arXiv subjects

Petr Kurfürst

Publications and source records attributed to Petr Kurfürst.

15 recordsLinked to original sources

arXiv2026

What is the reason for the lack of cool evolved stars near the center of the Galaxy?

The observed deficit of bright late-type giants in the central parsec of the Milky Way remains an open problem. We investigate whether repeated passages of red giants (RGs) through a past jet from Sgr$~$A$^*$ can modify their envelopes and apparent spectral types. Three-dimensional hydrodynamical simulations follow a $1\,M_\odot$, $100\,R_\odot$ RG through up to ten jet crossings at $10^{-3}\,\text{pc}$, using jet kinetic luminosities of $10^{42}$, $10^{44}$, and $10^{48}\,\text{erg}\,\text{s}^{-1}$. Each passage produces shocks, envelope ablation, and an asymmetric downstream tail. For jet luminosities up to $10^{44}\,\text{erg}\,\text{s}^{-1}$, the cumulative ablated mass evolves approximately as $ΔM\propto t^{1/2}$ and reaches about $10^{-4}\,M_\odot$ over a $10^5\,\text{yr}$ active phase. Repeated heating also raises the surface temperature from about $3600$ to $8500\,\text{K}$ during the first ten passages, potentially making an M-type giant appear as an A-type source. Jet feedback may therefore contribute to the apparent depletion of cool giants and the excess of hot stars in galactic nuclei (GN).

astro-ph.HE↗

arXiv2026

Evolution of Quasi-Periodic Eruptions in the post-TDE Accretion Disk Perturbed by an Orbiting Star

Quasi-periodic eruptions (QPEs) are a recently discovered class of highly variable X-ray bursts originating in galactic nuclei. These high-amplitude bursts exhibit periodicity ranging from tens of minutes to several days. QPEs are also characterized by variable peak amplitudes that can vary by a factor of few. While multiple physical models have been proposed to explain QPE light curves, none can fully account for all the observed features. A possible connection between QPEs and tidal disruption events (TDEs) has been suggested, particularly due to the past optical/UV outbursts that can be traced back for several sources, the long-term decay in the continuum luminosity, and the soft, thermal-dominated X-ray spectrum. Our primary goal is to verify whether the long-term decrease in eruption amplitudes detected for some QPE sources is consistent with the accretion disk being formed following a TDE. In this work, we adopt a simplified extreme mass ratio inspiral (EMRI) scenario, where a Solar-type star orbits a supermassive black hole (SMBH) and collides with an accretion disk twice per orbit, generating eruptions. We assume a post-TDE disk that follows a temporal power-law decline in mass accretion ($\propto t^{-p}$, $p>0$). As our aim is to develop a toy-model scenario, we have used purely analytical methods without considering all intervening processes in their full generality. Indications are that (i) the observed long-term decline in QPE amplitudes can be reproduced if the first monitored epoch occurs years to a few decades after the tidal disruption, (ii) stellar mass loss caused by ablation can play an important role in the evolution of QPE amplitudes in systems with heavy main-sequence (MS) stars.

astro-ph.HE↗

arXiv2026

Tidal disruption of a low-mass star in an active galactic nucleus as the origin of the PS16dtm outburst

The event PS16dtm, which occured in the center of the Narrow Line Seyfert 1 (NLS1) galaxy SDSS J015804.75-005221.8 (z = 0.080440), is one of the few candidates for a tidal disruption event in an already-acretting active galactic nucleus (AGN). We aim to shed light on the character of the tidal disruption event in this source since it exhibits unusual peculiarities, such as the double-peak optical/UV light curve and a low blackbody temperature with a lack of X-ray emission. We perform spectral analysis of the source before and during the event. We model the time evolution of the luminosity profile using a numerical code that describes the viscous evolution of the flow. From the combined spectral and timing studies, we interpret the event as the disruption of a $\sim 0.3 M_{\odot}$ main-sequence star, or gradual partial disruption of the low-mass giant star. The star is likely on a circular orbit, embedded in the accretion disc. The discussion of the evolution of the star rather suggests that the orbit is counter-rotating. We observe the system at a sufficiently large viewing angle that the actual disruption process is not directly observed. The disrupted star and inner disc are shielded from the observer by a gaseous envelope. Further observations of the system returning to the previous NLS1 state, particularly in the X-ray band, are needed to confirm the proposed scenario and to put constraints on the return to a regular NLS1 state.

astro-ph.HE↗

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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arXiv2025

High-cadence observations of galactic nuclei by the future two-band UV-photometry mission QUVIK

