Doppler shift, interstellar

The location of an observed molecular radio transition in its energy level scheme and its measured interstellar intensity contain important information concerning the physical state of the molecular cloud in which the transition is observed. It will therefore be an important task for future interstellar molecular research to observe and measure as many transitions of any one molecule in any one particular cloud. Doppler shifts, i.e. the difference in frequency between the rest frequency (known from laboratory measurements) and the observed interstellar line frequencies, provide information on the large scale motion of the molecular clouds while the linewidths indicate the turbulence within the clouds. 

Particle velocity effects. Particle velocity can cause Doppler broadening of spectral lines. The effect is extremely small for interstellar clouds at 10 K but is appreciable for clouds near high temperature stars. Outflows of gas from pulsing stars exhibit a red Doppler shift when moving away at high speed and a blue shift when moving toward us. 

We have described a selective heterodyne radiometer potentially useful in the detection of remote species such as pollutants and interstellar molecules. The system operates on the basis of the difference frequency between two radiated lines which, for closely spaced lines, is relatively insensitive to Doppler shift. This allows for the sensitive detection of known species moving at unknown velocities. The two frequencies may be obtained from individual transitions or... 

One very important area is the identification of molecular species in astronomical sources. A surprising number of polyatomic molecules and ions, a selection of which are listed in Table 7.1, have been identified in the so-called molecular clouds, which appear to contain regions with moderate effective temperatures (tens or even hundreds of Kelvins) and which may represent an early stage in the condensation of interstellar matter into stars. Parts of an emission spectrum from such a source arc shown in Figure 7.8. Doppler shifts of the frequencies of lines in these rotation spectra give accurate estimates of the velocities of the gas clouds relative to Earth. 

A gamma-ray line at 0.511 MeV results from the mutual annihilation of an electron and a positron, a particle-antiparticle pair. A number of radioactive decay chains (see Table I) result in the emission of a positron as a decay product, which will annihilate upon first encounter with an electron. Also of astrophysical importance is the production of electrons and positrons via the photon-photon pair-creation process. Such pair plasmas are found in the vicinity of compact objects, such as neutron stars and black holes, that are associated with heated accretion disks and relativistic flows and jets, within which particle acceleration is known to occur. Thus, relatively narrow lines of 0.511-MeV annihilation radiation are expected to arise in the interstellar medium through the decay of dispersed, nucleosynthetic radionuclides, while broadened, Doppler-shifted, and possibly time-variable lines may occur in the high-energy and dense environments associated with compact objects. 

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