arXiv subjects

Yu. A. Fadeyev

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

25 records · Page 2Linked to original sources

arXiv2009

Instability of LBV-stars against radial oscillations

In this study we consider the nonlinear radial oscillations exciting in LBV--stars with effective temperatures 1.5e4 K <= Teff <= 3e4 K, bolometric luminosities 1.2e6 L_odot <= L <= 1.9e6 L_odot and masses 35.7 M_odot <= M <= 49.1 M_odot. Hydrodynamic computations were carried out with initial conditions obtained from evolutionary sequences of population I stars (X=0.7, Z=0.02) with initial masses from 70M_odot to 90 M_odot. All hydrodynamical models show instability against radial oscillations with amplitude growth time comparable with dynamical time scale of the star. Radial oscillations exist in the form of nonlinear running waves propagating from the boundary of the compact core to the upper boundary of the hydrodynamical model. The velocity amplitude of outer layers is of several hundreds of km/s while the bolometric light amplitude does not exceed 0.2 mag. Stellar oscillations are not driven by the kappa-mechanism and are due to the instability of the gas with adiabatic exponent close to the critical value Gamma_1 = 4/3 due to the large contribution of radiation in the total pressure. The range of the light variation periods (6 day <= P <= 31 day) of hydrodynamical models agrees with periods of microvariability observed in LBV--stars.

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arXiv2008

Mass-luminosity relation and pulsational properties of Wolf-Rayet stars

Evolution of Population I stars with initial masses from 70M_\odot to 130M_\odot is considered under various assumptions on the mass loss rate \dot M. The mass-luminosity relation of W-R stars is shown to be most sensitive to the mass loss rate during the helium burning phase \dot M_{3α}. Together with the mass-luminosity relation obtained for all evolutionary sequences several more exact relations are determined for the constant ratio f_{3α}=\dot M/\dot M_{3α} with 0.5 \le f_{3α} \le 3. Evolutionary models of W-R stars were used as initial conditions in hydrodynamic computations of radial nonlinear stellar oscillations. The oscillation amplitude is larger in W-R stars with smaller initial mass or with lower mass loss rate due to higher surface abundances of carbon and oxygen. In the evolving W-R star the oscillation amplitude decreases with decreasing stellar mass M and for M < 10M_\odot the sufficiently small nonlinear effects allow us to calculate the integral of the mechanical work W done over the pulsation cycle in each mass zone of the hydrodynamical model. The only positive maximum on the radial dependence of W is in the layers with temperature of T\sim 2e5K where oscillations are excited by the iron Z--bump kappa-mechanism. Radial oscillations of W-R stars with mass of M > 10M_\odot are shown to be also excited by the kappa-mechanism but the instability driving zone is at the bottom of the envelope and pulsation motions exist in the form of nonlinear running waves propagating outward from the inner layers of the envelope.

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arXiv2004

The structure of radiative shock waves. V. Hydrogen emission lines

We considered the structure of steady-state plane-parallel radiative shock waves propagating through the partially ionized hydrogen gas of temperature T_1 = 3000K and density 1e-12 gm/cm^3 <= ρ_1 <= 1e-9 gm/cm^3. The upstream Mach numbers range within 6 <= M_1 <= 14. In frequency intervals of hydrogen lines the radiation field was treated using the transfer equation in the frame of the observer for the moving medium, whereas the continuum radiation was calculated for the static medium. Doppler shifts in Balmer emission lines of the radiation flux emerging from the upstream boundary of the shock wave model were found to be roughly one-third of the shock wave velocity. The gas emitting the Balmer line radiation is located at the rear of the shock wave in the hydrogen recombination zone where the gas flow velocity in the frame of the observer is approximately one-half of the shock wave velocity. The ratio of the Doppler shift to the gas flow velocity of 0.7 results both from the small optical thickness of the shock wave in line frequencies and the anisotropy of the radiation field typical for the slab geometry. In the ambient gas with density of ρ_1 >= 1e-11 gm/cm^3 the flux in the H-alpha frequency interval reveals the double structure of the profile. A weaker H-beta profile doubling was found for ρ_1 >= 1e-10 gm/cm^3 and U_1 <= 50 km/s. The unshifted redward component of the double profile is due to photodeexcitation accompanying the rapid growth of collisional ionization in the narrow layer in front of the discontinuous jump.

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arXiv2002

The Structure of Radiative Shock Waves. IV. Effects of Electron Thermal Conduction

We considered the structure of steady-state radiative shock waves propagating in the partially ionized hydrogen gas with density rho1 = 1e-10 gm/cm^3 and temperature 3000K <= T1 <= 8000K. The radiative shock wave models with electron thermal conduction in the vicinity of the viscous jump are compared with pure radiative models. The threshold shock wave velocity above of which effects of electron thermal conduction become perceptible is of U1=70 km/s and corresponds to the upstream Mach numbers from M1= 6 at T1=8000K to M1=11 at T1=3000K. In shocks with efficient electron heat conduction more than a half of hydrogen atoms are ionized in the radiative precursor, whereas behind the viscous jump the hydrogen gas undergoes the full ionization. The existence of the electron conductive precursor leads to the enhancement of the Lyman continuum flux trapped in the surroundings of the discontinuous jump. For upstream velocities ranged within 70 km/s <= U1 <= 85 km/s the partially ionized hydrogen gas of the radiative precursor undergoes the additional ionization (<= 5%), whereas the total radiave flux emerging from the shock wave increases by 10% <= delta(FRad) <= 25% .

