arXiv subjects

Yu. A. Fadeyev

Publications and source records attributed to Yu. A. Fadeyev.

At least 19 recordsLinked to original sources

arXiv2026

Evolutionary variations of superficial iron and calcium abundance in main sequence A stars

Main sequence stellar evolution models were computed together with solution of the equations of atomic diffusion for 16 elements from hydrogen to nickel. The grid of evolutionary tracks comprises the models with stellar masses ranged from 1.4 to $2.5M_\odot$ computed for initial helium and metal abundances $Y=0.28$ and $Z=0.02$, respectively. The calculations were done for the mass loss rates $10^{-15}M_\odot/\textrm{yr}\le\dot M\le 10^{-12}M_\odot/\textrm{yr}$ as well as for $\dot M=0$. The high superficial abundance of iron in Am stars is shown to be due to the radiative acceleration acting on the atoms of iron. The significantly smaller absorption coefficient of calcium is responsible for its gravitational settling and accumulation above its opacity maximum at $T\sim 10^6\:\textrm{K}$. Recover of the superficial calcium abundance is due to plunge of the outer convection zone bottom to layers with its excessive abundance. A significant role in the evolutionary variations of superficial abundances of chemical elements belongs to the intermediate convection zone arising for the first $\lesssim 300$ Myr due to accumulation of the atoms of iron and nickel in the layers with temperature $T\sim 2\times 10^5\:\textrm{K}$. In stars with mass $M\le 1.9M_\odot$ both the outer and intermediate convection zones merge due to evolutionary descend of the bottom of the outer convection zone so that overabundant iron and nickel are transported to the outer layers by convection. The merging of the convective zones is responsible for considerable variations of superficial abundances of calcium and iron with duration ranging from a quarter to a half of the main--sequence lifetime depending on the stellar mass. Therefore, Am stars as well as slowly rotating nonmagnetic A main--sequence stars have the common origin, whereas appearance of their chemical anomalies depend of the stellar mass and age.

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arXiv2025

Models of long-period variables of the globular cluster 47 Tuc

Stellar evolution computations were carried out for stars with a main sequence mass $M_\mathrm{ZAMS}=0.86M_\odot$ and initial metal abundance $Z=0.003$ and $Z=0.004$. Selected models of evolutionary sequences were used for calculation of radial pulsations in the RGB, eAGB and TP-AGB evolutionary stages. Not all pulsating red giants of the globular cluster 47 Tuc are shown to belong to the Mira variables because the lower limit of pulsation periods at the TP-AGB stage is $\approx 70$ day, whereas during the eAGB evolutionary stage the periods of radial oscillations range from $\approx 5$ to $\approx 40$ day. Periods and luminosities of hydrodynamic models of eAGB and TP--AGB pulsating stars locate along the common period-luminosity relation. Small masses of Mira variables in the globular cluster 47 Tuc ($0.54M_\odot\le M\le 0.70M_\odot$) is the main reason for irregular large-amplitude oscillations and the dynamical instability of outer stellar layers at pulsation periods $Π> 200$ day.

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arXiv2025

Elusive helium stars in the gap between subdwarfs and Wolf-Rayet stars~II. Nonlinear pulsations of stripped helium stars

It is shown for the first time that the stripped helium stars with masses 2 to 7 solar mass which are formed in close binary systems in the so-called case B of mass-exchange and retained low-mass hydrogen-helium envelopes, experience nonlinear radial pulsations. Pulsations are excited by the kappa-mechanism due to helium ionization. The region of pulsation instability extends over Hertzsprung-Russel diagram from the red giants branch to the region of effective temperatures from about 30,000 K to about 50,000 K. Variations of stellar luminosity should be observed mostly in the ultraviolet. The amplitudes of pulsations of the studied models reach 0.8 stellar magnitude and increase, as the stellar radii decrease. Pulsation periods of stars with effective temperatures exceeding 10,000 K range from 0.17 to 8.5 day and decrease with decreasing radii. The stars have substantially larger effective temperatures than their companions, which could be Be-stars. They are components of relatively wide binaries with orbital periods up to several years. The number of pulsating moderate-mass stripped helium stars in the Galaxy is about 1000.

