CO J = 3-2 and J = 2-1 observations of NGC 7027

Using the 15 m James Clerk Maxwell Telescope, we have mapped the molecular envelope surrounding the planetary nebula NGC 7027 in the J = 3-2 line of ¹²CO and have observed the central position in the J = 3-2 line of ¹³CO and the J = 2-1 lines of ¹²CO and ¹³CO. The J = 3-2 ¹²CO map shows substantial deviations from spherical symmetry. The mass-loss rate along the nebula's polar axis appears to be about half that in the equatorial directions. Bright line wings are caused by a bipolar flow of molecular gas along the low-density axis. This gas has an outflow velocity of 23 km s^-1, larger than the 15 km s^-1 expansion velocity of the bulk of the envelope. This flow is apparently driven by the pressure of the ionized region. A marked asymmetry in the J = 3-2 line profile toward the central position is probably due to a radiative transfer effect, which is more important in NGC 7027 than in other circumstellar envelopes due to the large velocity dispersion of neutral gas near the ionized region. We have compared the data with CO J = 1-0 observations and derive new estimates for envelope parameters from a detailed model. The kinetic temperature is 30 ± 3 K at a radius of r = 8″ and varies with radius approximately like r^-0.85. The total mass-loss rate is 1-2 × 10^-4 M_⊙ yr^-1, but the mass loss is aspherical. The CO:H₂ abundance ratio is 2_-1⁺² × 10^-4, and the ¹²COr:¹³CO abundance ratio is 90 ± 25. The radial extent of the CO emission is smaller than is consistent with the assumption of constant mass-loss rate and constant CO:H₂ ratio. This may be due to dissociation of the CO by the interstellar radiation field at characteristic radii of about 35″ in equatorial directions and about 25″ in polar directions. Alternatively, the mass-loss rate and/or the CO:H₂ ratio may have been smaller 8000 yr ago than they were in the more recent past.