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Discovery Of An Unbound Hyper-Star In The Milky Way Halo: SDSS J090745.0+024507

29/11/2011

If you’ve ever gazed at the night sky, you’ve probably wished upon a shooting star (which are really meteors). However, pioneering work by Brown et al. (2005) has shown that shooting stars do exist, and they’re as rare as one in one hundred million. They discovered that a hot blue horizontal branch star, SDSS J090745.0+024507, was leaving the Galaxy at an unprecedented rate. Currently situated at approximately 55kpc from the centre of our Galaxy, SDSS J090745.0+024507 is hurtling away at some 850 kilometres per second; 10 times faster than most ordinary stars orbit the centre of our Galaxy.

Corrected to the local standard of rest and for the solar reflex motion, the Galactic rest frame velocity of this star is +709 kms-1. The observed radial velocity is only a lower limit to the star’s true space velocity, but the radial velocity alone substantially exceeds the escape velocity from the Galaxy. SDSS J090745.0+024507 is entering into the intergalactic abyss.

In Brown et al.’s (2005) survey of faint blue horizontal branch candidates in the Galactic halo, they discovered a star, SDSS J090745.0+024507 from the Sloan Digital Sky Survey catalog, traveling with a heliocentric radial velocity of +853 ± 12 kms-1. This is a very unusual result given the age and proper motion of stars in the Galactic halo.

Unlike the galactic disc, the halo seems to be free of dust. In further contrast, stars in the galactic halo are of Population II, much older and with much lower metallicity than their Population I galactic disc counterparts (Benjamin et al. 2005). Thus, it seems something of a rarity to see an unbound blue star moving extremely fast through a sea of red stars. So how would such a star form and why is it traveling at this unprecedented rate?

The Milky Way, as well as most nearby elliptical galaxies and disk galaxies with spheroids, is believed to house a super-massive black hole (SMBH) at its center (Schodel et al. 2002, 2003; Genzel et al. 2003; Eisenhauer et al. 2003; Ghez et al. 2003a, 2003b, 2003c; Gebhardt et al. 2003). The deep gravitational potential well close to the BH enables relativistic phenomena, such as the relativistic motion of jets associated with some active galactic nuclei (Begelman, Blandford & Rees 1984).

Hills (1988) suggested that a stellar binary interaction with the Milky Way’s central black hole could eject one member of the binary with a velocity greater than 1000 kms-1. Yu & Tremaine (2003) further develop Hill’s analysis and suggest two additional mechanisms to eject “hyper-velocity” stars (hyper-stars) from the Galactic center: close encounters of two single stars and three-body gravitational interactions between a single star and a binary black hole. Yu & Tremaine (2003) predict production rates for all three mechanisms. Even the discovery of a single hypervelocity star can place important constraints on the formation mechanism and the nature of the Galactic center.

Such mechanisms slingshot these stars into highly eccentric orbits around the central SMBH. However, as Yu & Tremaine (2003) outlined in their paper, it may be possible from the black hole at the centre of the galaxy to throw stars out at speeds greater than the escape velocity of the Milky Way: . Hence, as Hills (1988) first pointed out, this deep gravitational potential of the SMBH will result in stars being expelled from the vicinity of the SMBH with extremely high velocities; with SDSS J090745.0+024507 being a very likely candidate for just such a journey.

Journal References: 

  • Yu, Q. & Tremaine, S. (2003) Ejection Of Hyper-Velocity Stars By The Black Hole In The Galactic Centre. The Astrophysical Journal, 599 (1): pp.1129–1138.
  • Brown, W.R. et al. (2005) Discovery Of An Unbound Hyper-Velocity Star In The Milky Way HaloThe Astrophysical Journal, 622 (1): pp. L33-L36.

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