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Anomalously broad diffuse interstellar bands (DIBs) at 5780.5, 5797.1,6196.0, and 6613.6 A are found in absorption along the line of sight toHerschel 36, the star illuminating the bright Hourglass region of the H IIregion Messier 8. Interstellar absorption from excited CH+ in the J=1 level andfrom excited CH in the J=3/2 level is also seen. To our knowledge, neitherthose excited molecular lines nor such strongly extended DIBs have previouslybeen seen in absorption from interstellar gas. These unusual features appear toarise in a small region near Herschel 36 which contains most of the neutralinterstellar material in the sight line. The CH+ and CH in that region areradiatively excited by strong far-IR radiation from the adjacent infraredsource Her 36 SE. Similarly, the broadening of the DIBs toward Herschel 36 maybe due to radiative pumping of closely spaced high-J rotational levels ofrelatively small, polar carrier molecules. If this picture of excitedrotational states for the DIB carriers is correct and applicable to most DIBs,the 2.7 degree cosmic microwave background may set the minimum widths (about0.35 A) of known DIBs, with molecular processes and/or local radiation fieldsproducing the larger widths found for the broader DIBs. Despite the intenselocal UV radiation field within the cluster NGC 6530, no previously undetectedDIBs stronger than 10 mA in equivalent width are found in the optical spectrumof Herschel 36, suggesting that neither dissociation nor ionization of thecarriers of the known DIBs by this intense field creates new carriers witheasily detectable DIB-like features. Possibly related profile anomalies forseveral other DIBs are noted.

astrophysical journal/the astrophysical journal

Anomalous Diffuse Interstellar Bands in the Spectrum of Herschel 36. I.  Observations of Rotationally Excited CH and CH+ Absorption and Strong,  Extended Redward Wings on Several DIBs