Theoretical models of planetary nebulae

Theoretical models are attempted for eight moderate excitation planetary nebulae which have been intensively observed with the image-tube scanner at Lick and in other investigations. The models are uniform density structures, often truncated, with central star energy distributions and chemical compositions so adjusted as to give an optimum fit with the observations. Although a satisfactory representation can be obtained for most of the stronger, usually observed lines, those arising from the 3p ³ configuration, specifically the nebular transitions of [Sii], [Cliii], and [Ariv], often give difficulties. A comparison with results of model predictions with those from simple, traditional procedures generally shows a good agreement, except in those instances where different judgments are made in interpreting emissions of ions of 3p ⁿ configurations.     

Theoretical models of planetary nebulae

Sets of theoretical models of spherically symmetrical gaseous nebulae are calculated for a fixed chemical composition and with central star, energy distributions given by Cassinelli (1971) and by Kunasz, Mihalas and Hummer (1975). A central region of constant densityN _H=100 cm^–3 is surrounded by a broken shell consisting of zones of different density and optical thickness. It is found that for a fixed chemical composition and central star flux, a considerable range can be found in the emitted fluxes of certain lines such as 4686 which are often used to fix the level of excitation of a nebula. Although it is possible to produce models yielding a variety of nebular line ratios in this way, these truncated inhomogeneous shell models have not reproduced the integrated spectra of nebulae that simultaneously show extreme features of high and low excitation with great strength-e.g. [Nii], [Oii] and 4686 Heii.     

Evolution du spectre de la Nova Delphini 1967 en 1971 et 1972

Continued spectroscopic observations of Nova Del 67 during 1971 and 1972 show a general decrease of the nebular and coronal emission with respect to the local continuum. The continuous spectrum exhibits a strong Balmer emission (figure 1). Equivalent widths of Hi, Hei, Heii, Oi, [Oi], Oii, [Oii], Oiii, [Oiii], Niii, [Sii], [Aiii], [Neiii], [Nev], [Fevi], [Fevii], [Fex], [Fexi], [Fexiv], [Nixvi], [Nixv] are listed in tables 1 and 2.Figures 5 to 10 show several line profiles, which can be interpreted in terms of a model proposed by Hutchings (1972). Kinematical properties of the nova envelope do not seem to have changed in the 1968–1972 time interval. However, a study of the [Oiii] (4959 Å) line indicates that the physical conditions in the polar blobs in 1972 (T _e=10 660 K,N _e=5,5×10⁵ cm^–3) are different from those prevailing in equatorial rings (T _e=9×100 K,N _e=7×10⁵ cm^–3).

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Le matérial d'observation utilisé pour cette étude a été obtenu à l'aide de téléscopes de 120 cm, 152 cm et 193 cm de l'Observatoire de Haute-Provence (CNRS).     

A proposed correction to the solar abundances of carbon and oxygen utilizing new and accurate theoretical forbidden transition probabilities

Previously published solar abundances of oxygen and carbon can be corrected to be logN(O) = 8.93 and logN(C) = 8.60 on the hydrogen log-scale when new accurate forbidden electric quadrupole transition probabilities A _Q(s^–1) are used. Such A _Q's, based on the new atomic structure and electron correlation theory, developed recently by Sinanolu and coworkers, are reported for the (¹ S ₀-¹ D ₂) lines of [C i], [N ii, [O i] and [O iii] and the (² P-² D) lines of [N i] and [O ii]. The available experimental values are also given for comparison.Work supported by Grant No. GP-29471 from the U.S. National Science Foundation.     

Excitation class of nebulae—An evolution criterion?

