Ga. Wade et al., LTE spectrum synthesis in magnetic stellar atmospheres - The interagreement of three independent polarised radiative transfer codes, ASTRON ASTR, 374(1), 2001, pp. 265-279
With the aim of establishing a benchmark for the detailed calculation of th
e polarised line profiles of magnetic stars, we describe an intercomparison
of LTE Stokes profiles calculated using three independent, state-of-the-ar
t magnetic spectrum synthesis codes: Cossam, Invers10 and Zeeman2. We find,
upon establishing a homogeneous basis for the calculations (identical defi
nitions of the Stokes parameters and the magnetic and stellar reference fra
mes, identical input model stellar atmosphere, identical input atomic data,
and identical chemical element abundances and magnetic field distributions
), that local and disc-integrated Stokes IQUV profiles of Fe II lambda 4923
.9 calculated using the three codes agree very well. For the illustrative c
ase of disc-integrated profiles calculated for abundance log n(Fe)/n(tot) =
4.60, dipole magnetic field intensity B-d = 5 kG, and projected rotational
velocity v(e) sin i = 20 km s(-1), Stokes I profiles (depth similar to 40%
of the continuum flux I-c) agree to within about 0.05% rms of I-c, Stokes
V profiles (full amplitude similar to 10%) to within about 0.02% rms of I-c
, and Stokes Q and U profiles (full amplitudes similar to2%) at the sub-0.0
1% rms level. These differences are sufficiently small so as to allow for c
ongruent interpretation of the best spectropolarimetric data available, as
well as for any data likely to become available during the near future. Thi
s indicates that uncertainties in modeling Stokes profiles result overwhelm
ingly from uncertainties in input atomic and physical data, especially the
state and structure of model stellar atmospheres.