Department of Theoretical, Atomical an Optical Physics

Transition probabilities have a significant interest in the astrophysical field. The aim of this experiment is to extend the present data base of measured Ar II transition probabilities to the ultraviolet (UV) region and to improve the quality of some of the already existing data. Despite
all the efforts made to assemble an accurate set of transition probabilities (Aki), some of the data recommended by the National Institute of Standards and Technology (NIST) for Ar II
spectral lines in the UV region have uncertainties around 50 per cent. We measured relative intensities of spectral lines emitted from a low-pressure-pulsed discharge lamp which generates an argon plasma. Excitation temperatures of 19 000–22 000 K are determined from the Boltzmann-plot of 11 Ar II spectral lines for which accurate Aki data are available from the literature. The transition probabilities of these 11 spectral lines were used for calculating new
Aki-values. Electron densities (Ne) of 3.5–9.0 × 1022 m−3 are determined by two-wavelength interferometry method. The measurements yield to a collection of 43 atomic transition probabilities of Ar II lines in the spectral region of 294–386 nm, 11 of which are new, at least up to the authors knowledge, and 22 improved Aki-values for which the existing data in the literature have uncertainties around 50 per cent. Comparison with previous data shows how our
measurements have good agreement with the most accurate Aki-values from the bibliography, whereas some of the values recommended by NIST present a significant disagreement.

In this paper, we present results of experimental determination of Stark parameters of Ar II
spectral lines. We report 126 half-width and 111 shift results, 53 half-widths and 44 shifts
of which are presented here for the first time. The obtained data may be of astrophysical
interest. The results were compared with available experimental results of other authors and
with calculated ones in the cases where it was possible. A low-pressure pulsed arc, filled with
pure argon, was used as a plasma source. All measurements were done under electron density
which ranges from 0.20 × 1023 to 1.15 × 1023 m−3 and electron temperature from 16 000 to
26 000 K.

Fourier Transform spectroscopy is able to provide high accuracy atomic parameters needed by many ongoing galactic surveys. Our laboratory has carried out a study of the neutral iron spectrum over the last years to measure oscillator strengths much needed for the calculation of chemical abundances. The main aim of this contribution is to encourage further dialogue with astronomers regarding their current necessities of spectroscopic data, as this would help spectroscopists to prioritise present-day needs within the field.

WD J204713.76–125908.9 is a new addition to the small class of white dwarfs with helium-dominated photospheres that exhibit strong Balmer absorption lines and atmospheric metal pollution. The exceptional abundances of hydrogen observed in these stars may be the result of accretion of water-rich rocky bodies. We obtained far-ultraviolet and optical spectroscopy of WD J204713.76–125908.9 using the Cosmic Origin Spectrograph on-board the Hubble Space Telescope and X-shooter on the Very Large Telescope, and identify photospheric absorption lines of nine metals: C, O, Mg, Si, P, S, Ca, Fe, and Ni. The abundance ratios are consistent with the steady-state accretion of exo-planetesimal debris rich in the volatile elements carbon and oxygen, and the transitional element sulphur, by factors of 17, 2, and 4, respectively, compared to the bulk Earth. The parent body has a composition akin to Solar system carbonaceous chondrites, and the inferred minimum mass, 1.6 × 1020 g, is comparable to an ...