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Diffuse interstellar bands in the spectra of massive YSOs

We have compared the B-V colour excess, E(B-V), obtained for a sample of five optically visible massive YSOs both from diffuse interstellar bands (DIBs) in their spectra and from their optical continuum slopes. Our targets are HD 200775, BD+40 4124, MWC 1080, MWC 297 and MWC 349A. First, E(B-V) towards each of the targets is derived by dereddening the observed continua to match those of B-type standard stars. A survey of DIBs in the spectra of the massive YSOs, and a control field star, then reveals that the DIBs are significantly weaker in the former than would be expected based on the total E(B-V) values. This result is strengthened by the finding that the DIBs in the control field star, HD 154445, have on average the strength expected from its continuum E(B-V). A rough estimate of the foreground reddening of intervening diffuse interstellar medium shows it to be smaller than the DIB-E(B-V), implying that at least part of the DIB carriers are formed within the parental molecular clouds in which the YSOs are embedded. The formation efficiency of the DIBs varies strongly however from cloud to cloud. The DIB-E(B-V) compares favourably with the total E(B-V) towards BD+40 4124, but is almost negligible in the line of sight towards MWC 297. Despite this general, but not unexpected, deficit we provide evidence that the DIB at 5849 Angstrom is a good tracer of total extinction in these lines of sight.