Westerlund 1 Ignacio Negueruela Santiago 2006 Starburst in our backyard.

Westerlund 1

Ignacio Negueruela

Santiago 2006

Starburst in our backyard

This work is carried out in collaboration with

Paul A. Crowther, Simon P. GoodwinUniversity of Sheffield (UK)

Michael MunoUniversity of California, Los Angeles (USA)

Wolfgang BrandnerMax-Planck-Institut für Astronomie (Germany)

Sean M. DoughertyDominion Radio Astrophysical Observatory (Canada)

Rens WatersAstronomical Institute ``Anton Pannekoek'‘ (The Netherlands)

J. Simon ClarkOpen University (UK)

Outline

Introduction: modes of star formation in the Milky Way and other galaxies.The observations: finding Westerlund 1The 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.

Figure from Mamajek et

al. 1997 (ApJ 516, L77)

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)

Image by Lynne Hillenbrand, Caltech

Many such groups contain one intermediate-mass Herbig Ae/Be star and several T Tauri stars.

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 proto-clusters, but ~90% of the proto-clusters disperse before becoming optically visible (Lada & Lada 2003, ARA&A 41,57)

Images from Lada & Lada (2003)

In most clusters the IMF has a broad maximum around ~0.6 M

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

Images from HST archive (NASA)

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.

Image by David Hanon

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.

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).

Image taken from class by James Schombert, University of Oregon

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.

The most massive clusters in the disk of the Milky Way have typical masses approaching (but not quite reaching) 104 Min stars.

Massive clusters

These clusters are born with several (10-20) O-type stars and later evolve into supergiant-rich 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).

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.

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).

It doesn’t have to be like this ...

Image from HST archive (NASA)

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.


The central region of R136 is so dense that it was believed to be a single super-massive star until resolved by HST.

Similar stellar densities are observed in young globular clusters, such as NGC 1850.

Starburst phenomena

Starburst irregular galaxy NGC 4214

Spiral starburst galaxy NGC 3310


Spiral starburst galaxy NGC 4314

Enhanced star formation is seen in a wide variety of galaxies, both irregular and spiral.

Starburst activity in the Antennae: over one thousand massive clusters being formed

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

Image of M51 from HST archive (NASA)

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)

Star formation in the pastThere 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?

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, ApJ 597, 957)

Galactic Centre clustersQuintuplet, 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).


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 regionAgain, 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!

Foreground

Wd 1

V/(V-I) diagram (from Westerlund ‘87)

Westerlund used photographic plates for photometry and derived absorption from low-resolution 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)

The role of the 1.52-m in the VLT era

Clark & Negueruela (2002; A&A 396, L25)

WN stars WC stars

1 arcmin

A

B

DG

K

J

L

C

E F

H

WC starsWN starsClark & 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

Blue luminous object with narrow emission lines


3-cm radio map

Local population

AGB

Wd 1

NTT/SUSI2 data

Clark et al. (2005; A&A 434, 949)

Intermediate resolution spectroscopy

Taken on June 2002, 7thNTT+EMMI

Red arm Grating 6

0.36 A/pixel 8200-8900

Grating 7 0.84 A/pixel 6300-7800

Hypergiant candidatesThe 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 LBVWd-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 starsWeird early B emission-line objects (quiescent LBVs?)

The infrared view

NTT+SOFIThree-colour image built with JHK filters

The infrared viewVLT+NACOThree-colour image built with JHK filters

The ISAAC spectra identify O7-8 V stars in the cluster.

The initial mass function

Brandner et al., in prep.

The IMF is close to Salpeter’s for the 3-30 M

Stars are hitting the MS at ~2 M

Parameters of Wd 1Our 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 standardAge 4 MyrNo less than 250 and likely more than 400 massive starsFor a standard Kroupa IMF, M>105M

Muno et al. (2006; ApJ 636, L41)Muno et al., in prepClark et al., in prep

The X-ray view

Muno et al. (2006; ApJ 636, L41)

The X-ray view

CXO J164710.2-455216 is a 10.6 s pulsar!

Muno et al., in prepClark et al., in prep

The X-ray view

A population of interacting wind binaries?

ConclusionsThe 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-heavyThere 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.

Westerlund 1 Ignacio Negueruela Santiago 2006 Starburst in our backyard.

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