Hypervelocity Stars
Everyone knows that shooting stars are just meteors entering the atmosphere, right? If you didn’t, congratulations—you just failed the fourth grade. What some people don’t know, however, is that real shooting stars exist as well; they’re called hypervelocity stars. These are big, fiery balls of gas rocketing through space at millions of miles per hour.
When a binary star system is gobbled down by the supermassive black hole (that’s the scientific term, by the way) at the center of a galaxy, one of the two partners is consumed, while the other is ejected at high speed. Just try to imagine a huge ball of gas, four times the size of our sun, hurtling out from our galaxy at millions of miles per hour.
We consider the process of stellar binaries tidally disrupted by a supermassive black hole. For highly eccentric orbits, as one star is ejected from the three-body system, the companion remains bound to the black hole. Hypervelocity stars (HVSs) observed in the Galactic halo and S-stars observed orbiting the central black hole may originate from such mechanism. In this paper, we predict the velocity distribution of the ejected stars of a given mass, after they have travelled out of the Galactic potential. We use both analytical methods and Monte Carlo simulations. We find that each part of the velocity distribution encodes different information. At low velocities < 800 km/s, the Galactic Potential shapes universally the observed distribution, which rises towards a peak, related to the Galactic escape velocity. Beyond the peak, the velocity distribution depends on binary mass and separation distributions. Finally, the finite star life introduces a break related to their mass. A qualitative comparison of our models with current observations shows the great potential of HVSs to constrain bulge and Galactic properties. Standard choices for parameter distributions predict velocities below and above ~800 km/s with equal probability, while none are observed beyond ~700 km/s. This may indicate either a deficit in tight binaries compared to those with wider separations, or a full loss-cone regime for disrupted binaries, as could be achieved by a more efficient relaxation processes than two body scattering.
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