1. Possible evolutionary sequence for high-mass star formation
IR-dark clump (20 K, >1 pc, cm-3)
undetected at near-IR, mid-IR
detected at (sub)mm
IR-luminous clump (50 K, >1 pc, 105 cm-3)
detected from mm to IR
Hot molecular core (>100 K, <0.1 pc, 107 cm-3)
inside IR-luminous clumps
UC HII region (0.1 pc)

2. High-mass star
forming region
0.5 pc

3. Clump
UC HII
HMC
Core

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

5. Jets/outflows and the conditions for high-mass star formation
Massive outflows are signatures of high-mass (proto)stars
HCO+ and SiO are jet/outflow tracers
HCO+ and SiO survey towards 50 massive (>100 MO) IR-dark and IR-luminous clumps
High-mass stars form if clump Σ > 0.6 g cm-2 in agreement with theoretical prediction (Krumholz & McKee 2008)
IR-dark clumps are not necessarily pre-stellar: jet/outflow activity decreases during evolution

6. Lòpez-Sepulcre et al. (2010): 100% outflow detection rate
for Σ > 0.6 g cm-2
in both IR-dark and
IR-luminous clumps

7. Jets/outflows and the conditions for high-mass star formation
Massive outflows are signatures of high-mass (proto)stars
HCO+ and SiO are jet/outflow tracers
HCO+ and SiO survey towards 50 massive (>100 MO) IR-dark and IR-luminous clumps
High-mass stars form if clump Σ > 0.6 g cm-2 in agreement with theoretical prediction (Krumholz & McKee 2008)
IR-dark clumps are not necessarily pre-stellar: jet/outflow activity decreases during evolution

8. Lòpez-Sepulcre et al. (2011): jet/outflow (i.e. SiO) strength
decreases with age (i.e. Lstar/Mgas)
Note: LSiO/Lbol and Lbol/Mgas
are distance independent
evolution

9. High-mass star formation: models
Competitive accretion: massive stars grow up at cluster center at expenses of low-mass stars; accretion boosted by gravitational well of cluster (Bonnell et al. 2004)
Monolithic collapse: massive star accretes from turbulence supported core (McKee & Tan 2002; Krumholz et al. 2003)
Bondi-Hoyle accretion: accretion boosted by gravitational field of star itself (Keto 2003)
 all imply (need?) disk formation

10. For all models disk + outflow may be the solution:
Outflow  channels stellar photons 
 lowers radiation pressure
Disk  focuses accretion   boosts ram pressure
 Disks solve radiation pressure problem in OB stars (Krumholz et al. 2007, Kuiper et al. 2010)

11. competitive accretion
1 pc clump collapse
competitive accretion
Bonnell (2005)

12. disk core accretion in 0.2 pc clump Krumholz et al. (2007) Zoom in
time
core accretion
in 0.2 pc clump
Krumholz et al. (2007)
disk

13. density & velocity of gas around O9 star (Keto 2007)
ionized gas
molecular gas
50 AU

14. 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…?

15. Where to search for?
disk?
0.5 pc

16. 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
outflow
disk
core

17. Maser lines Continuum Thermal lines TRACER PROs CONTRAs
High angular & spectral resolution
Unclear geometry & kinematics
Continuum
Sensitivity (and resolution)
No velocity info
Confusion with free-free and/or envelope
Thermal lines
Kinematics and geometry of outflow and disk
Limited angular resolution and sensitivity (but see ALMA and SKA)

18. Results of disk search Two types of rotating objects found:
Toroids
M > 100 MO
R ~ AU
L > 105 LO (O stars)
Disks
M < 10 MO
R ~ 1000 AU
L ~ 104 LO (B stars)

19. Examples of rotating toroids:

20. Beltran et al. (2004, 2011)

21. Beltran et al. (2011)
Codella et al. (2011) A2 A1

22. A1 A2 Beltran et al. (2011) A2 seems origin of outflow
Outflow close to plane of sky? A1 A2
Beltran et al. (2011)

23. Beltran et al. (2011) Vig et al. (2009) rotation + expansion
data model
Beltran et al. (2011) A2 A1
Vig et al. (2009)

24. First result:
velocity gradients perpendicular to bipolar outflows  rotating toroids

25. hypercompact HII + dust
O9.5 (20 MO) MO
absorption
HC HII A2 A1
outflow axis

26. Beltran et al.
(2006)
outflow axis

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

28. 7mm free-free & H2O masers
Hypercompact HII region
Moscadelli et al. (2007)
Beltran et al. (2007)
7mm free-free & H2O masers
500 AU

29. 7mm free-free & H2O masers
Hypercompact HII region
Moscadelli et al. (2007)
Beltran et al. (2007)
7mm free-free & H2O masers
30 km/s

30. 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?!?

