arXiv subjects

Viktor Khalack

Publications and source records attributed to Viktor Khalack.

12 recordsLinked to original sources

arXiv2026

Study of photometric and spectral variability of the roAp star HD~210684

This paper studies photometric and spectropolarimetric variability of HD210684 in order to derive its magnetic properties, rotational period, evolutionary stage, and global stellar parameters. The Discrete Fourier Transform is used to measure frequencies and amplitudes of periodic signals present in light curves of HD210684. Evolution models are calculated with MESA, while roAp type pulsations are simulated with GYRE. The values of $T_{eff}$, log(g), $v\sin{i}$, and radial velocity (RV) are derived from the best fit of Balmer line profiles using FITSB2. The Least Square Deconvolution (LSD) method is applied to available Stokes I&V spectra to measure the mean longitudinal magnetic field and RV. Detailed analysis of photometric variability reveals rotational modulation with period P=5.02188$\pm$0.00005d and splitting of high-overtone pulsations that corresponds to the same rotational period. Derived measurements also show periodic variability with P=5.02188d, but this variability appears to be different from the one predicted by a centred magnetic dipole model. We have determined the inclination angle between the line of sight and the rotation axis to be $i = 31^{\circ} \pm 2^{\circ}$, and the angle between the rotation and magnetic dipole axes as $\beta= 77^{\circ} \pm 3^{\circ}$. Simulations of stellar pulsations constrain $\log T_{eff}$<3.85 and show that our best fitting model depends on whether the observed modes are $\ell = 1$ or $\ell =2$. Considering the derived value of $\beta$, we prefer the best fit model with $\ell =2$ mode, which predicts that HD210684 lies on the main sequence with an age of approximately 1.45Gyr. The best fit of Balmer line profiles has resulted in higher values of $T_{eff}$ and log(g) for spectra acquired at rotational phases $\varphi$= 0.14 - 0.39 suggesting visibility of an area with higher surface temperature.

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arXiv2026

Pushing the Limit of Asteroseismic Detection for Cool Dwarfs using TESS and Deep Learning

Asteroseismology provides a powerful probe of stellar interiors by detecting stellar oscillations, including solar-like oscillations, which are stochastically excited by near-surface convection. While thousands of solar-like oscillators have been identified in evolved stars, only a limited number of main-sequence cool dwarfs have confirmed oscillations due to the low amplitudes of their signals. In this work, we train a convolutional autoencoder on TESS two-minute light curves to automatically identify solar-like oscillation features in cool dwarf main sequence and sub-giant stars. Using catalogs of confirmed oscillators for training and validation, our network achieves a classification accuracy of 99.8% on the test set, along with Precision of 0.945, Recall of 0.998, and F1 Score of 0.971. From the Asteroseismic Target List, our model identifies 3463 potential solar-like oscillators (probability greater than 0.5). After further analysis, we find a list of 24 candidate stars that have the potential to exhibit solar-like oscillations. Notably, several of these candidates occupy regions of the color-magnitude diagram that are accessible only through more resource-intensive radial velocity observations, thereby has the potential of extending the detection frontier of TESS-based asteroseismology. Our candidate catalog provides a valuable foundation for follow-up efforts aimed at expanding the sample of cool-dwarf solar-like oscillators. This will ultimately improve our understanding of stellar structure and evolution across the lower main sequence and strengthen the evidence for using deep learning techniques to study stellar light curves.

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arXiv2026

Variability classification of TESS targets in LOPS2, the first long-term pointing field of PLATO. Version 1 of the public variability catalogue

The PLAnetary Transits and Oscillations of stars (PLATO) mission is expected to launch in January 2027. A total of 8\% of its data rate will be dedicated to complementary science targets selected from approved Guest Observer proposals. We seek to provide an open-source catalogue of variable stars in PLATO's first long-term observing field, LOPS2. We want to use existing observations from the Transiting Exoplanet Survey Satellite (TESS), which has observed many stars in LOPS2. We classified 38 million calibrated aperture light curves from the TESS-Gaia Light Curve pipeline (TGLC, $G\lesssim17$) for 6 million unique sources in LOPS2 with two machine learning frameworks -- a deep neural network and a feature-based gradient-boosted decision-tree ensemble. We combined their predictions to create this first version of the LOPS2 variability catalogue, performed manual vetting of a sub-sample classified light curves, and a statistical analysis of the results to validate our methodology and to assess the variability properties and parameters of the stars in the catalogue. Our classification resulted in the identification of approximately 72% of the light curves having dominant instrument- or pipeline-induced signal, with the remaining 28% representing 3.6 million individual candidate variable stars, including pulsating, rotating, and eclipsing stars. Candidate pulsators exhibit varied behaviour in terms of their frequencies, amplitudes, rotation, and fundamental parameters. To ensure purity of the samples, filtering on colour, luminosity, the dominant frequency and its amplitude, and presence of close neighbours is helpful. We provide the first version of our PLATO LOPS2 variability catalogue to the community for further study and scrutiny. It is to date one of the largest catalogues of variable stars from an automated classification pipeline.

