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Chandra Observations of the Galactic Super Star Cluster Westerlund 1

Chandra Observations of the Galactic Super Star Cluster Westerlund 1. Michael Muno (UCLA/Hubble Fellow). Super Star Clusters. M82 with HST; McCrady et al. 2003. Each “point source” is a 10 5 -10 6 M sun star cluster. These are the building blocks for the starburst phenomenon.

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Chandra Observations of the Galactic Super Star Cluster Westerlund 1

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  1. Chandra Observations of the Galactic Super Star Cluster Westerlund 1 Michael Muno (UCLA/Hubble Fellow)

  2. Super Star Clusters M82 with HST; McCrady et al. 2003 Each “point source” is a 105-106 Msun star cluster. These are the building blocks for the starburst phenomenon.

  3. Super Star Clusters M82 with Subaru • As building blocks of starbursts, we can study how feedback modifies the ISM and IGM. • As coeval stellar populations, we can study which stars leave behind black holes and neutron stars. M82 with Chandra

  4. The Unusual Stellar Population in Westerlund 1 • Over 25 Wolf-Rayet stars. • One confirmed LBV. • Several red supergiants. • Five yellow hypergiants. • Over 80 OB supergiants. • Main sequence 06 stars. 1 pc (e.g., Westerlund 1987, Clark et al. 2005) VRI from 2.2m MPG/ESO+WFI Clark et al. (2005)

  5. A Galactic Super Star Cluster? • 150 stars with M>35 Msun • Mass: 105 Msun • Extent: ~6 pc across • Distance: 5 kpc • Age: 4 +/- 1 Myr The cluster is coeval, and old enough to have produced supernovae. Est. rate: 1 per 10,000 years! 1 pc VRI from 2.2m MPG/ESO+WFI Clark et al. (2005)

  6. Chandra Observations 1 pc VRI from 2.2m MPG/ESO+WFI Clark et al. (2005) Chandra ACIS We see diffuse X-rays from the cluster wind and unresolved pre-main-sequence stars, point-like emission from colliding wind binaries, and black holes.

  7. Chandra Observations pulsar 1 pc VRI from 2.2m MPG/ESO+WFI Clark et al. (2005) Chandra ACIS We see diffuse X-rays from the cluster wind and unresolved pre-main-sequence stars, point-like emission from colliding wind binaries, and a pulsar!

  8. Pulsar CXO J164710.2-455216 • Period: 10.6107(1) s • Spin-down: <2x10-10 s s-1 • LX = 3x1033 erg s-1 (not a radio pulsar) • Spectrum: kT = 0.6 keV blackbody (not a cooling NS) • No IR counterpart, so K>18.5 (Mcount. < 1Msun; not an X-ray binary) This pulsar is almost certainly a magnetar.

  9. The Progenitor Was >40 Msun • The Pulsar is in Wd 1 (99.95% confidence) • A search of 300 archival Chandra and XMM fields reveals no new 5-30 s pulsars, so there is a <0.5% chance of finding one in any field (Nechita, Gaensler, Muno, et al. in prep). • The pulsar is well within the cluster, with a <10% chance of being an unrelated X-ray source. Position of pulsar Expected density of interlopers (dashed line, very small number)

  10. Other Neutron Stars with >30 Msun Progenitors 1E 1048.1-5937 SGR 1806-20 • A HI shell around 1E 1048.1-5937 was interpreted as the wind-blown bubble from a 30-40 Msun progenitor (Gaensler et al. 2005) • SGR 1806-20 is the member of a star cluster ~3 Myr old, and so had a ~50 Msun progenitor (Figer et al. 2005; also Vrba et al. 2000 for SGR 1900+14).

