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1.
Yu. A. Fadeyev 《Astrophysics and Space Science》1982,86(1):143-155
The nonlinear self-excited oscillations of the envelopes of low-massive highly luminous stars are described. The parameters for these models wereM=0.8M
,M
bol=–5.5, –5.84 mag,T
eff=4500, 5000, 5500 K. The oscillations have been found to consist of the standing wave pulsation near the envelope bottom and running waves in outer layers. The ratio of the standing wave frequency
s
to the average frequency of the running waves
r
increases with the stellar luminosity:
s
/
r
=1.7 whenM
bol=–5.5 mag and
s
/
r
=2.4 whenM
bol=–5.84 mag. The frequency of oscillations near the photosphere is found to be in close agreement with the critical frequency for running waves. Mass loss from these stars is caused by shocks. It has been shown that agreement between FG Sge's period change observed during the last decade and the period-luminosity relation for double shell stars takes place when FG Sge's luminosity isM
bol=–5.96 mag. 相似文献
2.
Astronomy Letters - Evolutionary calculations of population I stars with initial masses M0 = 1 M⊙, 1.5 M⊙ and 2 M⊙ were carried out up to the stage of the proto-planetary nebula.... 相似文献
3.
Yu. A. Fadeyev 《Astronomy Letters》2003,29(11):775-782
Based on a self-consistent solution of the equations of gas dynamics, kinetics of hydrogen atomic level populations, and radiative transfer, we analyze the structure of a shock wave that propagates in a partially ionized hydrogen gas. We consider the radiative transfer at the frequencies of spectral lines by taking into account the effects of a moving medium in the observer's frame of reference. The flux in Balmer lines is shown to be formed behind the shock discontinuity at the initial hydrogen recombination stage. The Doppler shift of the emission-line profile is approximately one and a half times smaller than the gas flow velocity in the Balmer emission region, because the radiation field of the shock wave is anisotropic. At Mach numbers M1?10 and unperturbed gas densities σ1=10?10 g cm?3, the Doppler shift is approximately one third of the shock velocity U1. The FWHM of the emission-line profile δ ? is related to the shock velocity by δ ? ≈ k ? U1, where k ? = 1, 0.6, and 0.65 for the Hα, Hβ, and Hγ lines, respectively. 相似文献
4.
Yu. A. Fadeyev 《Astronomy Letters》2011,37(6):403-413
Instability of population I (X = 0.7, Z = 0.02) massive stars against radial oscillations during the post-main-sequence gravitational contraction of the helium core
is investigated. Initial stellar masses are in the range 65M
⊙ ≤ M
ZAMS ≤ 90M
⊙. In hydrodynamic computations of self-exciting stellar oscillations we assumed that energy transfer in the envelope of the
pulsating star is due to radiative heat conduction and convection. The convective heat transfer was treated in the framework
of the theory of time-dependent turbulent convection. During evolutionary expansion of outer layers after hydrogen exhaustion
in the stellar core the star is shown to be unstable against radial oscillations while its effective temperature is T
eff > 6700 K for M
ZAMS = 65M
⊙ and T
eff > 7200 K for M
ZAMS = 90M
⊙. Pulsational instability is due to the κ-mechanism in helium ionization zones and at lower effective temperature oscillations decay because of significantly increasing
convection. The upper limit of the period of radial pulsations on this stage of evolution does not exceed ≈200 day. Radial
oscillations of the hypergiant resume during evolutionary contraction of outer layers when the effective temperature is T
eff > 7300 K for M
ZAMS = 65M
⊙ and T
eff > 7600 K for M
ZAMS = 90M
⊙. Initially radial oscillations are due to instability of the first overtone and transition to fundamental mode pulsations
takes place at higher effective temperatures (T
eff > 7700 K for M
ZAMS = 65M
⊙ and T
eff > 8200 K for M
ZAMS = 90M
⊙). The upper limit of the period of radial oscillations of evolving blueward yellow hypergiants does not exceed ≈130 day.
Thus, yellow hypergiants are stable against radial stellar pulsations during the major part of their evolutionary stage. 相似文献
5.
