R. H. Garstang

Atoms in high magnetic fields (white dwarfs)

High magnetic fields have been of interest for some time in solid-state physics. Recent spectroscopic experiments on highly excited barium atoms and the discovery of very large magnetic fields in white dwarf stars and neutron stars have stimulated renewed interest in the spectroscopy of free atoms in high magnetic fields. The various spectroscopic regimes ranging from the low-field Zeeman effect to the high-field Landau regime are reviewed. Calculations of the energy levels of hydrogen, helium and their isoelectronic ions in fields of any strength are surveyed. Observations of the quadratic Zeeman effect in the alkalis and high-field effects in the neutral barium spectrum are described. Finally, recent discoveries of continuum polarization in white dwarf stars, its interpretation in terms of the presence of high magnetic fields, the confirmation of such fields spectroscopically, and some remaining mysteries in these remarkable stellar spectra are reviewed.

MODEL FOR ARTIFICIAL NIGHT-SKY ILLUMINATION.

A model has been constructed to allow calculation of the night-sky brightness caused by a city at its center and outside the city, and at arbitrary zenith distances. A circular city of uniform brightness is assumed, with the total brightness proportional to the population. Molecular scattering and aerosol scattering are included, with the amount of aerosols being an adjustable parameter, and different scale heights being adopted for molecules and aerosols. The reflectivity of the ground and the fraction of light radiated above the horizontal are taken as parameters. Applications are given to several cities, to the general population-distance relations, to brightness-distance relations, and to the city center brightness-population relations.

NIGHT SKY BRIGHTNESS AT OBSERVATORIES AND SITES

A model previously constructed for night-sky brightness calculations has been modified to allow for the curvature of the earth. The model has been applied to calculate the brightness at the following observatories: Mount Wilson, Lick, Mount Palomar, Kitt Peak, Sacramento Peak, Mauna Kea, McDonald, San Pedro Martir, Mount Hopkins, Mount Lemmon, Lowell (Mars Hill), Lowell (Anderson Mesa), Fick, Iowa, Van Vleck, David Dunlap, Anglo-Australian, Haute Provence, and Cerro Tololo. Calculations have also been carried out for the following prospective observatory sites: Junipero Serra, Mount Graham, Charleston Peak, Wheeler Peak, Miller Peak, San Benito Mountain, Lowell (Hutch Mountain), Lowell (Saddle Mountain), and South Baldy (New Mexico). The model is extended to calculate magnitudes in the B photometric band.

Hyperfine Structure and Intercombination Line Intensities in the Spectra of Magnesium, Zinc, Cadmium, and Mercury*

The transition probabilities of the lines 1S0–3P0 and 1S0–3P2 of the s2–sp arrays in Mg i, Zn i, Cd i, and Hg i, which arise by the interactions of the nuclear moments with the electrons, are computed for each odd isotope individually and for the naturally occurring isotopic mixture. The individual hyperfine structure component transition probabilities are given for the 1S0–3P2 lines. Part of the intensity of the lines arises from the interaction of 3P0 or 3P2 with 3P1 and part from the interaction of 3P0 or 3P2 with 1P1, the former being more important for 1S0–3P0 and the latter for 1S0–3P2. Spin-spin and spin-other-orbit interactions in Mg i are calculated for the 3s3p3P term and compared with observations.

Transition Probabilities in the Ar i Spectrum

Relative line strengths have been computed for the transition arrays of 3p54s–3p54p and 3p54s–3p55p in Ar i. Intermediate-coupling theory has been used, starting from LS coupling, and using the experimental energy levels as data for determining the parameters of the theory by least-squares adjustment of the calculated energies. The results have been placed on an absolute scale using the Coulomb approximation. The Lande g factors computed agree well with the observed values. The calculated transition probabilities are in fairly satisfactory agreement with various experimental determinations, and this agreement is better than that obtained using either LS- or jl-coupling calculations. The results for the 4s–4p array appear to be more accurate than those for the 4s–5p array.

