VELOcities of CEpheids (VELOCE). I. High-precision radial velocities of Cepheids

We present the first data release of VELOcities of CEpheids ( dedicated to measuring the high-precision radial velocities (RVs) of Galactic classical Cepheids (henceforth, Cepheids). The first data release ( DR1) comprises RV measurements of bona fide classical Cepheids on both hemispheres collected mainly between 2010 and 2022, along with observations of stars, most of which had previously been misclassified as Cepheids. The median per-observation RV uncertainty for Cepheids is and reaches $2\,$ for the brightest stars observed with Non-variable standard stars were used to characterize RV zero-point stability and to provide a base for future cross-calibrations. We determined zero-point differences between and 31 literature data sets using template fitting, which we also used to investigate linear period changes of Cepheids. In total, 76 spectroscopic binary Cepheids and 14 candidate binary Cepheids were identified using data alone, which are investigated in detail in a companion paper ( II). DR1 provides a number of new insights into the pulsational variability of Cepheids, most importantly: a) the most detailed description of the Hertzsprung progression based on RVs to date; b) the identification of double-peaked bumps in the pulsation curve; and c) clear evidence that virtually all Cepheids feature spectroscopic variability signals that lead to modulated RV variability at the level of tens to hundreds of and that cannot be satisfactorily modeled using single-periodic Fourier series. We identified stars exhibiting such modulated variability, of which 4 also exhibit orbital motion. Linear radius variations depend strongly on pulsation period and a steep increase in slope of the $ R/p$ versus $ P -$relation is found near $10$\,d. This effect, combined with significant RV amplitude differences at fixed period, challenges the existence of a tight relation between Baade-Wesselink projection factors and pulsation periods. We investigated the accuracy of RV time series measurements and RV amplitudes published by third data release ( \ DR3) and determined an offset of $0.65 relative to Whenever possible, we recommend adopting a single set of template correlation parameters for distinct classes of large-amplitude variable stars to avoid systematic offsets in among stars belonging to the same class. The peak-to-peak amplitudes of exhibit significant ($16$) dispersion. Potential differences of RV amplitudes require further inspection, notably in the context of projection factor calibration.