Yu. A. Fadeyev 《Astronomy Letters》2010,36(5):362-369
Hydrodynamic calculations of nonlinear radial oscillations of LBV stars with effective temperatures 1.5 × 104 K ⩽ T
eff ⩽ 3 × 104 K and luminosities 1.2 × 106
L
⊙ ⩽ L ⩽ 1.9 × 106
L
⊙ have been performed. Models for the evolutionary sequences of Population I stars (X = 0.7, Z = 0.02) with initial masses 70M
⊙ ⩽ M
ZAMS ⩽ 90M
⊙ at the initial helium burning stage have been used as the initial conditions. The radial oscillations develop on a dynamical
time scale and are nonlinear traveling waves propagating from the core boundary to the stellar surface. The amplitude of the
velocity variations for the outer layers is several hundred km s−1, while the bolometric magnitude variations are within ΔM
bol ⩽ 0·
m
2. The onset of oscillations is not related to the κ-mechanism and is attributable to the instability of a self-gravitating envelope gas whose adiabatic index is close to its
critical value of Γ1 = 4/3 due to the dominant contribution of radiation in the internal energy and pressure. The interval of magnitude variation
periods (6 days ≤ II ≤ 31 days) encompasses all currently available estimates of the microvariability periods for LBV stars,
suggesting that this type of nonstationarity is pulsational in origin. 相似文献
6.
Yu. A. Fadeyev 《Astrophysics and Space Science》1983,95(2):357-368
The results of calculations of graphite grain formation in the atmospheres of R CrB stars are given. The parameters for the models wereM=1M ⊙,M bol=?6 mag. The effective temperature ranged from 5300K to 8300K. The chemical composition corresponded to the hydrogen-deficient carbon rich mixture:X=0,Y=0.9,Z c=0.1. The results obtained show the existence of a critical mass loss rate which is ranged fromM *≈10?6 M ⊙yr?1 forT eff=5300 K toM *≈10?5 M ⊙ yr?1 forT eff=8300 K. As soon as the rate of mass loss exceedsM * by 3–5 times the degree of condensation of carbon changes from 0 to 0.7. The finite radii of grains are about from 0.01 μm to 0.6 μm depending on the density near the condensation point, the velocity of matter outflow, and the stellar effective temperature. The duration of grain growth should amount to some dozens of days. It is supposed that the most probable explanation of dust-shell formation around R CrB stars is graphite condensation behind a shock wave arising from nonlinear stellar pulsation. 相似文献
7.
We have performed hydrodynamic calculations of the radial pulsations of helium stars with masses 10M⊙ ≤ M ≤ 50M⊙, luminosity-to-mass ratios 5 × 103L⊙/M⊙ ≤ L/M ≤ 2.5 × 104L⊙/M⊙, and effective temperatures 2 × 104 K ≤ Teff ≤ 105 K for helium and heavy-element mass fractions of Y=0.98 and Z=0.02, respectively. We show that the high-temperature boundary of the instability region for radial pulsations at L/M ? 104L⊙/M⊙ extends to Teff≈105 K. The amplitude of the velocity variations for outer layers is several hundred km s?1, while the brightness variations in the B band of the UBV photometric system are within the range from several hundredths to half a magnitude. At constant luminosity-to-mass ratio, the radial pulsation period is determined only by the effective temperature of the star. In the ranges of luminosity-to-mass ratios 104L⊙/M⊙ ≤ L/M ≤ 2 × 104L⊙/M⊙ and effective temperatures 5 × 104 K ≤ Teff ≤ 9 × 104 K, the periods of the radial modes are within 6 min ?Π?103 min. 相似文献
8.
A global iteration method to determine the self-consistent structure of steady plane-parallel radiative shock waves is shown
to converge to the stable solution with upstream front velocities of 15 km/s ≤ U
1≤ 60 km/s and for hydrogen gas of unperturbed temperature T= 3000 K and density ρ = 10−10gcm−3.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
9.
10.
The nonlinear self-excited pulsations of population-II stars with mass 0.6M
and luminosities from 128 to 1280L
are studied. The pulsation periods are found to be in the range of 1.3 to 19 days. An increase of the stellar luminosity is shown to be accompanied by an increasing nonadiabaticity and decreasing efficiency of the radiative damping region. This leads to both an increase of the growth rate while pulsations are exciting and an increase of the oscillation amplitude of the limit cycle. In the models withL800L
the efficiency of the radiative damping region becomes so small that amplitude growth ceases due to a dissipation of the mechanical energy by shocks in the stellar atmosphere. The models with periods of from 1.3 to 3 days show the bump on their light curves. The bump is connected with a travelling pulse generated at the antinode of the second overtone at maximum compression. The time delays estimated for the pulses reflected of the stellar core are in a good agreement with the pulse resonance condition proposed by Aikawa and Whitney (1983). The model with the period of 2.1 days revealed double resonance 0 = 22, 20 = 31 causing alternating oscillations with slightly different periods and amplitudes. The models with period of 10 days and longer reveal the resonance 0 = 21. This resonance causes the flat top on the light curve at a period of about 10 days and appearance of a shallow alternating minimum at longer periods, as is observed in RV Tau variables. The theoretical period-luminosity relation proposed for population-II cepheids is in good agreement with that obtained from observations. 相似文献