Transcript Disk

Disks around young O-B (proto)stars:
Observations and theory
R. Cesaroni, D. Galli, G. Lodato, C.M. Walmsley, Q. Zhang
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The importance of disks in massive (proto)stars
The search for disks: methods and tracers
The result: “real” disks found in B (proto)stars
The stability of disks  accretion rate onto star
The apparent lack of “real” disks in O stars:
observational bias, short lifetime, or different star
formation scenario?
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…?
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
Which tracer…?
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 Keto et al., Cesaroni et al., Zhang et al.,
1500 AU Shepherd & Kurtz, Olmi et al., Sandell et al.,
Chini et al., Gibb et al., Beltràn et al.,
Beuther et al., …
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
SMA and ALMA)
Thermal lines Kinematics and
geometry of outflow
and disk
Results of disk search
Two types of objects found:
Toroids
Disks
• 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, circumcluster structures
• 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:
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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:
Sako et al. (2005)
M17
0.01 pc
Chini et al. (2004)
disk 2.2 micron
13CO(1-0)
0.07 pc
IRAS 20126+4104
M*=7Cesaroni
MO
et al.
Hofner et al.
Moscadelli et al.
H2O masers prop. motions
IRAS 20126+4104
Edris et al. (2005)
Sridharan et al. (2005)
NIR & OH masers
disk
DISKS IN MASSIVE (PROTO)STARS
Ltot Mdisk Rdisk Mstar (dM/dt)out tout
104LO 1-15MO
6-25MO
a few103AU
10-4MO/yr 104yr
 Disks do exist in B-type (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:
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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:
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
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.)
G192.16-3.82
Shepherd & Kurtz (1999)
2.6mm cont.
disk
CO outflow
G192.16-3.82
Shepherd & Kurtz (1999)
Shepherd et al. (2001)
3.6cm cont. & H2O masers
Simon et al. (2000): TTau stars
Velocity maps (CO J=21)
Fuente et al. (2003):
mm continuum in
Herbig 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 HW2
Torrelles et al. (1998)
Patel et al. (2005)
… but see Comito & Schilke
for a different interpretation
IRAS 18089-1732
Beuther 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)
CH3CN(12-11)
Gibb et al. (2002)
Olmi et al. (2003)
Beltran et al. (2005)
Olmi et al. (1996)
Beltran et al. (2004)
1200 AU
Disks & Toroids
O stars
M*
(MO)
7
6-10
15-20
40
20…
-
B stars
L
Mdisk
Ddisk
(LO) (MO)
(AU)
IRAS20126 104
4
1600
G192.16
3 103
15
1000
M17
?
>110
20000
NGC7538S 104 100-400 30000
G24.78 (3) 7 105 80-250 4000-8000
G29.96
9 104 300
14000
G31.41
3 105 490
16000