The Interstellar Medium toward the Galactic Center Source 2MASS
  J17470898-2829561
  We describe and discuss remarkable infrared spectra, covering key portions of
the $2-5$ $\mu$m wavelength interval, of the probable OH/IR supergiant 2MASS
J17470898$-$2829561 (2M1747), located in direction of the Sgr B molecular cloud
complex within the Central Molecular Zone (CMZ) of the Galaxy. This star was
originally singled out for examination based on its suitability for
spectroscopy of lines of H$_3^+$ in the CMZ. Analysis of the spectra shows that
2M1747 is deeply embedded within Sgr B1, with A$_V$ $\gtrsim$ 100 mag, making
it the only star within Sgr B for which infrared spectra have been obtained at
present, and thereby a unique infrared probe of the dense interstellar medium
within the CMZ. Despite the high extinction, spectra of 2M1747 reveal a veiled
photosphere in the $K$ band and circumstellar gas in the $M$ band, giving clues
as to its nature. Its $ 3.5-4.0$ $\mu$m spectrum contains the strongest
absorption lines of H$_3^+$ observed toward any object to date. The $4.5-4.8$
$\mu$m spectrum has impressively deep and wide absorption lines of interstellar
CO, most of which arise in dense gas within Sgr B1. The $3-5$ $\mu$m spectrum
also contains several solid state absorption features, which are characteristic
of both dense and diffuse clouds, and which raise questions about the
identifications of some of these features. We discuss the nature of the star,
the extinction to it, the extinction law for dust in the CMZ, and the
identifications of the various solid-state features and where they are produced
along this complex line of sight.
