He II Emission from Wolf-Rayet Stars as a Tool for Measuring Dust
  Reddening
  We calibrated a technique to measure dust attenuation in star-forming
galaxies. The technique utilizes the stellar-wind lines in Wolf-Rayet stars,
which are widely observed in galaxy spectra. The He II 1640 and 4686 features
are recombination lines whose ratio is largely determined by atomic physics.
Therefore they can serve as a stellar dust probe in the same way as the Balmer
lines are used as a nebular probe. We measured the strength of the He II 1640
line in 97 Wolf-Rayet stars in the Galaxy and the Large Magellanic Cloud. The
reddening corrected fluxes follow a tight correlation with a fixed ratio of
7.76 for the He II 1640 to 4686 line ratio. Dust attenuation decreases this
ratio. We provide a relation between the stellar E(B-V) and the observed line
ratio for several attenuation laws. Combining this technique with the use of
the nebular Balmer decrement allows the determination of the stellar and
nebular dust attenuation in galaxies and can probe its effects at different
stellar age and mass regimes, independently of the initial mass function and
the star-formation history. We derived the dust reddening from the He II line
fluxes and compared it to the reddening from the Balmer decrement and from the
slope of the ultraviolet continuum in two star-forming galaxies. The three
methods result in dust attenuations which agree to within the errors. Future
application of this technique permits studies of the stellar dust attenuation
compared to the nebular attenuation in a representative galaxy sample.
