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. 
Nicholas S. Hakobian and Richard M. Crutcher
astro-ph.GA astro-ph.SR