The 2175 Angstrom Interstellar Extinction Bump: Is the Wavelength Variable?

TL;DR

This study investigates the variability of the 2175 Angstrom interstellar extinction bump's wavelength, finding it to be more variable than previously thought, especially when considering different lines of sight and the suitability of Herbig Ae/Be stars for such measurements.

Contribution

The paper provides a physical analysis of the UV extinction curve, revealing that the bump wavelength varies around 2200 Angstrom and is less invariant than earlier studies suggested.

Findings

Wavelengths are around 2200 Angstrom with smaller scatter than previous studies.

The scatter in bump wavelength is still larger than the canonical 2175 Angstrom.

Herbig Ae/Be stars may not be ideal for interstellar extinction studies.

Abstract

The most striking characteristics of the mysterious 2175 Angstrom extinction bump, the strongest spectroscopic absorption feature seen on the interstellar extinction curve, are the invariant central wavelength and variable bandwidth: its peak position at 2175 Angstrom is remarkably constant while its bandwidth varies from one line of sight to another. However, recent studies of the lines of sight toward a number of Herbig Ae/Be stars have revealed that the extinction bump exhibits substantial shifts from the canonical wavelength of 2175 Angstrom. In this work we revisit these lines of sight and take a physical approach to determine the ultraviolet (UV) extinction curve for each line of sight. It is found that the wavelengths of the derived UV extinction bumps are around 2200 Angstrom and the scatters are considerably smaller than that of the previous study based on the same set of Herbig Ae/Be stars, consistent with the conventional wisdom. Nevertheless, the scatters are still appreciably larger than that associated with the nominal bump position of 2175 Angstrom. This is discussed in the context that Herbig Ae/Be stars are not well-suited for interstellar extinction studies.