A principally new, quantitative system of the classification of the spectra of planetary nebulae is proposed. Spectral class of excitation class of the nebulap is determined according to the relative intensities of emission lines (N ₁+N ₂) [OIII]/4686 HeII and (N ₁+N ₂) [OIII]/H (Table I, Figure 1). The excitation classes are obtained for 142 planetary nebulae of all classes—low (p=1–3), middle (p=4–8), and high (p=9–12⁺) (Tables II, III, and IV). An empirical relationship between excitation classp and mean radius of nebulae is discovered (Figure 2). This relationship as well as excitation classp, as an independend parameter, admit an evolutionary interpretation. It is shown that after reaching the highest class of excitationp=12⁺ the nebulae decrease their class of excitation with the further increases of sizes. The diagram of this relationship has two nearly-symmetric branches — rising and descending with the apogee onp=12⁺ (Figure 2).     

Abundances of [WC] central stars and their planetary nebulaee

We review elemental abundances derived for planetary nebula (PN) WCcentral stars and for their nebulae. Uncertainties in the abundances of[WC] stars are still too large to enable an abundance sequenceto be constructed. In particular it is not clear why the hotter [WCE]stars have C and O abundances which are systematically lower than those oftheir supposed precursors, the [WCL] stars. This abundance differencecould be real or it may be due to unaccounted-for systematic effects inthe analyses. Hydrogen might not be present in [WC] star winds asoriginallysuggested, since broad pedestals observed at the base of nebular lines canplausibly be attributed to high velocity nebular components. It isrecommended that stellar abundance analyses should be carried out withnon-LTE model codes, although recombination line analyses can provideuseful insights. In particular, C II dielectronic recombinationlines provide a unique means to determine electron temperatures in cool[WC] star winds. We then compare the abundances found for PNe which have [WC] central starswith those that do not. Numerous abundance analyses of PNe have beenpublished, but comparisons based on non-uniform samples and methods arelikely to lack reliability. Nebular C/H ratios, which might be expected todistinguish between PNe around H-poor and H-rich stars, are rather similarfor the two groups, with only a small tendency towards larger values fornebulae around H-deficient stars. Nebular abundances should be obtainedwith photoionization models using the best-fitting non-LTE modelatmosphere for the central star as the input. Heavy-metal line blanketingstill needs to be taken into consideration when modeling the central star,as its omission can significantly affect the ionizing fluxes as well asthe abundance determinations. We discuss the discrepancies between nebularabundances derived from collisionally excited lines and thosederived from optical recombination lines, a phenomenon that may havelinks with the presence of H-deficient central stars.     

Planetary nebula with a peculiar spectrum in the ultraviolet: CN 3-1

We have obtained IUE ultraviolet spectra of the low-excitation planetary nebula CN 3-1. The recordings show the well-known doublet 2800 MgII and resonance line 2852 MgI as strong absorption lines. We show that these lines cannot be of interstellar origin and that they may be formed in two envelopes surrounding the main nebula: in the transition zone (doublet 2800 MgII) and in the neutral envelope (line 2852 MgI). These envelopes possess an important property: they contain dust particles, and even a moderate amount of such particles may influence the strength of the absorption lines of MgII and MgI.The emission line 4686 HeII has no relation to the nebula CN 3-1 and belongs to its nucleus, a star of type WR. It is very probable that the nucleus of this nebula is a binary system with a WR component (T _*=50 000 K), exciting the helium lines, and a star of B0 or B2 type (T _*=26 000 K) exciting the nebular linesN ₁+N ₂ [OIII], 3727 [OII], hydrogen lines, etc.     

Spectroscopic studies of four planetary nebulae with emission-line nuclei

Spectroscopic observations of four planetary nebulae (PNe) with emission-line central stars of different spectral types are presented: Cn 1-5, Pe 1-1, NGC 5873, and M1-19. The interstellar extinction, physical conditions (n _e , T _e ), and abundances of several elements (He, N, O, Ne, S, Ar, Cl) have been determined for all nebulae. The nebula Cn 1–5 with fairly high abundances of helium and nitrogen is shown to belong to type I PNe. Possible variability of the intensities of low-excitation emission lines in NGC 5873 has been found; it can be related to variations of the stellar wind from the central star. The measured α-element abundance ratios (S/O, Ne/O, Ar/O, Cl/O) are in good agreement with those typical of HII regions.     

