Lawrence H. Aller

Physics of Thermal Gaseous Nebulae

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A spectroscopic study of moderately bright planetary nebulae

Spectroscopic results are presented for most of the nebulae observed in a programme involving some forty planetaries measured with an image tube scanner at Lick Observatory and with a photo-electric spectrum scanner at Mt. Wilson Observatory. The spectroscopic range normally covered, λ 3700 to λ 8700 is supplemented for a few objects with appropriate photographic data calibrated photoelectrically for the region λ 3120-λ 3800. Available published photoelectric data are also employed to improve the calibration of the finally adopted system of intensities.The main thrust of the programme is to investigate weaker lines, arising from less abundant ions, or due to less favorable excitation conditions. Information pertaining to additional ions gives further diagnostic data on the nebular plasma.For a given nebula, different diagnostics suggest different values of the electron temperature and density. These are believed to represent actual variations in physical conditions from point to point within the nebula, although in some instances, they may reflect inadequacies in atomic parameters, particularly collisional cross-sections. The spectroscopically most interesting nebulae show a wide variation in physical conditions, indicated by lines ranging in excitation from those of Mgi to [Fevii]. Relatively cool clouds of denser material appear to be immersed in a high excitation plasma excited by a hot central star.

The spectrum of the variable planetary nebula IC 4997

The compact, dusty, presumably young planetary nebula (PN) IC 4997 has been studied extensively since the variability of the lambda 4363/lambda 4340 ratio was established in 1956. Since 1938, other nebular lines have shown changes. IC 4997 is also unique because of the great density range revealed by its spectrum which goes in excitation from Mg I to (Ar IV). We present a detailed listing of spectral lines from 360 to 1005 nm. The diagnostic diagram shows that the spectrum can be interpreted only in terms of strata with a huge density gamut. Essential spectral features can be reproduced approximately by a model consisting of a geometrically thin shell of density around 10(exp 7) atoms cm(exp -3), surrounded by a much larger shell with a density of about 10(exp 4) atoms cm(exp -3). The actual, certainly more complex structure can be evaluated only when high resolution spatial imaging is at hand. The usual method of getting abundances from N(ion)/N(H(+)) and ionization correction factors (ICFs) cannot be applied here. It is argued that a reasonable theoretical model that represents the spectrum provides a valid initial approximation to nebular abundances. We propose that the chemical composition of IC 4997 does not differ greatly from that of the Sun. The finally adopted model suggests that the ejection of the material destined to form the inner shell occurred between 1900 and 1960, but observational evidence of such an ejection event is lacking. Perhaps the shell was accelerated. A need for further study is emphasized, especially the role of dust which appears to contribute 2% of the total mass. More attention to this object is recommended. An accurate measurement of its distance is especially desirable.

The optical and ultraviolet spectrum of the planetary nebula NGC 2440

New measurements of the optical and ultraviolet emission-line intensities of the high-excitation planetary nebula NGC 2440 in the wavelength range 1240--8578 A are analyzed with the aid of photoionization models. The observed (O III) and (N II) temperatures (13,800 K and 10,000 K, respectively) differ by more than the models predict. Inclusion of charge-transfer reactions at published rates improves agreement between calculated and observed abundances of many ions, but worsens the agreement for several highly ionized ions. Nitrogen shows a larger overabundance N(N)/N(O)roughly-equal1.0, whereas ultraviolet lines give a carbon abundance close to the solar value. The refractory elements magnesium, calcium, and iron have similar gas-phase depletions of approx.1.5 dex. The abundances of oxygen, neon, sodium, sulfur, chlorine, potassium, and argon are roughly solar.

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 3p3 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 3pn configurations.

The chemical compositions of gaseous nebulae

Ascertainment of the chemical compositions of gaseous nebulae has been a goal of many investigations since the pioneering studies of Bowen and Wyse (1939) and of Wyse (1942). At that epoch it was widely believed that the elemental abundances in all stars and nebulae were essentially the same. The concept of chemical evolution in galaxies was yet to be introduced.

Types of Gaseous Nebulae

A gaseous nebula is an ionized low-density plasma, very large in size compared with laboratory or even stellar dimensions. There exist a great variety of these objects. Densities range from a few ions and electrons/cm3 to perhaps 1012/cm3, beyond which point the plasma resembles the photosphere or chromosphere of a stellar atmosphere. We restrict our attention to ionized plasmas that emit observable spectral lines and continua. The gas kinetic temperatures of these objects, as measured by speeds of ions and electrons, range from a few thousand degrees in galactic diffuse nebulae to several hundred thousands or even millions in the highly attenuated bubbles associated with supernova events. Our main attention, however, will be devoted to objects with gas kinetic temperatures between 5000°K and 30,000°K.

The Chemical Composition of Planetary Nebulae

The problem of the determination of the chemical compositions of planetary and other gaseous nebulae constitutes one of the most exasperating problems in astrophysics. On the one hand, the problem appears to be conceptually simple — the mechanisms of excitation of the various lines appear to be well understood and the necessary physical parameters can be obtained by quantum mechanical theory. Yet the task is a difficult one and we want to explore some of the significant features.

Variable Spectra of IC 4997 and NGC 6572

Variability of the [0III]4363/4340 H γ ratio in IC 4007 was established in 1956 by William Liller and L.H. Aller who attributed the changes to a gradual decrease of electron density with time. Subsequent 4363/4340 ratio fluctuations negated this explanation. Ferland pointed out that small changes in the radiative flux of the Planetary nebula nucleus (PNN) could explains the variations. Our pervious study emphasized IUE observations, here we compare high dispersion spectra obtained with the Hamilton Echelle Spectrograph with previous measurements to asses line intensity variations. Emission line variability in PNN spectra as noted by Mendez et al. (1988) and by other for HeII 4686 in NGC 6572 may offer significant clues. PNN 4686 appeared by 1990 in IC 4997. Possibly both of these PNN may be evolving into Wolf-Rayet objects, but this development does not necessarily imply that the nebular excitation will increase with time.

Collisionally Excited Lines and Plasma Diagnostics

To this point we have discussed lines which are excited by radiative processes, mostly photoionization followed by cascade. Collisional excitation can play an Important role in many instances. If mechanical energy or magnetic energy from hydromagnetic waves are dissipated in a gas, the temperature may be raised and atomic levels excited by collisions with electrons. Collisional excitation of hydrogen, in particular, could become important. In a low-density gas which is ionized by radiation, collisional excitation of hydrogen may be less important, but that of low-lying levels of many other atoms and ions can become significant.