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 3 × 10-4 M⊙ yr-1, and the abundances of CO, HCO+, and HCN of 1 × 10-4, 3 × 10-8, and 0.9 × 10-8 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. In the former case, the cosmic-ray ionization rate is increased by a factor of 10-100, whereas the gasdust heat exchange rate should be larger by a factor of 3 than the average interstellar rate.