Molecular envelope of the planetary nebula NGC 7027.

Strong emission of HCO(+) J = 1-0 has been detected in the molecular envelope of planetary nebula NGC 7027. A modeling of the envelope including a balance of thermal heating and cooling of the gas leads to a mass-loss rate of 0.0003 solar mass/yr, and the abundances of CO, HCO(+), and HCN of 0.0001, 3 x 10 to the -8th, and 0.9 x 10 to the -8th per H2, respectively. It is suggested that the large observed abundance of HCO(+) can be explained either by the ionization due to soft X-rays from the central star or a high gas-dust heat exchange rate.