Profile of the Unidentified Interstellar Absorption Band at 6180 Å at Different Galactic Longitudes

Abstract

THERE is evidence¹ that the profile of the unidentified interstellar absorption band at 4430 Å is constant over a range of galactic longitudes. This band is the strongest of a number of apparently similar bands centred on wavelengths of 6180 Å, 4890 Å and 4760 Å. The band at 6180 Å was first noticed by Wilson², who speculated as to an interstellar origin. The second band, at 4890 Å, was discovered independently by Wilson³ and by Butler and Seddon⁴, while the third was tentatively identified as interstellar by Merrill in private correspondence with Beals⁵. The only evidence presented so far about the interstellar nature of these bands, apart from studies of correlation of strengths with those of known interstellar absorptions, comes from Merrill and Humason⁶, who have shown that the position of the 4430 Å band is stationary with respect to stellar absorption lines in the spectrum of an early type binary star. Strength correlation is difficult for all these bands because of their weakness and width, and has only been attempted for the band at 4430 Å. For the same reasons profile determination is difficult, but may well be crucial in the identification of their origin.