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Title a Self-Consistent Photoionization-Dust A self-consistent photoionization-dust continuum-molecular line Title transfer model of NGC 7027 Author(s) Volk, K; Kwok, S Citation The Astrophysical Journal, 1997, v. 477 n. 2 pt. 1, p. 722-731 Issued Date 1997 URL http://hdl.handle.net/10722/179686 Rights Creative Commons: Attribution 3.0 Hong Kong License THE ASTROPHYSICAL JOURNAL, 477:722È731, 1997 March 10 ( 1997. The American Astronomical Society. All rights reserved. Printed in U.S.A. A SELF-CONSISTENT PHOTOIONIZATIONÈDUST CONTINUUMÈMOLECULAR LINE TRANSFER MODEL OF NGC 7027 KEVIN VOLK AND SUN KWOK Department of Physics and Astronomy, University of Calgary, Calgary, Alberta, Canada T2N 1N4 Received 1996 June 20; accepted 1996 September 23 ABSTRACT A model to simulate the entire spectrum (1000Ó to 1 cm) of the high-excitation young planetary nebula NGC 7027 is presented. The ionized, dust, and molecular components of the object are modeled using geometric parameters obtained from visible, radio, infrared, and CO data. The physical processes considered include recombination lines of H and He, collisional excited lines of metals, bf and † contin- uum radiations, two-photon radiation, dust continuum radiation, and molecular rotational and vibra- tional transitions. The dust component is assumed to be heated by a combination of direct starlight and the line and continuum radiation from the ionized nebula. The molecular component of the nebula is coupled to the dust component through the stimulated absorption of the dust continuum radiation. Spe- ciÐcally, we compare the predicted Ñuxes of the CO rotational lines and the 179.5 km water rotational line to those observed by the Infrared Space Observatory satellite. From the observed ratio of these lines, we derived a gas phase water-to-H abundance ratio of 3.2 ] 10~7 assuming that the water is uniformly mixed with the CO. 2 Subject headings: dust, extinction È ISM: individual (NGC 7027) È ISM: molecules È molecular processes È radiative transfer 1. INTRODUCTION oxygen present. Since the optical spectrum of NGC 7027 suggests a C-to-O ratio greater than 2, one expects only NGC 7027 is a planetary nebula that more than any carbon-based molecules in the gas. The near-infrared spec- other object, has been responsible for revolutionizing our trum of NGC 7027 is dominated by emission features com- understanding of the planetary nebula phenomenon. While monly attributed to the polycyclic aromatic hydrocarbon it is relatively innocuous in its optical appearance, it was (PAH) molecules(Gillett, Forrest, & Merrill 1973), and in found to have a very large mid-infrared excess due to dust the mid-infrared there is no sign of the 9.7 km silicate emission(Woolf 1969; Gillett, Merrill, & Stein 1972). It is feature commonly observed in oxygen-rich circumstellar also one of the strongest thermal radio continuum sources envelopes. It is therefore safe to assume that the dust in in the sky, suggesting the presence of an ionized region of NGC 7027 is carbon-based. very high emission measure(Terzian, Balick, & Bignell It was a surprise, therefore, when the Infrared Space 1974). The detection of the J 1 ] 0 rotational line of CO \ Observatory (ISO) detected water in NGC 7027 (ISO press in NGC 7027 byMufson, Lyon, & Marionni (1975) came as release 1996). The ISO Long Wavelength Spectrometer a surprise, for it was not expected that molecules could exist (LWS) spectrum from 157 to 192 km shows a nebular emis- in or around an ionized environment. Since this detection, sion line due to C II at 157.7 km, three CO rotational lines NGC 7027 has been found to be a rich molecular source, (16 ] 15 at 162.8 km, 15 ] 14 at 173.6 km, 14 ] 13 at 186.0 with many carbon-based molecules discovered in its km), and the 179.5 km rotational line(2 ] 1 ) of ortho- massive ([1M ) neutral molecular envelope. Mapping of _ water. The observations of these highly excited12 01 CO lines by the molecular envelope by millimeter interferometry sug- ISO therefore provide new constraints to the molecular gests that it is located outside of the small (D10A) ionized model of NGC 7027, which previously has been based on region(Bieging, Wilner, & Thronson 1991). The dust the Ðtting of the lowest CO rotational lines. The expanded envelope coexists with the molecular gas and is responsible spectral coverage o†ered by ISO Ðlls a previous gap for the large extinction(A D 3 mag) seen in the nebula. The V between the infrared and the submillimeter observations, central star is clearly hotter than average for planetary and provides for the Ðrst time the opportunity to create a nebulae, with hydrogen and helium Zanstra temperatures comprehensive model of NGC 7027 covering the entire of 500,000 and 430,000 K, respectively, having been sug- spectrum from the UV to radio. gested (see Table VII-6 ofPottasch 1984). However, direct