1. Uli Heber Bad Honnef, 8.12.2006 Hyper-velocity stars in the Milky Way

2. Outline  Galactic structure  Run-away stars  hyper-velocity stars - massive black holes as slingshots - an old helium star - a young main sequence star - an even younger giant  hyper-velocity star sample

3. Components of the Galaxy Buser Dark Halo Very old stars Old stars young & old stars

4. Stellar populations High mass, short-lived Low mass, Long-lived from Moehler

5. M 15 UV

6. Hot subluminous stars SdB + sdO stars: Extreme Horizontal Branch stars EHB HB sdB sdO Dorman et al. (1993, ApJ 419, 596)

7. Faint Blue stars at high galactic latitudes  UV-excess surveys aim at QSO - photometric: PG (Palomar Green) EC (Edinburgh Cape) - objective prism: HS (Hamburg Schmidt) HE (Hamburg ESO)  Population of faint blue stars: white dwarfs, hot subdwarfs, BHB, pAGB...

8. Apparently normal B stars at Galactic latitudes Greenstein & Sargent (1972) Significant fraction of faint blue stars at high galactic latitudes may be normal B-stars HS 1914+7135 Mass: 6-10 M o distance: 6.5-7.5 kpc Heber, et al. (1995, A&A 303, L33)

9.  Why is it difficult to distinguish a main sequence star from a blue horizontal branch (BHB) star? HRD T eff -log g sdB sdO BHB /MS? Hunger & Heber (1987)

10. Apparently normal B stars at high Galactic latitudes  Massive B stars and blue Horizontal Branch stars: similar T eff and log g, different mass distances!  How to distinguish a massive B star from a BHB star? BHB: - low helium - weird metal abundance pattern - slow rotators massive B stars: - normal abundance pattern - fast rotation

11. Run-away stars  Normally massive stars are found in the Galactic plane ejection scenario: born in the plane and ejected (Blaauw, 1961)  Calculate path and time of flight: - radial velocities, distances & proper motion - orbit integrator: Odenkirchen & Brosche (1992) - Galactic potential: Allen & Santillan (1991)

12. Supernovae in binary systems Massive binaries: primary explodes as Supernova neutron star secondary is released at orbital velocity: <200km/s

13. Dynamical ejection scenario  Dynamical interaction of a binary with a single star or another binary can lead to ejection at velocities of a few hundred km/s (Leonard & Duncan 1988, 1990)

14. Apparently normal blue stars at high galactic latitude About 100 analysed : Almost all can be explained by ejection from the plane - ejection velocities typically 100 – 200 km/s - T flight < T evol - cluster origin has been proven for a few stars from Hipparcos parallaxes Dynamic ejection and binary supernova scenarii are in good shape !

15. „Hyper-velocity“: speed limits in space 500 km/s How fast can a „run-away“ star travel? May a star leave the Galaxy? Exceed the Galactic escape velocity: Solar-neighbourhood: Galactic Escape Speed: 544 km/s (498...608 km/s) (Smith et al. 2006, astroph/0611671) 300 km/s

16. The massive black hole in the centre of the Galaxy Star S2: - orbital period: 15 yrs -d min =120 AU -V max =5000 km/s -M BH =2.6 10 6 M sun Schödel et al. (2003, ApJ 596, 1015)

17. Tidal disruption of a binary Hills (1988):  Disruption of a binary near the SMBH releases companion at up to 1000 km/s or more.  Detection of a single HVS: evidence for a SMBH Yu & Tremaine (2003)

18. Numerical predictions Slingshot mechanism for the MBH in the Galactic centre:  HVS production rate: 1 HVS/100000yrs (Yu & Tremaine, 2003) Halo: 2000 HVS  binary MBH (Yu & Tremaine, 2003; Levin, 2005; Baumgart et al., 2006; Sesana et al. 2006) HVS production rate: 10 times larger - single stars can also be ejected

