arXiv subjects

P. Walczak

Publications and source records attributed to P. Walczak.

At least 19 recordsLinked to original sources

arXiv2026

Asteroseismology of the multiperiodic field SX Phe pulsator BL Camelopardalis

BL Camelopardalis (BL Cam) is the most metal-poor SX Phoenicis star known in the Galactic field, making it an excellent test bed for studies of Population II stellar structure and evolution, as well as for investigating the formation channels of blue straggler stars. We aim to constrain fundamental stellar parameters and pulsational properties of BL Cam and identify the nature of its observed oscillation modes. We analysed high-precision space-based data from the TESS mission and long-term ground-based observations from the Zwicky Transient Facility (ZTF) survey. The ZTF data were primarily used to derive the orbital parameters of the system, while the TESS light curves enabled us to extract a rich oscillation spectrum. We identified multiple pulsation frequencies and performed seismic modelling using a Bayesian approach. We detected numerous pulsation frequencies, including a dominant radial mode and additional non-radial components. We predicted all fitted modes in our seismic models as excited. The models reproduced the two highest amplitude-independent frequencies (i.e. the radial fundamental mode and a dipole mode) and suggested the possible presence of additional radial overtones. The inferred stellar parameters confirm the extremely low metallicity of BL~Cam. We also confirm its binary nature, with an orbital period of 144 days. BL Cam provides a valuable benchmark for testing stellar models of metal-poor pulsators. The combination of space-based photometry and Bayesian seismic modelling enables robust constraints to be placed on its internal structure and pulsation properties.

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arXiv2025

Asteroseismology of blue stragglers in $\omega$ Centauri: the case of five double-mode radial SX Phoenicis pulsators

We present the first extensive seismic modelling of SX Phe stars in the stellar system $\omega$ Cen. First, using the new values of reddening $E(B-V)$ and distance modulus $(m-M)_V$, and bolometric corrections from Kurucz model atmospheres, we determine the effective temperatures and luminosities of all SX Phe variables in $\omega$ Cen with available $(B-V)$ colours. Next, we carefully select SX Phe stars that have a frequency ratio strongly suggesting excitation of two radial modes, and, in addition, their preliminary pulsational models have the values of $(T_{\rm eff},~ L)$ consistent with observational determinations. For five double-mode radial pulsators, we perform an extensive seismic modeling using the Bayesian analysis based on Monte Carlo simulations. We study the effect of opacity tables and helium abundance. With the OPAL data and $Y=0.30$, we obtained masses in the range (1.0,~1.2) M$_{\odot}$, metallicity $Z\in(0.0007,~0.0029)$ and the age of about (1.9,~3.8) Gyr. The OP and OPLIB seismic models have always higher metallicites, sometimes outside the allowed range for $\omega$ Cen. In the case of three stars, we find seismic models within the observed range of $(T_{\rm eff},~L)$ with all three opacity tables. In the case of two stars, with the highest metallicity, seismic models computed with the OP and OPLIB tables are located far outside the observed error box. The OPAL seismic models follow the age$-$metallicity relation known for $\omega$ Cen from the literature.

astro-ph.SR↗

arXiv2023

Asteroseismology of double-mode radial $δ$ Scuti stars: AE Ursae Majoris and RV Arietis

We construct complex seismic models of two high-amplitude delta Sct stars, AE UMa and RV Ari, each pulsating in two radial modes: fundamental and first overtone. The models reproduce, besides the frequencies of two radial modes, also the amplitude of bolometric flux variations (the parameter f) for the dominant mode. Applying the Monte Carlo-based Bayesian analysis, we derive strong constraints, on the parameters of the model as well as on the free parameters of the theory. A vast majority of seismic models of the two stars are just at the beginning of hydrogen-shell burning and a small fraction is at the very end of an overall contraction. The stars have a similar age of about 1.6 Gyr for the hydrogen-shell burning phase. Both stars have unusual low overshooting from the convective core; about 0.02 and 0.004 of the pressure scale height for AE UMa and RV Ari, respectively. This result presumably indicates that overshooting should vary with time and scale with a decreasing convective core. The efficiency of convection in the envelope of both stars is rather low and described by the mixing length parameter alphaMLT of about 0.3-0.6. The third frequency of RV Ari, confirmed by us in the TESS photometry, can only be associated with mixed nonradial modes l=1, g4-g8 or l=2, g10-g12. We include the dipole mode into our Bayesian modelling and demonstrate its huge asteroseismic potential.

