Accuracy of radial-velocity measurements for early-type stars - II. Investigations of spectrum mismatch from high-resolution observations

The accuracy with which radial velocities of early-type stars can be measur ed is limited in practice by the existence of asymmetrical differences betw een object and template spectrum, constituting "spectrum mismatch". Our stu dies of the magnitude of spectrum-mismatch errors, commenced in Paper I (Ve rschueren et al. 1999) on the basis of synthetic spectra having different a ttributes of effective temperature (T-eff) and log g, are continued here in a complementary approach that employs observed spectra. From over 60 de-ar chived observations rye derive accurate wavelength scales for the spectra o f 16 dwarfs of spectral types B8-F7, and examine the results of cross-corre lating the spectra against different (observed) template spectra. We also t est the effects of (a) truncating the spectra at different levels below the continuum, (b) adding rotational broadening to enforce a visual match of l ine-width between object and template, (c) applying rotational broadening t o exacerbate a rotational mismatch, and (d) neglecting the presence of fain t 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 p ossible to measure radial velocities to an accuracy considerably better tha n 1 km s(-1) for slowly-rotating stars in the range of spectral types exami ned, and a careful discussion of the nature and sources of the random and s ystematic errors that become significant in work of this nature enables us to specify conditions that are important for achieving such accuracy routin ely. We find that both rotational broadening, and the star-to-star variatio ns in line strengths that are so prevelant among A-type spectra, can give r ise to more deleterious mismatch shifts (RV errors) than do differences in T-eff alone, even for DeltaT(eff) as great as 300 - 400 K. By intercomparin g the results given by wide regions of spectrum (similar to 800 Angstrom) w ith those obtained by isolating small groups of features in very narrow win dows (similar to 30 Angstrom), we have been able to designate a window near lambda 4570 Angstrom, that should be particularly reliable for high-accura cy results; and we propose further studies at very high S/N ratio in that s pecific window to complement and extend the results of the present paper.