Probing the 9.7\mu m Interstellar Silicate Extinction Profile through the Spitzer/IRS Spectroscopy of OB Stars

TL;DR

This study uses Spitzer/IRS spectra of OB stars to analyze the 9.7μm interstellar silicate extinction profile, revealing consistent peak positions but varying widths, and provides a parameterization for IR extinction.

Contribution

It offers the first detailed observational characterization of the 9.7μm silicate extinction profile across multiple lines of sight and introduces an analytical formula for IR extinction.

Findings

The 9.7μm extinction profile peaks at ~9.7μm across different stars.

Profiles show narrow (~2.0μm) and broad (~3.0μm) features with no environmental dependence.

The average profile resembles that of the diffuse interstellar medium.

Abstract

The 9.7$\mu$m interstellar spectral feature, arising from the Si--O stretch of amorphous silicate dust, is the strongest extinction feature in the infrared (IR). In principle, the spectral profile of this feature could allow one to diagnose the mineralogical composition of interstellar silicate material. However, observationally, the 9.7$\mu$m interstellar silicate extinction profile is not well determined. Here we utilize the Spitzer/IRS spectra of five early-type (one O- and four B-type) stars and compare them with that of unreddened stars of the same spectral type to probe the interstellar extinction of silicate dust around 9.7$\mu$m. We find that, while the silicate extinction profiles all peak at ~9.7$\mu$m, two stars exhibit a narrow feature of FWHM ~2.0$\mu$m and three stars display a broad feature of FWHM ~3.0$\mu$m. We also find that the width of the 9.7$\mu$m extinction feature does not show any environmental dependence. With a FWHM of ~2.2$\mu$m, the mean 9.7\mu m extinction profile, obtained by averaging over our five stars, closely resembles that of the prototypical diffuse interstellar medium along the lines of sight toward Cyg OB2 No.12 and WR 98a. Finally, an analytical formula is presented to parameterize the interstellar extinction in the IR at $0.9\mu {\rm m} < \lambda < 15\mu {\rm m}$.