Diffuse interstellar bands in the spectra of massive young stellar objects

We have compared the B-V colour excess, E(B-V), obtained for a sample of fiveoptically visible massive YSOs both from diffuse interstellar bands (DIBs) intheir spectra and from their optical continuum slopes. Our targets are HD200775, BD+40 4124, MWC 1080, MWC 297 and MWC 349A. First, E(B-V) towards eachof the targets is derived by dereddening the observed continua to match thoseof 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 weakerin the former than would be expected based on the total E(B-V) values. Thisresult 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). Arough estimate of the foreground reddening of intervening diffuse interstellarmedium shows it to be smaller than the DIB-E(B-V), implying that at least partof the DIB carriers are formed within the parental molecular clouds in whichthe YSOs are embedded. The formation efficiency of the DIBs varies stronglyhowever from cloud to cloud. The DIB-E(B-V) compares favourably with the totalE(B-V) towards BD+40 4124, but is almost negligible in the line of sighttowards MWC 297. Despite this general, but not unexpected, deficit we provideevidence that the DIB at 5849 Angstrom is a good tracer of total extinction inthese lines of sight.Comment: MNRAS in press; 8 pages LaTeX, 4 ps figure