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ABSTRACT: The coupled radiative transfer and statistical equilibrium equations for multilevel ionic structures in the atmospheres of early-type stars are solved. Both lines and continua are treated consistently; the treatment is applicable throughout a transonic wind, and allows for the presence of background continuum sources and sinks in the transfer. An equivalent-two-level-atoms approach provides the solution for the equations. Calculations for simplified He (+)-like model atoms in parameterized isothermal wind models indicate that subordinate line profiles are sensitive to the assumed mass-loss rate, and to the assumed structure of the velocity law in the atmospheres.
02/1978;
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ABSTRACT: A numerical method is presented of solving the radiative transfer equation in the comoving frame of a spherically symmetric expanding atmosphere in which both the line and the electron-scattering source function can depend on frequency (i.e., when there is partial frequency redistribution in the scattering process). This method is used to assess the adequacy of various assumptions regarding frequency redistribution in the comoving frame and to discuss the effects of electron scattering more accurately than previously possible. The methods developed here can be used in realistic model atmospheres to account for the (major) effects of electron scattering upon emergent flux profiles.
01/1977;
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ABSTRACT: The effects of partial frequency redistribution in the scattering process for lines formed in moving atmospheres are analyzed using a general method that allows the transfer equation to be solved in the comoving frame of the gas. The same chromospheric and atomic model studied by Cannon and Vardavas (1974) is employed in the calculations, but a depth scale with logarithmically spaced points is adopted. It is found that in both static and moving atmospheres, the profiles obtained with complete and partial frequency redistribution are virtually identical. The large differences in profiles obtained by Cannon and Vardavas when they used complete and partial redistribution are shown to be spurious (and physically unreal) effects resulting from angle averaging in the observer's frame instead of the comoving frame.
05/1976;
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ABSTRACT: To examine the effect of the radial flow of atmospheric material on the
temperature distribution in a stellar atmosphere, a picket-fence model
with Gaussian lines is formulated and solved numerically in the comoving
frame of the gas, which is assumed to move with a prescribed velocity
law. Extensive results have been obtained for both static and dynamical
models, with planar and moderately extended spherical geometries. For
static models, the effect of lines on the temperature distribution is
virtually independent of extension. When a large-scale velocity field is
imposed, significant surface heating and additional back-warming are
found; the magnitude of these effects increases with the extension of
the atmosphere. If a significant flow velocity persists to sufficient
depth, the enhanced escape probability can lead to a cooling in the
deeper layers, which competes with the back-warming. The results
obtained here suggest that the deposition of energy arising from the
intrusion of line opacity into the continuum, caused by velocity
gradients, could influence the dynamics of the flow.-
The Astrophysical Journal 01/1976; 203:647-659. · 6.02 Impact Factor
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ABSTRACT: A method is presented for treating radiative transfer in resonance lines, allowing for the full angle and frequency dependence of redistribution in the scattering process, as seen in the laboratory frame. The case of an equivalent-two-level-atom source function is considered; the problem to be treated is then linear in the radiation field. We apply this method to the Ca II lines in the solar atmosphere, using a redistribution function which takes into account a mixture of coherence in the atom's frame, with Doppler redistribution in the laboratory frame (for atoms which have not suffered an elastic collision), and of complete redistribution in the laboratory frame (for atoms that are collisionally perturbed during the emission process). Both the angle-averaged approximation and the full angle-dependent solution were obtained, and were compared to assess, differentially, the effects of angular redistribution upon the computed line profile and its center-to-limb behavior. For the Ca II line in a homogeneous solar chromosphere the angle-dependent effects are found to be negligible, indicating that one may use angle-averaged redistribution functions when studying partial redistribution effects in line profiles.
12/1975;
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ABSTRACT: A method for solving the line-formation problem using the full
comoving-frame formulation of the radiative-transfer equation is
presented for the case of spherically symmetric atmospheres expanding
with arbitrarily large velocities. A stable differencing scheme and a
frequency-by-frequency elimination procedure are developed to solve the
partial differential equations that describe the radiation field in the
comoving frame. It is noted that this method allows computation of the
radiation field from a given model atmosphere which must specify the
depth dependence of all the relevant physical variables. Numerical
results obtained for several models involving line formation by
two-level atoms, electron scattering, and continuous absorption are
discussed which simulate situations in the stellar winds of hot stars
and similar objects. The force exerted by radiation on the gas is
examined in a number of situations, and flux profiles are described for
very high-velocity flows with very weak or nonexistent continuum and
electron-scattering opacities. It is concluded that the mechanism
proposed by Noerdlinger and Rybicki (1974) for the destruction of
radially driven envelopes in planar geometries becomes inoperative even
in the case of slightly extended spherical configurations.
