Sequentially Triggered Star Formation in OB Associations
Thomas Preibisch & Hans ZinneckerMax-Planck-Institute for Radioastronomy, Bonn, Germany
Astrophysical Institute Potsdam,Germany
Upper Scorpius
Upper Centaurus - Lupus
Oph cloud
age = 17 Myr
generation I
age = 5 Myr
generation II
age <= 1 Myr
generation III
ISO
OB associations: (Ambartsumian 1947)
- Unbound stellar groups containing O–B2 stars, Ø ~ 20 ... 50 pc
- Density < 0.1 M pc-3 unstable against galactic tidal forces < 30 Myr old
Blaauw (1964, 1991): Many OB associations consist of distinct sub-groups
with different ages sequential (triggered ?) formation
Ori OB 1
Sco OB 2
Per OB 2
Summary of recent results on OB associations: Protostars and Planets V chapter by Briceno et al. (2006; astro-ph/0602446)
Many observations show star formation near massive stars:
Q: Was the formation of the YSOs triggered, or did YSOs form prior to the arrival of the shock waves ?
A: Determine ages of the YSOs and compare to shock arrival time
OB associations show the result of a recently completed star formation process,
star formation history and initial mass function allow a
quantitative comparison to models
Star formation in irradiated globules:"radiatively driven implosion"
Star formation in swept-up shells:"collect & collapse" model
Trifid NebulaHST, Hester et al. (1999)
O star
OB stars
RCW 79blue: H, red: 8 mZavagno et al. (2006, A&A 446,171)
YSOs
YSOs
Theoretical models for triggering mechanisms in OB associations:
1. Sequentially triggered formation of OB subgroups (Elmegreen & Lada 1977, Lada 1987)
Predictions: - Bimodal star formation: low-mass stars form independently are on average older, show large age spread - IMF variations: younger OB subgroups should have larger fractions of low-mass stars
Age spread among low-mass stars: [8....12] Myr [4....12] Myr [0....12] Myr
IMF variations: defict excess of low-mass stars
star formation terminated
Theoretical models for triggering mechanisms in OB associations:
2. Radiation-driven implosion of globules near OB stars (Bertoldi 1989; Lefloch & Lazareff 1994; Kessel-Deynet & Burkert 2003)
Predictions:
- OB stars form first, are older than low mass stars - Age gradients: stars close to the O star are older than those further away
(see also next talk by W.P. Chen)
Hester & Desh (2005)
Ages of the low-mass stars: 7 , 5 , 3 , 1 Myr
Theoretical models for triggering mechanisms in OB associations:
3. Supernova shock wave compression of cloud (e.g. Foster & Boss 1996, ApJ 468, 784; Vanhalla & Cameron 1998, ApJ 508, 291)
At suitable distances of ~ 20 ... 100 pc,
where vshock = 20 ... 50 km/sec,
cloud collapse can be triggered by supernova
shock waves
Predictions:
- High- and low-mass stars have same age
- Small age spread (since vshock > 20 km/sec)
- Age difference of ~ 5...10 Myr between subgroups
NEXT: Predictions versus observations
D= 144 pc49 B-stars
Upper Scorpius D = 142 pc 66 B-stars
Upper Centaurus - Lupus
D = 116 pc 42 B-stars
Lower Centaurus - Crux
10
The nearest OB association: Scorpius - Centaurus (Sco OB2)
Hipparcos revealed B to F starsde Zeeuw et al (1999, AJ 117, 354)
de Bruijne (1999, MNRAS 310, 585)
25 pc
Sco
= 5 MyrUpper Scorpius = 17 Myr
Upper Centaurus - Lupus
= 16 MyrLower Centaurus - Crux
10
The nearest OB association: Scorpius - Centaurus (Sco OB2)
25 pc
Sco
Ages of the massive stars from MS turnoff
What about the low-mass stars ?de Geus et al. (1989, A&A 216,44)
Mamajek et al.(2002, AJ 124,1670)
Upper Scorpius
10
25 pc
Sco
Huge field star confusion in extended OB associations:
Young low-mass members must be individually identified
by spectroscopy (6708Å Lithium lines)
- X-ray selected candidates: Walter et al (1994, AJ 107,692) Preibisch et al (1998, A&A 333,619)
- Survey with multi-object spectrograph 2dF: Preibisch et al (2002 AJ 124, 404):
250 low-mass members (representative sample)
+ 114 higher-mass stars from Hipparcos:
364 known members
SpT = B0.5 ... M6 , M = 20 M ... 0.1 M
Statistically robust & well defined sample
Individual spectral types and extinctions known:
derive IMF and star formation history from HRD
The stellar population of Upper Scorpius
HR Diagram for Upper Sco
5 Myr isochrone
High- mass and
low-mass stars
are coeval
Spread of isochronal agesPreibisch et al (2002, AJ 124, 404)
Reasons for spread of isochronal ages:
- distance spread: D / D = 30 pc / 145 pc ~ ± 0.25 mag
