The interstellar Ca?II distance scale

摘要

Aims. We attempt to extend the relation between the strengths of the interstellar Ca IIlinesand the distances to early-type stars to objects beyond1kiloparsec, with the line saturation taken into account. Methods. We measure the Ca II K and Ca IIH equivalent widths, and compute Ca IIcolumn densities for 262lines of sight towards early-type stars with available Hipparcos parallaxes().The targets are located within a few hundred parsecs of the Galacticplane, and span all the range of Galactic longitudes. We fit the - parallax relation with a function of the form ,using a maximum-likelihood approach to take account of errors in bothvariables. We use the resultant formula to estimate distances to starsin OBassociations and clusters, and compare them to those foundin the literature, usually estimated by spectrophotometric methods. Results. For lines of sight with 1.3$ -->EW(K)/EW(H) > 1.3, we obtain the following approximate formula for the distance: ,where the equivalent widths EW(K) and EW(H) are inm?, and the distancein parsecs. The errors in,resulting from the uncertainty in the fit parameters and errors in the equivalent widths, are typically about15of the distance. We can also expect the equation not to hold forobjects situated farther than a few hundred parsecs from the Galacticplane. We find several cases of significant column density differencesbetween association or cluster members, especially notable in theTrumpler16 cluster, indicating either a localcontribution to the Ca IIcolumn density, or background/foreground stars being confused with members. The ratioappears to depend on the Galactic longitude, being highest in the range 70< l< 120and lowest for 200< l< 300.This effect may be due to large-scale structure being present in the Ca IIlayer, or to the nonmember confusion being enhanced in these directions. Key words: ISM: lines and bands - stars: distances - stars: early-type - open clusters and associations: general