The Quick Ultra-VIolet Kilonova surveyor (QUVIK), a two-band UV space telescope approved for funding as a Czech national science and technology mission, will focus on detecting early UV light of kilonovae (Werner et al., 2024). In addition, it will study the UV emission of stars and stellar systems (Krtička et al., 2024) as well as the intense and variable emission of active galactic nuclei (AGN) or galactic nuclei activated by tidal disruption events (Zajaček et al., 2024). In this contribution, we describe the role of this small ($\sim 30$-cm diameter) UV telescope for studying bright, nearby AGN. With its NUV and FUV bands, the telescope will perform high-cadence ($\sim 0.1$-$1$ day) two-band photometric monitoring of nearby AGN ($z<1$), which will allow us to probe accretion disk sizes/temperature profiles via photometric reverberation mapping. Thanks to its versatility, QUVIK will be able to perform a moderately fast repointing ($<20$ min) to target candidates for tidal disruption events (TDEs). Early detection of the UV emission following a TDE optical flare, in combination with the subsequent two-band UV monitoring performed simultaneously with other observatories, will enable us to infer the time delay (or its lack of) between the optical, UV, and X-ray emission. In combination with theoretical models, it will be possible to shed more light on the origin of the UV/optical emission of TDEs. Furthermore, the two-band monitoring of nuclear transients will be beneficial in distinguishing between TDEs (nearly constant blue colour) and supernovae (progressive reddening) in the era of intensive wide-field surveys.

astro-ph.GA↗

arXiv2025

Red giant - jet collisions in galactic nuclei I: 3D hydrodynamical model of a few stellar orbits

Several models have been proposed to explain missing red giants (RGs) near the Galactic centre. Recently, a scenario has been suggested that predicts, among other processes, a long-term ablation of the surface layers of RGs during their repetitive passages through the Galactic jet (Zajaček et al., 2020). In this study, we perform detailed three-dimensional numerical modelling of this phenomenon. We calculate the ablation rate of the surface layers of a RG orbiting the supermassive black hole (SMBH) as it passes through the nuclear jet. In particular, we model the jet-star interaction for approximately 10 passages for the closer orbital distance of $10^{-3}\,\text{pc}$ and 2 passages for $10^{-2}\,\text{pc}$. We find that the mass loss due to ablation by the jet behaves with time as $ΔM_{\star}\propto \sqrt{t}$ and the total ablated mass during a single active galactic nucleus (AGN) phase ($10^5$ years) is $\sim 10^{-4}\,M_{\odot}$. We arrive at similar rates of the stellar ablation for the relatively smaller jet luminosity $10^{42}\,\text{erg}\,\text{s}^{-1}$ as in the previous analytical calculations. For larger jet luminosities of $10^{44}$ and $10^{48}\,\text{erg}\,\text{s}^{-1}$, the ablation rates inferred from $\sim 10$ interactions as well as extrapolated power-law fits are significantly lower than analytical values. Overall, the mass ablation rate per interaction and the extrapolated cumulative mass loss during the jet activity are comparable to the stellar-wind mass loss. For the smallest orbital distance of $10^{-3}\,\text{pc}$, we also track the thermal behaviour of the stellar surface layer, whose temperature appears to grow rapidly during the first 10 passages from $\sim 3600\,{\rm K}$ (spectral type M) to $\sim 8500\,{\rm K}$ (spectral type A). RG-jet interactions can thus lead to observable changes in the nuclear stellar population during the jet existence.

astro-ph.HE↗

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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arXiv2025

Revealing EMRI/IMRI candidates with quasiperiodic ultrafast outflows

The first detection of the quasiperiodic ultrafast outflow in the ASASSN-20qc system was reported by Pasham et al. (2024). The outflow is revealed in the soft X-ray spectra as an absorption feature, which is enhanced periodically every $\sim 8.3$ days. The repetitive nature of the ultrafast outflow is tentatively explained by an orbiting massive perturber, possibly an intermediate-mass black hole (IMBH), trajectory of which is inclined with respect to the accretion flow around the primary supermassive black hole (SMBH). In this scenario, the orbiting body pushes the disc gas into the outflow funnel, where it is accelerated by the ordered magnetic field (Suková et al. 2021). Quasiperiodic ultrafast outflows (a.k.a. QPOuts) are thus a novel phenomenon that can help reveal new extreme-/intermediate-mass ratio inspiral (EMRI/IMRI) candidates. These then would be prime candidate sources for a simultaneous detection and monitoring in electromagnetic as well as gravitational wave domains.

