Publications
of the
MPIfR
Optical & Infrared
Interferometry Group
T. Blöcker, Y. Balega, K.-H. Hofmann,
and G. Weigelt:
Bispectrum speckle interferometry
observations and radiative transfer
modelling of the red supergiant NML Cyg:
Multiple dust-shell structures evidencing previous superwind phases
Astronomy and Astrophysics 369, 142-154 (2001)
Abstract.
NML Cyg is a highly evolved OH/IR supergiant,
one of the most prominent
infrared objects due to its strong obscuration by dust, and supposed to
be
among the most luminous supergiants in the galaxy.
We present the first diffraction-limited
2.13µm observations of NML Cyg with
73mas resolution. The speckle interferograms were obtained with the 6m
telescope at the Special Astrophysical Observatory, and the
image reconstruction is based on the bispectrum speckle-interferometry
method.
The visibility function declines towards the diffraction limit to ~0.6.
Radiative transfer calculations have been carried out to model the
spectral
energy distribution, given by ground-based photometry and ISO
spectroscopy,
and our 2.13µm visibility function. Additionally,
mid-infrared visibility functions at 11µm were considered.
The observed dust shell properties do not appear to be in accordance
with
standard single-shell (uniform outflow) models but seem to require
multiple components. Considering previous periods of
enhanced mass-loss, various density enhancements in the dust shell were
taken
into account. An extensive grid of models was calculated for different
locations and strenghts of such superwind regions in the dust shell.
To match the observations from the optical to the
sub-mm domain requires at least two superwind regions embedded in the
shell.
The best model includes a dust shell with a temperature of 1000K at its
inner radius of 6.2 Rstar, a close embedded superwind shell extending
from 15.5 Rstar to 21.7 Rstar
with an amplitude (factor of density enhancement) of 10,
and a far-out density enhancement at 186 Rstar with an amplitude of 5.
The angular diameters of the central star and of the inner rim
of the dust shell amount to 16.2mas and 105mas, resp.
The diameter of the embedded close superwind region extends from
263mas to 368mas, and the inner boundary of the distant superwind
region
has a diameter of 3.15".
In the near-infrared the dust condensation zone is limb-brightened
leading
to a corresponding ring-like intensity distribution.
The grain sizes, a, were found to be in accordance with
a standard distribution function,
n(a)~a^-3.5, with a ranging between
amin=0.005µm and amax=0.15µm.
The bolometric flux amounts to
Fbol=3.63 10^-9 Wm^-2
corresponding to a central-star luminosity of L/Lsol=1.13 10^5
(d/kpc)^2.
Within the various parts of the dust shell, 1/r^2 density distributions
could be maintained differing only in their amplitude A.
A slight improvement of the far-infrared properties can be obtained if
a shallower density distribution of rho~1/r^1.7 is considered
in the distant superwind region.
The present-day mass-loss rate was determined to be
Mdot=1.2 10^-4 Msol/yr.
The inner embedded superwind shell corresponds to a phase
of enhanced mass-loss (with amplitude 10)
in the immediate history of NML Cyg which began ~59yr ago and
lasted for ~18yr.
Correspondingly, the outer superwind region is due to
to a high mass-loss period (amplitude 5) which terminated 529yr ago.
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