3D photoionization models of Planetary Nebulae
CTIO REU/PIA project by Katherine M. Guenthner
Advisors: Hugo Schwarz, Hektor Monteiro




Abstract

We attempt to build a more realistic model of the ringed bipolar planetary nebula, Mz 1, using a three-dimensional self-consistent photoionization code. The code divides the gaseous region of the nebula into numerous cubic cells. The code then calculates the physical conditions in each cell of the cube representing the nebula. From this we obtain electronic temperature and density, ionic fractional abundances, and emission-line luminosities. From observation taken at the Cerro Tololo Interamerican Observatory (CTIO) we create spectrophotometric line images and density maps and compare them with our model results. From this we obtain for the first time the luminosity and temperature of the ionizing source for Mz1. The results of this model will also help to get a broader understanding of the processes taking place in the shaping of planetary nebulae and their evolution.


I. Planetary Nebulae

When we observe planetary nebulae, we are in fact taking a glimpse into the late stages of stellar death. Stars with 8-12 times the mass of our Sun, eventually expell the material in their envelopes in an expanding shell of heated gas. This is what's known as a planetary nebula. The shell of material has expansion velocities of 10 to 30 kilometers per second. The central star compresses to a very dense and hot star, a white dwarf, that photoionizes the gas cloud. This illuminates the cloud via ultraviolet radiation in the same way an electric current lights up neon in a sign. The glowing material in the shell are various types of ionized elements. The greenish tint is due to the emission from doubly ionized oxygen (OIII). The reddish tint of planetary nebulae is due to the emission of Hydrogen Balmer beta lines.








There are many different morphologies of planetary nebulae. There have been many attempts to classify differnt PNe. The some of the well studied are the ring-shape structure, like the Helix Nebula above, and the bipolar structure seen below. But, morphology classifications only give apparent structures, not intrinsic structures. That is, the apparent shape of a PN depends on the orientation of the PN towards our line of sight. In the same way that a cylinder looks like a circle at a particular angle. This is one of the greatest problems in PN research, the physical origin of the different morphologies and how they evolve into such forms. One way to investigate this is to employ 3D models of PN.

Also, to better understand the origin and evolution of this late stage of stellar evolution, careful analysis of the spectra of PN must be made. This analysis can be performed by computer codes known as photoionization models. The models contain large amounts of atomic data in the code and by using characteristics of the central star and surrounding gas as input parameters,line spectrum can be calculated and then compared to spectroscopic observations.


II. Objectives: Menzel 1 (Mz 1)

Menzel 1 (Mz 1) is a planetary nebula with a bipolar/ellipsoidal shape and an equatorial ring of enhanced emission. To understand this better, we attempt to obtain a more realistic model Mz 1, using a self-consistent 3D photoionization model. The model yields a theoretical value of the temperature and luminosity of the central star.


III. 3D Photoionization Models of Planetary Nebulae

  • Photoionization models simulate a collection of physical properties of the PN, and shows the interaction between UV radiation of the central star (white dwarf) and how it is absorbed and processed by the surrounding nebula.
  • The model allows the determination of physical conditions at each point of the photoionized gaseous region.
  • We put aside the assumption that 1D models make of spherical symmetry in the PN, that it is a sphere with a central point as the star, and therefore we can model PN with other types of geometry.
  • The gaseous region is divided into cubic cells, thermal and ionization equilibria are assumed in order to obtain physical conditions, and the model calculates primary and secondary radiation transer.


  • Input parameters: ionizing radiation spectrum, spatial density distribution, elemental abundances
  • Output: physical conditions and emission line luminosities for each cell



IV. Steps to Modelling Mz1



V. Conclusions

At the end of the 10 weeks at CTIO, I had run over 100 models, of 30, 50, and 80 cubic cells. We hope to obtain a model within 20% error in all the line fluxes. Preliminary results show a central star temperature of T(star) ~ 10e^5 and a central star luminosity of L(star) ~ 500L(sun). We are still working on the modeling of Mz1. To further constrain our model, we wanted to know if dust is present and are now working with polarimetry data of Mz1.


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