[WR] nuclei of planetary nebulae: Inferences from statistical studies of the nebulae

There are about 50 galactic planetary nebulae know to have [WR] type nuclei. We have compared their nebular properties with those of the other planetary nebulae in the Galaxy. We have found that the nebular morphological types are similarly distributed in the two groups. Bipolar nebulae constitute only 20% of the total in each group. The distribution of the nebular electron densities and abundance ratios N/O, He/H and C/O are the same in the two groups. The only marked difference is that nebular expansion velocities are larger in the group of planetary nebulae with [WR] central stars. We argue that the WR phenomenon does not preferentially occur in more massive central stars of planetary nebulae, contrary to what has been suggested in some former studies. We demonstrate that, for most of the observed [WR] type objects, the WR phenomenon cannot be triggered by a late helium shell flash event.The results of our investigation are published inAstronomy & Astrophysics 303, 893 (1995) and in the proceedings of the 2nd International Colloquium on Hydrogen-deficient Stars, C.S. Jeffery & U. Heber (eds), Astronomical Society of the Pacific Conference Series, Vol. 96, p. 209 (1996).     

Spectral evolution of Nova V400 Per (1974) and Nova V373 Sct (1975)

Photographic and spectroscopic observations of the two galactic novae, V400 Per and V373 Sct, which appeared in 1974 and 1975, have been carried out at Asiago. The light curves of the two novae were characterized by the presence of brightness oscillations during the early decline. The spectral evolution was quite normal: the spectra showed at first, over a relatively strong continuum, wide emission bands of moderate excitation, accompanied by blueshifted absorptions, with radial velocities of –1760 km s^–1 (Nova Per) and –1260 km s^–1 (Nova Sct). Later, after the novae entered the nebular stage, the continuum weakened, the absorption disappeared and the novae displayed the usual emission spectrum, with permitted and forbidden lines of high excitation ([Oiii], Niii, Hei, Heii). Forbidden lines of Fevi and Fevii-and in Nova Sct, also Fex and Ax-were present for a time, but they soon disappeared, so that at the end the spectrum was dominated by the [Oiii] nebular lines, even stronger than H.     

On coronal Fe abundances and temperatures from XUV emission lines

Data obtained by the OSO-7 spectroheliograph on strong XUV lines of five, different Fe ions from the outer equatorial corona are presented. Interpretation of the data with a spherically symmetric model atmosphere gives average ion abundances for lines of sight at 0.3R from the limb. Fe xvi is usually more abundant than Fe xv, xiv, xii and ix, but there are times when Fe xii is more abundant than the other ions. The deviation of measured relative abundances of Fe xii, xiv, and xvi from predictions of ionization equilibrium at one temperature seems to indicate that there are appreciable temperature variations along lines of sight. Element abundances are very uncertain since they appear to depend so heavily on likely but unknown density irregularities along lines of sight.     

Self-consistent photoionization models of planetary nebulae luminescence: stellar ionizing continuum,nebular abundances,and evolution of the envelopes

The modern self-consistent photoionization model of planetary nebula luminescence is described. All of the processes which play an important role in the ionization and thermal equilibrium of the nebular gas are taken into consideration. The diffuse ionizing radiation is taken into account completely. The construction of the model is carried out for the radial distribution of gas density in the nebular envelope which is consistent with isophotal map of the nebula. The application of the model is illustrated on the example of the planetary nebulae BD+30°3639 and NGC 7293. It is shown that the continuum of the central star at 912 Å does not correspond to the blackbody spectrum but agrees with the spectrum of corresponding non-LTE model atmosphere. The radial distributions of electron density, electron temperature, and other parameters in the nebular envelopes are found.The evolution of the radial distribution of gas density in the planetary nebulae envelopes is investigated. Approximative analytical expression which describe both such distribution and its change with time is adjusted. It is shown that the nebular envelope is formed as a result of quiet evolution of the slow stellar wind of star-precursor, and the formation of the envelope begins from the decrease of star-precursor's mass loss rate. Obtained radial distributions of gas density in the envelopes of young nebulae rule out the idea that the planetary nebula is formed as a result of a rapid ejection of clear-cut envelope. So, there is no necessity for the superwind which is used for this purpose in theoretical calculations.A new method of the determination of planetary nebulae abundances is proposed. Unobserved ionization stages are taken into account with aid of the correlations between relative abundances of various ions which had been obtained from the grid of the photoionization models of planetary nebulae luminescence. Simple approximative expressions for the determination of He/H, C/H, N/H, O/H, Ne/H, Mg/H, Si/H, S/H, and Ar/H are found. The chemical composition of 130 Galactic planetary nebulae is revised. A comparative analysis of the abundances in the Galactic disk, bulge, and halo nebulae is carried out.     

