Galactic extinction laws: II. Hidden in plain sight, a new interstellar
  absorption band at 7700 \r{A} broader than any known DIB
  We have detected a broad interstellar absorption band centred close to 7700
\r{A} and with a FWHM of 176.6$\pm$3.9 \r{A}. This is the first such absorption
band detected in the optical range and is significantly wider than the numerous
diffuse interstellar bands (DIBs). It remained undiscovered until now because
it is partially hidden behind the A telluric band produced by O$_2$. The band
was discovered using STIS@HST spectra and later detected in a large sample of
stars of diverse type (OB stars, BA supergiants, red giants) using further STIS
and ground-based spectroscopy. The EW of the band is measured and compared with
our extinction and K I $\lambda\lambda$7667.021,7701.093 measurements for the
same sample. The carrier is ubiquitous in the diffuse and translucent Galactic
ISM but is depleted in the environment around OB stars. In particular, it
appears to be absent or nearly so in sightlines rich in molecular carbon. This
behaviour is similar to that of the $\sigma$-type DIBs, which originate in the
low/intermediate-density UV-exposed ISM but are depleted in the high-density
UV-shielded molecular clouds. We also present an update on our previous work on
the relationship between $E(4405-5495)$ and $R_{5495}$ and incorporate our
results into a general model of the ISM.
