High resolution UV stellar spectroscopy with the HST/GHRS, challenges and opportunities for atomic physics

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Abstract
The Goddard High Resolution Spectrograph (GHRS) on the Hubble Space Telescope (HST) is producing ultraviolet spectra of unprecedented resolution and photometric quality. We illustrate this with observations of the ultra-sharp-lined, chemically peculiar B-type star, χ Lupi. The photosphere of this star is rich in heavy elements, and its UV spectrum is replete with transitions of species not commonly observed in astrophysical spectra. It is an excellent "laboratory" for atomic spectroscopy. Wavelengths measured with the GHRS are accurate to ±1–2 mÅ, requiring Fourier-Transform-Spectrometer (FTS) quality laboratory wavelengths for line identifications and spectrum synthesis. We are able to discern evidence of isotope shifts/hyperfine splitting in lines of Hg, Pt, and possibly other species. Kurucz's comprehensive calculations of transition probabilities for the iron-group and other elements provides a reasonable starting point for theoretical spectrum synthesis. However, we have found evidence of large systematic errors in his data base for transitions from particular energy levels in Fe II, Cr II, Co II, and other ions, and its coverage of elements outside the range 20 ≤ z ≤ 28 is insufficient. We have relied on the direct participation of many atomic spectroscopists from around the world, who have provided new laboratory measurements and theoretical calculations, enabling us to accurately interpret these observations.