Radiative transfer
In the outer parts of stellar atmosphere radiative rates often dominate
over collisional rates between atomic energy levels. The equilibrium state
for the population densities will deviate from Local Thermodynamic Equilibrium
(LTE) and the full statistical equilibrium equations have to be solved.
A code was written to efficiently tackle such problems. This code,
MULTI ,
has been publicly available since 1986. I have been involved in a number
of projects in radiative transfer using MULTI:
Non-LTE line formation and consequences for abundance analysis
Lithium and Boron
A couple of papers address the details of line formation for lines that
are used for abundance analysis. Non-LTE modelling has been performed for
a grid of stellar atmospheres with different effective temperature, gravity
and metal abundance. The line formation is discussed in detail and tables
are given that enables a first order correction of abundances that have
been determined with classical model atmospheres under the erroneous
assumption of LTE. The papers are:
Carlsson, M., Rutten, R.J., Bruls, J.H.M.J., Shchukina, N.G.: 1994,
The non-LTE formation of LiI lines in Cool Stars.
Kiselman, D., Carlsson, M.: 1996,
The NLTE formation of neutral boron lines in cool stars
Formation of UV emission lines
Many emission lines in the UV are difficult to explain from standard
plane parallel modelling, even when non-LTE effects are included. The
excitation may typically be dominated by other processes than
collisional excitation. Examples are formation by recombination and
by optical pumping from another emission line. Papers addressing such
formation "puzzles" in the UV include:
Wahlstro m, C., Carlsson, M.: 1994,
The Formation of the Solar HeII 1640.4 Å Emission Line.
We find that the line is formed by the photoionization from the ground
state of He II, mainly by the incident coronal radiation, followed by
cascade recombination into the n = 3 levels.
Carlsson, M., Judge, P.G.: 1993,
O I lines in the Sun and Stars: I. Understanding the Resonance Lines.
The use of O I lines as spectral diagnostics of conditions in the
chromospheres of the sun and cool stars is addressed, focusing on the
resonance lines. It is shown how the O I line flux densities depend
simply and sensitively on the radiative transfer solution for
hydrogen. Approximate analytical formulas are derived for the O I line
flux densities in terms of the hydrogen number densities.
Formation of Rydberg lines
Another class of line formation problems concerns lines in the IR from
transitions between high lying levels. The prime example is the formation
of the emission lines from neutral magnesium close to 12 micrometer.
Were these lines formed in the corona, chromosphere or photosphere? Since
they showed very pronounced magnetic splitting it was important to find
out where they are formed. We addressed this problem in 1991 (paper came
out in 1992) with a follow up paper on the Rydberg lines of hydrogen:
Carlsson, M., Rutten, R.J., Shchukina, N.G.: 1992,
The formation of the MgI emission features near 12 mu.
The formation of two Mg I 12-micron emission features in the solar
spectrum is explained using plane-parallel non-LTE
modeling with a radiative-equilibrium model atmosphere
without chromosphere. It is shown that these emissions are a natural
consequence of population depletion by line photon losses followed by
population replenishment from the ionic reservoir in the highly
excited levels. The results confirm the suggestion by Lemke and
Holweger (1987) that the 12-micron lines are formed in the photosphere.
Carlsson, M., Rutten, R.J.: 1992,
Solar hydrogen lines in the infrared.
We study recently observed H I lines in the infrared solar spectrum,
employing detailed NLTE modeling to explain their formation and to
evaluate their diagnostic merits. The formation heights
of the H I lines range from the deep photosphere for near-infrared
line wings to the chromosphere for line cores with wavelengths greater
than 10 microns; these features provide valuable diagnostics of the
thermal structure of the solar atmosphere.
Mats Carlsson
Last modified: Fri Nov 14 12:14:58 MET