astro-ph.HE↗

arXiv2024

Models of hot star decretion disks

Massive stars can during their evolution reach the phase of critical (or very rapid, near-critical) rotation when further increase in rotation rate is no longer kinematically allowed. The mass ejection and angular momentum outward transport from such rapidly rotating star's equatorial surface may lead to formation and supports further existence of a circumstellar outflowing (stellar decretion) disk. The efficient mechanism for the outward transport of the mass and angular momentum is provided by the anomalous viscosity. The outer supersonic regions of the disks can extend up to a significantly large distance from the parent star, the exact radial extension is however basically unknown, partly due to the uncertainties in radial variations of temperature and viscosity. We study in detail the behavior of hydrodynamic quantities, i.e., the evolution of density, radial and azimuthal velocity, and angular momentum loss rate in stellar decretion disks out to extremely distant regions. We investigate the dependence of these physical characteristics on the distribution of temperature and viscosity. We also study the magnetorotational instability, which we regard to be the source of anomalous viscosity in such outflowing disks and to some extent we provide the preliminary models of the two-dimensional radially-vertically correlated distribution of the disk density and temperature. We developed our own two-dimensional hydrodynamic and magnetohydrodynamic numerical code based on an explicit Eulerian finite difference scheme on staggered grid, including full Navier-Stokes viscosity. We use semianalytic approach to investigate the radial profile of magnetorotational instability, where on the base of the numerical time-dependent hydrodynamic model we analytically study the stability of outflowing disks submerged to the magnetic field of central star.

astro-ph.SR↗

arXiv2021

Supernovae in colliding-wind binaries: observational signatures in the first year

When a core-collapse supernova explodes in a binary star system, the ejecta might encounter an overdense shell, where the stellar winds of the two stars previously collided. In this work, we investigate effects of such interactions on supernova light curves on time-scales from the early flash ionization signatures to approximately one year after the explosion. We construct a model of the colliding-wind shell in an orbiting binary star system and we provide an analytical expression for the shell thickness and density, which we calibrate with three-dimensional adaptive mesh refinement hydrodynamical simulations probing different ratios of wind momenta and different regimes of radiative cooling efficiency. We model the angle-dependent interaction of supernova ejecta with the circumstellar medium and estimate the shock radiative efficiency with a realistic cooling function. We find that the radiated shock power exceeds typical Type IIP supernova luminosity only for double red supergiant binaries with mass ratios $q \gtrsim 0.9$, wind mass-loss rates $\dot{M} \gtrsim 10^{-4} M_\odot\,\text{yr}^{-1}$, and separations between about 50 and 1500 AU. The required $\dot{M}$ increases for binaries with smaller $q$ or primaries with faster wind. We estimate that $\ll 1\%$ of all collapsing massive stars satisfy the conditions on binary mass ratio and separation. Recombination luminosities due to colliding wind shells are at most a factor of 10 higher than for an otherwise unperturbed constant-velocity wind, but higher densities associated with wind acceleration close to the star provide much stronger signal.

astro-ph.HE↗

arXiv2019

Modeling of interactions between supernovae ejecta and aspherical circumstellar environments

Massive stars are characterized by a significant loss of mass either via spherically symmetric stellar winds or pre-explosion pulses, or by aspherical forms of circumstellar matter (CSM) such as bipolar lobes or outflowing circumstellar equatorial disks. A significant fraction of most massive stars end their lives by a core collapse; supernovae (SNe) are always located inside circumstellar envelopes created by their progenitors. We study the dynamics and thermal effects of collision between expanding ejecta of SNe and CSM that may be formed during, for example, a sgB[e] star phase, a luminous blue variable phase, around PopIII stars, or by various forms of accretion. For time-dependent hydrodynamic modeling we used our own code built with a finite volumes method. The code is highly efficient for calculations of shocks and physical flows with large discontinuities. The initial geometry of the disks corresponds to a density structure of a material that orbits in Keplerian trajectories. We examine the behavior of the density, pressure, velocity of expansion, and temperature structure in the interaction zone under various geometrical configurations of dense equatorial disks. Our `low density' model shows significant asphericity in the case of the disk mass-loss rate $\dot{M}_\text{csd}=10^{-6}\,M_\odot\,\text{yr}^{-1}$. The models also show the zones of overdensity in the SN - disk contact region and indicate the development of Kelvin-Helmholtz instabilities within the zones of shear between the disk and the more freely expanding material outside the disk.

astro-ph.SR↗

arXiv2017

Two-dimensional modeling of density and thermal structure of dense circumstellar outflowing disks