19. The first hyper-velocity star Sample auf HBA stars from SDSS (Brown et al. 2005, ApJ 622, L33) Vrad = 853km/s (hel.) > 709km/s (gal.); pm=0 Late B-type (B=19.8 m ) d=40kpc (if HB) d=110kpc (if MS) Unbound to Galaxy

20. sdO stars from SDSS candidates selected from all releases according to colour: u-g<0.2 (0.4) g-r<0.1  11000 spectra:  40 sdO + 43 He sdO Radial velocities HVS

21. The second hyper-velocity star Spectrum from Keck I +LRIS Hel. RV=708km/s Gal. RV=751km/s ( (pm=0) Helium star (sdO) - Low mass: 0.5M sun - distance: 20kpc Hirsch, Heber, O´Toole & Bresolin (2005, A&A 441, L61) Old helium star

22. US 708: Keck LRIS spectrum T eff = 45500K, log g = 5.23, mass = 0.5 M o B=19.0 mag Distance: 19 kpc

23. Kinematics of US 708  vgal = 751km/s < v esc = 430km/s unbound  Can be traced back to the Galactic Center: - proper motion required: pm RA = -2.3 mas/yr pm DE = -2.4 mas/yr - t flight =32 Myrs < t evol = 100 Myrs MBH slingshot ejection from the Galactic Centre is plausible: - US 708 was in binary, disrupted by tidal interaction with MBH

24. HE 0437-5439 = HVS 3 VLT-UVES: v rad = 723+-3 km/s v gal > 563 km/s (pm=0) Teff = 20400 K Log g = 3.8 normal Helium B=16.2 mag Edelmann, Napiwotzki, Heber, Christlieb & Reimers (2005, ApJ 633, L181) : VLT UVES

25. HE 0437-5439: metals & rotation V rot sini =54 km/s metals: solar (to within a factor of 3) Main sequence star … = 1/3 solar, --- = 3*solar, full drawn= solar

26. Mass, distance and age Comparison to evolutionary tracks for ms stars: Mass = 8 M o Distance: 60 kpc Age = 25 Myr v gal > 563 km/s > v esc = 317 km/s unbound to Galaxy 25Myrs

27. Kinematics Time of flight to GC: 100 Myrs = 4 times T evol !! Alternatives: - Blue Straggler - Other formation channel: not from Milky Way Galactic plane

28. Origin in the LMC ?  Star is beyond LMC  Closer to LMC (18kpc) than to Galaxy  Can reach present position within T evol: V eject =600km/s (unbound to LMC) pmRA=2mas/yr (relative to LMC) Where is the massive black hole?

29. Eight HVS Heber et al. in prep. Edelmann et al., in prep. 4 more HVS discovered by Harvard survey (Brown et al. 2006, ApJ 640, L35; Brown et al. 2006, astro-ph/0604111 ): HVS 4-7 HVS 8: Edelmann et al. in prep.

30. Lifetime vs travel time Assuming MS-distances: Star d/kpc | t flight /Myrs | t evol /Myrs ---------------------------------------------------- HVS 4 72 | 130 | 140 HVS 5 38 | 55 | 240 HVS 6 51 | 105 | 220 HVS 7 96 | 240 | 160 Errors: +-20% HVS 4-6: o.k. HVS 7: ?????

31. OM 88 = HD 271791 V=12.3 T eff = 17800K log g = 3.0 normal He/H v rad (hel.) = 440 km/s ESO 2.2m: FEROS

32. OM 88: metal lines & rotation v rot sin i =124km/s solar metals Massive giant star

33. OM 88: mass, distance & age Mass: 11.5 M o solar metals Distance: 24 kpc Age: 17 Myrs Proper motions: Hipparcos, UCAC2, USNO-B1, ATC,... HIP: μ α = -1.0 mas/yr μ δ = +7.0 mas/yr Time of flight from GC: 90Myr 17Myrs