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arXiv2024

Models of Mira variables of the Large Magellanic Cloud

Consistent stellar evolution and nonlinear radial stellar pulsation calculations were carried out for models of asymptotic giant branch stars with initial masses $1.5M_\odot\le M_\mathrm{ZAMS}\le 3M_\odot$ and initial metal abundance $Z=0.006$. All the models are shown to be either the fundamental mode or the first overtone pulsators. The lower limit of the first overtone period increases with increasing mass of the Mira model from $Π_{1,\min}\approx 80$ days for $M=1.3M_\odot$ to $Π_{1,\min}\approx 120$ days for $M=2.6M_\odot$. The upper limit of the first overtone period and lower limit of the fundamental mode period depend on the stellar structure during mode switching and range from $Π_{1,\max}=130$, $Π_{0,\min}=190$ days for $M=0.96M_\odot$ to $Π_{1,\max}=210$, $Π_{0,\min}=430$ days for $M=2.2M_\odot$. The slope of the theoretical period--luminosity relation of Mira variables perceptibly increases with decreasing $Z$. Fourier spectra of the kinetic energy of twelve hydrodynamic models show a split of the fundamental mode maximum into several equidistant components. Frequency intervals between split components fall within the range $0.03 \le Δν/ν_0 \le 0.1$. The superposition of radial oscillations with the fundamental mode splitting leads to the long-term amplitude variations with the cycle length from 10 to 30 times longer than the fundamental mode period. A more thorough analysis of hydrodynamic models is required for understanding the origin of the principal pulsation mode splitting.

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arXiv2024

Revision of calcium and scandium abundances in Am stars based on NLTE calculations and comparison with diffusion stellar evolution models

The homogeneous data sets for the calcium and scandium abundances accounting for departures from LTE were obtained for a sample of 54 metallic-line (Am) stars. The Ca and Sc abundances were found to correlate with effective temperature Teff, the abundance growth with increasing Teff being higher in stars with surface gravity log g < 4 than in those with log g > 4. No correlation was found between Ca or Sc abundances and the iron abundance or the velocity of axial rotation. Am stars exhibit on average the higher values of [Ca/H] than those of [Sc/H] as well as the abundance ratio [Ca/Sc] = 0.41 +/- 0.30. However, at Teff > 9500 K there is an allusion to the systematic difference between Am stars with surface gravity log g > 4 and log g < 4. The iron excess is nearly the same in the range 7200 K <= Teff <= 10030 K. Evolution diffusion models computed with the code MESA for stars with masses from 1.5 to 2Msun show the surface abundances that are in good agreement with Ca and Fe abundances observed in Am stars of the three open clusters with the age > 600 Myr. Additional mechanisms of chemical separation should be considered for explanation of the Am phenomenon in young stars of the Pleiades cluster. We tested the published diffusion stellar evolution models. The diffusion models by Richer et al. (2000) and Hui-Bon-Hoa et al. (2022) are shown to agree with observations of Am stars in the open clusters at large values of the free turbulence parameter: omega=1000 for Ca and Fe, omega=500 for Sc. There is no model with the mass and age of the Am-type star Sirius that could reproduce its surface abundances from He to Ni. The results presented in the paper may be of importance for understanding the chemical peculiarity of Am stars.

