Radiation continuum

The black body radiation model for the continuum radiation from stars works well but it is not quite right. Careful consideration of the radiation profile shows deviations from the curves shown in Figure 2.1 due to the structure of the star itself. These deviations form the basis of a more detailed analysis including the effects of circulation within the star and will be left to others to explain we shall use black body radiation as our model for stars. 

The bulk of stellar radiation comes from the surface layers or atmosphere of a star, more particularly the photosphere , which is defined as the region having optical depths for continuum radiation between about 0.01 and a few. The optical depth ti is measured inwards from the surface and represents the number of mean free paths of radiation travelling vertically outwards before it escapes from the star. It is related to the geometrical height z above some arbitrary layer by... 

Background emission by the flame (Figure 8.23) includes contributions from molecular species and continuum radiation from incandescent particles and depends upon the combination of fuel and support gases used. The sample solvent and matrix will further augment background radiation. 

There are the further advantages that rotational lines can be studied and that fluorescent substances can be investigated by the inverse Raman effect. Benzene and other molecular liquids have been studied by this method by McQuillan and Stoicheff 232) jhe required continuum radiation was anti-Stokes emission produced by passing the laser beam in liquid toluene. 

GAS COOL Cooling from 2500 C -ve MgO (solid) CO COjrN OjH O Na(g)- Na(s)-i Na(l) Na- Na20 Continuum radiation from hot particles vis. radiation ceases 900 C Some emission due to energy transfer from zone 4 Mg droplets bum out Main cooling rone... 

COMBUSTION About 2500 C to2700 C ++ve Mg(l,g) MgO(s,l) Na(g) 0,0,Hj0,0H CO,COyNj,NO Visible emission zone from Na, Mg, MgO plus continuum radiation from hot particles High temperatures e ite Na atoms to emit at 590nm. Mg droplets move across rone Mg bums rapidly in diffusion flame surrounding liquid droplets... 

If the effective temperature of our defined system is less than the universal radiation background temperature of 2.7 K, transitions between the two levels can be observed in absorption. This is the case with interstellar formaldehyde. Alternatively absorption can be observed against the continuum radiation from a nearby bright source. Spontaneous emission will always occur provided the upper of the two levels is populated, and can be observed if the populations are different. There are, in addition, examples of the exceptional situation in which N2 > N the result of this population inversion is that stimulated emission dominates, and maser emission is observed. Interstellar OH and SiO provide diatomic examples of this unusual situation, as also does interstellar H2O we shall describe the results for OH later in this chapter. Departures from local thermodynamic equilibrium are very common, and the concept of temperature in interstellar gas clouds is not simple this is a major part of astrophysics which is, however, beyond the scope of this book. 

In radio astronomy multichannel or autocorrelation (Fourier) spectrometers are used which simultaneously cover the whole line profile. Consider a molecular cloud observed against a source of continuum radiation of a given brightness temperature. The continuum brightness temperature is the sum of the 2.7 °K isotropic background radiation Tbh of a continuum source (such as an HII region or a supernova remnant) which may be in the line of sight and located behind the molecular cloud. A specific molecular transition with optical... 

Here, the first term represents the line emission proper from the molecular cloud. The second term describes the attenuation of the continuum radiation due to absorption by the molecules. 

Fig. 20 shows the observed interstellar molecular lines of various isotopic species of formaldehyde, H2CO, as detected by Gardner et al., 1971. This particular line, the lowest asymmetry-doublet transition 110 — lu, is seen in absorption in the continuum radiation of the strong radio source Sgr B2, which is located behind the molecular gas cloud. Frequency is plotted along the abscissa and the ordinate is intensity, expressed in the ratio of line-to-continuum antenna temperatures. For all three formaldehyde isotopes the continuum temperature is Tc T >b Tex- This is the case because the formaldehyde molecules are in approximate equilibrium with the microwave background... 

The determination of plasma parameters using He-like spectra is based on a self-consistent modeling of the theoretical spectra. The following variables take part in the variation procedure based on least-squares fitting electron and ion temperatures, toroidal plasma velocity, concentrations of H-, He-and Li-like ions. In addition, a background function was used to subtract the plasma background from the experimental spectra. The background consists of continuum radiation from the plasma and detector noise. 

Plasma Appearance and Spectra The typical plasma has a very intense, brilliant white opaque core topped by a flame-like tail. The core, which extends a few millimeters above the tube, produces a spectral continuum with the atomic spectrum of argon superimposed. The continuum is typical of ion-electron recombination reactions and bremsstrahlung, which is continuum radiation produced when charged particles are slowed or stopped. 

Dust Dust has three major effects on the H II region spectrum. First, dust grains mixed with the ionized gas absorb Lyman-continuum radiation. Second, obscuration by dust is typically patchy differential extinction between stars and gas can affect the emission line equivalent widths. Third, dust can affect the heating and cooling by emitting and recombining with photoelectrons. 

Apart from the atomic and ion lines of the species present in a plasma source an emission spectrum has a continuum on which the emission lines are superimposed. This extends over the whole spectrum. It is due to the interactions between free electrons ( Bremsstrahlung ) and to the interaction of free and bound electrons ( recombination continuum ). The former is particularly important in the UV spectral region, whereas the latter is important at longer wavelengths. The spectral intensity distribution for the continuum radiation is given by ... 

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