Download presentation
Presentation is loading. Please wait.
Published byBernard Sullivan Modified over 9 years ago
1
Uli Heber Oxford, 23.5.2006 Hyper-velocity stars
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 stellar population of the Galactic centre
6
Components of the Galaxy Buser Dark Halo Very old stars Old stars young & old stars
7
Stellar populations High mass short-lived Low mass Long-lived
8
M 15 UV
9
Hot subluminous stars SdB + sdO stars: Extreme Horizontal Branch stars EHB
10
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...
11
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
12
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?
13
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
14
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 V rad = -39 km/s
15
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)
16
Supernovae in binary systems Massive binaries: primary explodes as Supernova neutron star secondary is released at orbital velocity: <200km/s
17
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)
18
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 !
19
„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
20
The supermassive black hole in the center of the Galaxy Schödel et al. (2003)
21
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
22
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
23
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
24
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)
25
US 708: Keck LRIS spectrum T eff = 45500K, log g=5.23, mass = 0.5 M o B=19.0 mag Distance: 19 kpc
26
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
27
Formation of US 708 accelerated by SMBH in Galactic Center: - US 708 was in binary, - disrupted by tidal interaction with SMBH
28
HE 0437-5439 = HVS No.3 VLT-UVES: v rad = 723+-3 km/s v gal = 563 km/s (pm=0) B=16.2 mag Teff = 20400 K Log g = 3.8 normal Helium Edelmann, Napiwotzki, Christlieb & Reimers (2005) VLT-UVES: VLT UVES
29
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
30
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
31
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
32
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?
33
OM 88 = HD 271791 V=12.3 T eff = 17800K log g = 3.0 normal He/H v rad = 440 km/s ESO 2.2m: FEROS
34
OM 88: metal lines & rotation v rot sin i =124km/s solar metals Massive giant star
35
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
36
The SAO HVS survey Brown et al. (2006, ApJ ): Brown et al. 2006, astro-ph/0604111
37
Eight HVS
38
Why are the HVS blue? The S-stars in the Galactic centre: (Eisenhauer et al 2005): Helium lines Hot blue stars
39
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?
40
Blue stars in the center of M31
41
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
42
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
43
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)
44
Binary Population Synthesis (BPS) Han et al. (2003) a: 1. CE ejection b: 1. stable RLOF c: 2. CE ejection d: merger merger
45
BPS Han et al: Binary population synthesis a) Without GK selection b) With GK selection merger
46
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
47
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
48
Kinematics of US 708
49
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
Similar presentations
© 2024 SlidePlayer.com. Inc.
All rights reserved.