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Westerlund 1
Starburst in our backyard
Ignacio Negueruela
Santiago 2006
This work is carried out in collaboration with
J. Simon Clark
Open University (UK)
Paul A. Crowther, Simon P. Goodwin
University of Sheffield (UK)
Michael Muno
University of California, Los Angeles (USA)
Wolfgang Brandner
Max-Planck-Institut für Astronomie (Germany)
Sean M. Dougherty
Dominion Radio Astrophysical Observatory (Canada)
Rens Waters
Astronomical Institute ``Anton Pannekoek'‘ (The Netherlands)
Outline
Introduction: modes of star formation in
the Milky Way and other galaxies.
The observations: finding Westerlund 1
The implications: the size of
Westerlund 1 and what we are learning
from it.
Star formation in the solar
neighbourhood
The field stellar population in the solar
neighbourhood has a wide range of ages.
We observe star formation
taking place preferentially
in relatively small
molecular clouds (such as
Bok globules), which form
small unbound groups of
intermediate and low-mass
stars.
This mode of star formation
appears dominant in local
star-forming regions,
such as the Taurus-Auriga
and  Ophiuchi clouds.
Image from ISO press release (ESA)
Figure from
Mamajek et al.
1997 (ApJ 516, L77)

Many such
groups contain
one
intermediatemass Herbig
Ae/Be star
and several T
Tauri stars.
Image by Lynne Hillenbrand, Caltech
Massive stars are not generally produced
in these small groups.
Open clusters
Clusters in the solar
neighbourhood (Orion
Spur, Perseus and
Sagittarius Arms)
generally have masses in
the 102 - 103 M range.
Battinelli & Capuzzo-Dolcetta (1991, MNRAS 249, 76),
from analysis of 100 clusters within 2kpc, find that
the typical mass of a Galactic cluster is 500 M.
The number of embedded
clusters suggests that ~90% of
stars are formed in protoclusters, but ~90% of the protoclusters disperse before
becoming optically visible
(Lada & Lada 2003, ARA&A 41,57)
In most clusters the IMF
has a broad maximum
around ~0.6 M
Images from Lada & Lada (2003)
There appears to be a good
statistical correlation between
the total mass of the cluster and
the mass of the most massive star
(Elmegreen 1983, MNRAS 203, 1011).
Such correlation is believed to stem from the size of the
molecular clouds and could be a statistical rather than
physical effect (Larson 1982, MNRAS 200, 159).
Lada & Lada (2003) estimate a
formation rate for open clusters in the
solar neighbourhood
R = (2-4) kpc-2 Myr-1
In order to form some massive
stars, a total mass  103 M is
necessary. The classical
example is the Orion Nebula
Cluster, where the Trapezium
contains four massive stars,
including 1 Ori C (O6 Vpe).
The total mass in the
molecular cloud associated
with M42 is  105 M, implying
that only a small fraction of
the gas is being turned into
stars.
Images from HST archive (NASA)
As massive stars are known
to be very disruptive for
their parental clouds, it is
generally believed that
they form relatively late
compared to low-mass
stars in the same cluster.
Image by David Hanon
Likewise, it is believed that clusters containing OB
stars form in the outskirts of massive clouds and
then give rise to unbound OB associations via
triggered star formation (e.g, Elmegreen 1983).
This is generally
assumed to be the
reason why, though
star formation
proceeds on a
typical scale
comparable to the
size of a giant
molecular cloud
(~80 pc, Efremov
1995, AJ 100, 2757),
Milky Way massive
clusters tend to be
much smaller.
Image taken from class by James Schombert, University of
Oregon
Massive clusters

