Wavelengths and oscillator strengths of Xe II from the UVES spectra of four HgMn stars

Abstract

In spite of large overabundances of Xe II observed in numerous mercury-manganese (HgMn) stars, Xe II oscillator strengths are only available for a very limited number of transitions. As a consequence, several unidentified lines in the spectra of HgMn stars could be due to Xe II. In addition, some predicted Xe II lines are redshifted by about 0.1 A from stellar unidentified lines, raising the question about the wavelength accuracy of the Xe II line data available in the literature. For these reasons we investigated the Xe II lines lying in the 3900-4521 A, 4769-7542 A, and 7660-8000 A spectral ranges of four well-studied HgMn stars. We compared the Xe II wavelengths listed in the NIST database with the position of the lines observed in the high-resolution UVES spectrum of the xenon-overabundant, slowly rotating HgMn stars HR 6000, and we modified them when needed. We derived astrophysical oscillator strengths for all the Xe II observed lines and compared them with the literature values, when available. In this framework, we performed a complete abundance analysis of HD 71066, while we relied on our previous works for the other stars. We find that all the lines with wavelengths related to the 6d and 7s energy levels have a corresponding unidentified spectral line, blueshifted by the same quantity of about 0.1 A in all the four stars, so that we identified these lines as coming from Xe II and modified their NIST wavelength value according to the observed stellar value. We find that the Xe II stellar oscillator strengths may differ from one star to another from 0.0 dex to 0.3 dex. We adopted the average of the oscillator strengths derived from the four stars as final astrophysical oscillator strength.