31. 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…?)

32. Example of rotating disk:

33. Keplerian rotation+infall:
IRAS
Cesaroni et al.
Hofner et al.
Sridharan et al.
Moscadelli et al.
Keplerian rotation+infall:
M*=10 MO
Image: 2µm cont.
--- OH maser
H2O masers
1000 AU
Moscadelli et al. (2010)
CH3OH
H2O
200 AU
jet
disk+jet
disk

34. Distance measurement to IRAS 20126+4104 with
H2O maser parallax (Moscadelli et al. 2010)
d = 1.64±0.05 kpc

35. Disks Toroids disks toroids M < a few 10 MO R ~ 1000 AU
L ~ 104 LO  B (proto)stars
large tacc/trot
 equilibrium, circumstellar structures
Toroids
M > 100 MO
R ~ AU
L > 105 LO  O (proto)stars
small tacc/trot
 non-equilibrium, circum-cluster structures
disks
Beltràn et al. (2010)
toroids

36. 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 - but distance is large (few kpc)
Orion I (450 pc) does have disk, but luminosity is unclear (< 105 LO???)
Difficult to detect disks in O (proto)stars. Why?
Observational bias or physical explanation?

37. Observational bias?

38. Assumptions: circumstellar disks HPBW = Rdisk/4 FWHMline = Vrot(Rdisk)
Mdisk  Mstar
same <Ncol> in all disks
TB > 20 K
obs. freq. = 230 GHz
5 hours ON-source
spec. res. = 0.2 km/s
S/N = 20
edge-on
i = 35°
Keplerian

39. Assumptions: circumstellar disks no stars HPBW = Rdisk/4
FWHMline = Vrot(Rdisk)
Mdisk  Mstar
same <Ncol> in all disks
TB > 20 K
obs. freq. = 230 GHz
5 hours ON-source
spec. res. = 0.2 km/s
S/N = 20
edge-on
i = 35°
no stars

40. Moreover… One should consider also: rarity of O stars
confusion with envelope
chemistry
confusion with outflow/infall
non-keplerian rotation
disk flaring
inclination angle …

41. Physical explanation?

42. O-star disks might be “hidden” inside toroids
O-star disk lifetime might be too short, i.e. less than rotation period:
photo-evaporation by O star (Hollenbach et al. 1994)
tidal destruction by stellar companions (Hollenbach et al. 2000)
In both cases we assume Mdisk=Mstar/2 and disk
surface density ~ R-1, i.e. Mdisk  Rdisk:

43. deeply embedded disk? rotating toroid CH3OH masers
CH3CN
1.3cm cont.
Furuya et al. (2008)
Sanna et al. (2010)

44. O-star disks might be “hidden” inside toroids
O-star disk lifetime might be too short, i.e. less than rotation period:
photo-evaporation by O star (Hollenbach et al. 1994)
tidal destruction by stellar companions (Hollenbach et al. 2000)
In both cases we assume Mdisk=Mstar/2 and disk
surface density ~ R-1, i.e. Mdisk  Rdisk:

45. Cesaroni, Galli, Lodato, Walmsley, Zhang (2007)
tidal destruction
rotational period
photo-evaporation

46. Disks in O (proto)stars might be shorter lived,
Photoionosation: inefficient disk destruction mechanism, for all spectral types (if Mdisk comparable to Mstar)
Tidal interaction with the stellar companions: more effective to destroy outer regions of disks in O stars than in B stars
Disks in O (proto)stars might be shorter lived,
and/or more deeply embedded than those
detected in B (proto)stars