Constraining Solar position and velocity with a Nearby Hypervelocity Star

TL;DR

This paper proposes a novel method to determine the Solar position and velocity in the Milky Way by analyzing the azimuthal angular momentum of nearby hypervelocity stars, utilizing Gaia data for improved galactic measurements.

Contribution

The paper introduces a new technique to estimate the Sun's galactocentric distance and azimuthal velocity using hypervelocity stars, enhancing precision with upcoming Gaia data.

Findings

Method can constrain R0 with ~0.27 kpc uncertainty.

Method can constrain Vsun with ~7.8 km/s uncertainty.

Applicable to future Gaia discoveries of nearby HVSs.

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.