NGC 6231: a young open cluster under X-rays *

NGC 6231: a young open cluster under X-rays*

H. Sana &

E. GossetY. NazeG. Rauw

J.-M. Vreux

& H. Sung

* an adaptation of the novel ‘An XMM-Newton view of the young open cluster NGC 6231’

20-Dec-200620-Dec-2006 NGC 6231: a cluster under X-raysNGC 6231: a cluster under X-rays

Contents

Generalities Generalities XMM-NewtonXMM-Newton NGC 6231NGC 6231

The X-ray emitters in NGC 6231 :The X-ray emitters in NGC 6231 : O-type starsO-type stars B-type starsB-type stars Optically faint X-ray emitters Optically faint X-ray emitters

20-Dec-200620-Dec-2006 NGC 6231: a cluster under X-raysNGC 6231: a cluster under X-rays 33

XMM-Newton

• ESA X-ray Multi-Mirror observatory: December 1999

• 3 X-ray telescopes containing 58 concentric mirrors each

• large effective area• high sensitivity• high spectral resolution• moderate spatial resolution


• 3 X-ray telescopes containing 58 concentric mirrors each

• large effective area• high sensitivity• high spectral resolution• moderate spatial resolution

• ESA X-ray Multi-Mirror observatory: December 1999

RGS: Reflection Grating Spectrometer (2 instr., Δλ= 0.04 Å )

EPIC: European Photon Imaging Camera (2 MOS + 1pn)

OM: Optical Monitor

XMM-Newton

X-ray spectro-photometry :

energy time of arrival

of every single X-ray photon


N

E

Scorpius

~5°

The young open cluster NGC 6231

NGC 6231

Sco OB1

2’

NGC 6231

N

E

• d ~ 1.6 kpc

• age ~ 3-5 Myr

• rich early-type star population

• core of the Sco OB1 association

• α = 16h54m, δ = -41°50’ (J2000)


The X-ray observing campaign

• September 2001 : XMM-Newton – 180 ks (~50h)

– splitted in six 30ks observations spread over 5 days

• One of the deepest exposures towards a young open cluster

RGS: Reflection Grating SpectrometerEPIC: European Photon Imaging CameraOM: Optical Monitor

not usable (too faint)

not usable (too bright)


The X-ray observing campaign

~ 30’ ~ 108 early-type stars * 92 B stars * 15 O stars * 1 WR

• September 2001 : XMM-Newton – 180 ks (~50h)

– splitted in six 30ks observations spread over 5 days

• One of the deepest exposures towards a young open cluster


610 X-ray sources 610 X-ray sources

> 520 optical/IR counterparts> 520 optical/IR counterparts

0.5-1.0 keV1.0-2.5 keV

2.5-10.0 keV

An XMM-Newton view of NGC 6231

Sana et al. 2006a,

A&A 454, 1047

Nature of the X-ray emitters in NGC 6231Nature of the X-ray emitters in NGC 6231 O-type starsO-type stars

B-type starsB-type stars

Optically faint X-ray emitterOptically faint X-ray emitter

Contamination by foreground/background sources ?Contamination by foreground/background sources ? ~ 20 field stars (mostly F and G dwarfs)~ 20 field stars (mostly F and G dwarfs)

~ 21 active stars~ 21 active stars

~ 16 extragalactic sources~ 16 extragalactic sources


O-type stars are hot, massive stars

O-type stars:– 30 kK < Teff < 45 kK– 15 M < M* < ?– 5×104 L < L* < 5×105 L

• Powerful stellar winds

– 10-7 M yr-1 < Ṁ* < 10-4 M yr-1 affect the star’s evolution

– v∞ ~ 2000-3000 km s-1 ( v = v∞ (1–R*/r)• Strong influence (radiative & kinetic energy, chemical input)

short life-time (a few 106 yr) few in number


O-type stars are hot, massive stars

O-type stars:– 30 kK < Teff < 45 kK– 15 M < M* < ?– 5×104 L < L* < 5×105 L

A (very) few hot topics:

• Upper mass limit : ~ 100-150 M • Formation process : coalescence vs. accretion

• Convective core but radiative enveloppe no surface B field

still a few O stars have one ! (e.g. Gagne et al. 2005)

fossile field ? / transported from the core to the enveloppe ?

