arXiv subjects

Brankica Kubátová

Publications and source records attributed to Brankica Kubátová.

9 recordsLinked to original sources

arXiv2026

Supernova interactions with aspherical circumstellar material I: calculations of light curves, AB magnitudes, spectra, and polarisation

We present an upgraded detailed numerical calculations of supernova (SN) interactions with significantly aspherical circumstellar matter (CSM), primarily formed as a disc or bipolar lobes. The circumstellar disc can arise as a result of, for example, mass transfer in a binary, while bipolar lobes can be the result of a violent pre-explosive ejection of matter, similar to the iconic cases of luminous blue variable stars (LBVs). We numerically simulate the radiation-hydrodynamic (RHD) behaviour of interaction processes using a 2D cylindrical version of the RHD code CASTRO. We then calculate light curves, spectral patterns, and polarisation profiles, all up to a relatively long time of two years after an SN shock breakout and from different directions, using the multidimensional Monte Carlo radiation transfer (MC-RT) codes SEDONA and SIROCCO. We calculated a total of five models for the two aforementioned configurations of the surrounding CSM, for stratified density levels, comparing the simulated hydrodynamic behaviour and differences in their observable properties. RHD models exhibit similar behaviour to previous adiabatic models, but with a significantly slower expansion velocity. The calculated light curves show a relatively smooth evolution in SN-disc interaction, and declines and brightening in SN-lobes interaction. Comparing models with real events with a presumed similar physical process provides guidance for selecting a more accurate CSM configuration when simulating real situations.

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arXiv2026

Spectral characteristics of fast rotating metal-poor massive stars

Low-metallicity massive stars are assumed to be progenitors of certain supernovae, gamma-ray bursts, and gravitational wave emitting mergers. These exotic phenomena contribute to their host galaxies through strong ionizing radiation and mechanical feedback. Here we investigate a certain type of very metal-poor (0.02 Zsun) hot massive single stars that rotate fast and evolve chemically homogeneously. Combining state-of-the-art theories of stellar evolution and stellar atmospheres modelling we predict synthetic spectra of these stars corresponding to different masses and evolutionary phases. The predicted spectra in early evolutionary phases is classified mainly as an early-O type giant or supergiant while in later evolutionary phases most of our model spectra are assigned to the WO-type spectral class. The Hubble Space Telescope's (HST) Ultraviolet Legacy Library of Young Stars as Essential Standards (ULLYSES) program will enable us to compare our predicted spectra with observations of stars of similar nature (e.g., metal-poor stars in Sextant A).

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arXiv2026

Progress towards a 3D Monte Carlo radiative transfer code for outflow wind modelling: II. 3-D applications

The massive hot stars play crucial role in the dynamics of galaxies. These stars influence their surroundings through strong winds which are highly structured processes. The theoretical study of the non-symmetric phenomena of the stellar winds are becoming important these days, mainly because 1-D models are not sufficient enough. We present a new version of our Monte Carlo radiative transfer code Andy Antares with improved treatment of the velocity field for arbitrary geometries. Our aim is to develop a numerical scheme that can incorporate a general velocity field defined at discrete points. Our main objective is to calculate radiative transfer in a general input hydrodynamic model. The Andy Antares code currently calculates pure radiative transfer. The input model is pre-calculated by another hydrodynamical code. The whole radiative transfer calculation is then processed in a Cartesian grid. Radiative transfer is solved using the Monte Carlo approach in 3-D regardless of the input hydrodynamical model's dimension. The velocity field at any given point is interpolated using the trilinear interpolation. The optical depth is then integrated numerically along the photon's path. We verified the accuracy of the numerical velocity interpolation by comparison with results obtained for analytical velocity fields, achieving successful outcomes. We also tested the radiative transfer solution on a 3-D model generated from a 2-D hydrodynamic model, and obtained emergent radiation. The code is suitable for the numerical solution of radiative transfer in 3-D with arbitrary velocity fields.

