Abstract. The accuracy with which radial velocities of early-type stars can be measured is limited in practice by the existence of asymmetrical differences between object and template spectrum, constituting "spectrum mismatch". Our studies of the magnitude of spectrummismatch errors, commenced in Paper I ) on the basis of synthetic spectra having different attributes of effective temperature (T eff ) and log g, are continued here in a complementary approach that employs observed spectra. From over 60 de-archived observations we derive accurate wavelength scales for the spectra of 16 dwarfs of spectral types B8−F7, and examine the results of cross-correlating the spectra against different (observed) template spectra. We also test the effects of (a) truncating the spectra at different levels below the continuum, (b) adding rotational broadening to enforce a visual match of line-width between object and template, (c) applying rotational broadening to exacerbate a rotational mismatch, and (d) neglecting the presence of faint companion spectra. We also cross-correlate pairs of spectra such that the differences between their T eff are minimal. We conclude that it will be possible to measure radial velocities to an accuracy considerably better than 1 km s −1 for slowly-rotating stars in the range of spectral types examined, and a careful discussion of the nature and sources of the random and systematic errors that become significant in work of this nature enables us to specify conditions that are important for achieving such accuracy routinely. We find that both rotational broadening, and the star-to-star variations in line strengths that are so prevelant among A-type spectra, can give rise to more deleterious mismatch shifts (RV errors) than do differences in T eff alone, even for ∆T eff as great as 300 − 400 K. By intercomparing the results given by wide regions of spectrum (∼800Å) with those obtained by isolating small groups of features in very narrow windows (∼30Å), we have been able to designate a window near λ 4570Å that should be particularly reliable for high-accuracy results, and we propose further studies at very high S/N ratio in that specific window to complement and extend the results of the present paper.

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 1−2 km sfor 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 (1985Fekel ( , 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.

ABSTRACT. This is the first paper in a series in which we study the kinematical structure and dynamical evolution of OB associations and young stellar clusters. By means of high-resolution high-signal-to-noise (S/N) spectroscopic observations, our goal is to obtain accurate stellar radial and rotational velocities and information on binarity in a selected number of such systems. In the present paper, we discuss the observations and data reduction for a sample of 156 early-type, established or probable, members of the Sco OB2 association. These stars form a subset of a larger sample that was observed by the HIPPARCOS satellite. The observations presented here were performed at the ESO 1.5-m telescope using the ECHELEC echelle CCD spectrograph. The formal S/N obtained is typically 70 at 3800 Â and 300 at 4050 Â. A new data-reduction package was developed. For the sake of quality control over the final spectra, we critically evaluated the performance of each procedure in the reduction. Emphasis is on techniques for detecting systematic errors in an empirical way and for maximally eliminating them, at least on a differential level. The accuracy of our reduced spectra is limited by intrinsic instrumental imperfections. Local random errors are given and the amplitude of systematic residuals that (may) occur in certain spectral regions is estimated in considerable detail. In anticipation of subsequent papers in this series, we finally show that the accuracy of differential radial velocities (for spectra of the same star) is essentially limited by centering and zero-point uncertainties amounting to ^ 1 km s -1 (rms).

Abstract. Owing to the typical morphology of spectra of early-type stars, random errors play a much more important role in the derivation of their radial velocities than in the case of late-type spectra. We derive a generalised lower bound of the random error on a cross-correlation derived radial velocity shift in the presence of random noise on both object and template spectrum, and discuss its dependence on spectral parameters and noise. In order to limit in practice the random error to this lower bound for earlytype spectra, we show that a number of specific crosscorrelation peak centering techniques are required. The influence of rotational mismatch between object and template on the random error is examined. Finally, the widely used error estimate based on the so-called r-statistic is critically evaluated in the context of early-type spectra.

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