Accuracy of radial-velocity measurements for early-type stars

I. Spectral-type mismatch in A-type synthetic spectra

Astrophysics Research Group, University of Antwerp (RUCA), Groenenborgerlaan 171, B-2020 Antwerpen, Belgium

Send offprint request to: W. Verschueren

Received: 29 April 1999
Accepted: 4 August 1999

Abstract

Measuring accurate radial velocities of rotating early-type (O-B-A) stars using cross-correlation techniques is hampered by the fact that object-template spectrum mismatch causes systematic errors that do not cancel out sufficiently in these spectra. This series of papers aims at quantifying those mismatch errors, understanding their astrophysical origin, and developing strategies to avoid them maximally. In this first paper, we employ synthetic spectra to study mismatch between A-type main-sequence stars caused solely by differences in T_eff and log g. We show that this spectral-type mismatch varies greatly throughout the spectrum, though with some degree of systematic dependence on rotational velocity and, to a lesser degree, on temperature. We propose a scheme for selecting spectral regions that should provide, for main-sequence A-type stars, accuracies better than 1 km s^-1 for v sin i ≤ 150 km s^-1 and between  km s^-1 for v sin i up to 300 km s^-1. The scheme includes sufficient spectral information to keep random errors conveniently small, but excludes all wavelength sub-intervals which produce systematic errors much larger than the above mentioned accuracy. Our predictions confirm the success of the methodology of Fekel (1985, 1999). We conclude that the proposed scheme needs further testing on a broad sample of real A-type spectra to see under what conditions of stellar individuality it may break down.