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arXiv2023

Theoretical period-radius and period-luminosity relations for Mira variables with solar metallicity

Evolutionary sequences of AGB stars with initial masses on the main sequence $M_\mathrm{ZAMS}=1.5M_\odot$, $2M_\odot$ and $3M_\odot$ were computed for the initial metallicity $Z=0.014$. Selected models of evolutionary sequences with envelopes under thermal equilibrium were used as initial conditions for calculation of nonlinear stellar pulsations. The hydrodynamic models of each evolutionary sequence are shown to concentrate along the continuous line in the period-radius and period-luminosity diagrams. The theoretical period-radius and period-luminosity relations differ from one another for different main-sequence star masses because the stellar luminosity of AGB stars depends on the degenerate carbon core mass which increases with increasing $M_\mathrm{ZAMS}$. In hydrodynamic models of evolutionary sequences $M_\mathrm{ZAMS}=2M_\odot$ and $M_\mathrm{ZAMS}=3M_\odot$ the periods of the first overtone pulsators are $86~\textrm{d}\leΠ\le 123~\textrm{d}$ and $174~\textrm{d}\leΠ\le 204~\textrm{d}$, whereas all models of the evolutionary sequence $M_\mathrm{ZAMS}=1.5M_\odot$ oscillate in the fundamental mode. Fairly regular radial oscillations exist in stars with pulsation periods $Π\lesssim 500$ d. In models with longer periods the amplitude rapidly increases with increasing $Π$ and oscillations become irregular.

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arXiv2021

Effects of metallicity on mode switching in Cepheids

The mode switching in Cepheids is studied using the methods of the nonlinear theory of stellar pulsation, depending on the main sequence mass $M_0$ and the abundance of elements heavier than helium $Z$. The grid of evolutionary and hydrodynamic models of core--helium burning Cepheids is represented by 30 evolutionary sequences of stars with initial masses $5.7M_\odot\le M_0\le 7.2M_\odot$ and $Z=0.014$, 0.018, 0.022. For considered values of $Z$ the periods of the fundamental mode and the first overtone at the oscillation mode switching are shown to depend on the mean density of the stellar matter. The upper limit of the period of the first overtone decreases witn increasing $Z$ from $\approx 6.9$ day for $Z=0.014$ to $\approx 4.1$ day for $Z=0.022$. The theoretical period--radius relation is independent of $Z$ and agrees well (within 2.5\%) with recent measurements of Cepheid radii based on the Baade--Wesselink method. The fundamental parameters of the short--period Cepheid CG Cas were derived with application of observational estimates of the period and the rate of period change. This star is shown to be the first--overtone pulsator.

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arXiv2020

Evolutionary status of W Vir pulsating variables

Stellar evolution calculations for population II stars with initial composition $Y_0=0.25$, $Z_0=10^{-3}$ and the initial stellar mass $M_0 = 0.82M_\odot$ were carried out from the main sequence to the white dwarf stage. Twelve AGB and post--AGB evolutionary sequences were computed with different values of the parameter in the Blocker mass loss rate formula ($0.01\leη_B\le 0.12$). Selected models of evolutionary sequences with masses $M=0.536M_\odot$, $0.530M_\odot$ and $0.526M_\odot$ that experience the loop in the Hertzsprung--Russel diagram due to the final helium flash were used as initial conditions for solution of the equations of hydrodynamics describing radial stellar oscillations. The region of instability to radial fundamental mode pulsations is shown to extend from the asymptotic giant branch to effective temperatures as high as $T_\mathrm{eff}\approx 6\times 10^3$ K. Pulsation periods of hydrodynamic models are in the range from 15 to 50 day and agree with periods of W~Vir pulsating stars. The models of intermediate spectral type fundamental mode pulsators with periods $Π> 50$ day locate in the upper part of the Hertzsprung--Russel diagram in the region of semiregular pulsating variables. We conclude that W~Vir pulsating variables are the low--mass post--AGB stars that experience the final helium flash.

