Uli Heber Oxford, Hyper-velocity stars

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  stellar population of the Galactic centre

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

Stellar populations High mass short-lived Low mass Long-lived

M 15 UV

Hot subluminous stars SdB + sdO stars: Extreme Horizontal Branch stars EHB

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...

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

 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?

Apparently normal B stars  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

HS (Heber, Moehler & Groote 1995) High projected rotation velocity: v rot sin i =260 km/s Mass: 6-10 M o distance: kpc V rad = -39 km/s

Run-away stars  Normally massive stars are found in the Galactic plane ejection scenario: born in the plane and ejected  Calculate path and time of flight: - radial velocities, distances & proper motion - orbit integrator: Odenkirchen & Brosche (1992) - Galactic potential: Allen & Santillan (1991)  Results for ~100 stars consistent with DES and/or BSE (e.g. Conlon et al. 1992, Martin 2005)

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

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)

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 !

„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: 300 km/s

The supermassive black hole in the center of the Galaxy Schödel et al. (2003)

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

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

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

The second hyper-velocity star Spectrum with Keck I +LRIS Hel. RV=708km/s Gal. RV=751km/s (pm=0) Spectral type: Helium star (sdO) Low mass: 0.5M sun Hirsch, Heber, O´Toole & Bresolin (2005)

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

Kinematics of US 708  vgal = 751 < v esc = 430km/s unbound to Galaxy  Can be traced back to the Galactic Center: - proper motion required: pmRA=-2.3 mas/yr pmDE=-2.4 mas/yr - time of flight: 32 Myrs - evolutionary life time (core helium burning): 100 Myrs

Formation of US 708 accelerated by SMBH in Galactic Center: - US 708 was in binary, - disrupted by tidal interaction with SMBH

HE = HVS No.3 VLT-UVES: v rad = km/s v gal = 563 km/s (pm=0) B=16.2 mag Teff = K Log g = 3.8 normal Helium Edelmann, Napiwotzki, Christlieb & Reimers (2005) VLT-UVES: VLT UVES

HE : 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

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

Kinematics Time of flight to GC: 100 Myrs = 3 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

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) Is there a massive black hole?

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

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

OM 88: mass, distance & age Mass: 12 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 17Myrs

The SAO HVS survey Brown et al. (2006, ApJ ): Brown et al. 2006, astro-ph/

Eight HVS

Why are the HVS blue? The S-stars in the Galactic centre: (Eisenhauer et al 2005): Helium lines Hot blue stars

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: HVS production rate: 10 times larger - single stars can also be ejected  How many „hyper-velocity“ stars are out there?

Blue stars in the center of M31

Summary Hyper-velocity stars - can not be formed by dynamical interaction or binary supernova mechanism - 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 8 HVS known form a class of star insight about the stellar population in the GC

Predictions Astrometry  US 708 (B=19): if ejected from GC proper motion: pmRA=-2.2mas/yr pmDE=-2.4mas/yr  HE (B=16): if ejected from GC: pm < 0.5 mas/yr if ejected from LMC: pm about 2 mas/yr (relative to LMC) Spectroscopy (UVES )  HE : abundances may discriminate between origin in Galaxy or LMC

The team Heinz Edelmann (Bamberg, Austin) Heiko Hirsch (Bamberg) Eva-Maria Pauli (Bamberg) Simon O´Toole (Bamberg, Sydney) Ralf Napiwotzki (Hatfield) Martin Altmann (Santiago) Uli Heber (Bamberg)

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

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

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 SPY: sdB & sdO

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

Kinematics of US 708

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