Interstellar Ca II line intensities and the distances of the OB stars

We show that the equivalent widths of the well-known interstellar Ca II H and K lines can be used to determine the distances to OB stars in our Galaxy. The equivalent widths, measured in the spectra of 147 early-type stars, are strongly related to the Hipparcos parallaxes of those objects. The lines fitted to the parallax-equivalent width data are given by the formulae pi = 1/[2.78EW(K) + 95] and pi = 1/[4.58EW(H) + 102], where pi is in arcseconds and EW is in milliangstroms. The form of the formulae, yielding a finite parallax even for zero absorption, shows that space within approximate to 100 pc of the Sun contains very little Ca II, which is in agreement with the known dimensions of the Local Bubble. Using Ca II lines for distance determination does not require the knowledge of the absolute magnitude of the object; it is thus well suited for targets for which the absolute calibration is either not precise (OB supergiants) or not available at all (peculiar objects). We also demonstrate that neither the reddening E(B - V) nor the equivalent widths of interstellar K I and CH lines are suitable candidates for distance estimation, their relation with parallaxes being far less tight than for Ca II.