Tatiana Ryabchikova

Multiwavelength modelling of the light variations in the chemically peculiar star CU Vir is presented. The modelling is based on the recent Doppler Imaging of CU Vir, which provides maps of the surface distribution of Si, Fe, He, and Cr. Intensity maps in both individual photometric filters and in the wide wavelength range from UV to NIR were calculated, taking into account the individual chemical abundances on the stellar surface. Comparison with observations revealed good agreement of both the light curves and their amplitude along the spectrum. Additionally, we analysed changes in the photometric period of the CU Vir from 1955 to 2022, including TESS measurements. The data of the last decades clearly indicate a gradual decrease in this period. Measurements of the CU Vir period over the next two decades will be crucial for verifying or refuting the periodic nature of its variations.

We present the results of Doppler Imaging of the Ap star CU Vir in the silicon lines over the 1985–2011 time span, as well as multi-element imaging in the 2009/2011 epoch. The surface distribution of silicon in CU Vir exhibits stability over the approximately 26 years studied: the number, shape, and mutual distribution of the overabundance spots have remained unchanged. The modelling of the light curve based on the surface elemental distribution obtained with DI did not reveal any significant changes in the shape of the light curve that could explain the photometric phase shift observed in CU Vir. Consequently, the phase shifts and changes in the photometric period of CU Vir are caused by the rigid longitudinal drift of the surface-abundance structures. We performed simulations of the Tayler instability of the background magnetic field of CU Vir, and discuss the possibility of explaining the period variations by the drift of surface instability modes.

Aims. We aim to accurately determine bound-to-bound transition wavelengths and energy levels of the high-lying open-shell configurations 4f³7s, 4f³6d, and 4f³5f of doubly ionised neodymium (Nd III, Z = 60) through high-resolution spectroscopy and semi-empirical calculations. This study is motivated by lanthanide atomic data requirements in astronomy, such those involved in investigations of kilonova spectra. Methods. Fourier transform spectra of Nd Penning and hollow cathode discharge lamps were recorded within the region 32500–54000 cm⁻¹ (3077–1852 Å) and grating spectra of Nd vacuum sliding sparks were recorded within the regions 820–1159 Å and 1600–3250 Å. New energy levels were found using the observed wavelengths measured accurate to a few parts in 10⁸ in Fourier transform spectra and to a few parts in 10⁷ in grating spectra. Atomic structure and transition probability calculations of Nd III were carried out using the Cowan codes, where energy parameters were adjusted to fit all known Nd III levels. Finally, Nd-rich stellar spectra were also used to aid in the analysis. Results. In total, 355 transitions were classified from observed spectra involving 116 previously experimentally unknown energy levels of the 4f³7s, 4f³6d, and 4f³5f configurations of Nd III, all reported here for the first time. Three newly identified levels of the 4f³5d configuration and one newly identified 4f⁴ level are also reported. Typical level energy uncertainties are 0.01 cm⁻¹ for the 4f³7s and 4f³6d levels and 0.3 cm⁻¹ for the 4f³5f levels. In addition, calculated energy levels up to 130 936 cm⁻¹ are presented, including eigenvector composition and calculated level lifetimes. Calculated transition probabilities and wavelengths between 1900–50 000 Å are also given. Using newly established levels of the 4f³7s configuration and the recently established levels of the 4f³6s configuration, the ionisation energy of Nd III has been estimated at 178 090 ± 330 cm⁻¹. This result offers up to twice the accuracy of the most recently published value.

The combination of the surface magnetic and abundance inhomogeneities in chemically peculiar Ap/Bp stars is responsible for their rotationally modulated variability. Within the framework of the fossil field hypothesis, these inhomogeneities are considered to be essentially stable over the main sequence timescale. However, a small group of Ap/Bp stars show rotational period changes that are not fully understood. Here, we present results of Doppler imaging of the rapidly rotating Ap star 56 Ari whose changes in period had previously been detected. A reconstruction of the surface distribution of silicon in 56 Ari reveals its complex spot pattern, which is responsible for the rotationally light variability and is correlated with a magnetic field modulation. Comparisons of abundance maps obtained over the unprecedentedly long (for such studies) interval from 1986 to 2014 confirms the stability and rigid rotation of the spot pattern. Thus, we find the period change in 56 Ari is not caused by a rearrangement of the surface magnetic structures and/or atomic diffusion operating on short timescale. It is also unlikely to be explained by the visibility changes of the spots due to the free-body precession of the stellar rotational axis. We briefly discuss possible alternative explanations of the period variability.

The TESS light curve of the silicon Ap star MX TrA was modeled using the observational surface distribution of silicon, iron, helium, and chromium obtained previously with the Doppler Imaging technique. The theoretical light curve was calculated using a grid of synthetic fluxes from line-by-line stellar atmosphere models with individual chemical abundances. The observational TESS light curve was fitted by a synthetic one with an accuracy better than 0.001 mag. The influence of Si and Fe abundance stratification on the amplitude of variability was estimated. Also, the wavelength dependence of the photometric amplitude and phase of the maximum light was modeled showing the typical Ap Si star behavior with increased amplitude and anti-phase variability in far ultraviolet caused by the flux redistribution.

