On the Relation Between Distance and Intensity For Interstellar Calcium and Sodium Lines

Abstract

The results of spectrophotometric measurements of the intensities of the interstellar K and D lines carried out at the Dominion Astrophysical Observatory in the period 1928–1946 are presented in tabular form. The data relate to 1 055 medium-dispersion spectra of 182 stars, mostly of O and early B types, and include also measures of radial velocity for stellar and interstellar lines in most of the stars. The interstellar intensities are correlated with stellar distance, a variety of criteria being used for estimating the latter, including trigonometric parallaxes, parallactic motions, luminosities of eclipsing binaries, spectroscopic absolute magnitudes, cluster parallaxes and galactic rotation effects. The various criteria are weighted according to the intrinsic reliability of each method and to the number of stars concerned. The distance–intensity relations best satisfying the observations are : r =34·83 K =30·75 D , where r is the distance in parsecs, K the equivalent width of the calcium K line in km/sec and D the mean equivalent width of the sodium lines D1 and D2 in km/sec. Distances assigned to individual stars by using these relations should rarely be more than 25 per cent in error. The scatter is attributed to irregularity in distribution of the discrete clouds of interstellar gas and to uncertainties in the distance estimates. The distance–intensity relations found are compared with those of other observers.