Emission lines from nebulae

These methods, using well-defined band passes, are capable of a better internal precision than high-resolution spectroscopy (as well as being vastly quicker), but they give (in general) only one abundance parameter and they need high-resolution spectroscopy for calibration. 

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)  

The excess energy (liv — 13.6 eV) of the ionizing photons supplies heat to the ionized gas, which is cooled chiefly by the emission of collisionally excited lines 

Intensities of collisionally excited lines relative to hydrogen lines depend on the ionic abundance and on the balance between excitation by electron collisions and de-excitation by both electron collisions and radiation. The emission rates per unit volume are given respectively by  

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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. 

Similar considerations apply to the observation of absorption and emission lines from other objects, such as supernova ejecta, planetary nebulae, and interstellar clouds. Some methods and results were already presented inO Sect. 12.3.3. 

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. 

The abundances of carbon relative to the other elements has been measured in stars and in the interstellar clouds and ionized nebulae. These measurements derive from high-resolution spectroscopy of spectra from stars, from the strength of forbidden line emissions and of ultraviolet lines from ions in nebulae, and from millimeter radio spectroscopy in interstellar clouds (see 12C, Astronomical measurements for a summary). 

Figure 10.40. N = 1 — - 0 rotational emission lines of CN observed from the Orion nebula [114]. The two lines correspond to the two transitions marked with asterisks in figure 10.41.

The microwave rotational spectrum of the CN radical has been elusive in the laboratory, and the first observations were made by astronomers in 1970 [113], with more extensive studies four years later [114, 115]. Figure 10.40 shows two emission lines observed from the Orion nebula, the initial assignment being based upon constants obtained from the electronic spectrum. Three years later the first observations of the... 

Herbig-Haro objects emission-line nebulae which are produced by shock waves in the supersonic outflow of material from young stars also referred to as Herbig-Haro nebulae. 

Roughly half of young T Tauri stars with ages <10Myr are observed to have optically thick disks of gas and dust with masses of 0.001-IM (Beckwith et al., 1990 Strom, 1994). These disks have spectra containing absorption features caused by the presence of water ice and silicates. Ultraviolet and visible emission lines indicate that the central stars are accreting mass from their disks at rates of 10 -10 Mo yr (Hartmann et al., 1998). Optically thick circumstellar disks are not observed around stars older than —10 Myr (Strom, 1995), which provides an approximate upper limit for the lifetime of the Sun s protoplanetary nebula. 

Stasinska Szczerba (2001) also point out that if, as expected, dielectronic recombinations for high level states strongly enhance the emissivities of recombination lines, the presence of small grains in filamentary planetary nebulae would boost the emission of recombination lines from the diffuse component, principally in the inner zone. Therefore, small grains could solve in a natural way both the temperature fluctuation problem and the ORL/CEL discrepancy. 

For abundance work in ionized nebulae, we need to understand better the effects of dust on the thermal and ionization balance. The calculated ionizing spectra from massive stars are still in a state of flux, as more physics and opacity are included this is an area that will continue to require attention as computing power grows. The effects of inhomogeneous structure on the observed emission-line spectrum of H II regions also needs to be addressed. 

Class III objects are weak line T Tauri stars (T Tauri stars in which the characteristic emission lines are only weakly observed in their optical spectra) and have little or no evidence of a disk. At this stage of solar nebula evolution, which may last between 3 and 30 Ma, the sun has formed, and the material of the nebula is being dissipated by solar winds in the inner part. In the outer part of the nebula material is dissipated by photo-evaporation caused by UV radiation from the solar wind. A positive pressure gradient near the inner edge of the nebula facilitates planetesimal formation. 

The solar system began to form about 4.55 Ae ago from a rotating disk of dust and gas (the solar nebula) which had sepatated from a larger molecular cloud a little earlier. The original composition of this disk can be obtained from two sources. The first of these are the spectral data from the sun. Rare earth abundances may be obtained both from photospheric absorption lines and coronal emission lines. These data are given in table 6 as atomic abundances relative to Si = 10 atoms (Aller 1987) and give the composition of the outer layers of the sun. 

FIGURE 21 Far-UV spectra, obtained with lUE, of diffuse nebulae, (a) Orion nebula, a typical H II region, showing continuum due to dust scattering of UV starlight, with nebular emission lines. [Reprinted with permission from Torres-Peimbert, S., et al. (1980). Astrophys. J. 293, 133.] (b) Cygnus Loop, a supernova remnant, showing emission lines due to shock-heated gas. [Reprinted with permission from Raymond, J. C., et al. (1980). Astrophys. J. 238, 881.]... 

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