IFU spectroscopy of Southern Planetary Nebulae IV: A Physical Model for IC 418

Abstract

We describe high spectral resolution, high dynamic range integral field spectroscopy of IC418 covering the spectral range 3300-8950{\AA} 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; $\log T_{\mathrm eff} = 4.525$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.