Extinction

The reddening in and around emission nebulae is characterized by the extinction AV and the ratio R of the absolute-to-selective absorption. Both are usually derived from the photometry of a single star or cluster stars that are associated with an emission nebula. Using the parameterized reddening relation published by Cardelli et al. (1989, ApJ, 345, 245), we show that AV and R can be derived with good precision from the observation of a set of common upper level Paschen-Balmer hydrogen line ratios. The use of common upper level line ratios has the advantage of being nearly independent of the excitation condition of the nebula (ne, Te). The line ratio method can be applied in regions where no stars are available for photometry