The Astrophysical Journal 11/1975; 202:465-489. · 6.02 Impact Factor
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11/1975;
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ABSTRACT: Spherical static non-LTE model atmospheres are presented for stars from
30 to 60 solar masses at various points on their evolutionary tracks,
and for some nuclei of planetary nebulae at two points of a modified
Harman-Seaton sequence. The method of Mihalas and Hummer was employed,
which uses a parametrized radiation force multiplier to simulate the
force of radiation arising from the entire line spectrum. However, in
the present work the density structure computed in the LTE models was
held fixed in the calculation of the corresponding non-LTE models; in
addition, the opacity of an average light ion was taken into account.
The effects of sphericity, as distinct from those arising from a density
structure modified by a large radiation force, were investigated by
computing a few planar models using the same parametrized radiation
force multiplier as for the spherical models. Extensive tables are given
of monochromatic magnitudes, continuum jumps and gradients,
Stroemgren-system colors, monochromatic extensions, and the profiles and
equivalent widths of the hydrogen lines for all models.
The Astrophysical Journal 10/1975; 202:92-113. · 6.02 Impact Factor
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ABSTRACT: A generalized formulation is given for treating partial redistribution effects in transfer problems in resonance lines with common upper states. The formulation allows explicitly for the possibility that several spectral lines may arise in transitions from a given upper level to several sharp lower levels, including, for example, the ground state and metastable states. Line profiles for the Ca II H and K lines have been calculated, accounting for the partial frequency coherence of scattered photons. These profiles are compared with calculations made with identical atomic and atmospheric models but assuming complete redistribution. Very significant differences between the profiles obtained using these two different physical descriptions of the scattering process are found, and it is now apparent that the assumption of complete redistribution is a serious oversimplification of the actual physical situation. The results question the validity of equating brightness temperatures observed at K(sub 1) in stellar spectra with minimum temperatures in stellar chromospheres; it appears likely that such a procedure will systematically underestimate the value of T-min.
09/1975;
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02/1975; 7:360.
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ABSTRACT: A method is presented that makes possible, for the first time, the
calculation of extended spherical non-LTE model stellar atmospheres in
hydrostatic and radiative equilibrium. This method is a generalization
of the complete-linearization technique of Auer and Mihalas. Models have
been obtained for a star with 60 solar masses, 1,000,000 solar
luminosities, and 24 solar radii, whose atmosphere is characterized by
an effective temperature of 39,500 K and a surface gravity log g = 3.45,
i.e., with a spectral type near O6. These models are differentiated by
the magnitude and radial dependence of a radiation force multiplier that
is inserted into the equation of hydrostatic equilibrium to simulate the
effect of radiation force on opacity sources which have not been
included explicitly in the calculation. Models have been obtained very
close to the limit at which the radiation force and gravity balance; as
this condition is approached, the atmospheres become more and more
extended.
The Astrophysical Journal Supplement Series 09/1974; 28:343-372. · 13.46 Impact Factor
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ABSTRACT: Line profiles for the Mg II h and k lines have been calculated,
accounting for the partial frequency redistribution of photons. These
profiles are compared with the complete redistribution calculations
under identical assumptions for the atomic and atmospheric models, and
the inadequacy of the latter approximation is demonstrated. In
particular, the temperature as deduced from the intensities at h1 and k1
under the assumption of complete redistribution appears to be a lower
limit for the chromospheric temperature minimum. The partial
redistribution profiles are in substantially better agreement with
observation than complete redistribution results in describing the
wavelength position of h1 and k1, and the relative behavior of the h and
k profiles.
The Astrophysical Journal 08/1974; 192:769-776. · 6.02 Impact Factor
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The Astrophysical Journal 01/1973; 179:827-845. · 6.02 Impact Factor
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The Astrophysical Journal 09/1967; 150:L57. · 6.02 Impact Factor
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Monthly Notices of the Royal Astronomical Society 147:339-354. · 4.90 Impact Factor
