Interstellar Propagation of Electromagnetic Signals

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The application of the model to the dark molecular clouds is considered in Section 3.

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In Section 4 a brief outlook is given for the model considered here in juxtaposition with other models of wave gas-dynamical processes in dark interstellar clouds which could affect the broadening of the molecular lines. As was mentioned earlier, the cyclotron waves characterize a high-frequency branch of collective oscillatory behaviour of electrons in interstellar magnetoplasma. The model under consideration focuses on the low-frequency magneto-electron waves which originate from the transport of the magnetic flux density by thermal electrons, due to Hall electron conductivity. It is the major purpose of the remainder of this paper to show that interstellar magnetoplasma of molecular star-forming clouds can transmit low-frequency perturbations in the magnetic flux density by weakly damped helicoidal circularly polarized waves, which owe their existence to the Hall drift of the magnetic field by flows of thermal electrons.

To see that the wave motions in question bear a circularly polarized character, we omit for the moment the term of Ohmic diffusion. Schematic picture for oscillations of the magnetic field vector in the helicoidal magneto-electron wave. Therefore, the helicons may propagate fairly freely throughout the ISM that has the above parameters. The helicons might be relevant to the interstellar scintillations of the pulsar signals. The latter effect is customarily attributed to the scattering of radio waves on fluctuations in electron density Rickett ; Narayan From the above simple estimates it does not seem implausible that helicoidal magneto-electron waves might provide a contribution to the effect of interstellar scintillations of the pulsar radio signal.

On the other hand, highly ionized H ii regions of the warm interstellar medium occupy only 25 per cent of the Galactic volume Heiles Therefore they make a significant contribution to the dispersion measure for only a small fraction of pulsars. However, if at some stage of the star formation the thermal electrons reside in the central region of a molecular cloud, then their presence would be very difficult to detect.

Interstellar Propagation of Electromagnetic Signals

In this case, highly coherent electron gas-dynamical processes resulting in observational consequences might enable indirect searches for the thermal electrons. The evidence for the existence of large-scale motions in dark star-forming molecular clouds provides the widths of molecular lines. Therefore it is interesting to discuss a possible contribution of helicons to the broadening of molecular lines detected from dark interstellar clouds. In dark molecular clouds this latter frequency is dominated by electron-neutral collisions , where is the particle density of H 2 molecules Mouschovias The last statement is illustrated in Fig.

In Fig.

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The above estimates show that the intercloud medium can transmit the helicoidal magneto-electron waves without significant attenuation in the regime of the ISM motions when the effects of ambipolar diffusion are heavily suppressed. We therefore conjuncture that they can be responsible for the broadening of molecular lines. The group velocity V h of helicoidal wave as a function of period of magneto-electron oscillations P h.


Understanding gas-dynamical processes governing the structure and the evolution of dense molecular clouds is one of the outstanding challenges in the current development of star formation astrophysics. While the central role of magnetic fields in such processes was recognized many years ago, the major uncertainties regarding the motions follow from inadequate knowledge of the material composition of the intercloud medium.

Over the years, convincing evidence has been obtained that shows that the composition of dark molecular clouds is dominated by molecular hydrogen with some admixture of OH and CO molecules whose linewidths are found to exhibit the supersonic character of intercloud motions. The fact that the linewidths cannot be explained as a result of the propagation of isothermal sound waves has served as an impetus in searching for alternative models of interstellar gas dynamics and has led to the hypothesis of the presence in dark molecular clouds of a sizable fraction of charged particles primarily electrons and ions whose collective flows are strongly coupled with the intercloud magnetic field.

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On average, the model provides a fairly reasonable account of data in the CO regions of clouds where the temperature and the ionization factor are pretty high. The latter circumstance can be regarded as an indication that the composition and the character of the motions in cores of the molecular clouds might be quite different from those which are implied by using the single-component MHD model of interstellar gas dynamics.

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This sort of magnetically polarized, poorly conducting soft matter can be thought of as gas-based ferrocolloid consisting of tiny ferromagnetic grains suspended in the dense gas of molecular hydrogen capable of sustaining a long-range magnetic chains extending along the intercloud magnetic fields. Continuing the investigation in this direction, we have explored here a model of a pure electron interstellar gas dynamics; the propagation of spiral magneto-electron waves which owe their existence to the Hall drift of magnetic flux by thermal electrons.

The basic inference of this model is that in dark molecular clouds the helicons suffer negligible damping by Ohmic conductivity. A similar conclusion regarding the helicons in a dusty space plasma has recently been reached by Rudakov et al. Our numerical estimates for the group velocity of the helicons suggest that these waves could be responsible for the broadening of molecular linewidths detected from dark star-forming clouds or, at least, provide a sizable contribution to this effect. Oxford University Press is a department of the University of Oxford.

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