1) The importance of disks in massive (proto)stars

2) The search for disks: methods and tracers

3) The result: “real” disks found in B (proto)stars

4) The stability of disks accretion rate onto star

5) The apparent lack of “real” disks in O stars: observational bias, short lifetime, or different star formation scenario?

Disks around young O-B (proto)stars: Observations and theory

R. Cesaroni, D. Galli, G. Lodato, C.M. Walmsley, Q. Zhang

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)

Disks and high-mass star formation

Two relevant timescales in star formation:

accretion: tacc = Mstar/(dM/dt)acc

contraction: tKH = GMstar/RstarLstar

M > 8-14 MO tacc > tKH (Palla & Stahler 1993)High-mass stars reach ZAMS still accreting!Spherical symmetry:radiation pressure > ram pressure

stars > 8-14 MO should not form!??

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

The search for disks in massive YSOs

Disks are likely associated with outflows:

outflow detection rate = 40-90% in massive YSOs

(Osterloh et al., Beuther et al., Zhang et al., …) disks should be widespread!

BUT…

What to search for…?

outflowoutflow

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

coredisk

CH3OH masers 5 mas 15 AU

Norris et al., Phillips et al. Minier et al., Edris et al., Pestalozzi et al., …

OH masers 10 mas 30 AU

Hutawarakorn & Cohen, Edris et al.

SiO & H2O masers 1 mas 3 AU

Greenhill et al., Torrelles et al., Wright et al., Shepherd et al., …

NIR, mm & cm continuum

100 mas 300 AU

Gibb et al., Yao et al. Preibisch et al., Chini et al., Sridharan et al. Jiang et al., Puga et al., Shepherd et al., …

Thermal lines: NH3, C18O, CS, C34S, CH3CN, HCOOCH3, …

500 mas 1500 AU

Keto et al., Cesaroni et al., Zhang et al., Shepherd & Kurtz, Olmi et al., Sandell et al., Chini et al., Gibb et al., Beltràn et al., Beuther et al., …

Which tracer…?

TRACER PROs CONTRAs

Maser lines 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 SMA and ALMA)

Results of disk searchTwo types of objects found:

Toroids• M > 100 MO

• R ~ 10000 AU• L > 105 LO

• (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

• (dM/dt)star ~ 10-4 MO/yr• trot ~ 104 yr• tacc ~ M/(dM/dt)star ~ 105 yr

tacc >> trot

equilibrium, circumstellar structures

Examples of rotating toroids:

• G10.62-0.38 (Keto et al. 1988)

• G24.78+0.08 (Beltràn et al. 2004, 2005)

• G28.20-0.05 (Sollins et al. 2005)

• G29.96-0.02 (Olmi et al. 2003, Gibb et al. 2004)

• G31.41+0.31 (Beltràn et al. 2004, 2005)

• IRAS 18566+0404 (Zhang et al. 2005)

• NGC 7538 (Sandell et al. 2003)

Examples of rotating disks:

M17

Chini et al. (2004)

2.2 micron

0.01 pc

13CO(1-0)

0.07 pc

Sako et al. (2005)

disk

IRAS 20126+4104Cesaroni et al.Hofner et al.

Moscadelli et al.

M*=7 MO

H2O masers prop. motions

IRAS 20126+4104

Edris et al. (2005)Sridharan et al. (2005)

disk

NIR & OH masers

104LO 1-15MO 6-25MO

a few103AU 10-4MO/yr 104yr

Ltot Mdisk Rdisk Mstar (dM/dt)out tout

Disks do exist in B-type (proto)stars

DISKS IN MASSIVE (PROTO)STARS

Open questions…

1. Mdisk ~ Mstar Are disks stable?

2. Can disks sustain accretion rate onto star?

3. Are there disks in O-type stars?

Disk stability

• Stability: Toomre’s parameter Q > 1• Q(H/R,Mdisk/Mtot) with H=disk thickness and

Mtot= Mdisk+ Mstar

Fiducial values (e.g. IRAS 20126+4104):H/R = 0.4

Mdisk/Mtot = 4 MO /11 MO = 0.4 Q=2 the disk is stable to axisymmetric

perturbations, but…

Disk stability

• Stability: Toomre’s parameter Q > 1• Q(H/R,Mdisk/Mtot) with H=disk thickness and

