First Interstellar HCO$^+$ Maser
  A previously unseen maser in the J = 1 - 0 transition of HCO$^+$ has been
detected by the Combined Array for Millimeter-wave Astronomy (CARMA). A
sub-arcsecond map was produced of the 2 arcmin$^2$ region around DR21(OH),
which has had previous detections of OH and methanol masers. This new object
has remained undetected until now due to its extremely compact size. The object
has a brightness temperature of $>$ 2500 K and a FWHM linewidth of 0.497 km
s$^{-1}$, both of which suggest non-thermal line emission consistent with an
unsaturated maser. This object coincides in position and velocity with the
methanol maser named DR21(OH)-1 by \citet{plambeck90}. No compact HCO$^+$
emission was present in the CARMA data towards the other methanol masers
described in that paper. These new results support the theory introduced in
\citet{plambeck90} that these masers likely arise from strong outflows
interacting with low mass, high density pockets of molecular gas. This is
further supported by recent observations of a CO outflow by \citet{zapata12}
that traces the outflow edges and confirms that the maser position lies along
the edge of the outflow where interaction with molecular tracers can occur.
