2010A&A...518A..62G

The reddening in and around emission nebulae is characterized by the extinction A_V 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. (1989ApJ...345..245C), we show that A_V 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 (n_e, T_e). The line ratio method can be applied in regions where no stars are available for photometry.