High-resolution spectroscopy for Cepheids distance determinationV. Impact of the cross-correlation method on the p-factorand the y-velocities~(*,**)

摘要

Context. The cross correlation method (hereafter CC) is widely used to derive the radial velocity curve of Cepheids when the signalto noise ratio of the spectra is low. However, if it is used with an inaccurate projection factor, it might introduce some biases in theBaade-Wesselink (BW) methods of determining the distance of Cepheids. In addition, it might affect the average value of the radialvelocity curve (or y-velocity) important for Galactic structure studies.Aims. We aim to derive a period-projection factor relation (hereafter Pp) appropriate to be used together with the CC method.Moreover, we investigate whether the CC method can explain the previous estimates of the "K-term" of Cepheids.Methods. We observed eight galactic Cepheids with the HARPS~(***) spectrograph. For each star, we derive an interpolated CC radialvelocity curve using the HARPS pipeline. The amplitudes of these curves are used to determine the correction to be applied to thesemi-theoretical projection factor. Their average value (or y-velocity) are also compared to the center-of-mass velocities derived inprevious works.Results. The correction in amplitudes allows us to derive a new Pp relation: p = -0.08 ± 0.05 log P + 1.31 ± 0.06. We also finda negligible wavelength dependence (over the optical range) of the Pp relation. We finally show that the y-velocity derived from theCC method is systematically blue-shifted by about 1.0 ± 0.2 km/scompared to the center-of-mass velocity of star. An additionalblue-shift of 1.0 km s~(-I)is thus needed to totally explain the previous calculation of "K-term" Cepheids (around 2 km s~(-I).Conclusions. The new Pp relation we derived is a reliable tool for distance scale calibration, and especially to derive the distanceof LMC Cepheids with the infrared surface brightness technique. Further studies should be devoted to determining the impact of thesignal to noise ratio, the spectral resolution, and the metallicity on the Pp relation.