The Utrecht catalogue of Fraunhofer-line intensities.
M. G. J. Minnaert · The Astronomical Journal · 1947
The Photomet4c A tias of the Solar Spectrum was edited in 1940 by several cooperating astronomers of the Utrecht Observatory. A few copies were sent to America just before the interruption of international relations by the war. For the American observatories and for other countries which were inaccessible, a great number of copies have been kept in store. Since the international exchange of scientific literature is again resumed, it has been possible to send twenty-five atlases to America and more will follow soon. One of the first results to be derived from the atlas is the equivalent width of the various Fraunhofer lines. A catalogue of Fraunhofer-line intensities is being derived as a cooperative undertaking of Utrecht astronomers with the help of some other collaborators. This catalogue is already ~omplete for the first order records, extending from the infrared to about X65oo. Strong lines are measured with a planimeter. For lines fainter than 20 mA several sets of typical profiles were drawn, having different widths and depths. These profiles were photographed on a reduced scale and their areas carefully determined once for all. Glass copies of these curves are superposed on the spectrum and the equivalent widths are determined by comparison and interpolation. In order to analyze line blends, a rather elaborate investigation had to be undertaken. (i) The starting point was the precise examination of numerous single lines by Mr. H. C. van de Hulst. He approximated the atlas profiles by "Voigt profiles," giving all transitions between Doppler and damping curves. The instrumental profile was approximated in the same way. (2) Since a Voigt profile, obliterated by another Voigt profile, results in a new Voigt profile, Mr. van de Hulst was able to determine the true individual profiles and to show that they differ according to the elements and to the excitation potential. (3) Now these true profiles were theoretically combined into doublets, assuming a solar model intermediate between the Schwarzschild-Schuster and the Milne-Eddington model; a great number of combinations between lines of different intensities at several distances were made. (4) Finally the theoretical doublets were obliterated again by the instrumental profile. The doublet curves thus obtained are compared to the atlas doublets and used for the analysis into separate lines. Utrecht, Netherlands; temporarily at Verkes Observatory, Williams Bay, Wis.