Hypervelocity StarsEjected from the Galactic Center

STScI ColloquiumOct 3, 2007

Warren R. BrownSmithsonian Astrophysical

Observatory

Collaborators: Margaret Geller,

Scott Kenyon, Michael Kurtz

Radial Velocities from the MMT

The first “Hypervelocity Star”

Predictions

Hills, 1988, Nature: prediction

Hills, 1991, AJ: orbits

Yu & Tremaine, 2003, ApJ: rates

NY Times2/22/2005

“It’s high time someone found it.” - Jack Hills SF Chronicle, 2/11/2005

The Milky Way

Kaufmann

The Galactic Center

http://www.mpe.mpg.de/ir/GC/prop.html

Eisenhauer et al. 2003

Three-body exchange

Bromley 2005

An Unexpected Star

• B9 main sequence star.• Solar metallicity.• g=19.8 thus d=110 kpc.

• Travel time ~160 Myr.

Brown et al. (2005)

Our Search for more Hypervelocity Stars

Fukugita et al (1996)

Brown et al. (2006a,2006b, 2007a, 2007b)

Lowest Mass White Dwarf

Kilic et al (2007a,b)

Extremely Metal Poor Galaxy

Kewley et al. (2007)Brown et al. (2007c)

log(O/H)+12 = 7.44

Spectroscopic Observationsof an Unusual Parameter Space

Radial Velocities

Brown et al (2007b)

HVS: Ejection Model

Bromley et al (2006); Brown et al. (2007a)

HVS: Main Sequence Stars

Brown et al. (2007b)

Blue HB

MS

Kaufmann

Horizontal Branch HVSs

HVS: Locations and Travel Times

Brown et al. (2007b)

- 300 0 +300 km/s

HVS: Sky DistributionBrown et al. (2007a)

+90

60

30

0

-60

-30

-90

12060 180 240 300 360

HVS: Space Density

Brown et al. (2007b)

Theoretical Applications

• Dark Matter Potential:Gnedin et al (2005), Yu & Madau (2007)

• Binary Black Hole / origin:Baumgardt et al, Gualandris et al, Merritt, Levin, O’Leary & Loeb, Perets et al, Sesana et al., Lu et al., Svensson et al.

• Stars orbiting the BH:Ginsburg & Loeb

• Stellar Populations:Demarque & Virani, Kollmeier & Gould

Ginsburg & Loeb (2006)

LISA

Future Work• Discovery survey: MMT, Whipple 1.5m.

• Spectroscopic identifications: VLT (Heber), WHT (Keenan).

• Space velocities: HST (Gnedin).

• Variability: MDM (Stanek).

• Numerical simulations: (Bromley).

• Unusual objects: more to come!

Conclusions

• MBH = hypervelocity stars.

• First HVS: B star +850 km/s.

• Now 10 known HVSs.

• HVSs unique window on the Galactic Center:• Mass function of stars• In-fall history• Massive black hole (binary?)

NY Times

The Hypervelocity StarsID Type g’

(mag)

VminRF

(km/s)

d (kpc)

tGC

(Myr)

Citation

HVS1 B 19.8 +709 110 160 Brown et al. (2005)

HVS2 sdO 18.8 +717 19 32 Hirsch et al. (2005)

HVS3 B 16.2 +548 61 100? Edelmann et al. (2005)

HVS4 B 18.4 +558 75 140 Brown et al. (2006a)

HVS5 B 17.9 +638 55 90 Brown et al. (2006a)

HVS6 B 19.1 +508 75 160 Brown et al. (2006b)

HVS7 B 17.7 +423 55 120 Brown et al. (2006b)

HVS8 B 17.9 +430 45 100 new!

HVS9 B 18.6 +490 55 110 new!

HVS10 B 19.2 +432 85 190 new!

Mass Function of Stars

ArchesPredicted: 2000 HVSs(Yu & Tremaine)

We Observe:7 HVS in 6000 deg2

~50 3-4 Msun HVSs

16 3-4 Msun stars

~100 3-4 Msun stars

NASA HST

Salpeter

Halo Structure: Sgr Stream