Unusually Weak Diffuse Interstellar Bands toward HD 62542

As part of an extensive survey of diffuse interstellar bands (DIBs), we have obtained optical spectra of the moderately reddened B5 V star HD 62542, which is known to have an unusual UV extinction curve of the type usually identified with dark clouds. The typically strongest of the commonly cataloged DIBs covered by the spectra—those at 5780, 5797, 6270, 6284, and 6614 Å—are essentially absent in this line of sight, in marked contrast with other lines of sight of similar reddening. We compare the HD 62542 line of sight with others exhibiting a range of extinction properties and molecular abundances and interpret the weakness of the DIBs as an extreme case of deficient DIB formation in a dense cloud whose more diffuse outer layers have been stripped away. We comment on the challenges these observations pose for identifying the carriers of the diffuse bands.