The most recent e†ort in the modeling of NGC 7027 was imaging of the central star indicates a lower, but still high, made byHoare, Roche, & Clegg (1992), who took into temperature of D200,000È250,000 K (Jacoby 1988; Walton account both the ionization and the dust components. In et al.1989; Heap & Hintzen 1990; Hubeny, Heap, & Lanz this paper, we present a model which includes the radiative 1994). The high central-star temperature suggests that the interactions between the ionized, molecular, and dust com- star must be massive. The rapid evolution of the massive ponents in NGC 7207. Furthermore, we have made use of central star, together with the high density of the ionized the observed strength of the 179 km water line to derive a region, suggests that the nebula is very young, having left water abundance in NGC 7027. the asymptotic giant branch only D103 yr ago. Conventional circumstellar chemistry models assume THE MODEL that all of the available oxygen in a carbon-rich molecular 2. region is bound in the CO molecule and that all the rest of Planetary nebulae are complicated systems involving the molecular chemistry proceeds with essentially no free close radiative and dynamic interactions between the 722 LINE TRANSFER MODEL OF NGC 7027 723 central star and the nebula(Kwok 1994). On the radiative with the ionized gas. A second dust component, made up of side, the ionized gas component is heated by photoioniza- amorphous carbon, is assumed to be outside of the ionized tion from absorbing the Lyman continuum photons from component. The radiation transfer problem for the second the central star, and cooled by collisional-excited line emis- dust component (the neutral wind region) is solved self- sions. The dust component is heated by a combination of consistently, producing the continuum Ñuxes[ J (r)] at the visible continuum photons from the central star and the every radius. The molecular gas is assumed to bel mixed line photons (in particular Lya) from the ionized gas com- with this dust component, and the values ofJ (r) are used as ponent, and cooled by self-radiation. The molecular gas a source of background radiation availablel for stimulated component is heated by gas collisions and absorption of absorption by the molecules. The equation of transfer and near-infrared continuum photons, and cooled by molecular the equation of statistical equilibrium are then solved for rotational emission. Since the three components are ther- the relevant molecules (in our case CO andH O) to mally coupled, an analysis of the radiation transfer problem produce the emergent Ñuxes in each of the rotational2 tran- requires simultaneous solutions to the photoionization, sitions. A Ñow chart illustrating the interactions between dust continuum transfer, and the molecular line transfer various components is given in Figure 1. equations. The density proÐle for the gas in the ionized and neutral While this problem seems to be intractably complicated, regions has a three-component structure. The ionized certain approximations are possible. First, the ionized com- region is assumed to be a shell of constant density and ponent is heated by the central star alone and is not a†ected constant Ðlling factor, the values of which are taken to be by the neutral components. Second, since most of the dust free parameters with some guidance from the forbidden-line radiation is emitted in the far-infrared where the nebula is analyses available in the literature. The remainder of the optically thin, there is very little feedback of the dust emis- circumstellar shell is taken to consist of a high-density sion into the other components. Similarly, the molecular neutral shell of small angular width and a stellar wind gas emits only line radiation; although these lines can be region with the usual inverse-square density proÐle. The optically thick because of self-absorption, they do not con- two components of the neutral gas have been deduced from stitute a signiÐcant source of heating of the dust component. CO maps and line proÐle analysis (seeJaminet et al. 1991). However, near-infrared emission from the dust can be A schematic diagram of the geometry of the source struc- absorbed by the molecular gas via vibrational excitations, ture is shown in Figure 2. and the molecular line transfer problem is dependent on the output of the dust transfer problem. 2.1. T he Photoionization Model In this paper, we have set up the problem in the following The photoionization model was constructed using the way. First, the ionized gas component is treated indepen- CLOUDY photoionization code of G. Ferland (see Ferland dently. All the permitted lines with signiÐcant optical 1996 for a description of CLOUDY). For nebular elemental depths (e.g., the recombination lines of H and He) have their abundances the values ofMiddlemass (1990) were used.
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