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arXiv2022

Asteroseismology of the double-radial mode $δ$ Scuti star BP Pegasi

Using the ASAS data, we determine the pulsational frequencies of the high-amplitude $δ$ Sct star BP Pegasi. The analysis revealed only the two known, independent frequencies. On the basis of multicolour Strömgren photometry, we independently find that both frequencies can only be associated with radial modes which, according to the frequency ratio, are fundamental and first overtone modes. The models fitting the two frequencies depend strongly on the opacity data. For low values of the mixing length parameter $α_{\rm MLT}\approx 0.5$, only the OPAL seismic models in the post-main sequence phase of evolution are caught within the observed error box. Seismic OP and OPLIB models can only reach the error box if we increase $α_{\rm MLT}$ to at least 2.0. Then, including the non-adiabatic parameter $f$ into our seismic modelling, we constrain various parameters, employing Monte Carlo-based Bayesian analysis.

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arXiv2021

Mode identification and seismic study of $δ$ Scuti, the prototype of a class of pulsating stars

We present a seismic study of $δ$ Scuti based on a mode identification from multicoulor photometry. The dominant frequency can be associated only with a radial mode and the second frequency is, most probably, a dipole mode. The other six frequencies have more ambiguous identifications. The photometric mode identification provided also some constraints on the atmospheric metallicity [m/H]$\approx$+0.5 and microturbulent velocity $ξ_t\approx 4~\kms$.\\ For models reproducing the dominant frequency, we show that only the fundamental mode is possible and the first overtone is excluded. However, the location of $δ$ Scuti near the terminal age main sequence requires the consideration of three stages of stellar evolution. For the star to be on the main sequence, it is necessary to include overshooting from the convective core with a parameter of at least $α_{\rm ov}=0.25$ at the metallicity greater than $Z=0.019$. It turned out that the value of the relative amplitude of the bolometric flux variations (the nonadiabatic parameter $f$) is mainly determined by the position of the star in the HR diagram, i.e., by its effective temperature and luminosity, whereas the effect of the evolutionary stage is minor. On the other hand, the convective efficiency in the subphotospheric layers has a dominant effect on the value of the parameter $f$. %in the $δ$ Sct star models. Comparing the theoretical and empirical values of $f$ for the radial dominant mode, we obtain constraints on the mixing length parameter $α_{\rm MLT}$ which is less than about 1.0, independently of the adopted opacity data and chemical mixture. This value of $α_{\rm MLT}$ is substantially smaller than for a calibrated solar model indicating rather low to moderately efficient convection in the envelope of $δ$ Scuti.

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arXiv2020

Seismic analysis of the double-mode radial pulsator SX Phoenicis

We present the results of complex seismic analysis of the prototype star SX Phoenicis. This analysis consists of a simultaneous fitting of the two radial-mode frequencies, the corresponding values of the bolometric flux amplitude (the parameter $f$) and of the intrinsic mode amplitude $\varepsilon$. The effects of various parameters as well as the opacity data are examined. With each opacity table it is possible to find seismic models that reproduce the two observed frequencies with masses allowed by evolutionary models appropriate for the observed values of the effective temperature and luminosity. All seismic models are in the post-main sequence phase. The OPAL and OP seismic models are in hydrogen shell-burning phase and the OPLIB seismic model has just finished an overall contraction and starts to burn hydrogen in a shell. The OP and OPLIB models are less likely due to the requirement of high initial hydrogen abundance ($X_0=0.75)$ and too high metallicity ($Z\approx 0.004$) as for a Population II star. The fitting of the parameter $f$, whose empirical values are derived from multi-colour photometric observations, provides constraints on the efficiency of convective transport in the outer layers of the star and on the microturbulent velocity in the atmosphere. Our complex seismic analysis with each opacity data indicates low to moderately efficient convection in the star's envelope, described by the mixing length parameter of $α_{\rm MLT}\in (0.0,~0.7)$, and the microturbulent velocity in the atmosphere of about $ξ_{\rm t}\in(4,~8)~\kms$.

astro-ph.SR↗

arXiv2020

Rotational modulation and single g-mode pulsation in the B9pSi star HD174356?