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arXiv2001

The structure of radiative shock waves. III. The model grid for partially ionized hydrogen gas

The grid of the models of radiative shock waves propagating through partially ionized hydrogen gas with temperature 3000K <= T_1 <= 8000K and density 10^{-12} gm/cm^3 <= ρ_1 <= 10^{-9}gm/cm^3 is computed for shock velocities 20 km/s <= U_1 <= 90 km/s. The fraction of the total energy of the shock wave irreversibly lost due to radiation flux ranges from 0.3 to 0.8 for 20 km/s <= U_1 <= 70 km/s. The postshock gas is compressed mostly due to radiative cooling in the hydrogen recombination zone and final compression ratios are within 1 <ρ_N/ρ_1 \lesssim 10^2, depending mostly on the shock velocity U_1. The preshock gas temperature affects the shock wave structure due to the equilibrium ionization of the unperturbed hydrogen gas, since the rates of postshock relaxation processes are very sensitive to the number density of hydrogen ions ahead the discontinuous jump. Both the increase of the preshock gas temperature and the decrease of the preshock gas density lead to lower postshock compression ratios. The width of the shock wave decreases with increasing upstream velocity while the postshock gas is still partially ionized and increases as soon as the hydrogen is fully ionized. All shock wave models exhibit stronger upstream radiation flux emerging from the preshock outer boundary in comparison with downstream radiation flux emerging in the opposite direction from the postshock outer boundary. The difference between these fluxes depends on the shock velocity and ranges from 1% to 16% for 20 km/s <= U_1 <= 60 km/s. The monochromatic radiation flux transported in hydrogen lines significantly exceeds the flux of the background continuum and all shock wave models demonstrate the hydrogen lines in emission.

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arXiv1999

The structure of radiative shock waves. II. The multilevel hydrogen atom

Models of steady-state plane-parallel shock waves propagating through the unperturbed hydrogen gas of temperature T=6000K and density rho = 1e-10 gm/cm^3 are computed for upstream velocities from 15 km/s to 70 km/s. The shock wave structure is considered in terms of the self-consistent solution of the radiation transfer, fluid dynamics and rate equations for 2 <= L <= 4 atomic bound levels with a continuum. The radiative flux Frad emergent from the shock wave was found to be independent of the lower limit nu_L of the frequency range provided that nu_L < nu_2, where nu_2 is the Balmer continuum head frequency, At the same time the decrease of nu_L is accompanied by decrease of the Lyman continuum flux and leads to smaller heating and weaker ionization of the hydrogen gas in the radiative precursor. For all models the size of the radiative precursor is of 10^4 cm and corresponds to several mean free paths of photons at the frequency of the Lyman continuum edge nu_1. The compression ratio at the discontinuous jump gradually increases with increasing upstream velocity U_1, reaches the maximum of ρ^+/ρ^- = 3.62 at U_1 = 55 km/s and slowly decreases for larger U_1 due to the strong rize of the preshock gas temperature. The radiative flux from the shock wave was determined as a function of the upstream velocity U_1 and its ratio to the total energy flux in the shock wave C_2 was found to range within 0.18 < Frad/C_2 < 0.92 for 15 km/s <= U_1 <= 65 km/s. Thus, at upstream velocities U_1 > 60 km/s the shock wave losses more than 90% of its total energy due to radiation. For all shock wave models the role of collisional processes in both bound-bound and bound-free atomic transitions was found to be negligible in comparison with corresponding radiative processes.

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arXiv1998

The Structure of Radiative Shock Waves. I. The method of global iterations

The structure of steady plane-parallel radiative shock waves propagating through the hydrogen gas undergoing partial ionization and excitation of bound atomic states is investigated in terms of the self-consistent solution of the equations of fluid dynamics, radiation transfer and atomic kinetics. The shock wave model is represented by a flat finite slab with no incoming radiation from external sources at both its boundaries. The self-consistent solution is obtained using the global iteration procedure each step of which involves (1) integration of the fluid dynamics and rate equations for the preshock and postshock regions, consecutively, both solutions being fitted by the Rankine-Hugoniot relations at the discontinuous jump; (2) solution of the radiation transfer equation for the whole slab. The global iteration procedure is shown to converge to the stable solution which allows for the strong coupling of the gas flow and the radiation field produced by this flow. Application of the method is demonstrated for the shock waves with upstream velocities from 15 km/s to 60 km/s (that is with upstream Mach numbers from 2.3 to 9.3) and the hydrogen gas of unperturbed temperature T=3000K and density rho = 1e-10 gm/cm^3.

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