The structure of a shock front in atomic hydrogen |
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| Authors: | Angelo J Skalafuris |
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| Institution: | (1) Smithsonian Astrophysical Observatory, Cambridge, Mass.;(2) Dept. of Physics, City College of New York, New York, U.S.A. |
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| Abstract: | A shock wave passing through a stellar atmosphere disturbs the gas, and the consequent adjustment of the fluid is a redistribution of the shock's kinetic energy among the various degrees of freedom. This paper deals with the effects of the Lyman continuum on the shock front. The shock heated gas is cooled principally by ionizing collisions of ground state atoms. This process is followed by a large quasi-isothermal region in which radiative recombinations occur. A final cycle of processes consisting of ionization, photo-recombinations to upper-level and collisional de-excitation, gives way to a sequence of statistical balances as each degree of freedom in the fluid attains equilibrium. Our calculations show that to a great extent, the shock structure is separated into successive regions of internal and radiative relaxation by an intermediate layer of ionized gas appearing at high shock speeds. Numerical results are presented for a range of shock speeds typifying a cepheid atmosphere.Radiation field and gas motions in shock waves are coupled, but the gas reacts little to the radiation it produces. Only the Lyman continuum has an appreciable effect on the shock structure. The principal escape of energy from the shock wave is through continuum radiation produced in recombinations to upper levels; thus the continuum emission in the red is stronger than an equivalent black body. Lyman photons are trapped in the shock while 20–30% of the shock's kinetic energy escapes to the Balmer and Paschen continua after the Lyman continuum is in equilibrium. The post- and pre-shock lines, as well as the post-shock continuum above the Lyman constitute the only observable spectra which emanate from the shock wave. The shock structure is perturbed only by the radiation which is not observed, and its absence tends to distort the emission profile from a Planck distribution.This work was originally started at Smithsonian Observatory and was completed at City College New York under contract with NASA Institute for Space Studies, New York. |
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