Chemically peculiar (CP) stars of the upper main sequence are characterised by specific anomalies in the photospheric abundances of some chemical elements. The group of CP2 stars, which encompasses classical Ap and Bp stars, exhibits strictly periodic light, spectral, and spectropolarimetric variations that can be adequately explained by the model of a rigidly rotating star with persistent surface structures and a stable global magnetic field. Using observations from the Kepler K2 mission, we find that the B9pSi star HD 174356 displays a light curve both variable in amplitude and shape, which is not expected in a CP2 star. Employing archival and new photometric and spectroscopic observations, we carry out a detailed abundance analysis of HD 174356 and discuss its photometric and astrophysical properties in detail. We employ phenomenological modeling to decompose the light curve and the observed radial velocity variability. Our abundance analysis confirms that HD 174356 is a silicon-type CP2 star. No magnetic field stronger than 110G was found. The star's light curve can be interpreted as the sum of two independent strictly periodic signals with P1 = 4.04355(5)d and P2 = 2.11169(3)d. The periods have remained stable over 17 years of observations. In all spectra, HD 174356 appears to be single-lined. From the simulation of the variability characteristics and investigation of stars in the close angular vicinity, we put forth the hypothesis that the peculiar light variability of HD 174356 arises in a single star and is caused by rotational modulation due to surface abundance patches (P1) and g mode pulsation (P2).

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arXiv2020

The Pulsation Properties of $λ$ Bootis Stars I. The Southern TESS Sample

We analyse TESS light curves for 70 southern $λ$ Boo stars to identify binaries and to determine which of them pulsate as $δ$ Scuti stars. We find two heartbeat stars and two eclipsing binaries among the sample. We calculate that 81 percent of $λ$ Boo stars pulsate as $δ$ Sct variables, which is about twice that of normal stars over the same parameter space. We determine the temperatures and luminosities of the $λ$ Boo stars from photometry and Gaia DR2 parallaxes. A subset of 40 $λ$ Boo stars have 2-min TESS data, reliable temperatures and luminosities, and $δ$ Sct pulsation. We use Petersen diagrams (period ratios), échelle diagrams and the period--luminosity relation to identify the fundamental mode in 20 of those 40 stars and conclude that a further 8 stars are not pulsating in this mode. For the remaining 12, the fundamental mode cannot be unambiguously identified. Further mode identification is possible for 12 of the fundamental mode pulsators that have regular sequences of pulsation overtones in their échelle diagrams. We use stellar evolution models to determine statistically that the $λ$ Boo stars are only superficially metal weak. Simple pulsation models also better fit the observations at a metallicity of $Z=0.01$ than at $Z=0.001$. The TESS observations reveal the great potential of asteroseismology on $λ$ Boo stars, for determining precise stellar ages and shedding light on the origin(s) of the $λ$ Boo phenomenon.

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arXiv2020

Ensemble asteroseismology of pulsating B-type stars in NGC 6910

Asteroseismology offers the possibility of probing stellar interiors and testing evolutionary and seismic models. Precise photometry and spectroscopy obtained during multi-site campaigns on young open clusters allows discovering rich samples of pulsating stars and using them in a simultaneous seismic modelling called ensemble asteroseismology. The aim of this study is to obtain the age of the open cluster NGC 6910 by means of ensemble asteroseismology of the early-type pulsating members, to derive their stellar parameters, and to classify the excited modes. We used time-series analysis, performed photometric and spectroscopic mode identification, and calculated grids of evolutionary and seismic models to apply the procedure of ensemble asteroseismology for nine pulsating members of NGC 6910. With two iterations of the procedure of ensemble asteroseismology applied to nine pulsating stars we derived an age of 10.6$^{+0.9}_{-0.8}$ Myr for NGC 6910. Of the nine pulsating stars examined in the paper, eight are $β$ Cep stars, including three that are hybrid $β$ Cep and slowly pulsating B-type (SPB) pulsators, and one is an SPB star. Interestingly, the least massive $β$ Cep star, NGC 6910-38, has a mass of about 5.6 M$_\odot$. The present theory does not predict unstable $p$ modes in B-type stars with such a low mass. The $g$ modes with relatively high frequencies ($>3.5$ d$^{-1}$), observed in three members of the cluster, are also stable according to seismic modelling. Both findings pose a challenge for theoretical calculations and prompt a revision of the opacities. The procedure of ensemble asteroseismology was found to be successful for NGC 6910 and $χ$ Per on the basis of pulsating B-type stars and can therefore be applied to other young open clusters that are rich in such stars.

