A Revised Calibration of the MV-W(O I 7774) Relationship using Hipparcos Data: Its Application to Cepheids and Evolved Stars

A new calibration of the M_V-W(O I 7774) relationship has been calculated using better reddening and distance estimates for a sample of 27 calibrator stars of spectral types A to G, based on accurate parallaxes and proper motions from the Hipparcos and Tycho catalogues. The present calibration predicts absolute magnitude with accuracies of +/-0.38mag for a sample covering a large range of M_V, from -9.5 to +0.35 mag. The color term included in a previous paper has been dropped since its inclusion does not lead to any significant improvement in the calibration. The variation of the O I 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 a correction, we could increase the list of calibrators to 58 by adding M_V and O I 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 O I 7774 feature. We use our derived calibrations to estimate M_V for a set of evolved objects to be able to locate their positions in the HR diagram.