- unresolved binaries: <~ + 0.75 mag
- photometric variability <~ ± 0.3 mag
HRD consistent with no age spread, < 1-2 Myr
- Mass function is consistent with field star IMF
age of the high-mass stars: 5 Myr diameter: ~ 30 pc
age of the low-mass stars: 5 Myr 1D velocity dispersion: 1.3 km/sec
age spread < 1-2 Myr lateral crossing time ~ 25 Myr
age spread << crossing time
external agent coordinated onset of star formation over the full spatial extent
Implications on the star formation process
ScoCen is surrounded by several H I shells
De Geus (1992, A&A 262, 258)
Wind & supernova driven
expanding superbubble
from UCL crossed Upper Sco
~ 5 Myr ago
De Geus (1992, A&A 262, 258):
Scenario for the star formation history de Geus (1992, A&A 262, 258); Preibisch & Zinnecker (1999, AJ 117, 2381)
Supernova in USco
star formation triggered
star formation terminated
cloud fully dispersed
star formation in Oph triggered
Supernova & wind driven shock wave
from UCL crosses USco cloud
Wind & ionizing radiation of the
massive stars disperse the cloud
Supernova & wind driven shock wave
from USco reaches Oph cloud
Scenario for the star formation history de Geus (1992, A&A 262, 258); Preibisch & Zinnecker (1999, AJ 117, 2381)
Supernova in USco
star formation triggered
star formation terminated
cloud fully dispersed
star formation in Oph triggered
Supernova & wind driven shock wave
from UCL crosses USco cloud
"Any theory of star formation is incomplete without a corresponding theory of cloud formation" (Elmegreen & Lada 1977)
Hartmann et al (2001, ApJ 562,852) and others:
Molecular clouds are short-lived structures,
i.e.do not exist for > 10 Myr without forming stars
and "wait for a trigger"
Wind & ionizing radiation of the
massive stars disperse the cloud
Supernova & wind driven shock wave
from USco reaches Oph cloud
Rapid formation of molecular clouds and stars
Ballesteros-Paredes et al.(1999, ApJ 527,285); Hartmann et al.(2001 ApJ 562, 852); Clark et al.(2005, MNRAS 359,809)
Large-scale flows in the ISM
accumulate and compress gas
to form transient molecular clouds
Wind & supernova shocks waves create
coherent large-scale velocity fields,
formation of large structures in which
star formation can be triggered nearly
simultaneously
Hartmann et al. (2001 ApJ 562, 852)
Ballesteros-Paredes et al. (1999, ApJ 527,285)
OB star winds in UCL create
expanding superbubble (v ~ 5 km/sec)
Interaction with ISM flows
starts to sweep up clouds
Triggered cloud & star formation in ScoCen T = - 14 Myr
30 pc
UCL
After supernovae in UCL
added energy & momentum to
the expanding superbubble,
the shock wave (~ 30 km/sec)
crossed the USco cloud,
increased pressure triggered
star formation in USco
Triggered cloud & star formation in ScoCen T = - 5 Myr
60 pc
UCL
Triggered cloud & star formation in ScoCen
Lupus I cloudgeneration III
Shock wave from USco superbubble
triggers star formation
in Oph and Lupus I clouds
T = - 1 Myr
Upper Centaurus - Lupus
generation I
Upper Scorpius
generation II
Ophiuchus
generation III
Observation:
Determination of centroid spacemotions of the groups:
de Bruijne (1999, MNRAS 310, 585)
Upper Sco moves away from
UCL with v ~ 5 (±3) km/sec
Model Predictions I:
Stellar groups triggered in swept-up clouds
move away from the trigger source
Hipparcos proper motions of USco and UCL members
+
Centroid space motion in the rest-frame of UCL
de Zeeuw et al (1999, AJ 117, 354)
Model versus observations
adapted from:Mamajek & Feigelson (2001,in: Young Stars Near Earth,
ASP 244, p. 104; astro-ph/0105290)
Observation:Mamajek & Feigelson (2001)
Several young stellar groups:
- Cha cluster
- TW Hydra Association
- CrA cloud
move away from UCL with v ~ 10 km/sec
were located near the edge ofUCL 12 Myr ago (when SN exploded)
X (pc)
UCL
Y
(pc)
X (pc)
0
50 100 150
-100
-50
0
T = - 12 Myr
UCL
CrA
TW Hya
ChaY
(p
c)
0 50 100 150
-100
-50
0
T = 0 (today)
Model Predictions I:
Stellar groups triggered in swept-up clouds
move away from the trigger source
Model versus observations
USco
UCL
S
Observation:
Lupus I cloud located at
interface of USco and
UCL (post-SN, wind-driven)
superbubbles
Model Predictions II:
Elongated star forming clouds form at the
intersection of two expanding flows
Dust extinction map from Dobashi et al. 2005, PASJ 57,S1
Model versus observations
How general are these findings?