Periodicity in spectral variations of IC 4997?

The emission lines in the spectra of some planetary nebula experience variations with the time. These variations (due to evolution effects and by their nature) differ from the spectral changes which we usually observe in stars. It appears that the long-scale and systematic observations on the intensity behaviour of different emission lines of nebula may be used as an independent indicator to receive a principally new and quite unpredicted category of information related to the physics of nebulae and their nuclei. Particularly, the analysis of long standing observations carried out in relation to the forbidden lines 4363 [Oiii] andN ₁+N ₂ [Oiii] of double-ionized oxygen lead to a conception about the possible existence ofrelativistic electrons of moderate energy in planetary nebulae and the generation oftransition radiation as a result of electrodynamic interaction of these electrons with the dust particles in nebula (Gurzadyan, 1991). Just on the basis of the same observational material, the long-scale periodicity in the activity or variations of a star-like planetary nebula IC 4997 is discovered about which is the present note.     

Deep high spectral resolution spectroscopy and chemical composition of ionized nebulae

High spectral resolution spectroscopy has proved to be very useful for the advancement of chemical abundances studies in photoionized nebulae, such as H II regions and planetary nebulae (PNe). Classical analyses make use of the intensity of bright collisionally excited lines (CELs), which have a strong dependence on the electron temperature and density. By using high resolution spectrophotometric data, our group has led the determination of chemical abundances of some heavy element ions, mainly O⁺⁺, O⁺, and C⁺⁺ from faint recombination lines (RLs), allowing us to deblend them from other nearby emission lines or sky features. The importance of these lines is that their emissivity depends weakly on the temperature and density structure of the gas. The unresolved issue in this field is that recombination lines of heavy element ions give abundances that are about 2–3 times higher than those derived from CELs – in H II regions – for the same ion, and can even be a factor of 70 times higher in some PNe. This uncertainty puts into doubt the validity of face values of metallicity that we use as representative not only for ionized nebulae in the Local Universe, but also for star‐forming dwarf and spiral galaxies at different redshifts. Additionally, high‐resolution data can allow us to detect and deblend faint lines of neutron capture element ions in PNe. This information would introduce further restrictions to evolution models of AGBs and would help to quantify the chemical enrichment in s‐elements produced by low and intermediate mass stars. The availability of an échelle spectrograph at the E‐ELT will be of paramount interest to: (a) extend the studies of heavyelement recombination lines to low metallicity objects, (b) to extend abundance determinations of s‐elements to planetary nebulae in the extragalactic domain and to bright Galactic and extragalactic H II regions. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Nebular complex in the region of Cas OB2 association; ring nebula Sh157

This paper presents the results of monochromatic [Oiii], [Nii], and [Sii] observations of ring nebula Sh157 around the star (WR + B0III) HD 219460 belonging to the Ba 3 cluster. A stratification of radiation typical for photoionization excitation has been revealed. The observations suggest that the Sh157 ring structure may arise as a result of the HD 219460 stellar wind blowing the surroundingHii region, and the bubble age is found to bet(2–5)×10⁵ yr. Three outer envelopes have been distinguished: a weaker extended emission shell apparently blown out by the wind from B-stars of the Ba 3 cluster, and two dust shells which are likely to be associated with NGC 7510 and Cas OB2. The paper is also concerned with the discussion of young star aggregates Ba 3, NGC 7510, Cas OB1, OB2, OB4, OB5, OB7, and Cep OBI and the associatedHii regions, shells and supershells of gas and dust, molecular clouds, and supernova remnants which may be probable members of a single giant stellar complex where the star formation process is in progress.     