short life-time (a few 106 yr) few in number


X-ray emission from O-type stars

Berghöfer et al. 1997 Log Lbol

Log

LX

Rosat All Sky Survey

• 1978: launch of EINSTEIN: massive stars are‘intrinsic’ X-ray emitters



Berghöfer et al. 1997 Log Lbol

Log

LX

Rosat All Sky Survey

• 1978: launch of EINSTEIN: massive stars are‘intrinsic’ X-ray emitters

• Produced by shocks within the dense layers of the winds

• Strong but soft emission– Lx ~ 1031-1033 erg s-1 – Emission peak ≤ 1keV

• Harnden et al. 1979; Long &

White1980; Pallavicini et al. 1981; …

Canonical relation Lx ~ 10-7 Lbol


• Produced by shocks within the dense layers of the winds

• Strong but soft emission– Lx ~ 1031-1033 erg s-1 – Emission peak ≤ 1keV

• Harnden et al. 1979; …

Canonical relation:

Lx ~ 10-7 Lbol


Mass of star A larger than mass of star B

• Pollock 1987: WR binaries tend to be more X-ray luminous compared to equivalent single stars • Chlebowski & Garmany 1991: same trend for O-type stars

signature of a wind interaction


O-type X-ray emitters in NGC 6231



All the O-type stars are detected



To be fitted by optically thin thermal plasma (e.g. mekal models) flux Lx

All the O-type stars are detected as

bright and soft X-ray sources


Stars of spectral type O9 or earlier


To be fitted by optically thin thermal plasma (e.g. mekal models) flux Lx


Sana et al. 2006b, MNRAS 372, 661Filled : O-type

Open : B-type

Binary

Single

LPVs

III

V

I

CPD-41°7742 (10-3 cnt s-1)

Sana et al. 2005a,A&A 441, 213

HD 152248 (cnt s-1)

Sana et al. 2004, MNRAS 350,809


Stars of spectral type O9 or earlier


Filled : O-type Open : B-type

Binary

Single

LPVs

III

V

I

• dispersion ~ 40% much more limited than previously admitted

• within our sample, O-type star X-ray variability only seen for probable CWBs + only identified mechanism that produces a significant deviation from the ‘canonical’ relation

)153.0(912.6loglog bolX LL

Canonical relation for O starsO stars:

Sana et al. 2006b, MNRAS 372, 661



V

I


What have we learned from XMM-Newton ?

• The canonical relation is tightly constrainedbut still not understood on firm theoretical ground !

• Our sample is inhomogeneous in terms of spectral sub-type and luminosity class same mechanism for all

• Confirmed by a recent study by Anthokin et al. on Cyg OB2

• Very limited room for the B field


B-type stars

– 10 kK < Teff < 30 kK

– 3 M < M* < 15 M

• radiative enveloppe no dynamo no surface B field

• Weak winds weak shocks (if any)

No X-ray emission expected

but still … some B-type stars are associated to X-ray sources e.g. Berghofer et al. 1997 : 10% detection rate

hidden companion ? (e.g. Stelzer et al. 2003)

Berghöfer et al. 1997


B-type X-ray emitters in NGC 6231

• 92 B-type stars in the FOV

• ~ 20% are associated with an X-ray source





• fainter and harder than O stars

B star

O star






• 4/15 display flare-like activity

• spectral properties indistinguishable from those of other faint X-ray sources

B star

X-ray faint star








favours the hidden companion hypothesis








favours the hidden companion hypothesis but still…


Filled : O-type Open : B-type

Binary

Single

LPVs

III

V

I

• limited dispersion; linear correlation coefficient r ~ 0.75 significant at 0.99 confidence level

)4.2(8.22log)06.0(22.0log bolX LL

LX-Lbol relation for detected B-starsB-stars:

Sana et al. 2006b, MNRAS 372, 661



V

I



• The detected X-ray sources associated with B-type stars are probably ‘hidden’ companion

• There seems to be a link between Lx and Lbol if real, its origin is ????


Optically faint X-ray sources in NGC 6231

610 X-ray sources 610 X-ray sources

~ 60 foreground/background sources~ 60 foreground/background sources

~ 15 O-type stars + 1 WR~ 15 O-type stars + 1 WR

~ 18 B-type stars (if any)~ 18 B-type stars (if any)

What are the > 500 remaining sources ?What are the > 500 remaining sources ?

most of them have V~16-20 optical counterpartsmost of them have V~16-20 optical counterparts

• NGC 6231 age ~ 3-5 Myr NGC 6231 age ~ 3-5 Myr All stars with M All stars with M < 2 Msol are still in the PMS stage< 2 Msol are still in the PMS stage


Pre-Main Sequence (PMS) stars

The main kind of PMS are the T Tauri objects :

• Classical T Tauri (cTTs) : • emission in the Balmer lines• near-IR excess (accretion disk)

