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Astron. Astrophys. 363, 1055-1064 (2000)

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6. Conclusions

We formulated and solved a full set of equations for the LTE model stellar atmosphere in radiative and hydrostatic equilibrium, including both external irradiation and Compton scattering in model equations. Computer code ATM21 used here allowed us to compute precisely radiative transfer and the models with very large relative values of external irradiation fluxes [FORMULA] and energies of incident X-ray photons, approaching [FORMULA].

We believe, that the model assumptions and numerical results are relevant to accretion disks with hot irradiating coronae in AGN, and their spectral properties.

1. Numerical results presented above clearly demonstrate the nature and the origin of soft X-ray - EUV hump in atmospheres illuminated by external hard X-rays, and in AGN. We prove that the part of spectrum, in which most of the radiative energy is reemitted, is created by redistribution processes in the atmosphere. The redistribution includes either reprocessing by thermal absorption/emission, or Comptonization by electrons colder than the incoming X-rays. The latter process is critically important in stellar/disk atmospheres consisting of hydrogen and helium alone. The importance of thermal emission and absorption increases with increasing abundance of heavy elements (iron).

2. We demonstrate, that the hydrogen Lyman edge exhibits natural tendency to disappear, even for moderate external X-ray fluxes. This occurs in all our models, either with heavy elements (iron) or without them. Disappearance of the Lyman jump occurs in stellar atmospheres with LTE equation of state, and is due to excessive ionization of hydrogen in extremely hot gas. It is not related to possible NLTE effects (Hubeny & Hubeny 1998), but it is solely due to huge temperature rise in the external layers of our illuminated LTE atmospheres.

3. As pointed out by Koratkar & Blaes (1999), practically all spectral lines observed in AGN spectra are seen as emission lines (even without absorption parts). Unfortunately we were unable to compute spectral line profiles, except for hydrogen Lyman and Balmer lines. In most cases of our computations synthetic Lyman and Balmer lines exhibit strong emission cores, sometimes even without any surrounding absorption dips. Since practically all of the LTE b-f opacity jumps appear in emission, we believe that the lacking iron lines will also appear in emission. This is obviously related to very strong temperature inversions in our LTE model atmosphere.

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© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000
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