A revised calibration of the Mv-W(OI 7774) relationship using Hipparcos data: Its application to Cepheids and evolved stars

A new calibration of the Mv-W(OI 7774) relationship has been calculated using better estimates of reddenings and distances to a sample of 27 calibrator stars of A-G spectral types, based on accurate parallaxes and proper motions from the Hipparcos and Tycho catalogues. The present calibration predicts absolute magnitude with accuracies of $\pm$ 0.38 mag. for a sample covering a large range of Mv, from -9.5 to $+0.35$ mag. The colour term included in a previous paper has been dropped since its inclusion was not bringing any significant improvement to the calibration. The variation of the OI 7774 feature in the classical cepheid SS Sct has been studied. We calculated a phase-dependent correction to random phase OI feature strengths in Cepheids, such that it predicts mean absolute magnitudes using the above calibration. After applying such correction, we could increase the list of calibrators to 58 by adding Mv and OI triplet strength data for 31 classical Cepheids. The standard error of the calibration using the composite sample was comparable to that obtained from the primary 27 calibrators, showing that it is possible to calculate mean Cepheid luminosities from random phase observations of the OI7774 feature. We use our derived calibrations to estimate Mv for a set of evolved objects to be able to locate their positions in the H-R diagram.