De-reddening of optical spectra – Which extinction curve?

Spectral lines are used to determine a broad range of physical properties within HII regions and planetary nebulae (PNe). It is therefore important that we possess accurate intensities for the transitions, and have the means by which observed results may be accurately de-reddened.We point out that there are serious differences between the “standard” extinction curves, and that these may lead to errors in line ratios of as high as ∼80%. It is noted that the variation in Balmer line ratios in planetary nebulae is most consistent with the extinction curves of Whitworth [Whitworth, A.E., 1958. AJ 63, 201] and Ardeberg and Virdefors [Ardeberg, A., Virdefors, B., 1982. A&A 115, 347], and that these are likely to represent the most reliable functions for spectral de-reddening.