  11. WhichStars Form Black Holes? solar White Dwarf Metallicity Heger et al. 2003 metal-free 9 25 40 100 140 260 Initial Mass (Solar Masses)

  12. WhichStars Form Black Holes? Wd 1 solar White Dwarf Metallicity Heger et al. 2003 metal-free 9 25 40 100 140 260 Initial Mass (Solar Masses)

  13. WhichStars Form Black Holes? Wd 1 solar Cyg X-1 GX 301-2 White Dwarf Metallicity Heger et al. 2003 metal-free 9 25 40 100 140 260 Initial Mass (Solar Masses)

  14. Massive Progenitors to Neutron Stars • These pulsars show that massive stars can lose 95% of their mass: • Through winds (e.g., Heger et al 2003), • Via binary mass transfer (Wellstein & Langer 1999), • Or during supernovae (Akiyama & Wheeler 2005). • As magnetars, B-fields appear important: • Massive stars could produce rapidly-rotating cores (e.g., Duncan & Thomas 1992; Heger et al. 2005). • Or magnetars could form from highly-magnetic progenitors (e.g., Ferrario & Wickramasinghe 2005).

  15. Diffuse Emission from Wd 1 • After removing point sources, there is 3x1034 erg s-1 of diffuse emission.

  16. Spatial Distribution • Spatial distributions of point sources and diffuse emisison have 0.5 pc (20”) HWHM. • Diffuse emission also has an extended halo beyond 5 pc (3’).

  17. Contributions to Diffuse X-rays • Unresolved pre-main-sequence stars. • Stellar winds. • Supernovae. Orion Cas A Rosette Nebula

  18. Spectra of the Diffuse Emission Si S Ar Fe • Thermal plasma should exhibit lines of He-like Si, S, Ar, and Fe. • Those lines are weak in spectra from Westerlund 1, suggesting the emission is non-thermal.

  19. All You Need to Know About X-ray Spectra Colliding-wind binary: kT=2 keV Si S Ar Fe Si S Ar Fe • Note stronger lines from thermal plasma.

  20. Pre-Main-Sequence Stars Orion: Feigelson et al. 2005 Westerlund 1 • Age ~1 Myr. • 1398 optically-studied 0.3<M<3Msun stars. • LX = 1.2x1033 erg s-1, w/ line-rich, thermal spectra. • Age ~4 Myr. • LX = 3x1034 erg s-1, <20% thermal. • About 30,000 stars with 0.3<M<3Msun.

  21. Stellar Winds • With 25 WR stars, the power output is 1039 erg s-1, and the mass loss rate a few 10-4 Msun yr-1. • The entire diffuse flux from Westerlund 1 can be explained as a freely-expanding wind. • Most clusters are ~10x more luminous than would be expected from supersonic winds (Chevalier & Clegg 1985; Stevens & Hartwell 2003). • The density of the wind might be increased by entraining cooler material (Dorland & Montmerle 1987; Stevens & Hartwell 2003). • The pressure could be increased if the wind is confined by the ISM (Chu et al. 1995).

  22. The Rosette: Surrounded by ISM N. Outters Observatoire d’Orange Chandra/Townsley et al. 2003

  23. NGC 3603: Surrounded by ISM HST/Brandner et al. 2000 Chandra/Moffat et al. 2002

  24. Westerlund 1: ISM is Gone 2 pc 3.6, 4.5, and 8.0 mm: Spitzer/GLIMPSE (courtesy R. Indebetouw) Chandra

  25. Supernova Remnants? • Supernova rate is one per 10,000 yr. • At 1051 erg per SN, the power output is 3x1039 erg s-1. • Less than 10-5 of that is X-rays. 1 pc

  26. No Supernova Shocks in a Wind-Blown Bubble Westerlund 1 RCW 49 (Westerlund 2) 2 pc 3.6, 4.5, and 8.0 mm: Spitzer/GLIMPSE (courtesy R. Indebetouw) 3.6, 4.5, 5.8 and 8.0 mm: Spitzer/GLIMPSE (E. Churchwell et al.)

  27. The Diffuse Emission: A Puzzle • Pre-main-sequence stars are a minor (<20 %) contributor. • The lack of emission from colliding stellar winds is unexplained: • Does it emit out of the Chandra bandpass? • Does it interact with the ISM outside the Chandra field-of-view? • Does it fail to thermalize? • The non-thermal emission requires MeV electrons. Are these from SN or stellar winds?

  28. What’s Next: • More on Westerlund 1: • XMM observations: measure spin-down for the 10.6 s pulsar, search for more pulsars, and study more extended for diffuse X-rays. • Proposed pulsar search with Green Bank. • Other star clusters: • Chandra observations of clusters being discovered using 2MASS and Spitzer/ GLIMPSE.

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