Physical Sciences: Origin of the Diffuse Interstellar Band at 4430

MANNING 1 has suggested recently that the 4430 Å interstellar absorption line may be due to transitions in Fe 3+ ions in a silicate matrix. I have considered this idea from a slightly different point of view, and it may be fruitful to compare the lines of argument. An absorption band at 4430 Å had been found in demantoid garnet, Ca 3 Fe 2 (SiO 4 ) 3 , and a band at 4440 Å in chrysoberyl, BeAl 2 O 4 , both attributed to ferric iron 2 . Provided the coordination is octahedral the iron spectrum will be similar in many host lattices, and so it is not suggested that the interstellar particles must consist of either of these minerals. The possibility of silicates in space is indicated by the suggestion 3 that infrared absorption features at 9.7 µm and 10.6 µm are similar to those in the series of orthopyroxene inosilicates (Mg, Fe)SiO 3 . Gilman 4 has suggested that circumstellar grains of oxygenrich cool stars will be composed chiefly of refractory silicates such as Mg 2 SiO 4 and Al 2 SiO 5 .