Estimation of errors in the distances to intrinsically reddened stars

Abstract

We show that adding the intrinsic color excess ΔE _B-V of a star to the interstellar E _B-V can significantly change the estimate of its distance r. When ΔE _B-V changes from $0\mathop .\limits^m 0$ to $0\mathop .\limits^m 5$, the relative error δ in the estimated distance rapidly increases to 99.5%. For $\Delta E_{B - V} > 0\mathop .\limits^m 5$, r cannot be determined from the distance modulus. The error δ depends neither on absolute magnitude M _V, nor on apparent magnitude m _V, nor on interstellar extinction but depends only on the star’s intrinsic reddening ΔE _B−V . We provide quantitative estimates of the intrinsic reddening in stars. The mean values of $\overline {\Delta E_{B - V} }$ for B0 stars in open clusters and associations are $0\mathop .\limits^m 2$ and $0\mathop .\limits^m 35$, respectively; $\overline {\Delta E_{B - V} }$ for Be stars is $0\mathop .\limits^m 1 - 0\mathop .\limits^m 2$; the effect of cooler companions on the color indices of OB stars can vary. The individual values of ΔE _B-V for separate OB stars exceed $0\mathop .\limits^m 5$ and reach $0\mathop .\limits^m 6 - 0\mathop .\limits^m 7$. We show that the intrinsic reddening of cluster stars produced by circumstellar gas-dust envelopes can be separated from the interstellar reddening even for the same extinction law.