Astron. Astrophys. Suppl. Ser. 140, 107-123

Accuracy of radial-velocity measurements for early-type stars

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

W. Verschueren - M. David - R.E.M. Griffin

Send offprint request: W. Verschueren,
e-mail: verschue@ruca.ua.ac.be

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

Received April 29; accepted August 4, 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 $T_{\rm eff}$ and logg. 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 vsini $\leq$ 150 kms^-1 and between 1 - 2 kms^-1for vsini up to 300 kms^-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.

Key words: methods: numerical -- techniques: radialvelocities -- techniques: spectroscopic -- stars: early-type -- stars: kinematics