Several OB associations show subgroup sequences of three generations that provide evidence for triggered formation, e.g.
ScoCen: UCL USco Oph / Lupus I
Cep OB2: NGC 7160 IC 1396 VDB 142
Superbubbles in W3/W4 (Oey et al. 2005, AJ 129, 393)
BUT: Some associations also contain subgroups with similar ages
e.g.: ScoCen: UCL [17 Myr] - LCC [16 Myr]
Supernova-driven
expanding H I shell
v = 22 km/sec
Gorjian et al. (2004,ApJS 154, 275)
Extragalactic example of supernova triggering in OB associations: Hen 206 (LMC)
Triggered formation of several new OB subgroups
OB association NGC 2018: age ~ 10 Myr
80 pc Spitzer image blue: 3.6+4.5 m, cyan: 5.8 m, green: 8.0 m, red: 24 m
optical image
Explanation for subgroups with the same age (e.g., ScoCen: UCL [17 Myr] - LCC [16 Myr] )
How general are these findings?
Results for well investigated OB associations: (Sco OB2, Cep OB2, Ori OB1)
Briceno et al. (2006; Protostars & Planets V chapter; astro-ph/0602446)
- IMF of most OB subgroups consistent with field IMF, no good evidence for IMF variations
- Low- and high mass stars have the same ages, have formed together
- In most regions, age spreads are (much) smaller than the crossing time
consistent with models of large-scale triggering by shock waves
Supernova (+wind) driven shock waves play an important role, but other triggering mechanisms are also at work in some regions:
Cep OB 2 association:
HD 206267 (O6)
IC 1396 (4 Myr)
class 0 / class Iprotostars
NGC 7160(10 Myr)
IRAS 12 m
1
Spitzer 3.6+4.5 m, 5.8+8 m, 24 mReach et al (2004, ApJS 154, 385)
<1 Myr
VDB142
13 pc
Sicilia-Aguilar et al (2004, AJ 128, 805; 2005 AJ 130, 188)
Reach et al (2004, ApJS 154, 385)
VDB 142:Radiation-driven implosion of globule (no supernova triggering!)
The globule will form a small stellar group,but no OB subgroup!
Conclusions:
OB subgroups with well defined age sequences and small internal age spreads
suggest large-scale triggered formation scenarios.
(Supernova/wind driven shock waves)
Expanding bubbles coherent large-scale ISM flows new clouds
Supernova shock waves cloud compression
triggered formation of whole OB subgroups (several 1000 stars).
Other triggering mechanisms (e.g. radiation-driven implosion of globules)
may operate simultaneously, but seem to form only small groups of stars
(i.e. are secondary processes).
Note: Our Sun formed in an OB association !
Supernova shock wave injected short-lived radionucleids (e.g. 26Al).(Cameron & Truran 1977; Hester & Desh 2005)
THE END
Interpretation of the HR Diagram: Age spread or no age spread ??
Perfect world: no errors, no uncertainties
age histogram
simulated HRD
Coeval population of 5 Myr old stars
Interpretation of the HR Diagram: Age spread or no age spread ??
false impression of a large age spread
and an accelerating star formation rate
in an actually perfectly coeval population !
