Can Hyper-Velocity Stars Illuminate the Structure of the Galactic Halo?

[Garth]

Probing the Shape of the Galactic Halo with Hyper-Velocity Stars

Precise proper motion measurements (sigma_mu ~ 10 mkas/yr) of the recently discovered hyper-velocity star (HVS) SDSS J090745.0+024507 would yield significant constraints on the axis ratios and orientation of a triaxial model for the Galactic halo. Triaxiality of dark matter halos is predicted by Cold Dark Matter models of galaxy formation and may be used to probe the nature of dark matter. However, unless the distance to this star is determined to better than 10%, these constraints suffer from one-dimensional degeneracies, which we quantify. We show how proper motion measurements of several HVSs could simultaneously resolve the distance degeneracies of all such stars and produce a detailed picture of the triaxial halo. Additional HVSs may be found from radial velocity surveys or from parallax/proper-motion data derived from GAIA. High-precision proper-motion measurements of these stars using the Space Interferometry Mission (SIM PlanetQuest) would substantially tighten the constraints they yield on the Galactic potential.

Do they need a SMBH to sling shot them or would smaller masses do?

Garth

 

[George Jones]

Do they need a SMBH to sling shot them or would smaller masses do?

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.

As discussed by Brown et al. (2005), the velocity of the HVS greatly exceeds that plausible for a runaway star ejected from a binary in which one component has undergone a supernova explosion. The only known mechanism for a star to obtain such an extreme velocity is ejection from the deep potential of the massive black hole at the Galactic center, as a result of scattering with another star or tidal breakup of a binary (Hills 1988; Yu & Tremaine 2003). Only extremely close to the massive black hole, at r . 0.01 pc, can stars attain the required speeds v ≈ (2GMbh/r)1/2 & 1000 km s−1.

Regards,
George

 

[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 traveling 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 M_Solar ~ 2 x 10³³ gms.


then M_BH ~ 10¹⁶\frac{r}{2G} ~ 10⁶M_Solar .

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 10³M_Solar.

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

Garth