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

Abstract

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.

Catalog of radial velocity (and other observables) time series, the cross-correlation functions and our tailored correlation templates are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/631/A37

Partly based on observations made with ESO telescopes at Paranal and La Silla observatories under program IDs: 072.D-0419, 073.D-0136, 091.D-0469(A), 097.D60150(A) and 190.D-0237 for HARPS data; 098.D-0379(A), 0100.D-0397(A) and 0101.D-0551(A) for UVES data; and 073.D-0072(A), 074.D-0008(B), 075.D-0676(A), 084.B-0029(A) and 087.D-0603(A) for FEROS data. Partly based on observations made with the SOPHIE spectrograph at the Observatoire de Haute-Provence (CNRS, France), under program IDs PNPS.FRAN (12B), PNPS.GALL (13A, 14A, 15A), PNPS.KERV (13B, 16B, 17B), and PNPS.BORG (18A). Partly based on observations made with the CORALIE spectrograph on the Euler telescope (Swiss Observatory) at La Silla, Chile, under program numbers 1 and 756. Partly based on observations collected at the Telescope Nazionale Galileo in the framework of the OPTICON proposal 2015B/15 for HARPS-North data.