Spectral Line Radiation Research Articles

The galactic density wave, molecular clouds and star formation

A model has been devised for the orbits of molecular clouds in the Galaxy. The molecular clouds are assumed to be launched from the two-armed spiral-shock wave, to orbit in the Galaxy like ballistic particles with gravitational perturbations due to the density-wave spiral-potential and each cloud is assumed to produce an identical cluster of stars. A comparison of the model with observations suggests that each cloud radiates detectable 12C160 (J = 1 → 0) spectral line radiation from birth to an age of 30 million years and that stars are seen in the cloud 15 million years after its birth. The model has been tested by comparing its predicted velocity-longitude diagram for CO against the observed one for the Galaxy and by comparing the model's predicted surface brightness in the UBV photo metric bands against observed surface photometry for Sb and Sc galaxies.

The galactic density wave, molecular clouds, and star formation

A dynamical model has been devised for the galactic orbits of molecular clouds after their birth in the galactic density wave. This model suggests a picture in which young protoclusters or associations and their associated placental interstellar material actively radiate /sup 12/C/sup 16/O spectral line radiation at 115 GHz until about 30 million years after birth, at which time there is an abrupt cutoff of that spectral line radiation.A survey of 63 young open clusters reveals associated 115 GHz emission if the cluster or association contains main-sequence O stars, but the associated emission is not present for clusters whose earliest main-sequence star is B0 or later, confirming the cutoff predicted by the dynamical model.This association between /sup 1/2C/sup 16/O spectral line radiation and open clusters containing young stars has allowed us to use the surface density of /sup 12/C/sup 16/O to determine the surface density of young clusters, and we have used the dynamical model to determine their corresponding birth locations and birthrates. We also derive the birthrate of open clusters averaged over the Galaxy.In addition, it has been suggested that if supernovae in the cluster are capable of ridding the cluster of its interstellar gas, thereby stopping the COmore » spectral line radiation, then these observations give an upper mass limit for stars that become supernovae.Next we discuss recombination line observations of H II regions and compare the predictions of our dynamical model for them against the predictions for CO molecular clouds.Finally we compare our independent time scale from the dynamical model with conventional stellar evolution time scales. They agree if about 20 million years elapse between the time and interstellar cloud passes through the spiral-shock wave and the time when stars begin to form in the cloud.« less

Density-dependent Lines of One- and Two-electron Ions in Diagnostics of Laboratory plasma. I. The rates of collision relaxation of excited levels

Plasma devices with inertial plasma confinement such as laser produced plasmas, exploding wires, plasma focus, etc., which have been rapidly developed during recent years, appear to be very intensive sources of spectral line radiation in far UV and X-ray regions. Analysis of this radiation provides a good tool for plasma diagnostics with very high electron densities up to 1022 cm-3. In this work, consisting of two parts, we consider the mechanism of the formation of spectral lines in hot and dense plasma. The key point for density diagnostics is the fact that for some ion levels the rate of collisional relaxation has the same order of magnitude as the radiative decay. Thus the intensities of spectral lines arising from these levels show a strong dependence on electron density which makes diagnostics possible.In the first paper, emphasis is laid on the calculation of rates of transition between close ion levels induced by electron or ion impact, which usually gives the main contribution to the collisional relaxation constants. The influence of plasma polarization effects on the collision frequency in a dense plasma is also considered.In a second paper, the formation of spectral lines of hydrogen-like, helium-like and oxygen-like ions will be considered for electron density 1018 cm-3 ⩽ N ⩽ 1024 cm-3 and the ion charge 10 ⩽ Z ⩽ 19.

Migration of excitation in transfer of spectral-line radiation

A simple mathematical model is developed for the transfer of energy through a gas by the combined effect of radiative transfer and migration of excitation. The “excitation” is carried through the gas by a succession of atoms which experience resonant excitation exchange; it thus appears to random walk through the medium. The theory developed here is valid when the distance traveled by an atom while excited is much larger than the typical distance at which two atoms can exchange excitation (roughly 10 -6cm). The model is expressed in terms of a pair of coupled transport equations for the intensity of radiation and the density of excited atoms, which are solved by means of a generalized discrete-ordinate technique. Extensive numerical results are obtained and discussed in terms of characteristics lengths for the various transfer processes. Substantial effects of migration are seen in both the distribution of excited atoms near the cell windows and the line profile of the emergent radiation for typical laboratory conditions.

Transfer of Radiation in Spectral Lines

<i>Transfer of Radiation in Spectral Lines</i>

On the Force Associated with Absorption of Spectral Line Radiation

On the Force Associated with Absorption of Spectral Line Radiation

Time-dependent aerodynamic non-LTE radiative transfer

A method is developed for solving simultaneously in one dimension the equation of transfer for non-LTE spectral line radiation and the time-dependent equations specifying conservation of mass, energy and linear momentum. In particular, we illustrate the method on a ‘simple’ time-dependent problem in which a pulsating disturbance at some point in a model homogeneous atmosphere propagates towards the surface and steepens into a shock. The resulting emergent intensities show rather dramatic changes over very small time intervals due to the effect of the velocity, density and temperature distributions on the radiative absorption properties of the gas, and thus emphasises the need to solve the above-mentioned four basic equations if one is to obtain physically realistic model atmospheres experiencing initial disturbances.

