Instability of LBV stars against radial oscillations |
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Authors: | Yu A Fadeyev |
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Institution: | (1) Instituut voor Sterrenkunde, Celestijnenlaan 200D, 3001 Leuven, Belgium;(2) Department of Astrophysics (IMAPP), Radboud Universiteit Nijmegen, Heyendaalseweg 135, 6525, AJ, Nijmegen, The Netherlands;(3) Department of Physics and Astronomy, Aarhus University, Building 1520, Ny Munkegade, 8000 Aarhus C, Denmark;(4) University of Central Lancashire Centre for Astrophysics, Preston, PR1 2HE, United Kingdom |
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Abstract: | 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. |
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Keywords: | |
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