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arXiv2019

Complex asteroseismology of SX Phoenicis

We present seismic analysis of the prototype SX Phoenicis that aims at fitting the two radial-mode frequencies and the corresponding values of the bolometric flux amplitude (the parameter $f$), whose empirical values are derived from multi-coulor photometric observations. Seismic model that meets these conditions is of low mass, $M=1.05 M_\odot$, has moderately effective convection in the outer layers, described by the mixing length parameter $α_{\rm MLT} \approx 0.7$, and the microturbulent velocity in the atmospheres of about $ξ_{\rm t}\approx 8$km/s. Such seismic studies of stars like SX Phe are very important for deriving constraints on outer-layer convection, because the object is on the border between very effective and ineffective convection.

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arXiv2019

Extreme events in forced oscillatory media in 0, 1 and 2 dimensions

One of the open questions in the field of optical rogue waves is the relevance of the number of spatial dimensions in which waves propagate. Here we review recent results on extreme events obtained in 0, 1 and 2 spatial dimensions in the specific context of forced oscillatory media. We show that some dynamical scenarii can be relevant from 0 to 2D while others can take place only in sufficiently large number of spatial dimensions.

physics.optics↗

arXiv2017

The solitary g-mode frequencies in early B-type stars

We present possible explanations of pulsations in early B-type main sequence stars which arise purely from the excitation of gravity modes. There are three stars with this type of oscillations detected from the BRITE light curves: $κ$ Cen, a Car, $κ$ Vel. We show that by changing metallicity or the opacity profile it is possible in some models to dump pressure modes keeping gravity modes unstable. Other possible scenario involves pulsations of a lower mass companion.

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arXiv2016

Interpretation of the BRITE oscillation data of the hybrid pulsator $ν$ Eridani: a call for the modification of stellar opacities

The analysis of the BRITE oscillation spectrum of the main sequence early B-type star $ν$ Eridani is presented. Only models with the modified mean opacity profile can account for the observed frequency ranges as well as for the values of some individual frequencies. The number of the $κ$ modified seismic models is constrained by the nonadiabatic parameter $f$, which is very sensitive to the opacity changes in the subphotospheric layers where the pulsations are driven. We present an example of the model that satisfies all the above conditions. It seems that the OPLIB opacities are preferred over those from the OPAL and OP projects. Moreover, we discuss additional consequences of the opacity modification, namely, an enhancement of the efficiency of convection in the Z-bump as well as an occurrence of close radial modes which is a kind of avoided-crossing phenomenon common for nonradial modes in standard main sequence models.

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arXiv2016

Sound speed and oscillation frequencies for solar models evolved with Los Alamos ATOMIC opacities

Los Alamos National Laboratory has calculated a new generation of radiative opacities (OPLIB data using the ATOMIC code) for elements with atomic number Z=1-30 with improved physics input, updated atomic data, and finer temperature grid to replace the Los Alamos LEDCOP opacities released in the year 2000. We calculate the evolution of standard solar models including these new opacities, and compare with models evolved using the Lawrence Livermore National Laboratory OPAL (Iglesias and Rogers 1996) opacities. We use the solar abundance mixture of Asplund et al. (2009). The new Los Alamos ATOMIC opacities have steeper opacity derivatives than those of OPAL for temperatures and densities of the solar interior radiative zone. We compare the calculated nonadiabatic solar oscillation frequencies and solar interior sound speed to observed frequencies and helioseismic inferences. The calculated sound-speed profiles are similar for models evolved using either the updated Iben evolution code (see \cite{Guzik2010}), or the MESA evolution code (Paxton et al., 2015). The LANL ATOMIC opacities partially mitigate the "solar abundance problem".

