A New Method to Determine the Interstellar Reddening toward WN Stars

Abstract

The emission lines in Wolf-Rayet stars may be utilized to determine the absorption properties of their line-of-sight interstellar medium. One can measure the line strengths both in terms of the equivalent widths (in units of the continuum) and fluxes. In general, in a comparison of two lines, the equivalent width ratio is extinction independent; its value depends on intrinsic properties of the objects in question. The line flux ratio will differ by a constant factor and the absorption properties of the interstellar medium. As an example of this method, we compare the measurements of the He II emission features at λ 4686 and λ 1640 in a number of WN stars in the Galaxy and the Large Magellanic Cloud (LMC). We show that their equivalent width ratios are single valued and are independent of the spectral subtypes. We adopt an LMC extinction law and then utilize the line fluxes in the LMC WN stars to evaluate the expected line flux ratio in this purely empirical approach. We derive the reddening for the stars in the Galaxy using a galactic extinction law and their observed line flux ratios. There is very good agreement of our derived reddening with previous determinations. It is possible to generalize this method by using measurements of several lines in W-R stars at various wavelengths. One could then derive the extinction law itself in front of both WN and WC subtypes. This may have eventual application to determinations of the absorption properties of the interstellar medium in more distant galaxies.