http://arxiv.org/abs/2104.10174
We show that measuring the velocity components of hypervelocity stars (HVSs) can discriminate between Modified Newtonian Dynamics (MOND) and Newtonian gravity. HVSs are ejected from the Galactic center on radial trajectories with a null tangential velocity component in the reference frame of the Galaxy. They acquire tangential components due to the non-spherical components of the Galactic gravitational potential. Axisymmetric potentials only affect the latitudinal components $v_\theta$, and non-null azimuthal components $v_\phi$ originate from non-axisymmetric matter distributions. For HVSs with sufficiently large ejection speed, the azimuthal velocity components are proportionate to the deviation of the gravitational potential from axial symmetry. The ejection velocity threshold is $\sim$ 750 km s$^{-1}$ for 4 $M_{\odot}$ stars and increases with decreasing HVS mass. We determine the upper limit of $v_\phi$ as a function of the Galactocentric distance for these high-speed HVSs, if MOND, in its QUMOND formulation, is the correct theory of gravity and the triaxial Galactic bulge is the primary source of the azimuthal component $v_\phi$. In Newtonian gravity, values of $v_\phi$ larger than this limit can easily appear if the dark matter halo is triaxial or if the dark matter halo and the baryonic components are axisymmetric but their two axes of symmetry are misaligned. Therefore, even a limited sample of high-speed HVSs could in principle distinguish between the QUMOND scenario and the dark matter model if the azimuthal components $v_\phi$ of the observed HVSs do not meet the QUMOND upper limit. However, the uncertainties on the Galactocentric azimuthal velocity components of the currently known HVSs are still at least a factor of $\sim 10$ too large to make this test conclusive. Astrometric measurements with micro-arcsecond precision would make this test feasible.
S. Chakrabarty, L. Ostorero, A. Gallo, et. al.
Thu, 22 Apr 2021
37/44
Comments: 14 pages, 13 figures
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