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

H. Sana &. NGC 6231: a young open cluster under X-rays *. E. Gosset Y. Naze G. Rauw J.-M. Vreux. & H. Sung. * an adaptation of the novel ‘An XMM-Newton view of the young open cluster NGC 6231’. Contents. Generalities XMM-Newton NGC 6231 The X-ray emitters in NGC 6231 :

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

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  1. H. Sana & NGC 6231: a young open cluster under X-rays* E. Gosset Y. Naze G. Rauw J.-M. Vreux & H. Sung * an adaptation of the novel ‘An XMM-Newton view of the young open cluster NGC 6231’

  2. NGC 6231: a cluster under X-rays Contents • Generalities • XMM-Newton • NGC 6231 • The X-ray emitters in NGC 6231 : • O-type stars • B-type stars • Optically faint X-ray emitters

  3. NGC 6231: a cluster under X-rays 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

  4. NGC 6231: a cluster under X-rays XMM-Newton • ESA X-ray Multi-Mirror observatory:  December 1999 X-ray spectro-photometry :  energy  time of arrival of every single X-ray photon • 3 X-ray telescopes containing 58 concentric mirrors each • large effective area • high sensitivity • high spectral resolution • moderate spatial resolution RGS: Reflection Grating Spectrometer (2 instr., Δλ= 0.04 Å ) EPIC: European Photon Imaging Camera (2 MOS + 1pn) OM: Optical Monitor

  5. NGC 6231: a cluster under X-rays • α = 16h54m, δ = -41°50’ (J2000) Scorpius N N NGC 6231 E E 2’ • d ~ 1.6 kpc • age ~ 3-5 Myr • rich early-type star population • core of the Sco OB1 association Sco OB1 NGC 6231 ~5° The young open cluster NGC 6231

  6. NGC 6231: a cluster under X-rays 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 Spectrometer EPIC: European Photon Imaging Camera OM: Optical Monitor • not usable (too faint) • not usable (too bright)

  7. NGC 6231: a cluster under X-rays 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 • ~ 30’ ~ 108 early-type stars * 92 B stars * 15 O stars * 1 WR

  8. NGC 6231: a cluster under X-rays An XMM-Newton view of NGC 6231 • 610 X-ray sources • > 520 optical/IR counterparts • Contamination by foreground/background sources ? • ~ 20 field stars (mostly F and G dwarfs) • ~ 21 active stars • ~ 16 extragalactic sources • Nature of the X-ray emitters in NGC 6231 • O-type stars • B-type stars • Optically faint X-ray emitter 0.5-1.0 keV 1.0-2.5 keV 2.5-10.0 keV Sana et al. 2006a, A&A 454, 1047

  9. 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 • short life-time (a few 106 yr) • few in number • 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) NGC 6231: a cluster under X-rays

  10. 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 • short life-time (a few 106 yr) • few in number 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 ? NGC 6231: a cluster under X-rays

  11. Berghöfer et al. 1997 X-ray emission from O-type stars Rosat All Sky Survey • 1978: launch of EINSTEIN: •  massive stars are • ‘intrinsic’ X-ray emitters Log LX Log Lbol NGC 6231: a cluster under X-rays

  12. Berghöfer et al. 1997 X-ray emission from O-type stars 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 ~ 10-7Lbol Log LX Log Lbol NGC 6231: a cluster under X-rays

  13. X-ray emission from O-type stars • 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 ~ 10-7Lbol 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 NGC 6231: a cluster under X-rays

  14. NGC 6231: a cluster under X-rays O-type X-ray emitters in NGC 6231

  15. NGC 6231: a cluster under X-rays O-type X-ray emitters in NGC 6231 All the O-type stars are detected

  16. NGC 6231: a cluster under X-rays O-type X-ray emitters in NGC 6231 All the O-type stars are detected as bright and soft X-ray sources To be fitted by optically thin thermal plasma (e.g. mekal models)  flux  Lx

  17. NGC 6231: a cluster under X-rays O-type X-ray emitters in NGC 6231 Stars of spectral type O9 or earlier To be fitted by optically thin thermal plasma (e.g. mekal models)  flux  Lx

  18. O-type X-ray emitters in NGC 6231 HD 152248 (cnt s-1) III Sana et al. 2004, MNRAS 350,809 CPD-41°7742 (10-3 cnt s-1) Sana et al. 2005a,A&A 441, 213 Sana et al. 2006b, MNRAS 372, 661 Filled : O-type Open : B-type I V Binary Single LPVs Stars of spectral type O9 or earlier NGC 6231: a cluster under X-rays

  19. O-type X-ray emitters in NGC 6231 III Sana et al. 2006b, MNRAS 372, 661 Filled : O-type Open : B-type I V Binary Single LPVs Canonical relation for O stars: • 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 NGC 6231: a cluster under X-rays

  20. O-type X-ray emitters in NGC 6231 • What have we learned from XMM-Newton ? • The canonical relation is tightly constrained • but 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 I V NGC 6231: a cluster under X-rays

