Consistent radial velocities of classical Cepheids from the cross-correlation technique ★ ★★

摘要

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 ?. 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.