2019A&A...631A..37B

Context. Accurate radial velocities (v_rad) of Cepheids are mandatory within the context of Cepheid distance measurements using the Baade-Wesselink technique. The most common v_rad derivation method consists in cross-correlating the observed stellar spectra with a binary template and measuring a velocity on the resulting mean profile. Nevertheless, for Cepheids and other pulsating stars, the spectral lines selected within the template as well as the way of fitting the cross-correlation function (CCF) have a direct and significant impact on the measured v_rad.
Aims. Our first aim is to detail the steps to compute consistent CCFs and v_rad of Cepheids. Next, this study aims at characterising the impact of Cepheid spectral properties and v_rad computation methods on the resulting line profiles and v_rad time series.
Methods. We collected more than 3900 high-resolution spectra from seven different spectrographs of 64 Classical Milky Way (MW) Cepheids. These spectra were normalised and standardised using a single custom-made process on pre-defined wavelength ranges. We built six tailored correlation templates selecting unblended spectral lines of different depths based on a synthetic Cepheid spectrum, on three different wavelength ranges from 3900 to 8000 Å. Each observed spectrum was cross-correlated with these templates to build the corresponding CCFs, adopted as the proxy for the spectrum mean line profile. We derived a set of line profile observables as well as three different v_rad measurements from each CCF and two custom proxies for the CCF quality and amount of signal.
Results. This study presents a large catalogue of consistent Cepheid CCFs and v_rad time series. It confirms that each step of the process has a significant impact on the deduced v_rad: the wavelength, the template line depth and width, and the v_rad computation method. The way towards more robust Cepheid v_rad time series seems to go through steps that minimise the asymmetry of the line profile and its impact on the v_rad. Centroid or first-moment v_rad, that exhibit slightly smaller amplitudes but significantly smaller scatter than Gaussian or biGaussian v_rad, should therefore be favoured. Stronger or deeper spectral lines also tend to be less asymmetric and lead to more robust v_rad than weaker lines.