Mtot= Mdisk+ Mstar

Fiducial values (e.g. IRAS 20126+4104):H/R = 0.4

Mdisk/Mtot = 4 MO /11 MO = 0.4 Q=2 the disk is stable to axisymmetric

perturbations, but…

Lodato and Rice (2004):• thick (H ~ R/2) and massive (Mdisk ~ Mstar/2) disks with

Q > 1 develop non-axisymmetric instabilities (over trot ~ 104 yr) towards Q ~ 1 marginal stability

Lodato, Rice, and Armitage (2005):• disk cooling development of spiral structure or disk

fragmentation• upper limit to the transport of matter through the disk

by viscosity maximum accretion rate through the disk onto the star, for disk surface density ~ R-1:

(dM/dt)star = 0.38 (H/R)2 Mdisk/trot ~ 10-5 MO/yr

Disk accretion rate

Problem: (dM/dt)star=10-5 MO/yr too small

• Estimated mass accretion rate on star much smaller than accretion on disk:

(dM/dt)disk= Mcloud/tfree-fall = 10-4-10-3 MO/yr

• Star formation timescale too long:

tstar = Mstar/(dM/dt)star = 106-107 yr >> 105 yr

(e.g. Tan & McKee 2004).

Possible solutions:• For surface density ~ R-p

(dM/dt)star = 0.38/(2-p) (H/R)2 Mdisk/trot

may be very large for p ~ 2

• Massive disks (Mdisk ~ Mstar) strong episodes of spiral activity (dM/dt)star enhanced by 10 times over trot ~ 104 yr (Lodato & Rice 2005)

• Magnetised disks also develop enhancements of accretion rate (Fromang et al. 2004)

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 massive disks in O (proto)stars. Why?

Observational bias?For Mdisk= Mstar/2, a Keplerian disk in a 50 MO star can be detected up to: continuum sensitivity:

d < 1.7 [Mstar(MO)]0.5 ~ 12 kpc line sensitivity:

d < 6.2 Mstar(MO) sin2i/W2(km/s) ~ 8 kpc spectral + angular resolution:

d < 14 Mstar(MO) sin2i/[D(’’)W2(km/s)] ~ ~ 19 kpc

all disks detectable up to galactic center

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

On the other hand, if O protostars do not have disks, a physical explanation is required:

• O-star disks “hidden” inside toroids• O-star disk lifetime 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:

tidal destructionrotational period

photo-evaporation

Disks in O (proto)stars might be shorter lived,

and/or more deeply embedded than those

detected in B (proto)stars

• 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

Conclusions

• Found about ~10 disks 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.)

G192.16-3.82Shepherd & Kurtz (1999)

CO outflow

2.6mm cont.disk

G192.16-3.82Shepherd & Kurtz (1999)

Shepherd et al. (2001)

3.6cm cont. & H2O masers

Simon et al. (2000): TTau stars Velocity maps (CO J=21)

Fuente et al. (2003):

mm continuum inHerbig Ae/Be stars

(age ~ 106 yr)

Mdisk(B) << Mdisk(A)

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

Cep A HW2Torrelles et al. (1998)

… but see Comito & Schilkefor a different interpretation

Patel et al. (2005)

IRAS 18089-1732Beuther et al.(2004, 2005)

Gibb et al. (2002)Olmi et al. (2003)

Olmi et al. (1996)Furuya et al. (2002)Beltran et al. (2004)

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

Furuya et al. (2002)

Beltran et al. (2004)

Furuya et al. (2002)

Beltran et al. (2004)

Gibb et al. (2002)Olmi et al. (2003)

Beltran et al. (2005)

CH3CN(12-11)

Olmi et al. (1996)Beltran et al. (2004)

1200 AU

Disks & Toroids

L

(LO)

Mdisk

(MO)

Ddisk

(AU)

M*

(MO)

IRAS20126

104 4 1600 7

G192.16 3 103 15 1000 6-10

M17 ? >110 20000 15-20

NGC7538S 104 100-400

30000 40

G24.78 (3) 7 105 80-250 4000-8000

20…

G29.96 9 104 300 14000 -

G31.41 3 105 490 16000 -

O stars

B stars