Accepted articles

Reviews of topical problems


Hypervelocity stars: theory and observations

 a,  b,  b
a Institute of Astronomy, Russian Academy of Sciences, ul. Pyatnitskaya 48, Moscow, 119017, Russian Federation
b Russian Federal Nuclear Center E.N.Zababakhin All-Russia Scientific Research Institute ofTechnical Physics, PO Box 245, Snezhinsk, Chelyabinsk Region, Russian Federation

One of the peculiarities proper to objects of micro world (elementary particles) — relativistic velocity — now is a subject of scientific discussions on its applicability to macro-objects: stars, planets, asteroids, as well as white dwarfs, neutron stars and black holes of stellar mass. Awareness of the possibility became clear after Warren Browns discovery of HyperVelocity Stars (HVS) at the beginning of 21-st century. These stars were predicted by Jack Hills in 1988 in the frame of scenario of dynamic capture of a binary star by central Super Massive Black Hole (SMBH). The star acceleration mechanism can produce the HVS due to momentum exchange in the classical problem of three bodies when a binary passes in the vicinity of the SMBH with the second component of the binary captured by the SMBH. Today observations give the upper limit of the stellar kinematic anomaly about ~1700 km/s. This value is well reproduced by several scenarios alternative to Hills one. Hyper velocity stars may be generated as a result of stellar clusters collisions, supernova blasts in close binary star systems, orbital instability of triple and multiple star systems, capture of stars from another galaxies, and etc. But the most promising for extremely high velocities are the scenarios involving black holes masses of which vary from several solar masses to several billion masses of the sun. Hills scenario takes special place in the elucidation of the HVSs nature because being based on an idea of accidental capture of a binary in close vicinity of the SMBH it does not refer to the problem of Galactic centre population. This scenario forecasts that HVSs and stars captured on close to SMBH orbits (possibly identified with S-stars) should have correlated statistics. A discovery of S-stars played important role in investigations of the central part of the Galaxy — analysis of their dynamics provided for conclusive evidence of SMBH residence in the centre of the Milky Way. This review briefly concerns the history of discovery and investigation of HVSs and S-stars, provides information on their observational statistics and describes the numerical techniques of their modeling in the frames of classical problem of three bodies as well as N-body simulations. The limits of effective acceleration of stars in classical Hills scenario as well as in the modified one when it was supposed that one of the binary components is the black hole of intermediate mass or SMBH. The acceleration produced in the cross field of two black holes proved to be sufficient for a generation of stars with relativistic velocities (1/2c—2/3c). The issues of survivability of stars it the regions with extreme gradients of gravitational field as dependent on SMBH mass and pericenter distance were discussed. In the frame of self-consistent model unifying classical and modified Hill's scenarios the forecast of the probabilities of HVSs generation in the Galaxy and the generation of stars with relativistic velocities outside the Galaxy was made. The issues on searching for stars and asteroids with relativistic velocities during near future cosmic missions and getting new visions of the Universe were also discussed in the review.

Keywords: kinematic anomaly, dynamical capture, Hills scenario, S-stars, hypervelocity stars, relativistic velocity stars, supermassive black holes, galaxies
DOI: 10.3367/UFNe.2020.11.038892
Citation: Tutukov A V, Dryomova G N, Dremov V V "Hypervelocity stars: theory and observations" Phys. Usp., accepted

Received: 28th, July 2020, revised: 19th, November 2020, 30th, November 2020

:   ,   ,    « : » ,  ; DOI: 10.3367/UFNr.2020.11.038892

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