Jérôme Deprince

Context . Most hot subdwarfs (sdO/B) are low-mass core-helium-burning stars formed through binary interaction. A subgroup of intermediate He-rich sdOBs shows extreme heavy metal ( Z > 30) enrichments exceeding 10 ⁴ times solar, particularly in Zr or Pb. Aims . We analysed the first ultraviolet spectra of the heavy-metal subdwarfs LS IV–14°116 (Zr-rich) and EC 22536–5304 (Pb-rich) to determine their abundance patterns and test nucleosynthesis models. The targets probe contrasting evolutionary channels: the close binary EC 22536–5304 with an sdF companion and the apparently single LS IV–14°116, likely formed by a white dwarf merger. Methods . Both stars show exceptionally rich heavy-element spectra dominated by ions in stages III–VI , many transitions being absent from standard line lists. We compiled literature energy levels, wavelengths, and oscillator strengths and implemented them in the spectral synthesis code S YNSPEC . As data were unavailable, we computed the oscillator strengths for As III , Se III , Hf IV , and Tl IV . New photoionisation cross-sections for Pb III–VI enabled the first non-local thermal equilibrium (non-LTE) models of multiply ionised Pb. Results . In LS IV–14°116 we detected 16 light and 24 heavy metals (Ga–Bi); Br, Nb, Mo, Pd, In, Sb, Te, and Xe have been measured in an sdO/B star for the first time. EC 22536–5304 is even more enriched; we detected 13 light and 26 heavy metals, including first detections of La, Ce, Pr, Nd, Er, Yb, Lu, Hf, Ta, W, Os, Pt, Hg, Tl, and Bi, mainly in stages IV-V . LS IV–14°116 peaks at ~4.3 dex for Sr-Sn, declining to 3.1 dex at Pb and 2.3 dex at Bi, whereas EC 22536–5304 shows 1–3 dex enhancements from Ga-Sn, rising to ~4 dex for La–Tl and reaching 6.2 dex for Pb and 5.4 dex for Bi. Both stars are Fe-poor (−0.8 and −2.5 dex). Conclusions . The photospheres of both stars are strongly enriched in heavy elements relative to their initial compositions. The abundance patterns cannot be explained by atomic diffusion alone and retain a clear signature of nucleosynthesis. The distribution in EC 22536–5304 closely matches predictions of i -process nucleosynthesis during its formation, providing strong evidence for i -process self-enrichment in hot subdwarfs. The contrasting abundance patterns imply different evolutionary histories and i -process conditions: EC 22536–5304 likely formed via Roche-lobe overflow onto its sdF-type companion, whereas the single star LS IV–14°116 most likely originated from the merger of two low-mass white dwarfs. These results suggest that heavy metals in other He-rich sdOB and sdO stars are also self-synthesised and that i -process nucleosynthesis may be widespread among He-rich hot subdwarfs.

The discovery of gravitational waves from a neutron star merger in 2017 (GW170817) and the associated kilonova (AT2017gfo) confirmed these events as key sites for heavy element production through the r-process. Subsequent observations, including late-time spectra with JWST, have highlighted the need for accurate modeling of kilonova ejecta. In the photospheric phase, atomic level populations can be estimated under LTE using Boltzmann and Saha relations, but about a week after the merger the ejecta enters the nebular phase where non-LTE effects dominate. Modeling nebular spectra therefore requires a detailed treatment of radiative and collisional processes that affect the population of atomic levels. This work focuses on electron-impact excitation in Sr II, a heavy ion relevant for kilonova spectra. Two computational approaches are employed: the Plane Wave Born approximation within the pseudo-relativistic Hartree-Fock method, and a Distorted Waves method using AUTOSTRUCTURE. The resulting collision strengths are compared against reference R-matrix data to evaluate the accuracy of these approximations and their suitability for large-scale applications to all heavy elements. In addition, radiative parameters for forbidden transitions are computed. These results provide an essential benchmark of approximations that could be used to compute atomic data for nebular-phase kilonova modeling.

