On the Interstellar Extinction Curve toward HD 93222, A Sightline with an Exceedingly Narrow 2175 Angstrom Extinction Bump

TL;DR

This paper studies the interstellar extinction curve toward HD 93222, revealing an exceptionally narrow 2175 Angstrom bump and suggesting the presence of nano-sized graphitic grains to explain its features.

Contribution

It presents the observation of an extremely narrow 2175 Angstrom bump and introduces a dust model including nano-sized graphitic grains to explain this feature.

Findings

The 2175 Angstrom bump toward HD 93222 is among the narrowest observed.

A dust model with nano-sized graphitic grains explains the extinction curve.

The extinction characteristics require a mixture of silicate, graphite, and nano-sized grains.

Abstract

The 2175 Angstrom extinction bump, the most prominent spectral feature superimposed on the interstellar extinction curve, is widely seen in the interstellar medium (ISM) of the Milky Way and external galaxies, both near and far. While its central wavelength is remarkably stable and independent with environment, its width shows considerable variation and environmental dependence. Here we examine the extinction curve for the line of sight toward HD 93222, a young star located in the Carina nebula. It is found that the 2175 Angstrom bump is extremely sharp, which is among the narrowest ever found in the Milky Way and external galaxies. We model the derived extinction curve and find that, to explain the extinction characteristics of HD 93222, in addition to the conventional silicate and graphite dust mixture, an additional population of nano-sized graphitic grains is required.