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Post Info TOPIC: IC 418


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Posts: 131433
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RE: IC 418
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Title: IFU spectroscopy of Southern Planetary Nebulae IV: A Physical Model for IC 418
Author: M.A. Dopita, A. Ali, R.S. Sutherland, D.C. Nicholls, M. A. Amer

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; logT_eff=4.525K, logL_*/ solar luminosity=4.029, logg=3.5and using stellar evolutionary models we estimate an initial mass of 2.5<M/solar masses<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 ~2000 yr since the central star evolved off the AGB. The estimated current mass loss rate ( \dot M=3.8 x 10^-8 solar masses yr^-1) and terminal velocity (v_{\infty} ~ 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 x 10^-5 solar masses yr^-1 with outflow velocity ~14 km/s.

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Posts: 131433
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IC418
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Title: Excitation of emission lines by fluorescence and recombination in IC418
Authors: V. Escalante, C. Morisset, L. Georgiev

We compare calculated intensities of lines of CII, NI, NII, OI and OII with a published deep spectroscopic survey of planetary nebula IC 418. Our calculations use a self-consistent nebular model and a synthetic spectrum of the central star atmosphere to take into account line excitation by continuum fluorescence and electron recombination. We found that the NII spectrum of s, p and most d states is excited by fluorescence due to the low excitation conditions of the nebula. Many CII and OII lines have significant amounts of excitation by fluorescence. Recombination excites all the lines from f and g states and most OII lines. In the neutral-ionised boundary the NI quartet and OI triplet dipole allowed lines are excited by fluorescence, while the quintet OI lines are excited by recombination. Electron excitation produces the forbidden optical lines of OI, and continuum fluorescence enhances the NI forbidden line intensities. Lines excited by fluorescence of light below the Lyman limit thus suggest a new diagnostic to explore the inner boundary of the photodissociation region of the nebula.

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Posts: 131433
Date:
IC 418
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Title: Discovery of Multiple Shells Around the Planetary Nebula IC 418
Authors: Gerardo Ramos-Larios, Roberto Vazquez, Martin A. Guerrero, Lorenzo Olguin, R.A. Marquez-Lugo, Hector Bravo-Alfaro

We have analysed optical, near-, and mid-IR images of the bright planetary nebula (PN) IC 418. These images probe unambiguously for the first time a number of low surface brightness structures and shells around the bright main nebula, including radial filaments or rays, a system of three concentric rings, and two detached haloes with sizes ~ 150" and 220" x 250", respectively. The main nebula is slightly off-centred with respect to the elliptical outer halo. The time-lapse between the two haloes is 10,000-50,000 yr, whereas the time-lapse between the three concentric rings is ~ 630 yr. We emphasise the advantages of near- and mid-IR imaging for the detection of faint structures and shells around optically bright nebulae.

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