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arXiv2025

Low-metallicity massive single stars with rotation. III. Source of ionization and C-IV emission in I Zw 18

Chemically homogeneously evolving stars have been proposed to account for several exotic phenomena, including gravitational-wave emissions and gamma-ray bursts. Here we study whether these stars can explain the metal-poor dwarf galaxy I Zwicky 18. We apply our synthetic spectral models from Paper II to (i) establish a classification sequence for these hot stars, (ii) predict the photonionizing flux and the strength of emission lines from a IZw18-like stellar population, and (iii) compare our predictions to available observations of this galaxy. Adding two new models computed with PoWR, we report (i) these stars to follow a unique sequence of classes: O->WN->WO (i.e. without ever being WC). From our population synthesis with standard assumptions, we predict that (ii) the source of the UV C-IV and other emission bumps is a couple dozen WO-type Wolf-Rayet stars (not WC as previously assumed) which are the result of chem. hom. evolution, while these, combined with the rest of the O-star population, account for the He-II ionizing flux and spectral hardness. Contrasting our results against published optical and UV data and accounting for different aperture sizes and spatial regions probed by the observations, we find that (iii) our models are highly consistent with them. Since our "massive Pop II stars" might just as well exist in early star-forming regions, our findings have implications for upcoming JWST surveys; and given that our results apply for binary populations too as long as the same fraction (10%) of the systems evolves chem. homogeneously, we conclude that the stellar progenitors of gravitational waves may very well exist today in IZw18.

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arXiv2025

Polarization of light from fast rotating Wolf-Rayet stars: A Monte Carlo simulations compared to analytical formula

Fast-rotating Wolf-Rayet (WR) stars are potential progenitors of long gamma-ray bursts, but observational verification is challenging. Spectral lines from their expanding stellar wind obscure accurate rotational velocity measurements. Intrinsic polarization from wind rotation may help determine rotational speeds, requiring precise wind models. Our study aims to investigate the intrinsic polarization due to the rotational distortion of WR winds considering multiple scattering of photons and compare it to a single-scattering model, where we use an analytical expression of the polarization. We study the polarization signatures resulting from the prolate structure of rotating winds of two WR stars using a 3D Monte Carlo radiative transfer code Hyperion. We estimated the intrinsic polarization resulting from multiple scattering in WR winds for different rotational velocities, inclination angles, and mass-loss rates. Our results indicate that at a rotation rate of less than 50 % of the critical rate, the intrinsic polarization from multiple scattering is close to that of a single scattering model. However, at higher rotation velocities, the polarization from multiple scattering increases with inclination up to 40$^\circ$, while it decreases for inclinations higher than about 60$^\circ$. This dependence is inconsistent with the single-scattering model. We also discuss the effect of the mass-loss rate on the polarization and find that the polarization changes linearly with the mass-loss rate. However, it is important to note that the relationship between polarization and mass-loss rate may vary for different types of stars. The results have implications for future studies of stellar winds and mass loss and may help to improve our understanding of the complex environments of massive stars. Our research offers valuable information on the complex polarization patterns observed in stellar winds.

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arXiv2022

Progress towards a 3D Monte Carlo radiative transfer code for outflow wind modelling

Context: Radiative transfer modelling of expanding stellar envelopes is an important task in their analysis. To account for inhomogeneities and deviations from spherical symmetry, it is necessary to develop a 3D approach to radiative transfer modelling. Aims: We present a 3D Monte Carlo code for radiative transfer modelling, which is aimed to calculate the plasma ionisation and excitation state with the statistical equilibrium equations, moreover, to implement photon-matter coupling. As a first step, we present our Monte Carlo radiation transfer routines developed and tested from scratch. Methods: The background model atmosphere (the temperature, density, and velocity structure) can use an arbitrary grid referred to as the model grid (modGrid). The radiative transfer was solved using the Monte Carlo method in a Cartesian grid, referred to as the propagation grid (propgrid). This Cartesian grid was created based on the structure of the modgrid; correspondence between these two grids was set at the beginning of the calculations and then kept fixed. The propgrid can be either regular or adaptive; two modes of adaptive grids were tested. The accuracy and calculation speed for different propgrids was analysed. Photon interaction with matter was handled using the Lucy's macroatom approach. Test calculations using our code were compared with the results obtained by a different Monte Carlo radiative transfer code. Results: Our method and the related code for the 3D radiative transfer using the Monte Carlo and macroatom methods offer an accurate and reliable solution for the radiative transfer problem, and are especially promising for the inclusion and treatment of 3D inhomogeneities.

