English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools annual

Global: Zendy Free Plan

Global: Zendy Tools monthly

Global: Zendy Plus monthly

The cross correlation method (hereafter CC) is widely used to derive theradial velocity curve of Cepheids when the signal to noise of the spectra islow. However, if it is used with the wrong projection factor, it mightintroduce some biases in the Baade-Wesselink (hereafter BW) methods ofdetermining the distance of Cepheids. In addition, it might affect the averagevalue of the radial velocity curve (or gamma-velocity) important for Galacticstructure studies. We aim to derive a period-projection factor relation(hereafter Pp) appropriate to be used together with the CC method. Moreover, weinvestigate whether the CC method can explain the misunderstood previouscalculation of the K-term of Cepheids. We observed eight galactic Cepheids withthe HARPS spectrograph. For each star, we derive an interpolated CC radialvelocity curve using the HARPS pipeline. The amplitudes of these curves areused to determine the correction to be applied to the semi-theoreticalprojection factor derived in Nardetto et al. (2007). Their average value (orgamma-velocity) are also compared to the center-of-mass velocities derived inNardetto et al. (2008). The correction in amplitudes allows us to derive a newPp relation: p = [-0.08+-0.05] log P +[1.31+-0.06]. We also find a negligiblewavelength dependence (over the optical range) of the Pp relation. We finallyshow that the gamma-velocity derived from the CC method is systematicallyblue-shifted by about 1.0 +- 0.2km/s compared to the center-of-mass velocity ofthe star. An additional blue-shift of 1.0km/s is thus needed to totally explainthe previous calculation of the K-term of Cepheids (around 2km/s). The new Pprelation we derived is a solid tool for the distance scale calibration(abridged).

High resolution spectroscopy for Cepheids distance determination. V. Impact of the cross-correlation method on the p-factor and the gamma-velocities