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

摘要

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