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arXiv2022

New hydrodynamic solutions for line-driven winds of hot massive stars using Lambert $W$-function

Hot massive stars present strong stellar winds that are driven by absorption, scattering and re\-emission of photons by the ions of the atmosphere (\textit{line-driven winds}). A better comprehension of this phenomenon, and a more accurate calculation of hydrodynamics and radiative acceleration is required to reduce the number of free parameters in spectral fitting, to determine accurate wind parameters such as mass-loss rates and velocity profiles. We use the non-LTE model-atmosphere code CMFGEN to numerically solve the radiative transfer equation in the stellar atmosphere and to calculate the radiative acceleration $g_\text{rad}(r)$. Under the assumption that the radiative acceleration depends only on the radial coordinate, we solve analytically the equation of motion by means of the Lambert $W$-function. An iterative procedure between the solution of the radiative transfer and the equation of motion is executed in order to obtain a final self-consistent velocity field that is no longer based on any $β$-law. We apply the Lambert-procedure to three O supergiant stars ($ζ$-Puppis, HD~165763 and $α$-Cam) and discuss the Lambert-solutions for the velocity profiles. It is found that, even without recalculation of the mass-loss rate, the Lambert-procedure allows the calculation of consistent velocity profiles that reduce the number of free parameters when a spectral fitting using CMFGEN is performed. Synthetic spectra calculated from our Lambert-solutions show significant differences compared to the initial $β$-law CMFGEN models. The results indicate the importance of consistent velocity profile calculation in the CMFGEN code and its usage in a fitting procedure and interpretation of observed spectra.

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arXiv2021

Spherically symmetric model atmospheres using approximate lambda operators V. Static inhomogeneous atmospheres of hot dwarf stars

Context. Clumping is a common property of stellar winds and is being incorporated to a solution of the radiative transfer equation coupled with kinetic equilibrium equations. However, in static hot model atmospheres, clumping and its influence on the temperature and density structures have not been considered and analysed at all to date. This is in spite of the fact that clumping can influence the interpretation of resulting spectra, as many inhomogeneities can appear there; for example, as a result of turbulent motions. Aims. We aim to investigate the effects of clumping on atmospheric structure for the special case of a static, spherically symmetric atmosphere assuming microclumping and a 1-D geometry. Methods. Static, spherically symmetric, non-LTE (local thermodynamic equilibrium) model atmospheres were calculated using the recent version of our code, which includes optically thin clumping. The matter is assumed to consist of dense clumps and a void interclump medium. Clumping is considered by means of clumping and volume filling factors, assuming all clumps are optically thin. Enhanced opacity and emissivity in clumps is multiplied by a volume filling factor to obtain their mean values. These mean values are used in the radiative transfer equation. Equations of kinetic equilibrium and the thermal balance equation use clump values of densities. Equations of hydrostatic and radiative equilibrium use mean values of densities. Results. The atmospheric structure was calculated for selected stellar parameters. Moderate differences were found in temperature structure. However, clumping causes enhanced continuum radiation for the Lyman-line spectral region, while radiation in other parts of the spectrum is lower, depending on the adopted model. The atomic level departure coefficients are influenced by clumping as well.

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arXiv2016

Spectroscopy of close visual binary components of the stable shell star 1 Del

Stable shell stars are ideal objects for studying basic physical principles of the formation of disks in Be stars. If these stars have a close unresolved visual companion, its contribution toward the modelling of the disk cannot be overlooked, as is sometimes done. The study aims to spectroscopically resolve close visual binary Be (shell) star 1 Del, which up to now was only resolved by speckle or micrometric measurements. The integral field spectroscopy obtained by the SINFONI spectrograph at the VLT telescope in the European Southern Observatory (ESO) in the infrared region was used; we supplemented these observations with visual spectroscopy with the Perek Telescope at the Ondrejov Observatory. Spectra of 1 Del were successfully resolved, and, for the first time, spectra of 1 Del B were obtained. We found that 1 Del A is a Be/shell star, while 1 Del B is not an emission-line object.

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