Gas-phase models for the evolved planetary nebulae NGC 6781, M4-9 and NGC 7293

Ali, A.; Shalabiea, Osama M.; El-Nawawy, M. S.; Millar, T. J.

doi:10.1046/j.1365-8711.2001.04513.x

Cited by 20 publications

(32 citation statements)

References 14 publications

(17 reference statements)

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“…The CN/HCN ratio calculated by Hasegawa et al (2000) is in the range 1.4 to 6.6, depending on the volume hydrogen density and the kinetic temperature, in good agreement with our average value (∼2.6). The abundances of HCN and HNC calculated by Ali et al (2001) are in general agreement with the observations reported here. However their abundance of CN is too high by a factor of ∼3.…”

Section: Chemistry Of N-bearing Moleculessupporting

confidence: 90%

“…Nevertheless, this ratio seems again to be relatively lower in the two youngest objects (HNC/HCN < ∼ 0.2) than in some more evolved ones (HNC/HCN up to ∼1). These values are in the range of those previously observed in other nebulae (Bachiller et al 1997a) and estimated theoretically (Ali et al 2001).…”

Section: Column Densities and Their Evolutionsupporting

confidence: 89%

“…Howe et al (1992Howe et al ( , 1994 modelled the evolution of a dense clump similar to those of the Helix nebula. More recently, Hasegawa et al (2000) and Hasegawa & Kwok (2001) reported detailed models of the neutral envelope of NGC 7027, and Ali et al (2001) constructed time-dependent gas-phase models which reproduce well the molecular abundances in evolved PNe such as NGC 6781, M4-9 and the Helix, observed by Bachiller et al (1997a).…”

Section: Introductionmentioning

confidence: 88%

“…The chemistry of N-bearing molecules has been recently studied by Ali et al (2001) with a detailed time-dependent chemical model suited to the conditions of evolved PNe, and by Hasegawa et al (2000) and Hasegawa & Kwok (2001) for the particular case of NGC 7027. The CN/HCN ratio calculated by Hasegawa et al (2000) is in the range 1.4 to 6.6, depending on the volume hydrogen density and the kinetic temperature, in good agreement with our average value (∼2.6).…”

Section: Chemistry Of N-bearing Moleculesmentioning

confidence: 99%

“…However the survival of H + 3 ion is expected to be very short, because of its very effective dissociative recombination (Bachiller et al 1997a), so this process may not be efficient enough to explain the observed high abundances. It has been pointed out by Ali et al (2001) that X-ray emission from the central hot star of PNe may also contribute to a high rate of ionization. In fact, the relatively high X-ray fluxes have been observed in several PNe (e.g.…”

Section: Chemistry Of N-bearing Moleculesmentioning

confidence: 99%

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The chemistry of compact planetary nebulae

Josselin

Bachiller

2002

A&A

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Abstract. We report high-sensitivity millimetre observations of several molecular species ( 13 CO, HCN, HNC, CN, HCO + and N 2 H + ) in a sample of compact planetary nebulae. Some species such as HCO + and CN are particularly abundant compared to envelopes around AGB stars or even interstellar clouds. We have estimated the following average values for the column densities ratios: CN/HCN ∼ 2.6, HCO + /HCN ∼ 0.5, and HNC/HCN ∼ 0.4. Thus, the chemical composition of the molecular envelopes in these compact PNe appears somewhat intermediate between the composition of proto-PNe (such as CRL 2688 or CRL 618) and well evolved PNe (such as the Ring, M4-9, or the Helix). From observations of the CO isotopomers, we have estimated that the 12 C/ 13 C ratio is in the range 10 < ∼ 12 C/ 13 C < ∼ 40. These values are below those expected from standard asymptotic giant branch models and suggest non-standard mixing processes. The observed molecular abundances are compared to very recent modelling work, and we conclude that the observations are well explained, in general terms, by time-dependent gas-phase chemical models in which the ionization rate is enhanced by several orders of magnitude with respect to the average interstellar value. Thus, our observations confirm that the chemistry in the neutral shells of PNe is essentially governed by the high energy radiation from the hot central stars. The complexity of the chemical processes is increased by numerous factors linked to the properties of the central star and the geometry and degree of clumpiness of the envelope. Several aspects of the PN chemistry that remains to be understood are discussed within the frame of the available chemical models.