Ne-Hg and He-Hg Low-Pressure Arc Discharges with Positive Volt-Ampere Characteristics

Experimental studies have been made of the valt-ampere characteristics of Ne-Hg and He-Hg lowpressure arc discharges in tubes 320 mm in length and 30 mm in diameter, operated at 50 Hz AC currents between 0.1 and 5 amperes. The positive volt-ampere characteristics with strong radiation of He and Ne spectral lines have been obtained.Measurements have been made of the axial and radial distributions of radiation intensities of He, Ne, Hg and Hg ion spectral lines emitted from the positive column of positive characteristics. Within experimental accuracy the positive column was found to be axially homogeneous, and it was confirmed that Hg and Hg ion depletion occurred along the center of the tube.Luminous efficacies of the positive column with thin phosphor coatings were about 12-19 lumens per watt for Hg-20 Torr Ne mixtures, 5-12 lumens per watt for Hg-40 Torr Ne mixtures and 5-10 lumens per watt for Hg-6 Torr He mixtures.

An exact solution to the multi-dimensional line transfer equation

The theory of generalized analytic functions is used to obtain an exact closed form analytical solution to a transfer problem for spectral line radiation in a multi-dimensional atmosphere. The multi-dimensional full-space and half-space Green's functions so obtained are quite general and may be used, along with the corresponding orthogonality relationships, to obtain solutions to any general multi-dimensional radiative transfer problem involving model two-level atoms. An application of the method using perturbation techniques is illustrated.

Energy transfer by spectral-line radiation

Energy transfer by spectral-line radiation

A General Formulation of the Transfer Equation. II. Line Formation with General Redistribution

The multidimensional equation of transfer for spectral line radiation under a general redistribution law is studied. It is shown that the equation may be rewritten as a system of equations of the "Feautrier" form, which are known to be exceedingly stable and efficient under numerical reduction. It is also shown that the inclusion of a multidimensional differential macroscopic velocity field does not alter the functional form of the equations obtained and therefore may also be treated by the general Feautrier technique.

A new electron-photon coincidence method for the measurement of lifetimes in atoms and molecules

A new method of measurement of atomic and molecular lifetimes is described, in which delayed coincidences are recorded between inelastically scattered electrons and the subsequent decay photons. The main advantage of this method is that sources of systematic error, such as cascading, spectral line blending, radiation trapping and collisional de-excitation, can often be eliminated. The method is described in detail, together with measurements on three helium states: 3 3D(13.4+or-0.5)ns, 4 3S(63.5+or-1.2)ns and 4 1S(75+or-5)ns.

Spectra, Variability, Size, and Polarization of H 2 O Microwave Emission Sources in the Galaxy

Radio spectral line radiation of water molecules at a wavelength of 1.35 centimeters has been measured from eight sources in the galaxy. The sources are less than I arc minute in diameter, have extremely high brightness temperatures, and show many spectral features. Some spectral features are slightly polarized and some have changed greatly in intensity in a few weeks time.

A spectroscopic investigation of a helium plasma arc

The spectral line radiation from the Poiseuille plasma arc, operated at 100 A and 1 atm pressure in helium, has been investigated. The purpose of this investigation was to determine whether the state of the plasma can be described as being in local thermodynamic equilibrium with a single temperature or whether additional parameters are needed to describe the state of the plasma. The emission coefficients of six helium lines were measured and these results compared to measurements of the Stark broadening of a line.

Monochromatic photography in the extreme ultra-violet

A pinhole camera with a thin aluminium filter can be used for broadband photography in the wavelength region below 800 A. Two developments of this instrument are described which result in improved wavelength resolution. The first uses a camera with a reflection filter to provide sensitivity only in the waveband 170-800 A. The reflection filter consists of two mirrors used at 10° grazing angle to reject short wavelength radiation. The second instrument combines a pinhole camera with a plane diffraction grating used at grazing incidence to give increased wavelength resolution. This instrument has been used to obtain monochromatic photographs of a helium plasma in spectral line radiation at 304 A and 256 A.

Spectral Line Emission from a Transient Nonuniform Plasma

Some effects of spatial gradients and transient behavior on spectral line radiation from impurities in a low-energy θ-pinch plasma are discussed for a case where local thermodynamic equilibrium could not be assumed. The plasma was produced and preheated by a fast capacitor bank discharge, a controlled trapped magnetic field was generated by a slower discharge through the same coil, and magnetic compression was achieved by a fast rising reverse magnetic field. Measured carbon impurity line intensities were compared to predictions on the basis of magnetohydrodynamic calculations coupled with spectroscopic theory. Large errors are incurred in electron temperatures from relative intensities of lines from successive stages of ionization if the radial variation of ionization and excitation is ignored.

The Spectra of Ne IV, Ne V, and Ne VI: A Further Analysis

The investigation is a sequel to the spectral analysis of Kaufman, Hughes and Williams (1960). From spectrograms taken with the high-current discharge assemblies Sceptre III and IV, new lines of Ne IV, Ne V and Ne VI in the range between about 2200 A and 3000 A have been observed and classified. The lists of classified lines and term values, both new and revised, are given in tabular form. No intersystem combinations have been observed in any of the three spectra. A method of distinguishing the higher stages of ionization (Ne IV, Ne V, Ne VI) has been developed by varying the concentration of the neon added to the deuterium. The relative intensities of the spectra vary with the concentration in a characteristic manner and so the lines of a given spectrum can be picked out with comparative ease. A theoretical explanation of the effect is advanced on the assumption that the plasma is cooled by spectral-line radiation. Methods of identifying the stage of ionization when high-current discharges are used as spectral sources are appraised.