Perfect world: no errors, no uncertainties
Reality: - photometric variability & errors
- unresolved binaries
- spread of individual distancescenter:145 pc
front:130 pc
back:160 pc
Upper Sco
Analysis for Upper Sco: no detectable age spread, = 5 Myr (±1-2 Myr)
age histogram
simulated HRD
Coeval population of 5 Myr old stars
Ori OB 1C members ?(~ 5 Myr)
e.g. Orion Nebula Cluster:
Distributed T Tauri stars: ~ 2 -10 Myr
Trapezium clusterstars: ~ 1 Myr
BN complex, OMC-S protostars: <~ 0.1 Myr
To Earth
simulated side-view:
O'Dell 2001 ARAA 39, 99
Age sequences / spreads and projection effects
15 Myr 5 Myr 1 Myrline-of-sight
no observed age sequence
false impression of
a large age spread
1 ... 15 Myr
The Supergiant Shell Region in IC 2574 Cannon et al. (2005, ApJ 630, L37)Stewart & Walter (2000, AJ 120,1794)
U + V + I
Expanding shell triggers a second generation of OB associations on its rim
N
central OB associationtotal mass ~150 000 M
age: ~ 11 Myr
young OB associations
M ~ 5000 ... 300000 M
ages ~ 1 ... 4 Myr
cavity surrounded byexpanding shell
Ø ~ 800 pc, M ~ 106 M
v ~ 25 km/sec
N
Note:most massive clouds are at bubble intersection
Yamaguchi et al. (2001, ApJ 553, L185)
shell diameter 1.9 kpcv(exp) = 10 – 40 km/sec
center: 400 OB stars ages 9-16 Myr
stars at the rim are < 6 Myr
H image, green contours: CO open circles: > 10 Myr clusters, filled circles: < 10 Myr old clusters
The Superbubble LMC4
Only 11 of 73 EGGs have YSOs
< 100 stars will eventually form in the pillars,
much less than the stellar population of the
exciting OB cluster NGC 6611
HST optical image; Hester et al. (1996)
"Pillars of Creation" in the Eagle Nebula (M16)
VLT near-infrared image; McCaughrean & Andersen (2002)
Detection of evaporating gaseous globules
"EGGs"; sites of triggered star formation ?
Triggered massive star formation in RCW 79Zavagno et al. (2006, A&A 446,171):blue: H, red: Spitzer 8 m
Data consistent with
"collect & collapse model"
fragmentation of the shocked dense
layer around an expanding HII region
Whitworth et al (1994)
central OB cluster,including an O4 star (~60 M)
compact HII region,ionized by an O9 star (~20 M),
contains many class I protostars
Radius of HII region = 6.4 pc
for n ~ 2000 cm-3 dyn = 1.7 Myr
collected layer fragmented ~105 yr ago,
consistent with ages of YSOs
30 Dor – NGC 2070 ( 40 O3 stars, 60 000 M , age = 2-3 Myr )
1'
15 pc
Radiation of the massive stars in the
central cluster triggers star formation
in the nebular filaments
(Rubio et al. 1998)
"Two stage starburst"
216 pc
Ori association Ori (O8) + 60 B-type stars, ~ 6 Myr
Dolan & Mathieu (2001, AJ 121, 2124) Dolan & Mathieu (2002, AJ 123, 387)
- global IMF consistent with field IMF
- low-mass stars concentrate
- near Ori, ages ~ 1 – 6 Myr
no ongoing star formation, age spread probably real
- near the dark clouds B30+B35
continuing star formation, ages ~ 0 - 6 Myr
IRAS 12+24+100 m
star formation begins
Star formation history:
1 Myr ago, a supernova explosion terminated
the star formation process near the center,
but there is still ongoing star formation in the
dark clouds B30 and B35
Ori
B30
B35
200 OB stars,
age ~ 3 Myr
Rcavity = 60 pc
Maiz-Apellaniz et al. (2004, AJ 128,1196)NGC 604 (in M33)
Superbubbles in W3/W4 Oey et al. (2005, AJ 129,393)
IC 1805: 1-3 Myr
W4
W3"230 pc shell"
age ~ 6 – 10 Myr
W3 Main: 105 yr
W3 North: 105 yr
W3 OH: 105 yr
IC 1795: 3-5 Myr
UCL
Upper Sco
Observation:
Initial configuration for Upper Sco:
from proper-motion back-tracing, Blaauw (1991)
elongation as signature of a swept up cloud
Model Predictions I:
swept-up clouds should be elongated
along the direction of the shock front
Model versus observations
Ori OB 1 association Briceño et al (2005, AJ 129, 907)
identify 197 low-mass members
of Ori OB 1a and 1b
derived ages:
1a: ~10 Myr, 1b: ~5 Myr
High- and low-mass stars are coeval
1a (~ 10 Myr)
1b (~ 3 Myr)
1c (~ 4 Myr)
1d (= ONC)
Ages of the OB stars from Brown et al (1994, A&A 289, 101)
2
16 pc
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