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arXiv2015

Nonlinear random optical waves: integrable turbulence, rogue waves and intermittency

We examine the general question of statistical changes experienced by ensembles of nonlinear random waves propagating in systems ruled by integrable equations. In our study that enters within the framework of integrable turbulence, we specifically focus on optical fiber systems accurately described by the integrable one-dimensional nonlinear Schrödinger equation. We consider random complex fields having a gaussian statistics and an infinite extension at initial stage. We use numerical simulations with periodic boundary conditions and optical fiber experiments to investigate spectral and statistical changes experienced by nonlinear waves in focusing and in defocusing propagation regimes. As a result of nonlinear propagation, the power spectrum of the random wave broadens and takes exponential wings both in focusing and in defocusing regimes. Heavy-tailed deviations from gaussian statistics are observed in focusing regime while low-tailed deviations from gaussian statistics are observed in defocusing regime. After some transient evolution, the wave system is found to exhibit a statistically stationary state in which neither the probability density function of the wave field nor the spectrum change with the evolution variable. Separating fluctuations of small scale from fluctuations of large scale both in focusing and defocusing regime, we reveal the phenomenon of intermittency; i.e., small scales are characterized by large heavy-tailed deviations from Gaussian statistics, while the large ones are almost Gaussian.

physics.optics↗

arXiv2015

Lambda Bootis stars in the SuperWASP survey

We have analysed around 170 000 individual photometric WASP measurements of 15 well established lambda Bootis stars to search for variability. The lambda Bootis stars are a small group of late-B to early-F, Pop. I, stars that show moderate to extreme (surface) underabundances (up to a factor 100) of most Fe-peak elements, but solar abundances of lighter elements (C, N, O and S). They are excellent laboratories for the study of fundamental astrophysical processes such as diffusion, meridional circulation, stellar winds, and accretion in the presence of pulsation. From the 15 targets, eight are variable and seven are apparently constant with upper limits between 0.8 and 3.0 mmag. We present a detailed time series analysis and a comparison with previously published results. From an asteroseismologic study we conclude that the found chemical peculiarities are most probably restricted to the surface.

astro-ph.SR↗

arXiv2013

The $β$ Cep/SPB star 12 Lacertae: extended mode identification and complex seismic modelling

Results of mode identification and seismic modelling of the $β$ Cep/SBP star 12 Lacertae are presented. Using data on the multi-colour photometry and radial velocity variations, we determine or constrain the mode degree, $\ell$, for all pulsational frequencies. Including the effects of rotation, we show that the dominant frequency, $ν_1$, is most likely a pure $\ell=1$ mode and the low frequency, $ν_A$, is a dipole retrograde mode. We construct a set of seismic models which fit two pulsational frequencies corresponding to the modes $\ell= 0,$ p$_1$ and $\ell= 1,$ g$_1$ and reproduce also the complex amplitude of the bolometric flux variations, $f$, for both frequencies simultaneously. Some of these seismic models reproduce also the frequency $ν_A$, as a mode $\ell= 1,$ g$_{13}$ or g$_{14}$, and its empirical values of $f$. Moreover, it was possible to find a model fitting the six 12 Lac frequencies (the first five and $ν_A$), only if the rotational splitting was calculated for a velocity of $V_{\rm rot}\approx 75$ km/s. In the next step, we check the effects of model atmospheres, opacity data, chemical mixture and opacity enhancement. Our results show that the OP tables are preferred and an increase of opacities in the $Z-$bump spoils the concordance of the empirical and theoretical values of $f$.

astro-ph.SR↗

arXiv2009

Complex asteroseismology of the $β$ Cep/SPB pulsator $ν$ Eridani: constraints on opacities

We undertake another attempt towards seismic modelling of the most intensive studied main sequence pulsators of the early B spectral type, $ν$ Eridani. Our analysis is extended by a requirement of fitting not only pulsational frequencies but also the complex amplitude of the bolometric flux variation, $f$, related to each mode frequency. This approach, called {\it complex asteroseismology}, provides a unique test of stellar parameters, atmospheres and opacities. In particular, the concordance of the empirical and theoretical values of $f$ we obtained for the high-order g mode opens a new gate in seismic studies of the main-sequence hybrid pulsators. The most intriguing and challenging result is that whereas an agreement of the theoretical and empirical values of $f$ for the radial mode can be achieved only with the OPAL data, a preference for the OP tables is derived from the analysis of the high-order gravity mode.

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