Nova Delphini 1967. Observations du spectre nebulaire entre 5600 et 8800 Å en 1970

Five 230 Å/mm infrared spectra of Nova Delphini 1967 have been obtained from May to September 1970 on Kodak hypersensitized IN plates. Visual brightness has dropped by one magnitude between fall 1969 and fall 1970. The continuous spectrum remains strong and a mean value of 2.35 is found for the gradient in the region 6000–8000 Å, indicating a slight increase in temperature since 1969. Permitted lines ofHi,Oi, Hei, Feii are weakening, while intensities of forbidden transitions due [Nii], [Fevii], [Oi], [Oii], [Aiii], [Av], [Fevii], [Fex], [Fexi], [Fexi], [Nixv] are much more stable At the dispersion of our spectra, there is little evidence of activity in the nova shell during 1970. Temperature seems to be slightly increasing in the lower layers as well as in the external, low density regions.

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Les observations ont été effectuées à l'Observatoire de Haute-Provence (C.N.R.S.)     

A Study of the Emission-Line Spectra and the Stellar Populations of Starburst Galaxies

Based on optical–NIR spectra, we discuss the nebular properties and stellar populations of starburst nuclei. Starbursts are found to have higher electron densities and higher excitations than HII nuclei. The emission lines have been used to estimate the nebular oxygen abundances, ionization parameter and radiation softness parameter. From a study of the H_α emission-line equivalent widths and the CaII triplet absorption line equivalent widths, we infer the ages and stellar content of the starburst regions. Most of the nuclei show evidence for a composite population – a young, ionizing population co-existing with evolved, non-ionizing stars of about 5–7 Myr, which are evolving towards the peak of RSG distribution. This revised version was published online in July 2006 with corrections to the Cover Date.     

The N ii spectrum of the Orion nebula

The predicted emission spectrum of N  ii is compared with observations of permitted lines in the Orion nebula. Conventional nebular models show that the intensities of the more intense lines can be explained by fluorescence of starlight absorption with a N abundance that is consistent with forbidden lines. Lines excited mostly by recombination, on the other hand, predict high N abundances. The effects of stellar and nebular parameters and of the atomic data on the predicted intensities are examined.     

The electron temperature distributions and internal kinematics of seven diffuse nebulae

A Fabry-Pérot spectrophotometer is used to derive values of the intensity ratio H/[Nii] at 98 points in the seven bright diffuse nebulae M8, M20, M16, M17, NGC7000, M42, IC434. The fraction of nitrogen in the singly ionized state is estimated in the different objects, and is found to be sufficiently constant within any one nebula so that the above intensity ratio may be used to derive accurate electron temperature distributions. The position of the peak of the nebular line, its excess non-thermal width, its shape and relative intensity are used to derive kinematical models of these objects.It is found that values of H/[Nii]1 are representative of the bright central cores of these nebulae. Temperatures between 7000K and 12000K were derived in the different objects. Although some of this apparent variation is due to the different conditions of excitation in the various nebulae, it is shown that a convincing progression of temperature in M8, M16, M17 is supported by radio recombination line results. The temperature variation within any one object was generally significantly less than 1500K.No evidence was found for velocities of mass motion at more than twice the speed of sound. Relative radial velocities of generally less than 15 km sec^–1 characterized the velocity fields of M8, M20, M16, M42. The velocities in M17 were measured as about 20 km sec^–1. Motions in NGC 7000 and IC 434 were much lower (5 km sec^–1) although here the number of points taken was too small to construct meaningful kinematical models.It is concluded that the internal motions of radiatively ionizedHii regions of Pop. I will not significantly affect the results of existing surveys for determining the rotation of the galaxy with radial velocities deduced from nebular emission lines.