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
  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 to
Herschel 36, the star illuminating the bright Hourglass region of the H II
region Messier 8. Interstellar absorption from excited CH+ in the J=1 level and
from excited CH in the J=3/2 level is also seen. To our knowledge, neither
those excited molecular lines nor such strongly extended DIBs have previously
been seen in absorption from interstellar gas. These unusual features appear to
arise in a small region near Herschel 36 which contains most of the neutral
interstellar material in the sight line. The CH+ and CH in that region are
radiatively excited by strong far-IR radiation from the adjacent infrared
source Her 36 SE. Similarly, the broadening of the DIBs toward Herschel 36 may
be due to radiative pumping of closely spaced high-J rotational levels of
relatively small, polar carrier molecules. If this picture of excited
rotational states for the DIB carriers is correct and applicable to most DIBs,
the 2.7 degree cosmic microwave background may set the minimum widths (about
0.35 A) of known DIBs, with molecular processes and/or local radiation fields
producing the larger widths found for the broader DIBs. Despite the intense
local UV radiation field within the cluster NGC 6530, no previously undetected
DIBs stronger than 10 mA in equivalent width are found in the optical spectrum
of Herschel 36, suggesting that neither dissociation nor ionization of the
carriers of the known DIBs by this intense field creates new carriers with
easily detectable DIB-like features. Possibly related profile anomalies for
several other DIBs are noted.
