Interstellar 2175 Å extinction bump

Abstract

<p indent="0mm">The interstellar extinction curve contains important information about the chemical composition and size distribution of interstellar dust. The <sc>2175 Å</sc> extinction bump is the most prominent spectral feature on the interstellar extinction curve. Ever since its first detection in 1965, it is well recognized that although the strength of the <sc>2175 Å</sc> extinction bump varies with astrophysical environments, its peak wavelength is invariant and is independent of environmental conditions. We derived the UV extinction curves for 26 interstellar lines of sight based on data from the International Ultraviolet Explorer (IUE) and the Kurucz stellar model atmospheric spectra. It is found that the central wavelength of the <sc>2175 Å</sc> extinction bump of these interstellar lines of sight is not invariant around <sc>2175 Å.</sc> Instead, it exhibits considerable variation in the <sc>2130–2300 Å</sc> wavelength range, in stark contrast to the over-half-a-century-long, common belief of a stable peak wavelength of <sc>2175 Å</sc> for the extinction bump. The new observational property that the central wavelength of the <sc>2175 Å</sc> extinction bump is not constant imposes a new constraint on identifying the bump carrier.