Uli Heber Oxford, 23.5.2006 Hyper-velocity stars.
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Transcript of Uli Heber Oxford, 23.5.2006 Hyper-velocity stars.
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Uli Heber
Oxford, 23.5.2006
Hyper-velocity stars
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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
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Components of the Galaxy
Buser
Dark Halo
Very old stars
Old starsyoung &
old stars
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Stellar populations
High mass short-lived
Low massLong-lived
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M 15
UV
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Hot subluminous stars
SdB + sdO stars: Extreme Horizontal Branch stars
EHB
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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 ...
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Mix of spectral types
bright magnitudes: hot subluminous stars dominate: sdB, sdO Green et al. (1986)
white dwarfs and QSO at fainter magnitudes
sdB
sdO
WD
QSO
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Why is it difficult to distinguish
a main sequence star from a
blue horizontal branch (BHB) star?
HRD
Teff-log g
sdBsdO
BHB/MS?
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Apparently normal B stars
Massive B stars and blue Horizontal Branch stars: similar Teff 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
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HS 1914+7135
HS 1914+7135 (Heber, Moehler & Groote 1995)
High projected rotation velocity:
vrot sin i =260 km/s
Mass: 6-10 Mo
distance: 6.5-7.5 kpc
Vrad = -39 km/s
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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)
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Supernovae in binary systems
Massive binaries: primary explodes as Supernova neutron star secondary is released at orbital velocity: <200km/s
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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)
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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 - Tflight < Tevol - cluster origin has been proven for a few stars from Hipparcos parallaxes
Dynamic ejection and binary supernova scenarii are in good shape !
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„Hyper-velocity“:speed limits in space
500km/s
How fast can a „run-away“ star travel?
May a star leave the Galaxy?Exceed the Galactic escape velocity:
300km/s
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The supermassive black hole in the center of the Galaxy
Schödel et al. (2003)
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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
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The first hyper-velocity star
Sample auf HBA starsfrom SDSS(Brown et al. 2005)
Vrad = 853km/s (hel.) = 709km/s (gal.)Late B-type (B=19.8m)if HB: d=40kpcif MS: d=110kpc
Unbound to Galaxy
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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
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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: 0.5MsunHirsch, Heber, O´Toole & Bresolin (2005)
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US 708: Keck LRIS spectrum
Teff = 45500K,
log g=5.23,
mass = 0.5 Mo
B=19.0 mag
Distance: 19 kpc
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Kinematics of US 708
vgal = 751 < vesc = 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
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Formation of US 708
accelerated by SMBH in Galactic Center:
- US 708 was in binary,
- disrupted by tidal interaction with SMBH
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HE 0437-5439 = HVS No.3
VLT-UVES:
vrad = 723+-3 km/s
vgal = 563 km/s (pm=0)
B=16.2 magTeff = 20400 KLog g = 3.8normal Helium
Edelmann, Napiwotzki, Christlieb & Reimers (2005)
VLT-UVES:
VLT UVES
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HE 0437-5439: metals & rotation
Vrot sini =54 km/s
metals: solar
(to within a factor of 3)
Main sequence star
… = 1/3 solar, --- = 3*solar, full drawn= solar
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Mass, distance and age
Comparison to evolutionary tracks for ms stars:
Mass = 8 Mo
Distance: 60 kpcAge = 25 Myr
vesc = 317 km/s <
vgal = 563 km/s unbound to Galaxy
25Myrs
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Kinematics
Time of flight to GC: 100 Myrs = 3 times Tevol!!
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
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Origin in the LMC ?
Star is beyond LMC Closer to LMC (18kpc)
than to Galaxy Can reach present position
within Tevol:
Veject=600km/s (unbound to LMC) pmRA=2mas/yr (relative to LMC)
Is there a massive black hole?
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OM 88 = HD 271791
V=12.3
Teff = 17800K
log g = 3.0
normal He/H
vrad = 440 km/s
ESO 2.2m: FEROS
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OM 88: metal lines & rotation
vrot sin i =124km/s
solar metals
Massive giant star
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OM 88: mass, distance & age
Mass: 12 Mo
solar metals Distance: 24 kpcAge: 17 MyrsProper motions:
Hipparcos, UCAC2, USNO-B1, ATC,... HIP: μα = -1.0 mas/yr
μδ = +7.0 mas/yr
17Myrs
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The SAO HVS survey
Brown et al. (2006, ApJ ):
Brown et al. 2006, astro-ph/0604111
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Eight HVS
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Why are the HVS blue?
The S-stars in the Galactic centre:(Eisenhauer et al 2005):
Helium lines
Hot blue stars
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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?
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Blue stars in the center of M31
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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
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Predictions
Astrometry 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 (UVES 11.1.2006) HE 0437-5439: abundances may discriminate between origin in Galaxy or LMC
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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)
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Binary Population Synthesis (BPS)
Han et al. (2003)
a: 1. CE ejection
b: 1. stable RLOF
c: 2. CE ejection
d: merger
merger
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BPS
Han et al:
Binary population synthesis
a) Without GK selection
b) With GK selection
merger
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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
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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)
sdOHe sdO
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Kinematics of US 708
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Stellar masses and ages
Stellares masses: 1/10 .... 80 x solar masseMass – luminosity relation: L ~ M3.5
Massive stars are luminous and die young