Constraining Solar position and velocity with a Nearby Hypervelocity Star

Abstract

Gravitational 3-body interaction among binary stars and the supermassive black hole (SMBH) at the center of the Milky Way occasionally ejects a hypervelocity star (HVS) with a velocity of ~1000 km/s. Due to the ejection location, such a HVS initially has negligible azimuthal angular momentum Lz ~ 0 kpc km/s. Even if the halo is mildly triaxial, Lz of a recently ejected nearby HVS remains negligible, since its flight time from the Galactic Center is too short to accumulate noticeable torque. However, if we make a wrong assumption about the Solar position and velocity, such a HVS would apparently have noticeable non-zero azimuthal angular momentum, due to the wrong reflex motion of the Sun. Conversely, with precise astrometric data for a nearby HVS, we can measure the Solar position and velocity by assuming that the HVS has zero azimuthal angular momentum. Based on this idea, here we propose a method to estimate the Galactocentric distance of the Sun R0 and the Galactocentric Solar azimuthal velocity Vsun by using a HVS. We demonstrate with mock data for a nearby HVS candidate that the Gaia astrometric data, along with the currently available constraint on Vsun/R0 from the proper motion measurement of Sgr A*, can constrain R0 and Vsun with uncertainties of ~0.27 kpc and ~7.8 km/s (or fractional uncertainties of 3 percent), respectively. Our method will be a promising tool to constrain (R0, Vsun), given that Gaia is expected to discover many nearby HVSs in the near future.

ApJ accepted. 15 pages, 5 figures. We added Section 5.5, where the situation after this paper first appeared (on April 23, 2018) is described. Conclusion is unchanged. Comments welcome