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Section: Chemistry Of N-bearing Moleculessupporting

confidence: 90%

Section: Column Densities and Their Evolutionsupporting

confidence: 89%

Section: Introductionmentioning

confidence: 88%

Section: Chemistry Of N-bearing Moleculesmentioning

confidence: 99%

Section: Chemistry Of N-bearing Moleculesmentioning

confidence: 99%

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The chemistry of compact planetary nebulae

Josselin

Bachiller

2002

A&A

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Prebiotic Chemical Evolution in the Astrophysical Context

Ziurys

Adande

Edwards

et al. 2015

Orig Life Evol Biosph

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An ever increasing amount of molecular material is being discovered in the interstellar medium, associated with the birth and death of stars and planetary systems. Radio and millimeter-wave astronomical observations, made possible by high-resolution laboratory spectroscopy, uniquely trace the history of gas-phase molecules with biogenic elements. Using a combination of both disciplines, the full extent of the cycling of molecular matter, from circumstellar ejecta of dying stars - objects which expel large amounts of carbon - to nascent solar systems, has been investigated. Such stellar ejecta have been found to exhibit a rich and varied chemical content. Observations demonstrate that this molecular material is passed onto planetary nebulae, the final phase of stellar evolution. Here the star sheds almost its entire original mass, becoming an ultraviolet-emitting white dwarf. Molecules such as H2CO, HCN, HCO(+), and CCH are present in significant concentrations across the entire age span of such nebulae. These data suggest that gas-phase polyatomic, carbon-containing molecules survive the planetary nebula phase and subsequently are transported into the interstellar medium, seeding the chemistry of diffuse and then dense clouds. The extent of the chemical complexity in dense clouds is unknown, hindered by the high spectral line density. Organic species such as acetamide and methyl amine are present in such objects, and NH2CHO has a wide Galactic distribution. However, organophosphorus compounds have not yet been detected in dense clouds. Based on carbon and nitrogen isotope ratios, molecular material from the ISM appears to become incorporated into solar system planetesimals. It is therefore likely that interstellar synthesis influences prebiotic chemistry on planet surfaces.

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The Zanstra Temperatures Revisited

Gruenwald¹,

Viegas²

2003

Symp. - Int. Astron. Union

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The temperature of the central stars in planetary nebulae is generally obtained by the Zanstra method. This method provides two values for the temperature of a single star: one obtained from the H recombination lines, TZ(H), and another from the He recombination lines, TZ(He II). The ratio (ZR) of these two values can be different from unit, leading to the well-known “Zanstra discrepancy”. The discrepancy is higher for lower temperature stars and the method does not reproduce the high values of stellar temperatures suggested by stellar evolutionary models. In this work a careful analysis of the effect of the optical depth on the determination of the Zanstra temperature is made, using photoionization models. The effects due to deviations from a blackbody spectrum, as well as to the He abundance in the nebulae, are also discussed. We show, quantitatively, that the details of the distribution of the H and He II Zanstra temperatures are mainly explained by an optical depth effect; in particular, the fact that the discrepancy is larger for low stellar temperatures. The results also show that for high stellar temperatures both Zanstra temperatures underestimate the stellar temperature, even for high optical depths. The stellar temperature, as well as the optical depth, can be obtained from a plot of ZR vs. TZ(He II). Since for nebulae of very low optical depth and/or high stellar temperature this plot only provides lower limits for T*, we propose the use of the line intensity ratio He II/He I vs. TZ(He II) diagram for obtaining the stellar temperature, as well as the nebular optical depth. The results of this work has been published in ApJ 543, 889 (2000).

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Gas-phase models for the evolved planetary nebulae NGC 6781, M4-9 and NGC 7293

Cited by 20 publications

References 14 publications

The chemistry of compact planetary nebulae

The chemistry of compact planetary nebulae

Prebiotic Chemical Evolution in the Astrophysical Context

The Zanstra Temperatures Revisited

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