[astro-ph/0601580] A Successful Targeted Search for Hypervelocity Stars

Authors:  Warren R. Brown, Margaret J. Geller, Scott J.Kenyon, Michael J. Kurtz (Smithsonian Astrophysical Observatory)
Abstract:  Hypervelocity stars (HVSs) travel with velocities so extreme that dynamical ejection from a massive black hole is their only suggested origin. Following our discovery of the first HVS, we have undertaken a dedicated survey for more HVSs in the Galactic halo and present here the resulting discovery of two new HVSs: SDSS J091301.0+305120 and SDSS J091759.5+672238, traveling with Galactic rest-frame velocities at least +558+-12 and +638+-12 km/s, respectively. Assuming the HVSs are B8 main sequence stars, they are at distances ~75 and ~55 kpc, respectively, and have travel times from the Galactic Center consistent with their lifetimes. The existence of two B8 HVSs in our 1900 deg^2 survey, combined with the Yu & Tremaine HVS rate estimates, is consistent with HVSs drawn from a standard initial mass function but inconsistent with HVS drawn from a truncated mass function like the one in the top-heavy Arches cluster. The travel times of the five currently known HVSs provide no evidence for a burst of HVSs from a major in-fall event at the Galactic Center in the last \~160 Myr.
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Neal Dalal
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Joined: September 25 2004
Affiliation: CITA

[astro-ph/0601580] A Successful Targeted Search for Hypervel

Post by Neal Dalal » January 28 2006

This paper discusses new hyper-velocity stars recently detected with radial velocities of order 600 km/s. These are quite interesting, since it's believed that they were ejected by the supermassive black hole at the Galactic center. One use of such stars, discussed in astro-ph/0506739 by Gnedin et al., is to measure the triaxiality of our halo : if we know they came from the Galactic center, any deviation from purely radial motion comes from non-sphericity of the Galactic potential.

The thing which confuses me is the claim that these must be ejected by a supermassive BH (e.g. with M > 106 M). The idea is that the ejected stars originated in tight binaries which were tidally disrupted by the SMBH. If I'm reproducing the argument correctly, if the original binary had mass m and binary velocity v, then the ejected star can have a velocity at infinity of v = v sqrt(2) * (MBH/m)(1/6). At most, v can be the escape velocity at the star's surface, e.g. 300 km/s. But if it is this high, then I don't need BH masses of 106 to reach the observed velocities of 550−700 km/s – even intermediate mass BH's with 100−1000 M are sufficient to do the job. So why is there such certainty that they came from the Galactic center? Am I missing something?

We talked about this at our coffee at CITA, without a successful resolution, so I was wondering if other people knew the answer. Another reason this could be important is that a similar hypervelocity star was seen ejected from the LMC. If a SMBH is required there too, this would be the first evidence for a SMBH in the LMC, and I think it would violate the M-σ relationship.

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