Possible evolutionary sequence
for high-mass star formation
1. IR-dark clump (20 K, >1 pc, 104-105 cm-3)
• undetected at near-IR, mid-IR
• detected at (sub)mm
2. IR-luminous clump (50 K, >1 pc, 105 cm-3)
• detected from mm to IR
3. Hot molecular core (>100 K, <0.1 pc, 107 cm-3)
• inside IR-luminous clumps
4. UC HII region (0.1 pc)
High-mass star
forming region
0.5 pc
Clump
UC HII
HMC
Core
HMC
Clump
nH2  R-2.6
Fontani et al. (2002)
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) IRdark and IR-luminous clumps
1.
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
2.
Lòpez-Sepulcre et al. (2010):
100% outflow detection rate
for Σ > 0.6 g cm-2
in both IR-dark and
IR-luminous clumps
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) IRdark and IR-luminous clumps
1.
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
2.
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
High-mass star formation: models
1) 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)
2) Monolithic collapse: massive star accretes from
turbulence supported core (McKee & Tan 2002;
Krumholz et al. 2003)
3) Bondi-Hoyle accretion: accretion boosted by
gravitational field of star itself (Keto 2003)
 all imply (need?) disk formation
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)
1 pc clump collapse
competitive accretion
Bonnell (2005)
time
Zoom in
core accretion
in 0.2 pc clump
Krumholz et al. (2007)
disk
density & velocity of gas around O9 star (Keto 2007)
ionized gas
molecular gas
50 AU
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…?
Where to search for?
disk?
0.5 pc
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
core
disk
outflow
TRACER
PROs
CONTRAs
Maser lines
High angular &
spectral resolution
Unclear geometry &
kinematics
Continuum
Sensitivity (and
resolution)
No velocity info
Confusion with freefree and/or envelope
Limited angular
resolution and
sensitivity (but see
ALMA and SKA)
Thermal lines Kinematics and
geometry of outflow
and disk
Results of disk search
Two types of rotating objects found:
Toroids
Disks
• M > 100 MO
• R ~ 10000 AU
• L > 105 LO (O stars)
• M < 10 MO
• R ~ 1000 AU
• L ~ 104 LO (B stars)
Examples of rotating toroids:
Beltran et al. (2004, 2011)
Beltran et al. (2011)
Codella et al. (2011)
A2
A1
• A2 seems origin of outflow
• Outflow close to plane of sky?
A1
Beltran et al. (2011)
A2
rotation + expansion
data
model
Beltran et al. (2011)
A2
A1
Vig et al. (2009)
First result:
• velocity gradients perpendicular to bipolar
outflows  rotating toroids
absorption
HC HII
A2
hypercompact HII + dust
O9.5 (20 MO) + 130 MO
A1
Beltran et al.
(2006)
Second result:
• Red-shifted absorption in molecular line
towards HII region  infall towards star
 accretion onto star?
Hypercompact HII region
Moscadelli et al. (2007)
Beltran et al. (2007)
7mm free-free & H2O masers
500 AU
Hypercompact HII region
Moscadelli et al. (2007)
Beltran et al. (2007)
7mm free-free & H2O masers
30 km/s
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?!?
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…?)
Example of rotating disk:
IRAS 20126+4104
Cesaroni et al.
Hofner et al.
Sridharan et al.
Moscadelli et al.
Moscadelli et al. (2010)
Keplerian
rotation+infall:
M*=10 MO
Image: 2µm cont.
--- OH maser
H2O masers
disk+jet
disk
1000 AU
jet
200 AU
CH3OH
H2O
Distance measurement to IRAS 20126+4104 with
H2O maser parallax (Moscadelli et al. 2010)
d = 1.64±0.05 kpc
Disks
Toroids
• M < a few 10 MO
• R ~ 1000 AU
• L ~ 104 LO  B (proto)stars
• large tacc/trot
 equilibrium, circumstellar
structures
• M > 100 MO
• R ~ 10000 AU
• L > 105 LO  O (proto)stars
• small tacc/trot
 non-equilibrium, circumcluster structures
disks
Beltràn et al. (2010) toroids
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?
Observational bias?
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
Keplerian
Assumptions:
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
circumstellar
disks
Moreover…
One should consider also:
• rarity of O stars
• confusion with envelope
• chemistry
• confusion with outflow/infall
• non-keplerian rotation
• disk flaring
• inclination angle
• …
Physical explanation?
• 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:
rotating toroid
CH3CN
deeply embedded disk?
CH3OH masers
1.3cm cont.
Furuya et al. (2008)
Sanna et al. (2010)
• 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:
Cesaroni, Galli, Lodato, Walmsley, Zhang (2007)
photo-evaporation
rotational period
tidal destruction
• 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