SIMBAD references

We present for the Orion nebula long-slit spectroscopic observations of emission lines in the wavelength regions 10 000-11 000 A and 3700-7330 A. These wavelength regions contain the corresponding multiplet lines of common upper atomic level of hydrogen P₆-H₆ (= P_γ -H_δ) and P₇-H₇ (= P_δ-H_ε) and of [S II] at 10 300 A and 4072 A. From these lines the extinction A_V towards the Orion nebula is derived with minimal reliance on recombination line model calculations. We find good internal agreement for the extinction derived from the corresponding hydrogen and [S II] lines, except for deviations of the order of 20-30% for data based on H_ε and P_δ, where deblending of the H_ε line may be uncertain by this amount and the flux of the P_δ line contains a similar error. There exists good agreement with the extinction derived from the other Paschen and Balmer line combinations. We also discuss in particular the method of extinction determination proposed by Allen (1979). However, we find no advantage from this over the other methods based on Paschen-Balmer lines and corresponding [S II] lines, neither from a greater precision achieved nor from the observational requirements.