Radial Velocities Without Spectroscopy -Astrometric determination of stellar radial motion

Abstract: This work demonstrates the determination of absolute stellar radial velocities from geometrical principles, i.e. using astrometry. Such `astrometric radial velocities' are obtained independently of spectroscopic information. While spectroscopic radial velocities are affected by line shifts from astrophysical processes on the stellar surfaces, by gravitational redshift and various other effects, astrometric radial velocities are not. Comparison between these two types of radial velocities therefore makes it possible to study certain stellar phenomena if the accuracies are good enough. Astrometric radial velocities of sufficient accuracy have indeed come within reach for stars in the Hyades cluster and a few more moving clusters, thanks to the space astrometry mission Hipparcos. The thesis consists of two parts. The first gives a brief background and explanation of the method used to derive astrometric radial velocities, and highlights some of the results. The second part consists of six research papers briefly summarised below. In Papers I and II some early results and expectations are outlined. It is demonstrated that the parallaxes for the cluster stars are considerable improved by the method. The expected effects when comparing astrometric and spectroscopic radial velocities are described. Papers III, IV and V give a systematic exploration of the concept of astrometric radial velocities and their determination in practice. In Paper III, various possibilities for their current and future determination are discussed. Only in nearby open clusters OB associations is it possible to reach accuracies at the sub-km/s level with available astrometric data. In Paper IV the kinematic cluster modelling is further developed and described in detail. Possible biases are investigated using Monte Carlo simulations. The Hyades is used as an example in applying the method. Paper V presents results in the form of astrometric radial velocities and improved parallaxes for stars in five open clusters and five OB associations. In Paper VI, extensive Monte Carlo modelling of the Hyades cluster and its observation by Hipparcos is used to study the internal velocity dispersion and how it affects the accuracy of astrometric radial velocities obtained in Paper V. For the dynamic modelling of the cluster, the N-body code NBODY6 is used, including stellar evolution. The minimum internal standard error of the astrometric radial velocities is set by the velocity dispersion at around 0.2 km/s, while the total standard error, including the uncertainty of the bulk cluster motion, is around 0.6 km/s.