A Study of the Interstellar Diffuse Absorption Features

Abstract

At a resolution of 0.17 Å, the profile of λλ 5780, 5797 were found to be asymmetric and steeper to the blue. The FWHM of λ 5780 and λ 5797 are respectively 2.7 Å and 1.1 Å. There is also a broad, shallow feature shortward of λ 5780. Equivalent widths of λλ 5780, 5797 were measured at a resolution of 0.26 Å, and they present a very tight correlation. Therefore, it is highly likely that these two diffuse lines have a common origin. The correlation between the equivalent width of λ 5780, EW(λ 5780), and E(B — V) is good for normal field stars. But there is a systematic weakening of λ 5780 in the spectra of Be stars, and the reverse is true for stars in the Scorpius-Ophiucus region. Excluding Be stars, a least square fit gives EW(λ 5780) = 0.75E(B — V) Å mag.⁻¹. From the profiles and structures of the diffuse features, and the correlations with various parameters of the interstellar medium, it is suggested that: (1) the diffuse band λ 4430 may be produced by the preionization of H⁻ and, (2) λλ 5780, 5797 may be due to the pure electronic transitions of the impurity centers in solid grains.