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arXiv2022

HD 213258: a new rapidly oscillating, super-slowly rotating, strongly magnetic Ap star in a spectroscopic binary

We report about HD 213258, an Ap star that we recently identified as presenting a unique combination of rare, remarkable properties. Our study of this star is based on ESPaDOnS Stokes I and V data obtained at 7 epochs spanning a time interval slightly shorter than 2 years, on TESS data, and on radial velocity measurements from the CORAVEL data base. We confirm that HD 213258 is definitely an Ap star. We found that, in its spectrum, the Fe II {\lambda}6149.2 {\AA} line is resolved into its two magnetically split components. The mean magnetic field modulus of HD 213258, ~ 3.8 kG does not show significant variations over ~2 years. Comparing our mean longitudinal field determinations with a couple of measurements from the literature, we show that the stellar rotation period must likely be of the order of 50 years, with a reversal of the field polarity. Moreover, HD 213258 is a rapidly oscillating Ap (roAp) star, in which high overtone pulsations with a period of 7.58 min are detected. Finally, we confirm that HD 213258 has a mean radial velocity exceeding (in absolute value) that of at least 99% of the Ap stars. The radial velocity shows low amplitude variations, which suggests that the star is a single-line spectroscopic binary. It is also a known astrometric binary. While its orbital elements remain to be determined, its orbital period likely is one of the shortest known for a binary roAp star. Its secondary is close to the borderline between stellar and substellar objects. There is a significant probability that it may be a brown dwarf. While most of the above-mentioned properties, taken in isolation, are observed in a small fraction of the whole population of Ap stars, the probability that a single star possesses all of them is extremely low. This makes HD 213258 an exceptionally interesting object that deserves to be studied in detail in the future.

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arXiv2020

Revealing the nature of HD63401

HD~63401 is known magnetic chemically peculiar (mCP) star that shows a slow rotation and probably possesses a hydrodynamically stable stellar atmosphere. In the latter case the atomic diffusion mechanism enforced by the magnetic field can lead to stratification of elemental abundance with optical depth. HD~63401 was recently observed with the space telescope \textit{TESS}, and its light curve shows distinct variability, which is usually detected in the $α^2$ CVn type stars. Based on the analysis of the light curve we derived its rotational period $P$ = 2.414 $\pm$ 0.018~d and studied variability of the effective temperature with this period. The estimates of the effective temperature and surface gravity were obtained from the best fit of Balmer line profiles observed in seven high-resolution spectra acquired with the spectropolarimeter ESPaDOnS for HD~63401. The same spectra were used to perform an abundance analysis employing the modified ZEEMAN2 code. We have found that He, C, P, V, Y, and Dy are in deficit in stellar atmosphere of HD~63401, while Na, Al, Si, Fe, Zn, and Sr are significantly overabundant.

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arXiv2020

VeSElkA: Vertical Stratification of Element Abundances in CP stars

A portion of the upper main-sequence stars, called chemically peculiar (CP) stars, show significant abundance anomalies mainly due to atomic diffusion of chemical elements within the stellar atmospheres of these stars. Slowly rotating CP stars may have hydrodynamically stable atmospheres where the competition between the gravitational and radiative forces leads to atomic diffusion that can be responsible for the abundance peculiarities observed in CP stars. Recently, project VeSElkA (Vertical Stratification of Element Abundances) was initiated with the aim to detect and study the vertical stratification of element abundances in the atmospheres of CP stars. Some results from abundance analysis of several slowly rotating (V$\sin{i}$< 40 km/s) CP stars observed recently with ESPaDOnS are presented here. Signatures of vertical abundance stratification for several chemical elements have been found in the stellar atmospheres of HD~22920, HD~41076, HD~95608, HD~116235, HD~148330 and HD~157087.

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arXiv2020

The 10.5 year rotation period of the strongly magnetic rapidly oscillating Ap star HD 166473

How magnetic fields contribute to the differentiation of the rotation rates of the Ap stars and affect the occurrence of non-radial pulsation in some of them are important open questions. Valuable insight can be gained into these questions by studying some of the most extreme examples of the processes at play. The super-slowly rotating rapidly oscillating Ap (roAp) star HD 166473 is such an example. We performed the first accurate determination of its rotation period, (3836 +/- 30)d, from the analysis of 56 measurements of the mean magnetic field modulus based on high-resolution spectra acquired between 1992 and 2019 at various observatories and with various instrumental configurations. We complemented this analysis with the consideration of an inhomogeneous set of 21 determinations of the mean longitudinal magnetic field spanning the same time interval. This makes HD 166473 one of only four Ap stars with a period longer than 10 years for which magnetic field measurements have been obtained over more than a full cycle. The variation curves of and of are well approximated by cosine waves. The magnetic field of HD 166473 only seems to deviate slightly from axisymmetry, but it definitely involves a considerable non-dipolar component. Among the stars with rotation periods longer than 1000 d for which magnetic field measurements with full phase coverage are available, HD 166473 has the strongest field. Its magnetic field is also one of the strongest known among roAp stars. Overall, the magnetic properties of HD 166473 do not seem fundamentally distinct from those of the faster-rotating Ap stars. However, considering as a group the eight Ap stars that have accuractely determined periods longer than 1000 d and whose magnetic variations have been characterised over a full cycle suggests that the angles between their magnetic and rotation axes tend to be systematically large.

