Determinations of the interstellar reddening toward Wolf-Rayet stars of the nitrogen sequence from observed UBV color indices.

Abstract

We demonstrate that continuum discontinuities, or "jumps", are present in the spectra of some Wolf-Rayet stars and that current models of Wolf-Rayet stars predict such jumps and provide good fits to the observed continua. Using a relation between the size of the He II (n = 4) jump at 3645 A and the intrinsic (b - v)_0_ color found from the model atmospheres of Wolf-Rayet stars of the nitrogen sequence, we derive a formula for calculating the color excesses, E_b-v_, of WN stars. The method assumes that helium is the major opacity source in the Wolf-Rayet atmosphere; therefore it cannot be applied to WC stars. The method requires measurements of only the u - b and b - v color indices; it is independent of any assumed values of the intrinsic colors. We investigate the differences in the various photometric and calibration systems used in the past to determine magnitudes and colors of Wolf-Rayet stars and give correction factors which we use to put all previous values onto one consistent system. Using the corrected values of the u - b and b - v colors, we then calculate color excesses for WN stars in the Galaxy and the Large and Small Magellanic Clouds from our formula. We compare our results with previous determinations of color excesses for WN stars and find generally good agreement for Galactic stars. However, there are many LMC stars for which the photometric data are not accurate enough to yield reliable reddening values.