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arXiv2020

Evolutionary status and fundamental parameters of the Cepheid V1033 Cyg

On the basis of consistent stellar evolution and nonlinear stellar pulsation calculations the Cepheid V1033 Cyg is shown to be the post--main sequence star at the first crossing of the instability strip during gravitational contraction of the helium core. The observed light variability of V1033 Cyg is due to radial oscillations in the fundamental mode. The best agreement (within one percent) between recent observations and the theoretical estimate of the period change rate was obtained for the evolutionary sequence with stellar mass $M=6.3M_\odot$ and helium and heavier element fractional abundances $Y=0.28$ and $Z=0.022$, respectively. The age of the star, the luminosity, the radius, the effective temperature and the surface gravity are $t_\mathrm{ev}=5.84\times 10^7$ yr, $L=2009L_\odot$, $R=45.6R_\odot$, $T_\mathrm{eff}=5726 \mathrm{K}$, $\log g=1.92$.

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arXiv2020

Evolutionary and hydrodynamic models of short--period Cepheids

The evolutionary calculations for population I stars with masses on the main sequence 5 M_\odot <= M_0 <= 6.1 M_\odot and initial fractional abundances of helium Y_0=0.28 and heavier elements Z_0=0.02 were carried out to the stage of central helium exhaustion. Selected core helium--burning models were used as initial conditions for solution of the equations of hydrodynamics and time--dependent convection describing radial pulsations of Cepheids. In the Hertzsprung--Russel diagram the evolutionary tracks are shown to cross the red edge of the instability strip for M_0 > 5.1M_\odot. The grid of hydrodynamic models of core helium--burning Cepheids on the stages of the second and the third crossings of the instability strip was computed. The pulsation period Πand the rate of period change \dotΠwere determined as a continuous function of star age for each evolutionary sequence of first--overtone pulsators. Results of calculations agree with observational estimates of \dotΠrecently obtained for the short--period Cepheids V532 Cyg, BG Cru and RT Aur.

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arXiv2019

Radial pulsations of stars on the stage of the final helium flash

Stellar evolution calculations to the stage of the cooling white dwarf were done for population~I stars with masses on the main sequence $1M_\odot\le M_0\le 1.5M_\odot$. The final helium flash LTP is shown to occur in post--AGB stars with initial masses $1.3M_\odot\le M_0\le 1.32M_\odot$ for the overshooting parameter $f=0.016$. In the case of more effective overshooting ($f=0.018$) the final helium flash occurs at initial masses $1.28M_\odot\le M_0\le 1.3M_\odot$. Fivefold variations of the parameter responsible for the mass loss rate during the post--AGB stage do not affect occurrence of the final helium flash but lead to perceptible changes of the evolutionary time. Selected models of two evolutionary sequences with initial mass $M_0 = 1.3M_\odot$ computed with overshooting parameters $f=0.016$ and $f=0.018$ were used as initial conditions in solution of the equations of hydrodynamics describing radial oscillations of stars on the stage of the final helium flash at effective temperatures less than $10^4$ K. The maximum pulsation period $Π=117$ day determined for the evolutionary sequence $M_0=1.3M_\odot$, $f=0.016$ is in a good agreement with observational estimates of the period of FG Sge. The mass, the radius and the effective temperature of the star are $M=0.565M_\odot$, $R=126R_\odot$ and $T_\mathrm{eff}=4445$ K, respectively. At the same time the average period change rate of FG Sge from 1960 to 1990 is nearly three time larger than its theoretical estimate.

