Emission Nebulae

Fig. 2. VLA detection of 3He+ in the PN J 320. We have modeled the radio continuum and line emission using the radiative transfer code NEBULA [1], assuming an expanding shell of ionized gas. The dashed line is the model including the H171 7 and 3He+ transitions. The solid line shows the observed spectrum and only includes the 3He+ transition. The model fits the data reasonably well even though the morphology is bipolar as indicated by the HST image [6]...

In gas clouds containing one or more hot stars (7 cn > 30 000 K), hydrogen atoms are ionized by the stellar UV radiation in the Lyman continuum and recombine to excited levels their decay gives rise to observable emission lines such as the Balmer series (see, for example, Fig. 3.22). Examples are planetary nebulae (PN), which are envelopes of evolved intermediate-mass stars in process of ejection and... 

The size of a spherically symmetrical ionization-bounded nebula (known as a Stromgren sphere ) can be found by equating the total number of recombinations in Case B to the total emission rate of ionizing photons from the central star(s) ... 

D. E. Osterbrock and G. J. Ferland, Astrophysics of Gaseous Nebulae and Active Galactic Nuclei, University Science Books, Mill Valley, Cal., 2006, is another classic text, indispensable for studies of emission nebulae. 

Helium is the second most abundant element in the visible Universe and accordingly there is a mass of data from optical and radio emission lines in nebulae, optical emission lines from the solar chromosphere and prominences and absorption lines in spectra of hot stars. Further estimates are derived more indirectly by applying theories of stellar structure, evolution and pulsation. However, because of the relative insensitivity of Tp to cosmological parameters, combined with the need to allow for additional helium from stellar nucleosynthesis in most objects, the requirements for accuracy are very severe better than 5 per cent to place cosmological limits on Nv and better still to place interesting constraints on t] or One can, however, assert with confidence that there is a universal floor to the helium abundance in observed objects corresponding to 0.23 < Fp < 0.25. 

From the analyses of emission line profiles (Figure 2), K3-66 should be in distant site from us and Ml-5 seems to be intrinsically compact planetary nebula. 

Many carbon rich stars also present an important emission at 11.3 pm associated with solid carbon and some of them present nebulosity of reflection as a consequence of the scattering of the circumstellar grains. There are indications that in the material ejected by these stars, carbon must exist, apart from CO molecules and solid grains, in some other form or species until now unknown, fullerenes are a possibility. Unfortunately, there is very little information about the presence of molecules of intermediate size (between 10 and 106 atoms) in circumstellar regions. There are bands in carbon rich planetary nebulae, for example those of 3.3,6.2,7.7, 8.6 and 11.3 pm which have not been detected in carbon stars but are observable in transition objects evolving between the giant red phase and the planetary nebula as for example, the Egg Nebula (Fig. 1.5) and the Red Rectangle. These infrared bands are normally associated with the vibration modes of materials based on carbon, possibly PAHs. But until now it has not been possible to make a conclusive identification of the carrier. 

Evidence of a relation between carbon particles and DIBs can be found in the analysis of the Red Rectangle spectrum. This object is a losing mass carbon star probably evolving to a planetary nebula phase. Diverse spectroscopic studies have revealed the good agreement between the emission lines found at 5,797, 5,850, 6,379, and 6,614 A and some of the most intense diffuse bands of the interstellar medium. It is likely that the carrier of some of these interstellar bands is also present in the material ejected by this object. 

The interstellar light extiction curve at 2,175 A can be modeled using fulleranes with different degree of hydrogenation (Webster 1995 Cataldo 2003 Cataldo et al. 2009). The fraction of interstellar carbon abundance needed to produce fulleranes is high, but not prohibitively and carbon stars and planetary nebulae could provide these molecules in sufficient quantity. The experimental determination of optical and infrared spectra for laboratory-isolated fulleranes would be very valuable in assessing their role as carriers of the DIBs and of the unidentified infrared emission features. 

Calculated spectra for hydrogenated fullerenes have been published in comparison with the unidentified infrared emission bands (Webster 1991 Stoldt et al. 2001). The infrared spectrum of C60H36 has been compared also with the infrared features of other astrophysical objects like the proto-planetary nebulae (Cataldo 2003a, b). An inventory about fullerenes and hydrogenated derivatives in the interstellar... 

The best evidence for a relation between carbon-particles and the diffuse interstellar bands comes from analysis of the Red Rectangle. The Red Rectangle is an usual mass-losing carbon star which is probably in transition into becoming a planetary nebula. Schmidt et al. (1980) using 6-20 A resolution discovered intense optical emission bands longward of 5400 A. With a higher spectral resolution of 1 A,... 

With current instruments it is possible to make spatial maps of the emission from different species in the Red Rectangle. These maps might provide valuable clues to the origin of different spectroscopic features. For example, in the spectrum of the Red Rectangle, the emission features which correspond to the diffuse interstellar bands are concentrated in what appears to be two hollow cones oriented perpendicular to the plane of this bipolar system (Schmidt Witt 1991). This hollow cone is similar to that proposed by Jura Kroto (1990) to explain the observed (Nguyen-Q-Rieu et al. 1986) HC,N emission (see around AFGL 2688, the Egg Nebula ), a very well studied carbon-rich object that appears to be in transition from a red giant to a planetary nebula. 

Fig. 1 The infrared emission spectrum from the high excitation planetary nebula NGC...

Fig. 2. The infrared emission spectrum from the reflection nebula HD 44179, the Red Rectangle (a from Ref. [44], b from Ref. [15].)...

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