Statistics – Computation
Scientific paper
Oct 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002aipc..636..134k&link_type=abstract
AIP Conference Proceedings, vol. 636, iss. no. 1, p. 134-143
Statistics
Computation
17
Astronomical Spectroscopy, Atomic Structure, Electronic Structure, Energy Levels, Line Spectra, Planetary Atmospheres, Solar Radiation, Space Plasmas, Stellar Atmospheres, Emission, Absorption, And Scattering Of Electromagnetic Radiation, Stellar Atmospheres, Radiative Transfer, Opacity And Line Formation, Atomic And Molecular Data, Spectra, And Spectral Parameters
Scientific paper
We need a list of all the energy levels of all atoms and molecules that matter (qualifiers below). Except for the simplest species, it is impossible to generate accurate energy levels or wavelengths theoretically. They must be measured in the laboratory. From the list of energy levels can be generated all the lines. Given the low accuracy required, 1 - 10%, all the other data we need can eventually be computed or measured. With the energy levels and line positions known, one can measure gf values, lifetimes, damping, or one can determine a theoretical or semiempirical Hamiltonian whose eigenvalues and eigenvectors produce a good match to the observed data, and that can then be used to generate additional radiative and collisional data for atoms or molecules. For atoms and ions, we need all levels, including hyperfine and isotopic splittings, for n less-than-or-equal 9 below the lowest ionization limit and as much as practicable above. Lifetimes and damping constants depend on sums over the levels. Inside stars there are thermal and density cutoffs that limit the number of levels, but in circumstellar, interstellar, and intergalactic space, photoionization and recombination can populate high levels, even for high ions. We need all stages of ionization for elements at least up through Zn. In the sun there are unidentified asymmetric triangular features that are unresolved multiplets of light elements with n less-than-or-equal 20. Simple spectra should be analyzed up to n = 20. Levels that connect to the ground or to low levels should be measured to high n, say n = 80. The high levels are necessary to match line series merging into continua. All the magnetic dipole, electric quadrupole, and maybe higher-pole, forbidden lines are required as well. Most of the universe is low density plasma or gas. If the Hamiltonian is well determined, forbidden lines should be reliably computable. For molecules, we need all levels below the first dissociation limit and as much as is practicable above, especially levels of all states that connect to the ground state. Stars populate levels to high V and to high J. In the sun there are many broad bumpy features that are molecular bands that are not in the line lists. For the cooler stars we need all the diatomics among all the abundant elements, and, essentially, the hydrides and oxides for all elements (especially ScO, TiO, VO, YO, ZrO, LaO). For M stars triatomics also become important. Much more laboratory and computational work is needed for H2O. In the brown dwarfs and "planets" methane is important and it needs more laboratory and computational work. We can produce more science by investing in laboratory spectroscopy rather than by building giant telescopes that collect masses of data that cannot be correctly interpreted.
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