Small-scale structure of the interstellar medium towards rho Oph stars: diffuse band observations

TL;DR

This study investigates small-scale variations in diffuse interstellar bands (DIBs) towards rho Oph stars, revealing differences in DIB strengths over very small distances, which helps understand the distribution of molecules and dust in the interstellar medium.

Contribution

First detection of small-scale DIB strength variations between close binary stars, providing new insights into interstellar medium structure at AU scales.

Findings

DIB strengths vary by 5-9% over 344 AU separation.

Dense, molecule-rich regions show stronger specific DIBs.

DIB variations can exceed atomic and diatomic species differences.

Abstract

We present an investigation of small-scale-structure in the distribution of large molecules/dust in the interstellar medium through observations of diffuse interstellar bands (DIBs). High signal-to-noise optical spectra were recorded towards the stars rho Oph A, B, C and DE using the University College London Echelle Spectrograph (UCLES) on the Anglo-Australian Telescope. The strengths of some of the DIBs are found to differ by about 5-9 % between the close binary stars rho Oph A and B, which are separated by a projected distance on the sky of only ~344 AU. This is the first star-system in which such small-scale DIB strength variations have been reported. The observed variations are attributed to differences between a combination of carrier abundance and the physical conditions present along each sightline. The sightline towards rho Oph C contains relatively dense, molecule-rich material and has the strongest \lambda\lambda 5850 and 4726 DIBs. The gas towards DE is more diffuse and is found to exhibit weak C_2 (blue) DIBs and strong yellow/red DIBs. The differences in diffuse band strengths between lines of sight are, in some cases, significantly greater in magnitude than the corresponding variations among atomic and diatomic species, indicating that the DIBs can be sensitive tracers of interstellar cloud conditions.