Precision versus Accuracy: some astrophysical and operational limitations

Abstract

The measurement of accurate radial velocities for A- or B-type dwarfs poses a separate category of problems: the small number of suitable lines, the wide wavelength range over which such lines are distributed, thermal broadening, rotational broadening, line distortions caused by rotating spots, spectrum variations due to obvious duplicity, and low-level velocity variations due to undetected companion spectra. Moreover, the fact that A- and B-type spectra fall into very distinct groups, each with its own sub-set of the above problems, means that it may be both difficult and unsatisfactory to specify velocity standards, since intrinsic uncertainties caused by differences in spectral type may exceed the measuring errors. The investigation summarized here into the nature and magnitude of some of these intrinsic errors employs wide spans of high-resolution, high-dispersion spectra of Sirius and Vega as ‘natural’ templates. Attention is also drawn to systematic errors which may arise (a) when modelling a cross-correlation ‘dip’ and (b) whenever a spectrum or a cross-correlation dip is measured in the unavoidable presence of another such spectrum or dip.