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arXiv2018

Project VeSElkA: results of abundance analysis for HD53929 and HD63975

Project VeSElkA (Vertical Stratification of Element Abundances) has been initiated with the aim to detect and study the vertical stratification of element abundances in the atmosphere of chemically peculiar stars. Abundance stratification occurs in hydrodynamically stable stellar atmospheres due to the migration of the elements caused by atomic diffusion. Two HgMn stars, HD53929 and HD63975 were selected from the VeSElkA sample and analysed with the aim to detect some abundance peculiarities employing the ZEEMAN2 code. We present the results of abundance analysis of HD53929 and HD63975 observed recently with the spectropolarimeter ESPaDOnS at Canada-France-Hawaii Telescope. Evidence of phosphorus vertical stratification was detected in the atmosphere of these two stars. In both cases, phosphorus abundance increases strongly towards the superficial layers. The strong overabundance of Mn found in stellar atmosphere of both stars confirms that they are HgMn type stars.

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arXiv2018

Project VeSElkA: Search for vertical stratification of element abundances in HD157087

The new spectropolarimetric spectra of HD157087 obtained recently with ESPaDOnS at CFHT are analysed to verify the nature of this object. The fundamental stellar parameters $T_{\rm eff}$ = 8882 K, $\log{g}$=3.57 have been obtained for HD157087 from the analysis of nine Balmer line profiles in two available spectra. Comparison of the results of our abundance analysis with the previously published data shows a variability of average abundance with time for some chemical species, while the abundance of other elements remains almost the same. The abundance analysis also reveals evidence of a significant abundance increase towards the deeper atmospheric layers for C, S, Ca, Sc, V, Cr, Mn, Co, Ni and Zr. Together with the found enhanced abundance of Ca and Sc this fact contradicts the classification of HD157087 as a marginal Am star. Analysis of the available measurements of radial velocity results in detection of long periodic and short periodic variations. The long periodic variation supports the idea that HD157087 is an astrometric binary system with a period higher than 6 years. The presence of short periodic variation of $V_{\rm r}$, as well as the detection of temporal variation of average abundance suggest that HD157087 may be a triple system, where a short periodic binary rotates around a third star. In this case, the short periodic binary may consist of slowly rotating Am and A (or Ap with weak magnetic field) stars that have similar effective temperature and surface gravity, but different abundance peculiarities.

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arXiv2015

Project VeSElkA: Vertical Stratification of Element Abundances

A new research project on spectral analysis that aims to characterize the vertical stratification of element abundances in stellar atmospheres of chemically peculiar (CP) stars is discussed in detail. Some results on detection of vertical abundance stratification in several slowly rotating main sequence CP stars are presented and considered as an indicator of the effectiveness of the atomic diffusion mechanism responsible for the observed peculiarities of chemical abundances. This study is carried out in the frame of Project VeSElkA (Vertical Stratification of Elements Abundance) for which 34 slowly rotating CP stars have been observed with the ESPaDOnS spectropolarimeter at CFHT.

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arXiv2015

Abundance analysis of HD 22920 spectra

The new spectropolarimetric observations of HD 22920 with ESPaDOnS at CFHT reveal a strong variability of its spectral line profiles with the phase of stellar rotation. We have obtained Teff = 13640 K, logg=3.72 for this star from the best fit of its nine Balmer line profiles. The respective model of stellar atmosphere was calculated to perform abundance analysis of HD 22920 using the spectra obtained for three different phases of stellar rotation. We have found that silicon and chromium abundances appear to be vertically stratified in the atmosphere of HD 22920. Meanwhile, silicon shows hints for a possible variability of vertical abundance stratification with rotational phase.

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arXiv2015

Project VeSElkA: Analysis of Balmer line profiles in slowly rotating chemically peculiar stars

We present results for the estimation of gravity, effective temperature, and radial velocity of poorly studied chemically peculiar stars recently observed with the spectropolarimeter Echelle SpectroPolarimetric Device for Observations of Stars at the Canada-France-Hawaii Telescope in the frame of the Vertical Stratification of Element Abundances project. The effective temperature and surface gravity values are determined for the very first time for four of the stars from our sample (HD23878, HD83373, HD95608, and HD164584). Grids of stellar atmosphere models with the corresponding fluxes have been calculated using version 15 of the PHOENIX code for effective temperatures in the range of 5000-15,000 K, for the logarithm of surface gravities in the range of 3.0-4.5 and for the metallicities from -1.0 to +1.5. We used these fluxes to fit the Balmer line profiles employing the code FITSB2 that produces estimates of the effective temperature, gravity, and radial velocity for each star. When possible, our results are compared to those previously published. The physical characteristics of 16 program stars are discussed with the future aim to study the abundance anomalies of chemical species and the possible vertical abundance stratification in their stellar atmosphere.

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