# Hyper-Velocity Stars

1. Jan 27, 2006

### Garth

Probing the Shape of the Galactic Halo with Hyper-Velocity Stars
Do they need a SMBH to sling shot them or would smaller masses do?

Garth

2. Jan 27, 2006

### George Jones

Staff Emeritus
At first, I didn't understand your acronym - I could only come with solar for S, and I knew you didn't mean this. Then the lightbulb went on.

In any case, the authors seem to feel that a supermassive black hole is necessary, as indicated by their second paragraph.

Regards,
George

3. Jan 28, 2006

### Garth

Well, that paper is suggesting that the hyervelocity stars have to be ejected from close binaries disrupted by a SMBH.

The velocity of such stars are ~ 600 km.sec-1 whereas typical galactic orbital velocities < 300 km.sec-1, so they are travelling at twice this rate. That paper refers to one star with a velocity in galactic coordinates of +853 ± 12 km.sec-1, which is, at its location, twice the velocity of escape from the galaxy.

Now they say for a star to retain an escaped velocity of this magnitude the initial velocity of ejection must be > 1000 km.sec-1, which is derived from the Kelperian escape velocity of
$v$ ≈ $$\sqrt {\frac{2GM_{BH}}{r}}$$ > 1000 km.sec-1

As G ~ 7 x 10-8 c.g.s units and they suggest
r < 0.01 parsec ~ 3 x 1016 cms. and as
1000 km.sec-1 is 108 cm.sec-1 and MSolar ~ 2 x 1033 gms.

then MBH ~ 1016$$\frac{r}{2G}$$ ~ 106MSolar .

However r ~ 0.01 parsec ~ 103 AU and a BH could approach much closer than that.

If the binary system were approached to 1 AU then the BH would only have to have a mass of 103MSolar.

In other words an IMBH would do, rather than a SMBH, and there may be many more of them.

Garth

Last edited: Jan 28, 2006