The Central 300 pc of the Galaxy Probed by Infrared Spectra of H$_3^+$
  and CO: III. Locations of Sgr B2 and Star $\iota$
  Using a simple relation between the radial expansion velocity of diffuse gas
in the Central Molecular Zone (CMZ) of the Galaxy and its distance from Sgr
A$^\ast$ we estimate the physical depths within the CMZ of Star $\iota$ (2MASS
$J$17470898-2829561) and two Sgr B2 far-infrared continuum sources with respect
to the location of Sgr A$^\ast$. To do this we use velocity profiles of
mid-infrared absorption lines of H$_3^+$ and of far-infrared absorption lines
of H$_2$O$^+$, OH$^+$, and $^{13}$CH$^+$. The distances to Star $\iota$ and to
the Sgr B2 sources are found to be $\sim$90 pc greater than the distance to Sgr
A$^\ast$. Our conclusion that Sgr B2 lies toward the rear of the CMZ is
contrary to many previous models in which it has been placed shallower than Sgr
A$^\ast$.