Context. The production of elements heavier than iron in the Universe still remains an unsolved mystery. About half of them are thought to be produced by the astrophysical r-process (rapid neutron-capture process), for which neutron star mergers (NSMs) are among the most promising production sites. In August 2017, gravitational waves generated by a NSM were detected for the first time by the LIGO detectors (event GW170817), and the observation of its electromagnetic counterpart, the kilonova (KN) AT2017gfo, suggested the presence of heavy elements in the KN ejecta. The luminosity and spectra of such KN emission depend significantly on the ejecta opacity. Atomic data and opacities for heavy elements are thus sorely needed to model and interpret KN light curves and spectra. Aims. The present work focuses on large-scale atomic data and opacity computations for all heavy elements with Z ≥ 20, with a special effort on lanthanides and actinides, for a grid of typical KN ejecta conditions (temperature, density, and time post-merger) between one day and one week after the merger (corresponding to the local thermodynamical equilibrium photosphere phase of the KN ejecta). Methods. In order to do so, we used the pseudo-relativistic Hartree–Fock (HFR) method as implemented in Cowan’s codes, in which the choice of the interaction configuration model is of crucial importance. Results. In this paper, HFR atomic data and opacities for all elements between Ca (Z = 20) and Lr (Z = 103) are presented, with a special focus on lanthanides and actinides. In particular, we found increased lanthanide opacities compared to previous works. Besides, we also discuss the contribution of every single element with Z ≥ 20 to the total KN ejecta opacity for a given NSM model, depending on their Planck mean opacities and elemental abundances. An important result is that lanthanides are found to not be the dominant sources of opacity, at least on average. The impact on KN light curves of considering such atomic-physics-based opacity data instead of typical crude approximation formulae is also evaluated. In addition, the importance of taking the ejecta composition into account directly in the expansion opacity determination (instead of estimating single-element opacities) is highlighted. A database containing all the relevant atomic data and opacity tables has also been created and published online along with this work.

Aims. It is now well established that a large amount of heavy (trans-iron) elements are produced during neutron star (NS) mergers. These elements can be detected in the spectra of the kilonova emitted from the post-merger ejected materials. Due to the high level densities that characterize the complex configurations belonging to heavy elements, thus giving rise to millions of absorption lines, the kilonova ejecta opacity is of significant importance. The elements that contribute the most to the latter are those with an unfilled nd subshell belonging to the fifth and the sixth rows of the periodic table, and those with an unfilled nf subshell belonging to the lanthanide and actinide groups. The aim of the present work is to make a new contribution to this field by performing large-scale atomic structure calculations in three specific sixth-row 5d elements, namely hafnium, osmium, and gold, in the first four charge stages (I–IV), and by computing the corresponding opacities, while focusing on the importance of the atomic models used. Methods. The pseudo-relativistic Hartree–Fock (HFR) method, including extended sets of interacting configurations, was used for the atomic structure and radiative parameter calculations, while the expansion formalism was used to estimate the opacities. Results. Theoretical energy levels, wavelengths, and oscillator strengths were computed for millions of spectral lines in Hf I–IV, Os I–IV, and Au I–IV ions, the reliability of these parameters being assessed through detailed comparisons with previously published experimental and theoretical results. The newly obtained atomic data were then used to calculate expansion opacities for typical kilonova conditions expected one day after the NS merger; these are a density of ρ = 10⁻¹³ g cm⁻³ and temperatures ranging from T = 5000 K to T = 15 000 K. Some agreements and differences were found when comparing our results with available data, highlighting the importance of using sufficiently complete atomic models for the determination of opacities.

Aims. Neutron star (NS) mergers are thought to be a source of heavy trans-iron element production. The latter can be detected in the spectra of the ejected materials, from which bright electromagnetic radiation is emitted. This latter is due to the radioactive decay of the produced heavy r-process nuclei and is known as kilonova. Because of their complex atomic structures – characterized by configurations involving unfilled nd or nf subshells – the heavy elements of the kilonova ejecta often give rise to numerous absorption lines generating significant opacities. The determination of the latter, which are of paramount importance for the analysis of kilonova light curves, requires knowledge of the radiative parameters of the spectral lines belonging to the ions expected to be present in the kilonova ejecta. The aim of the present work is to provide new atomic opacity data for two representative 4d elements, niobium (Nb) and silver (Ag), in their first four charge states, namely for Nb I–IV and Ag I–IV. Methods. Large-scale calculations based on the pseudo-relativistic Hartree-Fock (HFR) method were performed to obtain the atomic structure and radiative parameters while the expansion formalism was used to estimate the opacities. Results. Wavelengths and oscillator strengths were computed for several million spectral lines in Nb I–IV and Ag I–IV ions. The reliability of these parameters was estimated by comparison with the few previously published experimental and theoretical results. The newly obtained atomic data were then used to calculate expansion opacities for typical kilonova conditions expected one day after NS merger, a density of ρ = 10⁻¹³ g cm⁻³, and temperatures ranging from T = 5000 K to T =15 000 K.

In the present work, we report an investigation of plasma environment effects on the atomic parameters associated with the K‐vacancy states in highly charged iron ions within the astrophysical context of accretion disks around black holes. More particularly, the sensitivity of K‐line X‐ray fluorescence parameters (wavelengths, radiative transition probabilities, and Auger rates) in Fe XVII–Fe XXV ions has been estimated for plasma conditions characterized by an electron temperature ranging from 10 ⁵ to 10 ⁷ K and an electron density ranging from 10 ¹⁸ to 10 ²² cm ⁻³ . In order to do this, relativistic multiconfiguration Dirac‐Fock atomic structure calculations have been carried out by considering a time averaged Debye‐Hückel potential for both the electron–nucleus and electron–electron interactions.