Context. Evolution of massive stars is affected by a significant loss of mass either via (nearly) spherically symmetric stellar winds or by aspherical mass-loss mechanisms, namely the outflowing equatorial disks. However, the scenario that leads to the formation of a disk or rings of gas and dust around massive stars is still under debate. Aims. We study the hydrodynamic and thermal structure of optically thick, dense parts of outflowing circumstellar disks that may be formed around various types of critically rotating massive stars, for example, Be stars, B[e] supergiant (sgB[e]) stars or Pop III stars. Methods. We specify the optical depth of the disk along the line-of-sight from stellar poles. Within the optically thick dense region we calculate the vertical disk thermal structure using the diffusion approximation while for the optically thin outer layers we assume a local thermodynamic equilibrium with the impinging stellar irradiation. We use two of our own types of hydrodynamic codes: two-dimensional operator-split numerical code and unsplit code based on the Roe's method. Results. Our models show the geometric distribution and contribution of viscous heating that begins to dominate in the central part of the disk. In the models of dense viscous disks the viscosity increases the central temperature up to several tens of thousands of Kelvins. The high mass-loss rates and high viscosity lead to instabilities with significant waves or bumps in density and temperature in the very inner disk region. Conclusions. The two-dimensional radial-vertical models of dense outflowing disks including the full Navier-Stokes viscosity terms show very high temperatures that are however limited to only the central disk cores inside the optically thick area, while near the edge of the optically thick region the temperature may be low enough for the existence of neutral hydrogen.

astro-ph.SR↗

arXiv2017

A search for peculiar stars in the open cluster Hogg 16

The study of chemically peculiar (CP) stars in open clusters provides valuable information about their evolutionary status. Their detection can be performed using the $Δa$ photometric system, which maps a characteristic flux depression at $λ\sim 5200$ Å. This paper aims at studying the occurrence of CP stars in the earliest stages of evolution of a stellar population by applying this technique to Hogg 16, a very young Galactic open cluster (about 25 Myr). We identified several peculiar candidates: two B-type stars with a negative $Δa$ index (CD-60 4701, CPD-60 4706) are likely emission-line (Be) stars, even though spectral measurements are necessary for a proper classification of the second one; a third object (CD-60 4703), identified as a Be candidate in literature, appears to be a background B-type supergiant with no significant $Δa$ index, which does not rule out the possibility that it is indeed peculiar as the normality line of $Δa$ for supergiants has not been studied in detail yet. A fourth object (CD-60 4699) appears to be a magnetic CP star of 8 M$_\odot$, but obtained spectral data seem to rule out this hypothesis. Three more magnetic CP star candidates are found in the domain of early F-type stars. One is a probable nonmember and close to the border of significance, but the other two are probably pre-main sequence cluster objects. This is very promising, as it can lead to very strong constraints to the diffusion theory. Finally, we derived the fundamental parameters of Hogg 16 and provide for the first time an estimate of its metal content.

astro-ph.SR↗

arXiv2014

Time-dependent modeling of extended thin decretion disks of critically rotating stars

During their evolution massive stars can reach the phase of critical rotation when a further increase in rotational speed is no longer possible. Direct centrifugal ejection from a critically or near-critically rotating surface forms a gaseous equatorial decretion disk. Anomalous viscosity provides the efficient mechanism for transporting the angular momentum outwards. The outer part of the disk can extend up to a very large distance from the parent star. We study the evolution of density, radial and azimuthal velocity, and angular momentum loss rate of equatorial decretion disks out to very distant regions. We investigate how the physical characteristics of the disk depend on the distribution of temperature and viscosity. We calculated stationary models using the Newton-Raphson method. For time-dependent hydrodynamic modeling we developed the numerical code based on an explicit finite difference scheme on an Eulerian grid including full Navier-Stokes shear viscosity. The sonic point distance and the maximum angular momentum loss rate strongly depend on the temperature profile and are almost independent of viscosity. The rotational velocity at large radii rapidly drops accordingly to temperature and viscosity distribution. The total amount of disk mass and the disk angular momentum increase with decreasing temperature and viscosity. The time-dependent one-dimensional models basically confirm the results obtained in the stationary models as well as the assumptions of the analytical approximations. Including full Navier-Stokes viscosity we systematically avoid the rotational velocity sign change at large radii. The unphysical drop of the rotational velocity and angular momentum loss at large radii (present in some models) can be avoided in the models with decreasing temperature and viscosity.

astro-ph.SR↗

arXiv2013

Models of extended decretion disks of critically rotating stars

Massive stars may during their evolution reach the phase of critical rotation when the further increase in rotational speed is no longer possible. The ejection of matter in the equatorial region forms the gaseous outflowing disk, which allows the star to remove the excess of its angular momentum. The outer part of the disk can extend up to large distance from the parent star. We study the evolution of density, radial and azimuthal velocity and angular momentum loss rate of equatorial decretion disk up to quite distant outer regions. We calculate the evolution of density, radial and azimuthal velocity from the initial Keplerian state until the disk approaches the final stationary state.

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