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arXiv2019

Evolution and pulsations of population I post-AGB stars

Evolutionary calculations of population I stars with initial masses $M_0=1M_\odot$, $1.5M_\odot$ and $2M_\odot$ were carried out up to the stage of the proto--planetary nebula. Selected models of post--AGB evolutionary sequences with effective temperatures $3.6\times 10^3\,\mathrm{K}\lesssim T_\mathrm{eff}\lesssim 2\times 10^4\,\mathrm{K}$ were used as initial conditions in calculations of self--escited stellar oscillations. For the first time the sequences of hydrodynamic models of radially pulsating post--AGB stars were computed using the self--consistent solution of the equations of radiation hydrodynamics and time--dependent convection. Within this range of effective temperatures the post--AGB stars are the fundamental mode pulsators with period decreasing as the star evolves from $Π\approx 300$ day to several days. Period fluctuations are due to nonlinear effects and are most prominent at effective temperatures $T_\mathrm{eff} < 5000$K. The amplitude of bolometric light variations is $ΔM_\mathrm{bol}\approx 1$ at $T_\mathrm{eff} \lesssim 6000$K and rapidly decreases with increasing $T_\mathrm{eff}$. The theoretical dependence of the pulsation period as a function of effective temperature obtained in the study can be used as a criterion for the evolutionary status of pulsating variables suspected to be post--AGB stars.

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arXiv2019

On period distribution of RR Lyr type variables in the globular cluster M3

Evolutionary calculations of population II stars with chemical composition of the globular cluster M3 were carried out under various assumptions about the initial stellar mass ($0.809M_\odot\le M_\mathrm{ZAMS} \le 0.83M_\odot$) and the mass loss rate parameter in the Reimers formula ($0.45\leη_\mathrm{R}\le 0.55$). In general, 30 evolutionary tracks of the horizontal branch stars were computed. Selected models of evolutionary sequences were used as initial conditions for solution of the equations of hydrodynamics that describe radial stellar oscillations. Hydrodynamic models of RR Lyr type stars were computed for the core helium burning stage as well as for the preceding pre--ZAHB stage. Analytic relations for the effective temperature of the instability strip edges as a function of stellar luminosity are obtained. Theoretical histograms of the period distribution of RR Lyr type variables were produced for each evolutionary sequence using Monte--Carlo simulations based on the consistent stellar evolution and nonlinear stellar pulsation calculations. A satisfactory agreement with observations (i.e. the greater number of RRab variables) was found for the evolutionary sequence $M_\mathrm{ZAMS} = 0.811M_\odot$, $η_\mathrm{R}=0.55$ with the number fraction of fundamental mode pulsators $\approx 75\%$. At the same time the mean period of fundamental mode pulsators ($\langleΠ\rangle_0=0.79$ day) is substantially greater compared to the observational estimate of $\langleΠ\rangle_\mathrm{ab}$.

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arXiv2018

Secular perion changes and fundamental parameters of long--period Cepheids

Hydrodynamic computations of nonlinear Cepheid pulsation models with periods from 20 to 100 day on the evolutionary stage of core helium burning were carried out. Equations of radiation hydrodynamics and time--dependent convection were solved with initial conditions obtained from selected models of evolutionary sequences of population I stars with initial masses from $8M_\odot$ to $12.5M_\odot$. For each crossing of the instability strip the pulsation period $Π$ and the rate of period change $\dotΠ$ were derived as a function of evolutionary time. Comparing results of our computations with observational estimates of $Π$ and $\dotΠ$ we determined fundamental parameters (the age, the mass, the luminosity and the radius) of seven long--period Cepheids. Theoretical estimates of the stellar radius are shown to agree with radius measurements by the Baade--Wesselink technique within 3\% for RS~Pup and GY~Sge whereas for SV~Vul the disagreement between theory and observations does not exceed 10\%.

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arXiv2018

Evolution and period change in RR Lyr variables of the globular cluster M 3

The grid of evolutionary tracks of population II stars with initial masses from $0.81M_\odot$ to $0.85M_\odot$ and chemical composition of the globular cluster M 3 is computed. Selected models of horizontal branch stars were used as initial conditions for solution of the equations of radiation hydrodynamics and time--dependent convection describing radial stellar oscillations. The boundaries of the instability strip on the Herztsprung--Russel diagram were determined using nearly 100 hydrodynamic models of RR Lyr pulsating variables. For each evolutionary track crossing the instability strip the pulsation period was determined as a function of evolutionary time. The rate of period change of most variables is shown to range within $-0.02\le\dotΠ\le 0.05~\mathrm{day}/10^6\mathrm{yr}$. Theoretical estimate of the mean period change rate obtained by the population synthesis method is $\langle\dotΠ\rangle=6.0\times 10^{-3}~\mathrm{day}/10^6\mathrm{yr}$ and agrees well with observations of RR Lyr variables of the globular cluster M 3.

