRAS
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. Furuya et al. (2002) Beltran et al. (2004) Beltran et al. (2005)
UC HII + dust
O9.5 (20 MO) MO

29. CH3OH masers Beltran et al. (2004,2005) Goddi et al. (in prep.)
Mdyn= 19 MO
Mdyn= 55 MO
= Mstar+Mgas
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. tidal destruction
rotational period
photo-evaporation

33. Conclusion: How do OB stars form?
Disks found 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. but HII exists  infall in disk!
infall and rotation!
(dM/dt)infall > (dM/dt)HIIquench
but HII exists  infall in disk!
Beltran et al. (2006)
outflow axis

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

38. Disk tracers Maser lines Continuum Thermal lines TRACER PROs CONTRAs
CH3OH, H2O, OH
High angular (VLBI) & spectral resolution
Unclear geometry & kinematics
Continuum
(sub)mm, IR
Sensitivity (and resolution)
No velocity info
Confusion with free-free and/or envelope
Thermal lines
NH3, C18O, CS, C34S, CH3CN, HCOOCH3, …
Kinematics and geometry of both outflow and disk
Limited angular resolution and sensitivity (but see SMA and ALMA)

39. Zero-age main sequence
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
D (pc)
M (MO)
nH2 (cm-3)
T (K)
Clump 1
1000
105 30
HMC
0.1
100
107

41. IRAS 20126+4104 NIR & OH masers disk 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. 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 ∼ 100 stars
(dMaccr/dt)star= 10-2 MO /yr /100 =
= 10-4 MO/yr over tSF=tff=105 yr
infalling
Clump
nR-3/2
nR-2
rotating
HMC
circumstellar disks

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

51. Bik & Thi (2005): CO first overtone in
four B5-O6 stars fitted with Keplerian disk

52. Cep A HW2 Torrelles et al. (1998) Patel et al. (2005)
… but see Comito & Schilke
for a different interpretation

53. IRAS
Beuther et al.
(2004, 2005)

54. Gibb et al. (2002)
Olmi et al. (2003)
Olmi et al. (1996)
Furuya et al. (2002)
Beltran et al. (2004)

55. Gibb et al. (2002) Olmi et al. (2003) Beltran et al. (2005)
CH3CN(12-11)
Gibb et al. (2002)
Olmi et al. (2003)
Beltran et al. (2005)

56. Disks & Toroids L (LO) Mdisk (MO) Ddisk (AU) M* IRAS20126 104 4 1600 7
G192.16
3 103 15
1000
6-10
M17 ?
>110
20000
15-20
NGC7538S
30000 40
G24.78 (3)
7 105
80-250
20…
G29.96
9 104
300
14000 -
G31.41
3 105
490
16000
B stars
O stars