The advent of high wavelength resolution spectrographs on ground-based and satelliteborne telescopes in the past 20 years has enabled high wavelength accuracy spectral observations of stars and galaxies. This increase in wavelength resolution enables one to identify transitions from different atoms, ions, and molecules and reduces the uncertainty due to mis-identification of a line or blending with other lines of similar wavelength. In particular, accurate wavelength calibration of astrophysical spectra has yielded a wealth of information on stellar dynamics, including the motion of stars in a galaxy and star rotation. Furthermore, one can determine if shifts in the wavelength of the line are present because of line-broadening phenomena such as magnetic fields in the star’s atmosphere. Moreover, one can select lines in the spectra of distant, old galaxies that are unaffected by local effects and use these transitions to test the validity of fundamental constants throughout the lifetime of the universe.
However, the increase in wavelength accuracy of astrophysical spectra has, in some cases, surpassed the accuracy of the laboratory atomic database, prompting a reassessment of the atomic parameters in the literature. In particular, Nave and Sansonetti tackle one of the most spectroscopically dominant elements in astrophysical spectra: neutral and singly ionized iron. Indeed, for every nanometer of the visible solar spectrum there are more than ten lines from neutral and singly ionized iron. The large number of iron lines results from the large number of possible transitions between the energy levels in neutral or singly ionized iron. As a result, the complex spectrum of neutral and singly ionized iron has been extensively studied in the laboratory and theoretically for fundamental atomic physics and for applications to astrophysical studies. However, small differences between the calibrated wavelength scale of different laboratory measurements have limited the accuracy of the absolute wavelength determined from these measurements.
Nave and Sansonetti re-examine published and unpublished data for iron and reanalyze the observed spectra to reduce the uncertainty in the analysis procedure. Possible errors in the measurement techniques are discussed and tested experimentally. The work follows a meticulous and analytical methodology that demonstrates the dedication of the National Institute of Standards and Technology to the highest level of precision and expertise in atomic standards.
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