DETECTION OF HCO+ J=9-]8 LINE EMISSION TOWARD ORION IRC2 - MOLECULAR-IONS IN HOT DENSE GAS

We have detected the HCO+ J = 9 --> 8 802.458 GHz transition toward IRc2 in Orion. The line width implies that the emission originates in the low-velocity flow/doughnut most likely 10" or less from IRc2. If the line is optically thin, the density in the line-forming region is 10(7) cm-3 or higher. If the HCO+ line arises in the same approximately 10" x 20" source as other lines with similar width, the total column density of HCO+ is 3 x 10(14) cm-2 or higher, implying a line-of-sight averaged abundance 2 x 10(-9) or more. This abundance is significantly greater than would be expected in a high-density region. Equilibrium chemical models predict an abundance more than an order of magnitude lower than observed. Enhancement of HCO+ by local ultraviolet and X-ray fields is probably also insufficient to explain the observations. Time-dependent enhancement of the HCO+ abundance in shocks may offer an explanation, if initial conditions are appropriate, but published models are not relevant to the conditions immediately around IRc2. Under some circumstances, mechanisms which lower the electron abundance may also be able to raise the HCO+ abundance to the observed value.