The most massive clusters in the disk of the Milky
Way have typical masses approaching (but not
quite reaching) 104 Min stars.
These clusters are born with
several (10-20) O-type stars
and later evolve into supergiantrich clusters, such as h Per. By
this stage, they have already
blown out all rests of its
parental cloud, as indicated by
the lack of differential reddening
(e.g., Marco & Bernabeu 2001, A&A
372, 477).
The total mass of h & 
Persei is 3700 + 2800 M
in stars more massive
than the sun (M > 1M),
with a normal Salpeter’s
IMF down to this range
(Slesnick et al. 2002,
ApJ 576, 880).
Interestingly, this sort of massive
clusters tend to appear in complexes,
such as IC 1805 & IC 1848, or the
Carina complex Tr 14/16 + Cr 228 +
Tr 15 + NGC 3293.
It doesn’t have to be like this ...
The 30 Dor star forming region in the LMC
is 1kpc across and contains the 30-Dor
cluster, spanning  40pc.
Its central region, the R136
cluster is 2.7 pc across and
contains 3600 stars more
massive than M > 2.4M
(Hunter et al. 1996, ApJ 459, L27),
indicating a mass of  4x104 M
in stars more massive than
M > 1M.
Image from HST archive (NASA)
The central region
of R136 is so dense
that it was believed
to be a single supermassive star until
resolved by HST.
Images from HST archive (NASA)
Similar stellar densities
are observed in young
globular clusters, such as
NGC 1850.
Starburst phenomena
Images from HST archive (NASA)
Enhanced star formation
is seen in a wide variety
of galaxies, both
irregular and spiral.
Starburst irregular
galaxy NGC 4214
Spiral starburst
galaxy NGC
3310
Spiral starburst
galaxy NGC 4314
Starburst
activity in the
Antennae: over
one thousand
massive clusters
being formed
Images from HST archive (NASA)
Enhanced
star
formation in
Stephen’s
quintet
Collisions and other forms
of galaxy interaction seem
to be related to large
bursts of star formation.
Super star clusters
Ultradense HII regions (Johnson 2002, ASP
Conf. 267, p. 307):
Masses up to a few 106 M.
Ionising luminosities N ~1053 s-1
Size appears correlated to
intensity of starburst.
A continuum of masses from typical
Galactic clusters to the most massive
super-clusters.
Larsen (2004, ASP Conf. 322, p.19)
Whitmore (2000, ASP Conf. 197, p. 315)
Image of M51 from
HST archive (NASA)
Star formation in the past
There are strong indications that star
formation was more widespread and
stronger in the distant past.
Stellar populations have mostly
originated in starburst.
How can
we learn
about it?
Images from HST
archive (NASA)
Searching for massive
clusters in the Milky Way

NGC 6303 - 6-8 x103 M.
A
very young and compact object
(Moffat et al. 1994, ApJ 436, 183),
containing O3f and WR-like stars.
Cyg OB2 - nearby area of star
formation with 4-10 x104 M. Very
massive, but very extended (60
pc). (Knödlseder 2000, A&A 360, 539;
Comerón et al. 2002, A&A 389, 874 ).
Perhaps not so massive (Hanson 2003,

Images from HST
archive (NASA)
ApJ 597, 957)
Galactic Centre clusters
Quintuplet, Arches. Massive and compact clusters.
Galactic Centre “cluster”
The Arches has many massive stars, some with M>
100 M. (e.g., Figer et al. 2002, ApJ 581, 258)
The Arches has a top-heavy IMF (Stolte et al. 2002,
A&A 394, 459).
Images from HST
archive (NASA)
Galactic Centre clusters



The Arches has a top-heavy IMF.
Severe depletion of intermediate- and low-mass
stars (Stolte et al. 2005, ApJ 628, L113)
Total mass M < 104 M.
Very obscured.
Special
conditions?
NACO three-colour image from Stolte et al. (2005)
The W49 star-forming region
Again, a very
extended area of star
formation.
Very massive, but
distributed in several
clusters.
The most massive one
has 100-140 O-type
stars (Alves & Homeier
2003, ApJL, 589, 45)
Westerlund 1




Discovered by Bengt Westerlund in 1961 (PASP,
73, 51) in red plates - very reddened
Westerlund later found several blue, yellow and red
supergiants, some very luminous
(1987; A&AS 70, 311)
Two solutions proposed:
1) DM 13 with AV  11 (Westerlund ‘87)
2) DM 11 with AV  13 (Piatti et al. 1998, A&AS
127, 423)
Clark et al. (1998; MNRAS 295, L43) found
extended radio emission associated with two stars
But looks like
something in the
infrared!
Westerlund 1




Discovered by Bengt Westerlund in 1961 (PASP,
73, 51) in red plates - very reddened
Westerlund later found several blue, yellow and red
supergiants, some very luminous
(1987; A&AS 70, 311)
Two solutions proposed:
1) DM 13 with AV  11 (Westerlund ‘87)
2) DM 11 with AV  13 (Piatti et al. 1998, A&AS
127, 423)
Clark et al. (1998; MNRAS 295, L43) found
extended radio emission associated with two stars
V/(V-I) diagram
(from Westerlund ‘87)
Foreground
Wd 1
Westerlund used photographic plates for
photometry and derived absorption from lowresolution spectra.
Piatti et al. obtained
integrated
spectroscopy. Their
spectrum shows the
signature of late-type
SGs, from where they
inferred an age ~8 Myr.
Image from Piatti et al. (1998)
Westerlund 1




