sequentially triggered star formation in ob associations
DESCRIPTION
Sequentially Triggered Star Formation in OB Associations. Thomas Preibisch & Hans Zinnecker. Max-Planck-Institute for Radioastronomy, Bonn, Germany. Astrophysical Institute Potsdam, Germany. ISO. r Oph cloud. ageTRANSCRIPT
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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
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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)
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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
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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
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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
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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
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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
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= 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)
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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
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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
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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):
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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
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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
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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)
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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
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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
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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
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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
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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
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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
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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]
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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] )
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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:
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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!
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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)
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THE END
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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
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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
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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
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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
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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
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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 ?
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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
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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"
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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
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200 OB stars,
age ~ 3 Myr
Rcavity = 60 pc
Maiz-Apellaniz et al. (2004, AJ 128,1196)NGC 604 (in M33)
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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
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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
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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