• Weak-line T-Tauri (wTTs) : • no significant IR excess• weak Balmer emission (if any) difficult to disentangle from field stars

X-ray is one of the best criterion


An XMM-Newton view of NGC 6231

0.5-1.0 keV1.0-2.5 keV

2.5-10.0 keVSana et al. 2007, MNRAS, submitted

X-ray + Hα candidateX-ray + Hα emitter

X-ray source

7.0M

0.2M

X-ray + Hα candidateX-ray + Hα emitter

X-ray source


An EPIC view of NGC 6231

0.5-1.0 keV1.0-2.5 keV

2.5-10.0 keV

7.0M

0.2M

10.0Myr

0.5Myr1.5Myr

20.0Myr

4.0MyrX-ray + Hα candidateX-ray + Hα emitter

X-ray source

Sana et al. 2007, MNRAS, submitted


An EPIC view of NGC 6231

0.5-1.0 keV1.0-2.5 keV

2.5-10.0 keV

O-type stars

2Myr

4Myr

6Myr

8Myr

Sana et al. 2007, MNRAS, submitted


V

I

PMS X-ray emitters in NGC 6231


• XMM has allowed to disentangle the PMS from the numerous field stars

• There is no spatial trends between the cTTs and the wTTs

• There is no very significant difference in their ages

• Star formation in NGC 6231 was most probably not a punctual event

• Formation scenario is compatible with the coalescence model for massive star formation


What have we learned from XMM-Newton

One of the deepest X-ray observations of a young open cluster, spread over 5 days Crowded FOV: 610 sources (mostly PMS)

star formation history in NGC 6231: not a punctual event

O-type stars display a limited dispersion around the canonical relation : within our sample, X-ray variability only seen for probable CWBs only identified mechanism that produces a significant deviation from the

‘canonical’ relation we need to compare the new ‘canonical’ relation with other relations derived

in a homogeneous way At least two exceptional O-type colliding wind binaries (CWBs) X-ray emission associated with B-type stars? PMS companions


Final remarks

X-ray observations supported by high quality optical data are a powerful tool across the HR diagram

Young open clusters are privileged laboratories - homogeneous population : age, distance, environment

Thank you for your attention


Colliding wind binaries (CWBs)• In a massive binary system

– The winds from the two stars collide Gas heated up: T ≈ a few 107 K

kT ≈ a few keV

substantial X-ray emission can display phase-locked modulations due :

• to the changing opacities along the line of sight• in an eccentric system, to the variations of the shock strength because

of the changing separation

– The geometry of the interaction depends on the balance between the ram pressure of the two winds the interaction region wraps around the star that has the weaker

wind


Traquer les propriétés physiques : la spectroscopie, un outil puissant


Traquer les propriétés physiques


O miroir mon beau miroir, quelle est la plus massive d’entre nous ?

Contraintes observationnellesContraintes observationnelles Amas des Arches Amas des Arches

Centre galactique

Amas des Arches : * Pop.: 2000 étoiles* Age ~ 2-2.5 Myr* D ~ 25 000 a.l.

Particules de haute énergie


O miroir mon beau miroir, quelle est la plus massive d’entre nous ?

Contraintes observationnellesContraintes observationnelles Déficit d’étoiles au dessus de 120-150 MDéficit d’étoiles au dessus de 120-150 M

Considération théoriqueConsidération théorique Ledoux : instabilité vibrationnelleLedoux : instabilité vibrationnelle

Mesures directes : difficiles à mettre en oeuvreMesures directes : difficiles à mettre en oeuvre R136R136 WR20aWR20a


Berghöfer et al. 1997

Emission X des étoiles O

Produite par des chocs dans les basses couches du vent

Emission X intense mais

relativement peu

énergétiqueLx ~ 1031-1033 erg s-1 Pic d’émission ≤ 1keV

Relation canonique* :

Lx ~ 10-7 Lbol

* large dispersion observée


Binaires à collision de vents

keV2.1k

K104.12,8

2,8

7

vT

vT


Binaires à collision de vents courbes de lumière X


Interaction d’un type particulier


Merci de votre attention

Conclusions :Conclusions :

* Cent mille à un million de fois plus * Cent mille à un million de fois plus lumineuses que le soleillumineuses que le soleil

* Au moins 5 à 6 fois plus chaudes, * Au moins 5 à 6 fois plus chaudes,

objets fascinants et variésobjets fascinants et variés

* Leur leitmotiv : "Vivre vite, mourir jeune …* Leur leitmotiv : "Vivre vite, mourir jeune …

……et faire un maximum de bruitet faire un maximum de bruit

NGC 6231: a young open cluster under X-rays *

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