We describe high spectral resolution , high dynamic range integral field spectroscopy of IC418 covering the spectral range 3300-8950Å and compare with earlier data .
We determine line fluxes , derive chemical abundances , provide a spectrum of the central star , and determine the shape of the nebular continuum .
Using photoionisation models , we derive the reddening function from the nebular continuum and recombination lines .
The nebula has a very high inner ionisation parameter .
Consequently , radiation pressure dominates the gas pressure and dust absorbs a large fraction of ionising photons .
Radiation pressure induces increasing density with radius .
From a photoionisation analysis we derive central star parameters ; \log T _ { \mathrm { e } ff } = 4.525 K , \log L _ { * } / L _ { \odot } = 4.029 , \log g = 3.5 and using stellar evolutionary models we estimate an initial mass of 2.5 < M / M _ { \odot } < 3.0 .
The inner filamentary shell is shocked by the rapidly increasing stellar wind ram pressure , and we model this as an externally photoionised shock .
In addition , a shock is driven into the pre-existing Asymptotic Giant Branch stellar wind by the strong D-Type ionisation front developed at the outer boundary of the nebula .
From the dynamics of the inner mass-loss bubble , and from stellar evolutionary models we infer that the nebula became ionised in the last 100 - 200 yr , but evolved structurally during the \sim 2000 yr since the central star evolved off the AGB .
The estimated current mass loss rate ( \dot { M } = 3.8 \times 10 ^ { -8 } M _ { \odot } yr ^ { -1 } ) and terminal velocity ( v _ { \infty } \sim 450 km/s ) is sufficient to excite the inner mass-loss bubble .
While on the AGB , the central star lost mass at \dot { M } = 2.1 \times 10 ^ { -5 } M _ { \odot } yr ^ { -1 } with outflow velocity \sim 14 km/s .