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
