Publications
of the
MPIfR
Optical & Infrared
Interferometry Group
J.M. Winters, J.J. Keady, A. Gauger,
and P.V. Sada:
Circumstellar dust shells around long-period
variables:
VIII. CO infrared line profiles from dynamical models for C-stars
Astronomy and Astrophysics 359, 651-662 (2000)
Abstract.
We present simplified non-LTE radiative transfer calculations for
CO (v 0-1) fundamental and (v 0-2) overtone lines, using the structure
of dynamical models for dust shells around carbon-rich long-period
variables, which result from a consistent treatment of hydro- and
thermodynamics, radiative transfer, chemistry and carbon grain
formation.
Owing to the saturation of the fundamental band absorption cores the
most useful information is provided by the P-Cygni emission component,
which can be used to constrain the dust optical depth in the 5µm
region.
The first overtone lines clearly reflect the shocked,
layered structure of the inner shell region by showing a
multi-component structure of the profiles. At least the lines of higher
rotational excitation (Jl > 10) are not saturated even
for mass
loss rates of the order of 10-5Moyr-1
and
thus can be used to constrain the mass loss rate. Furthermore, the
dynamics of the
dust formation process is reflected in the temporal evolution of
the synthetic CO first overtone line profiles resulting from the
models.
The formation of a new dust layer in the inner shell region leads to a
secondary (low velocity) absorption component which evolves on the time
scale of a dust formation event, usually longer than the pulsation
period of the star.
The synthetic overtone line profiles calculated on the basis of one
dust
shell model resemble the time variations of corresponding lines in the
observed high resolution CO line spectra of the carbon-Mira IRC+10216,
which thereby are interpreted as result of a dust formation event
taking
place on a time scale of approx. 10yr corresponding to 6 pulsation
periods of the star. However, the calculated fundamental line
absorption
cores are broader than observed and the first overtone line strengths
are too high, indicating that the density and thus probably the mass
loss rate of the model is too high by a factor of about 3 and should be
closer to the value of 1 10-5 Moyr-1
given for
IRC+10216 in the literature.
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