Accuracy of radial-velocity measurements for early-type stars

Abstract

The accuracy with which radial velocities of early-type stars can be measured is limited in practice by the existence of asymmetrical dierences between object and template spectrum, constituting \spectrum mismatch. Our studies of the magnitude of spectrum- mismatch errors, commenced in Paper I (Verschueren et al. 1999) on the basis of synthetic spectra having dierent attributes of eective temperature (Te )a nd logg, 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 dierent (observed) template spectra. We also test the eects of (a) truncating the spectra at dierent levels below the continuum, (b) adding rotational broad- ening to enforce a visual match of line-width between object and template, (c) applying rotational broadening to exacerbate a rotational mismatch, and (d) neglect- ing the presence of faint companion spectra. We also cross-correlate pairs of spectra such that the dierences between their Te 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 signicant in work of this nature enables us to specify conditions that are important for achieving such accuracy routinely. We nd that both rotational broadening, and the star-to-star variations in line strengths that are so prevelant among