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