34. Why are the HVS blue? The S-stars in the Galactic centre: Helium lines Hot blue stars Two young star disks in the central Parsec (Paumard et al, 2006 ApJ 643, 1011) (Eisenhauer et al 2005, ApJ, 628, 246)

35. Hyper-velocity stars - are not extreme run-away stars - are unbound to the Galaxy tidal disruption of a binary by a SMBH discovery of three hyper-velocity stars: - sdO star: could be ejected from SMBH in GC - massive B star: ejected from LMC ??? - young giant, eject from ?? HVS known form a class of star Origin in GC possible for the long-lived stars SMBH slingshot may not be the only mechanism!

36. Predictions Astrometry (NTT, 20-24. 12.2006)  US 708 (B=19): if ejected from GC proper motion: pmRA=-2.2mas/yr pmDE=-2.4mas/yr  HE 0437-5439(B=16): if ejected from GC: pm < 0.5 mas/yr if ejected from LMC: pm about 2 mas/yr (relative to LMC) Spectroscopy  HE 0437-5439: abundances may discriminate between origin in Galaxy or LMC HVS 7 (proposed, P79) Survey: Calar Alto 3.5m (3n, Feb. 2007)

37. The team Simon O´Toole Heinz Edelmann Heiko Hirsch Ralf Napiwotzki

38. The team Heinz Edelmann (Bamberg, Austin) Heiko Hirsch (Bamberg) Simon O´Toole (Bamberg, Sydney) Ralf Napiwotzki (Hatfield) Martin Altmann (Santiago) Fabio Bresolin (Hawaii) Uli Heber (Bamberg)

39. Kinematics Time of flight to GC: 100 Myrs = 4 times T evol !! Alternatives: - Blue Straggler = merger (?) of two lower mass MS stars Merger rate very low (Gualandis et al. 2005, ) - Other formation channel: not from Milky Way Galactic plane

40. Blue stars in the center of M31

41. Binary Population Synthesis (BPS) Han et al. (2003) a: 1. CE ejection b: 1. stable RLOF c: 2. CE ejection d: merger merger

42. HVS formed through encounters with stellar-mass black holes New model O´Leary & Loeb (2006, astroph/0609046)  Cluster of stellar-mass black holes orbiting Sgr*  scattering of stars with these black holes  production rates similar to slingshot mechanism  How many „hyper-velocity“ stars are out there?

43. BPS Han et al: Binary population synthesis a) Without GK selection b) With GK selection merger

44. Comparison to Han et al. (HPMM) sdBs: best match: models with correlated masses and low CEE efficiency Poor match: models with 100% CEE efficiency O-types: He-sdO: stars clump at 45000K, too hot for any HPMM simulation set sdO: scattered in (Teff, log g) diagram Ströer et al. 2005 SPY: sdB & sdO

45. SDSS-sdOs Atmospheric models: - NLTE: - H+He, no metals - PRO2 code (Dreizler &Werner) - improved He atomic models - temperature correction scheme (Dreizler, 2003) Hirsch (diploma thesis) sdO He sdO

46. The SAO HVS survey Brown et al. (2006, ApJ 640, L35): 2 HVS at about 600km/s Brown et al. (2006, astro-ph/0604111): 2 HVS at 500 & 600 km/s

47. Kinematics of US 708

48. Stellar masses and ages Stellares masses: 1/10.... 80 x solar masse Mass – luminosity relation: L ~ M 3.5 Massive stars are luminous and die young

49. Apparently normal B stars at Galactic latitudes HS 1914+7135 (Heber, Moehler & Groote 1995) High projected rotation velocity: v rot sin i =260 km/s Mass: 6-10 M o distance: 6.5-7.5 kpc

53. Mix of spectral types  bright magnitudes: hot subluminous stars dominate: sdB, sdO Green et al. (1986, ApJS 61,305)  white dwarfs and QSO at fainter magnitudes sdB sd O WDWD QSO