Aims. Our goal is to accurately determine bound-to-bound transition wavelengths and energy levels of the low-lying open-shell configurations 4f⁴, 4f³ 5d, 4f³6s, and 4f³ 6p of doubly ionised neodymium (Nd III) through high-resolution spectroscopy and semi-empirical calculations. Methods. The emission spectra of neodymium (Nd, Z = 60) were recorded using Penning and hollow cathode discharge lamps in the region 11 500-54000 cm⁻¹ (8695–1852 Å) by Fourier transform spectroscopy at resolving powers up to 10⁶. Wavenumber measurements were accurate to a few 10⁻³ cm⁻¹. Grating spectroscopy of Nd vacuum sliding sparks and stellar spectra were used to aid line and energy level identification. For the analysis, new Nd III atomic structure and transition probability calculations were carried out using the Cowan code parameterised by newly established levels. Results. The classification of 432 transitions of Nd III from the Penning lamp spectra resulted in the determination of 144 energy levels of the 4f⁴, 4f³ 5d, 4f³ 6s, and 4f³ 6p configurations of Nd III, 105 of which were experimentally established for the first time. Of the 40 previously published Nd III levels, one was revised and 39 were confirmed. Conclusions. The results will not only benchmark and improve future semi-empirical atomic structure calculations of Nd III, but also enable more reliable astrophysical applications of Nd III, such as abundance analyses of kilonovae and chemically peculiar stars, and studies of pulsational wave propagation in these stars.

We present the results of the spectroscopic study of a chemically peculiar star HD 152564. Using medium-resolution (R = 37000) observations obtained with the HRS spectrograph mounted on the South African Large Telescope we determined atmospheric parameters T_eff=11950±200 K and log g=3.6±0.2 dex. Abundance analysis revealed mild deficiency of the light elements and an overabundance of up to ∼2 dex of metals with greatest excess for the silicon. With these characteristics HD 152564 is a typical member of the silicon subgroup of Ap stars. Rotational modulation of the light curve and line profiles of HD 152564 are typical for inhomogeneous surface distribution of elements in its atmosphere. We performed multi-element Doppler imaging of the HD 152564 surface. Abundance maps constructed for He , O , Mg , Si , and Fe revealed concentration of these elements in a sequence of equatorial spots as well as in the circumpolar rings. The photometric maximum of the light curve coincided with the visibility of two most overabundant silicon spots. Abundances determined from the different ionisation stages of Fe and Si show clear evidence for vertical stratification of these elements in HD 152564 atmosphere. Meanwhile, the horizontal distribution of silicon reconstructed from the lines of different ionisation stages and excitation energies appeared to be identical with increasing average abundance deeper in atmosphere.

We present the results of the spectroscopic study of a chemically peculiar star HD 152564. Using medium-resolution ($R=37000$) observations obtained with the HRS spectrograph mounted on the South African Large Telescope we determined atmospheric parameters T$_{eff}$=11950$\pm200$ K and log g=3.6$\pm0.2$ dex. Abundance analysis revealed mild deficiency of the light elements and an overabundance of up to $\sim$2 dex of metals with greatest excess for the silicon. With these characteristics HD 152564 is a typical member of the silicon subgroup of Ap stars. Rotational modulation of the light curve and line profiles of HD 152564 are typical for inhomogeneous surface distribution of elements in its atmosphere. We performed multi-element Doppler imaging of the HD 152564 surface. Abundance maps constructed for He, O, Mg, Si, and Fe revealed concentration of these elements in a sequence of equatorial spots as well as in the circumpolar rings. The photometric maximum of the light curve coincided with the visibility of two most overabundant silicon spots. Abundances determined from the different ionisation stages of Fe and Si show clear evidence for vertical stratification of these elements in HD 152564 atmosphere. Meanwhile, the horizontal distribution of silicon reconstructed from the lines of different ionisation stages and excitation energies appeared to be identical with increasing average abundance deeper in atmosphere.

We present chemical composition and fundamental parameters (the effective temperature, surface gravity and radius) for four sharp-lined A-type stars γ Gem (HD 41705), o Peg (HD 214994), θ Vir (HD 114330) and ν Cap (HD 193432). Our analysis is based on a self-consistent model fitting of high-resolution spectra and spectrophotometric observations over a wide wavelength range. We refined the fundamental parameters of the stars with the sme package and verified their accuracy by comparing with the spectral energy distribution and hydrogen line profiles. We found T_eff/log g = 9190±130 K/3.56±0.08, 9600±50 K/3.81±0.04, 9600±140 K/3.61±0.12, and 10200±220 K/3.88±0.08 for γ Gem, o Peg, θ Vir and ν Cap, respectively. Our detailed abundance analysis employs a hybrid technique for spectrum synthesis based on classical model atmospheres calculated in local thermodynamic equilibrium (LTE) assumption together with the non-LTE (NLTE) line formation for 18 of 26 investigated species. Comparison of the abundance patterns observed in A stars of different types (normal A, Am, Ap) with similar fundamental parameters reveals significant abundance diversity that cannot be explained by the current mechanisms of abundance peculiarity formation in stellar atmospheres. We found a rise of the heavy element (Zn, Sr, Y, Zr, Ba) abundance excess up to +1 dex with T_eff increasing from 7200 to 10000 K, with a further decrease down to solar value at T_eff = 13000 K, indicating that stars with solar element abundances can be found among late B-type stars.