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 sam ple of five optically visible massive young stellar objects (YSOs) bot h from diffuse interstellar bands (DIBs) in their spectra and from the ir optical continuum slopes. Our targets are HD 200775, BD+40 degrees 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 t hose of B-type standard stars. A survey of DIBs in the spectra of the massive YSOs, and a control held 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 tha t the DIBs in the control field star, HD 154445, have on average the s trength 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 w hich the YSOs are embedded. The formation efficiency of the DIBs varie s strongly however from cloud to cloud. The DIB E(B - V) compares favo urably with the total E(B - V) towards BD+40 degrees 4124, but is almo st negligible in the line of sight towards MWC 297. Despite this gener al, but not unexpected, deficit we provide evidence that the DIB at 58 49 Angstrom is a good tracer of total extinction in these lines of sig ht.