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arXiv2017

Radial pulsations of red giant branch stars

We performed hydrodynamic computations of nonlinear stellar pulsations of population I stars at the evolutionary stages of the ascending red giant branch and the following luminosity drop due to the core helium flash. Red giants populating this region of the Hertzsprung--Russel diagram were found to be the fundamental mode pulsators. The pulsation period is the largest at the tip of the red giant branch and for stars with initial masses from 1.1M_\odot to 1.9M_\odot ranges from 254 day to 33 day, respectively. The rate of period change during the core helium flash is comparable with rates of secular period change in Mira type variables during the thermal pulse in the helium shell source. The period change rate is largest (\dotΠ/Π\approx -0.01 yr^{-1}) in stars with initial mass Mzams=1.1M_\odot and decreases to \dotΠ/Π\sim -0.001\ yr^{-1} for stars of the evolutionary sequence Mzams=1.9M_\odot. Theoretical light curves of red giants pulsating with periods Pi > 200 day show the presence of the secondary maximum similar to that observed in many Miras.

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arXiv2017

Pulsations of intermediate-mass stars on the asymptotic giant branch

Evolutionary tracks from the zero age main sequence to the asymptotic giant branch were computed for stars with initial masses 2M_\odot <= Mzams <= 5M_\odot and metallicity Z=0.02. Some models of evolutionary sequences were used as initial conditions for equations of radiation hydrodynamics and turbulent convection describing radial stellar pulsations. The early asymptotic giant branch stars are shown to pulsate in the fundamental mode with periods from 30 to 400 day. The rate of period change gradually increases as the star evolves but is too small to be detected (d ln P/dt < 1.e-5 yr^-1). Pulsation properties of thermally pulsing AGB stars are investigated on time intervals comprising 17 thermal pulses for evolutionary sequences with intial masses Mzams=2M_\odot and 3M_\odot and 6 thermal pulses for Mzams=4M_\odot and 5M_\odot. TP-AGB stars with initial masses Mzams<=3M_\odot pulsate either in the fundamental mode or in the first overtone, whereas more massive red giants (Mzams>=4M_\odot) pulsate in the fundamental mode with periods as long as 1.e3 day. Most rapid pulsation period change with rate -0.02 < d ln P/dt < -0.01 yr^-1 occurs during decrease of the surface luminosity after the maximum helium luminosity. The rate of subsequent increase of the period is d ln P/dt <= 5.e-3 yr^-1.

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arXiv2015

Evolutionary status of Polaris

Hydrodynamic models of short--period Cepheids were computed to determine the pulsation period as a function of evolutionary time during the first and third crossings of the instability strip. The equations of radiation hydrodynamics and turbulent convection for radial stellar pulsations were solved with the initial conditions obtained from the evolutionary models of population I stars (X=0.7, Z=0.02) with masses from 5.2 to 6.5 Msol and the convective core overshooting parameter 0.1 <= aov <= 0.3. In Cepheids with period of 4 d the rate of pulsation period change during the first crossing of the instability strip is over fifty times larger than that during the third crossing. Polaris is shown to cross the instability strip for the first time and to be the fundamental mode pulsator. The best agreement between the predicted and observed rates of period change was obtained for the model with mass of 5.4 Msol and the overshooting parameter aov=0.25. The bolometric luminosity and radius are L = 1.26e3 Lsol and R = 37.5 Rsol, respectively. In the HR diagram Polaris is located at the red edge of the instability strip.

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