Discovered by Bengt Westerlund in 1961 (PASP,
73, 51) in red plates - very reddened
Westerlund later found several blue, yellow and red
supergiants, some very luminous
(1987; A&AS 70, 311)
Two solutions proposed:
1) DM 13 with AV  11 (Westerlund ‘87)
2) DM 11 with AV  13 (Piatti et al. 1998, A&AS
127, 423)
Clark et al. (1998; MNRAS 295, L43) found
extended radio emission associated with two stars
The role of the 1.52-m in the
VLT era
WN stars
WC stars
Clark & Negueruela (2002; A&A 396, L25)
1 arcmin
A
L
K
B
J
C
H
D
E
WN stars
G
F
WC stars
Clark & Negueruela (2002; A&A 396, L25)
3-cm radio map
These data are courtesy of
Sean Dougherty.
Red Supergiant with broad
emission features
3-cm radio map
These data are courtesy of
Sean Dougherty.
Blue luminous object with
narrow emission lines
3-cm radio map
These data are courtesy of
Sean Dougherty.
NTT/SUSI2 data
Local
population
Wd 1
AGB
Clark et al.
(2005; A&A 434, 949)
Intermediate resolution
spectroscopy
Taken on June 2002, 7th
NTT+EMMI
Red arm
 Grating 6
 0.36 A/pixel
 8200-8900
 Grating 7
 0.84 A/pixel
 6300-7800
Hypergiant candidates
The four YHG candidates
from Westerlund (1987) are
indeed extremely luminous:
Wd-16 A3Ia
V=15.9, MV =-9.7
Wd-12 A7Ia
V=16.9, MV =-9.8
Wd-4
F2Ia
V=14.4, MV =-10.0
Wd-265 F5Ia
V=17.1, MV =-9.5
A candidate LBV
Wd-243
Variable
spectrum around
A0Ia (Clark &
Negueruela 2004,
A&A 413, L15)
The stellar content of Wd 1 as
seen from the NTT
• 3 radio-bright M SGs
• 2 A-type HGs
• 2 F-type HGs
• A few very luminous B, A
and F SGs
• 1 LBV candidate
• 1 emission-line something
• 2 bright Ofpe/WNL
• 17 WR stars
• 23 OB SGs
Clark et al.
(2005; A&A 434, 949)
Negueruela & Clark
(2005; A&A 436, 541)
VLT observations
FORS2, 1 night in June 2004
 ISAAC high resolution spectroscopy
mode, 2 nights in June 2004 and 3 nights
in June 2005
 NACO, a few hours in service mode 2003
 Others coming

As OB
supergiants do
FORS2 MXU
observations with
G1200R and G1028z
There are well over
100 OB supergiants
in Wd 1
WN stars
FORS2 MOS observations
with G150I
A rich variety of everything …
Very luminous late-B SG
Luminous B3Ia stars
Weird early B emission-line
objects (quiescent LBVs?)
The infrared view
NTT+SOFI
Three-colour
image built
with JHK filters
The infrared view
VLT+NACO
Three-colour image
built with JHK filters
The ISAAC spectra identify O7-8 V stars in the cluster.
The initial mass function
The IMF is close to Salpeter’s for
the 3-30 M
Stars are hitting the MS at ~2 M
Brandner et al., in prep.
Parameters of Wd 1
Our best guess at parameters:
 E(B-V)  4.6
 AV  13
 (M - m)0  13.0 (d  4 kpc)
Extinction is very variable and not standard
Age  4 Myr
No less than 250 and likely more than 400
massive stars
For a standard Kroupa IMF, M>105M
The X-ray view
Muno et al.
(2006; ApJ 636, L41)
Muno et al., in prep
Clark et al., in prep
The X-ray view
Muno et al.
(2006; ApJ 636, L41)
CXO J164710.2-455216 is a 10.6 s pulsar!
The X-ray view
Muno et al., in prep
Clark et al., in prep
A population of interacting wind
binaries?
Conclusions
The massive stellar content of Wd 1 is larger
than that of any other open cluster in the
Galaxy.
There are more than one hundred post-MS
massive stars, many in short-lived transitional
phases (YHGs, LBVs, WRs, etc.).
There are no less than 250 and likely more than
400 massive stars.
The IMF does not seem very top-heavy
There is a large population of X-ray sources,
including an X-ray pulsar
Implications
There is star formation happening in the Milky
Way on very large scales.
Special conditions do not seem to be required for
this to occur.
We have the opportunity to study massive star
evolution in context
A large population of WR stars.
Transitional objects within an evolutionary
sequence.
Implications
With deep adaptive-optics IR imaging, we can
reach the low-mass stellar population of Wd 1
and study in situ the effects of large numbers of
massive stars on the IMF.
Neutron stars can be descended from very
massive stars. Confirmation that the 10.6 s
pulsar is a magnetar could show light on the
connection with progenitor mass.
Radio and X-ray observations will allow
investigation of emission mechanisms and role of
the environment.