Uli Heber Bad Honnef, 8.12.2006 Hyper-velocity stars in the Milky Way.
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Transcript of Uli Heber Bad Honnef, 8.12.2006 Hyper-velocity stars in the Milky Way.
Uli Heber
Bad Honnef, 8.12.2006
Hyper-velocity stars in the Milky Way
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
Components of the Galaxy
Buser
Dark Halo
Very old stars
Old starsyoung &
old stars
Stellar populations
High mass, short-lived
Low mass,Long-lived
from Moehler
M 15
UV
Hot subluminous stars
SdB + sdO stars: Extreme Horizontal Branch stars
EHBHB
sdB
sdO
Dorman et al. (1993, ApJ 419, 596)
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 ...
Apparently normal B stars at Galactic latitudes
Greenstein & Sargent (1972)
Significant fraction offaint blue stars at high galactic latitudes may be normal B-stars
HS 1914+7135
Mass: 6-10 Mo
distance: 6.5-7.5 kpc Heber, et al. (1995, A&A 303, L33)
Why is it difficult to distinguish
a main sequence star from a
blue horizontal branch (BHB) star?
HRD
Teff-log g
sdBsdO
BHB/MS?
Hunger & Heber (1987)
Apparently normal B stars at high Galactic latitudes
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
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)
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 - Tflight < Tevol - 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
500km/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)
300km/s
The massive black hole in the centre of the Galaxy
Star S2:- orbital period: 15 yrs-dmin=120 AU-Vmax=5000 km/s-MBH=2.6 106 Msun
Schödel et al. (2003, ApJ 596, 1015)
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)
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
The first hyper-velocity star
Sample auf HBA starsfrom SDSS(Brown et al. 2005, ApJ 622, L33)
Vrad = 853km/s (hel.) > 709km/s (gal.); pm=0 Late B-type (B=19.8m) d=40kpc (if HB) d=110kpc (if MS)
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 11000 spectra: 40 sdO + 43 He sdO
Radial velocities
HVS
The second hyper-velocity star
Spectrum fromKeck I +LRIS
Hel. RV=708km/s Gal. RV=751km/s( (pm=0)
Helium star (sdO)
- Low mass:0.5Msun
- distance: 20kpc Hirsch, Heber, O´Toole & Bresolin (2005, A&A 441, L61)
Old helium star
US 708: Keck LRIS spectrum
Teff = 45500K,
log g = 5.23,
mass = 0.5 Mo
B=19.0 mag
Distance: 19 kpc
Kinematics of US 708
vgal = 751km/s < vesc = 430km/s unbound Can be traced back to the Galactic Center: - proper motion required: pmRA = -2.3 mas/yr pmDE = -2.4 mas/yr
- tflight =32 Myrs < tevol= 100 Myrs
MBH slingshot ejection from the
Galactic Centre is plausible:
- US 708 was in binary, disrupted by tidal interaction with MBH
HE 0437-5439 = HVS 3
VLT-UVES:
vrad = 723+-3 km/s
vgal > 563 km/s (pm=0)Teff = 20400 KLog g = 3.8normal Helium B=16.2 mag
Edelmann, Napiwotzki, Heber, Christlieb & Reimers (2005, ApJ 633, L181)
:
VLT UVES
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
Mass, distance and age
Comparison to evolutionary tracks for ms stars:
Mass = 8 Mo
Distance: 60 kpcAge = 25 Myr
vgal > 563 km/s >
vesc = 317 km/s unbound to Galaxy
25Myrs
Kinematics
Time of flight to GC: 100 Myrs = 4 times Tevol!!
Alternatives: - Blue Straggler - 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 Tevol:
Veject=600km/s (unbound to LMC) pmRA=2mas/yr (relative to LMC)
Where is the massive black hole?
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.
Lifetime vs travel time
Assuming MS-distances:
Star d/kpc | tflight /Myrs | tevol /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: ?????
OM 88 = HD 271791
V=12.3
Teff = 17800K
log g = 3.0
normal He/H
vrad (hel.) = 440 km/s
ESO 2.2m: FEROS
OM 88: metal lines & rotation
vrot sin i =124km/s
solar metals
Massive giant star
OM 88: mass, distance & age
Mass: 11.5 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
Time of flight from GC: 90Myr
17Myrs
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)
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!
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)
The team
Simon O´Toole
Heinz EdelmannHeiko Hirsch
Ralf Napiwotzki
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)
Kinematics
Time of flight to GC: 100 Myrs = 4 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
Blue stars in the center of M31
Binary Population Synthesis (BPS)
Han et al. (2003)
a: 1. CE ejection
b: 1. stable RLOF
c: 2. CE ejection
d: merger
merger
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?
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. 2005
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)
sdOHe sdO
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
Kinematics of US 708
Stellar masses and ages
Stellares masses: 1/10 .... 80 x solar masseMass – luminosity relation: L ~ M3.5
Massive stars are luminous and die young
Apparently normal B stars at Galactic latitudes
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
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
sdO
WD
QSO