THE STRENGTH OF THE EMISSION LINES IS DUE TO LASER ACTION

Gudzenko and Shelepin proposed that if the free electrons in plasma cool sufficiently rapidly, there can be a population inversion in the lower levels of the atom, and this can lead to laser action. Gudzenko, Filipov and Shelepin showed that such a rapid cooling can be achieved by expanding a plasma jet into a vacuum. The basic theory for obtaining the properties of a decaying plasma was given by Bates, Kingston and McWhirter and is called the collisional radiative model. A number of workers, using this model, have carried out calculations on the properties of a rapidly decaying monatomic plasma. The results obtained from a number of subsequent experiments for the recombination rates are for the most part in reasonable agreement with those calculated from the collisional model. Detailed theoretical calculations (Zemtsov and Yu, Bohn ) of the population densities of excited levels in a decaying hydrogen (and hydrogen-like ions) plasma flow predicted population inversions at electron densities (ne=10^13 to 10^16 cm^-3) and electron temperatures (Te=5000 to 10^5 deg. K) with magnitudes which are very close to those existing in stellar atmospheres. The prediction regarding the inversion in the atomic level populations has been amply confirmed in several laboratory experiments (Hoffmann, et al.). The subject has been reviewed by Gudzenko et al.

Also, it is well known that in certain types of stars (Wolf-Rayet, P Cygni), matter is ejected more or less continuously.

We have thus proposed the following realistic model of a quasar : A quasar is a star in which the surface plasma is undergoing rapid radial expansion giving rise to population inversion and laser action in some of the atomic species. The assumption of the ejection of matter from quasars at high speed is supported from the fact that the widths of emission spectral lines observed in quasars are typically of the order of 2000 - 4000 km/sec. We call the proposed model the plasma-laser star (PLS) model. Let us then examine the consequences of this model.

Detailed calculations on the properties of a rapidly decaying monatomic plasma have been carried out by a number of workers (Zemtsov and Yu, Bohn, Drawin). The population densities of the excited levels are functions of the electron density (ne), the electron temperature (Te), and the density of ground state atoms (n(1)). In stellar atmospheres , n(1) is a function of ne and Te. thus the state of plasma, after expansion, in a star can be represented by a point on a plot with ne and Te as axes.

Laser gain versus electron density and electron temperature. For a given transition in a given atom, strong population inversion takes place only within a narrow area in the ne, Te diagram.

This area is surrounded by a medium population inversion area, which in turn is surrounded by a weak population inversion area. On the high ne side, the boundary of the population inversion is rather steep. (Strong population inversion regions will give rise to strong lines, and similar statements hold for the medium and weak inversion regions.)

Contour diagram representing the laser gain regions of two wavelenghts, L1 and L2, arising from different transitions in different atoms. Solid line represents region of strong population inversion, dashed equals medium and dotted is weak.

We consider what will be observed if the emission-line region of a quasar corresponds to the different points shown on the diagram.

Point 1: L1 strong, L2 strong.
Point 2: L1 strong, L2 absent.
Point 3: L1 weak, L2 strong.
Point 4: L1 strong, L2 medium,
and so on...

Thus a whole range of relative intensities is possible. We next consider the observational evidence relevant to this point. We have carried out spectral classification of quasars. There are quasars which show two or more emission lines at practically the same wavelength; such quasars were put together in a group. In the redshift interpretation, quasars belonging to the same group tend to have the same redshift. We give here three examples in which wide variations in relative intensities have been recorded; we have restricted ourselves to only such cases where the two quasars were investigated by the same astronomer(s) using the same telescope.

3C 309.1 and 0957+00. Spectra of both quasars were obtained by Lynds and his coworkers (1966, 1967, 1968) on the Kitt Peak 84-inch telescope, and are reproduced in Fig.3. Both quasars show two emission lines at 3640 (±5) Å and 5337 (±5) Å respectively. 3640 Å is stronger than 5337 Å in 0957+00, but 3640 Å, is quite weak in 3C 309.1, in which 5337 Å is very strong.
3C 208 and 3C 204. Spectra of both quasars have been obtained by Schmidt on the Palomar telescope. 4030 Å is of medium strength in both the quasars, but 3275 Å is medium in 3C 208, and weak in 3C 204.
1508-05 and 2329-384. Spectrograms for both the quasars were obtained by Peterson and Bolton on the Mount Stromlo 74-inch telescope. 4185 Å is strong in both the quasars. On the other hand, another line, 3396 Å is strong in 2329-384, but weak in 1508-05.

It is obvious that these results are in strong support of the PLS model regarding the relative intensities of emission lines. Varshni and Lam have presented model calculations for laser action in He II 4686 Å within the framework of the PLS model.

More recently Varshni and Millette have performed computations on laser action in C IV, N V and O VI, Varshni and Nasser have performed the same computations on the levels of He I and C III. There is also an online thesis; Talbot (1994) based on the work by Millette on laser action in C IV.

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