Astrometric radial velocities. I. Non-spectroscopic methods for measuring stellar radial velocity

TL;DR

This paper explores three non-spectroscopic astrometric methods to measure stellar radial velocities, highlighting their potential, current limitations, and application to real data, offering alternatives to spectroscopic techniques.

Contribution

It analyzes and compares three astrometric methods for measuring stellar radial velocities, emphasizing their potential and current feasibility for future astrometric projects.

Findings

Method 1 currently lacks sufficient accuracy.

Method 2 is marginally feasible and applied to 16 stars.

Method 3 achieves high accuracy (<1 km/s) with existing data.

Abstract

High-accuracy astrometry permits the determination of not only stellar tangential motion, but also the component along the line-of-sight. Such non-spectroscopic (i.e. astrometric) radial velocities are independent of stellar atmospheric dynamics, spectral complexity and variability, as well as of gravitational redshift. Three methods are analysed: (1) changing annual parallax, (2) changing proper motion and (3) changing angular extent of a moving group of stars. All three have significant potential in planned astrometric projects. Current accuracies are still inadequate for the first method, while the second is marginally feasible and is here applied to 16 stars. The third method reaches high accuracy (<1 km/s) already with present data, although for some clusters an accuracy limit is set by uncertainties in the cluster expansion rate.