  21. Berghöfer et al. 1997 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 toX-ray sources e.g. Berghofer et al. 1997 : 10% detection rate  hidden companion ? (e.g. Stelzer et al. 2003) NGC 6231: a cluster under X-rays

  22. NGC 6231: a cluster under X-rays B-type X-ray emitters in NGC 6231 • 92 B-type stars in the FOV • ~ 20% are associated with an X-ray source

  23. NGC 6231: a cluster under X-rays 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

  24. NGC 6231: a cluster under X-rays 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 • 4/15 display flare-like activity • spectral properties indistinguishable from those of other faint X-ray sources B star X-ray faint star

  25. NGC 6231: a cluster under X-rays 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 • 4/15 display flare-like activity • spectral properties indistinguishable from those of other faint X-ray sources  favours the hidden companion hypothesis

  26. NGC 6231: a cluster under X-rays 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 • 4/15 display flare-like activity • spectral properties indistinguishable from those of other faint X-ray sources  favours the hidden companion hypothesis but still…

  27. B-type X-ray emitters in NGC 6231 III Sana et al. 2006b, MNRAS 372, 661 Filled : O-type Open : B-type I V Binary Single LPVs LX-Lbol relation for detected B-stars: • limited dispersion; linear correlation coefficient r ~ 0.75 • significant at 0.99 confidence level NGC 6231: a cluster under X-rays

  28. B-type X-ray emitters in NGC 6231 • What have we learned from XMM-Newton ? • 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 ???? I V NGC 6231: a cluster under X-rays

  29. Optically faint X-ray sources in NGC 6231 • 610 X-ray sources ~ 60 foreground/background sources ~ 15 O-type stars + 1 WR ~ 18 B-type stars (if any) • What are the > 500 remaining sources ? • most of them have V~16-20 optical counterparts • NGC 6231 age ~ 3-5 Myr  All stars with M < 2 Msol are still in the PMS stage NGC 6231: a cluster under X-rays

  30. 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 NGC 6231: a cluster under X-rays

  31. NGC 6231: a cluster under X-rays 7.0M 0.2M X-ray source X-ray source X-ray + Hα candidate X-ray + Hα candidate X-ray + Hα emitter X-ray + Hα emitter An XMM-Newton view of NGC 6231 0.5-1.0 keV 1.0-2.5 keV 2.5-10.0 keV Sana et al. 2007, MNRAS, submitted

  32. NGC 6231: a cluster under X-rays 7.0M 0.2M 0.5Myr X-ray source 1.5Myr 4.0Myr X-ray + Hα candidate 10.0Myr X-ray + Hα emitter 20.0Myr An EPIC view of NGC 6231 0.5-1.0 keV 1.0-2.5 keV 2.5-10.0 keV Sana et al. 2007, MNRAS, submitted

  33. NGC 6231: a cluster under X-rays O-type stars 2Myr 4Myr 6Myr 8Myr An EPIC view of NGC 6231 0.5-1.0 keV 1.0-2.5 keV 2.5-10.0 keV Sana et al. 2007, MNRAS, submitted

  34. PMS X-ray emitters in NGC 6231 • What have we learned from XMM-Newton ? • 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 I V NGC 6231: a cluster under X-rays

  35. NGC 6231: a cluster under X-rays 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

  36. NGC 6231: a cluster under X-rays 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

  37. 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 NGC 6231: a cluster under X-rays

  38. NGC 6231: a cluster under X-rays Traquer les propriétés physiques : la spectroscopie, un outil puissant

  39. NGC 6231: a cluster under X-rays Traquer les propriétés physiques

  40. NGC 6231: a cluster under X-rays O miroir mon beau miroir, quelle est la plus massive d’entre nous ? • Contraintes observationnelles • 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

  41. NGC 6231: a cluster under X-rays O miroir mon beau miroir, quelle est la plus massive d’entre nous ? • Contraintes observationnelles • Déficit d’étoiles au dessus de 120-150 M • Considération théorique • Ledoux : instabilité vibrationnelle • Mesures directes : difficiles à mettre en oeuvre • R136 • WR20a

  42. NGC 6231: a cluster under X-rays 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 énergétique • Lx ~ 1031-1033 erg s-1 • Pic d’émission ≤ 1keV • Relation canonique* :Lx ~ 10-7Lbol * large dispersion observée

  43. NGC 6231: a cluster under X-rays Binaires à collision de vents

  44. NGC 6231: a cluster under X-rays Binaires à collision de vents courbes de lumière X

  45. NGC 6231: a cluster under X-rays Interaction d’un type particulier

  46. NGC 6231: a cluster under X-rays Merci de votre attention Conclusions : * Cent mille à un million de fois plus lumineuses que le soleil * Au moins 5 à 6 fois plus chaudes,  objets fascinants et variés * Leur leitmotiv : "Vivre vite, mourir jeune … …et faire un maximum de bruit

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