DUST AND LIGHT POLLUTION

A model for the prediction of the brightness of the night sky due to man-made light pollution by the addition of an ozone layer is refined by the use of a more accurate representation of the atmospheric molecular density variation as a function of height, and by using a better mathematical representation of the scattering angular function of aerosols. Each of these modifications leads to a small reduction in the predicted brightness of the night sky. This model adds a thin layer of dust of arbitrary optical thickness and height above sea level. Dust clouds at various heights and of various optical thickness are studied. Most calculations have used scattering and absorption coefficients appropriate for volcanic clouds; a few calculations refer to desert dust. Light pollution is reduced by a dust cloud of moderate density whose altitude is below about 10 km (for the V band) and increased for dust clouds at greater altitudes. Observations from good sites are not likely to be greatly affected by the increases in light pollution caused by volcanic clouds at altitudes of order 20 km.

Transition Probabilities for Forbidden Lines

A compilation is given of transition probabilities of forbidden lines which occur in the spectra of gaseous nebulae.

Electron collisional excitation cross sections for Fe III and Fe VI and iron abundances in gaseous nebulae

We have calculated electron impact excitation cross sections between most of the low metastable levels of Fe III and Fe VI, using the close-coupling method. In addition, we have recalculated the electric quadrupole and magnetic dipole transition probabilities for these transitions in Fe VI. The statistical-equilibrium equations appropriate to a collisional/radiative model are solved to obtain level populations for each ion. Line emissivities have been calculated for the forbidden lines (Fe III) and (Fe VI). Comparison is made with with intensities in several nebulae and peculiar stars. The iron abundances in the Orion Nebula, NGC 6720, NGC 7009, and NGC 7662 are discussed. NGC 7662 seems to have an iron abundance an order of magnitude smaller than the sular abundance.

Magnetic-Quadrupole Radiation and Solar Coronal De-Excitation

Transition probabilities have been computed for a number of spectral lines due to magnetic-quadrupole radiation. Three such lines, in Fe ix, Fe xvii and Fe xxv, appear to be of importance in the de-excitation of excited atoms in the solar corona. Some years ago Mizushima (1964) to the possibility of magnetic-quadmpole transitions being allowed in the case of spectral lines for which A S = i 1. Such transitions can also take place in many cases by electric-dipole transitions between states perturbed by spin-orbit or nuclear spin-electron orbit interaction. Detailed calculations of the transition probabilities of magneticquadrupole lines were carried out by Mizushima (1966) and by Garstang (1967). In most of the cases which were examined magnetic-quadrupole radiation was found to be unimportant. The interesting exception is the line 3s2 15o - 3s3p p2 in Mg I, observed in the planetary nebula NGC 7027, for which Garstang showed that magnetic-quadrupole radiation is the dominant process. Garstang pointed out that in low-density sources magnetic-quadrupole radiation might be a significant de-excitation mechanism for certain 3p2 levels. In this paper we shall examine this suggestion. The Einstein spontaneous emission transition probability for magnetic-multipole radiation was given by Garstang (1967). For the case of magnetic-quadrupole radiation his result can be put in the form

OSCILLATOR STRENGTHS FOR NEUTRAL TECHNETIUM

Oscillator strengths are calculated for lines in the spectrum of neutral technetium which may be of importance in the study of stellar spectra. The calculations were performed by the setting up and diagonalization of the energy matrices of the 4d56S5s5p and 4d65D5p partial configurations for the 37 classified energy levels of odd parity arising from these configurations. Oscillator strengths obtained in the same manner for the corresponding transitions in neutral manganese support the accuracy of the Tc results for transitions from the a 6S level and the a 6D-z 6D and a 6D-y 6P multiplets, and indicate that the results for the a 6D-z 6F multiplet are probably overestimated by about a factor of 2.6.