Publications
of the
MPIfR
Optical & Infrared
Interferometry Group
K.-H. Hofmann, Y. Balega, M. Scholz and
G. Weigelt:
Diffraction-limited Speckle Masking
Observations of the
Mira Variables R Cas and R Leo with the 6 m SAO Telescope
Abstract (poster) for
IAU Symp. 191, Asymptotic Giant Branch Stars, Aug 27th - Sep 1rst,
1998,
Montpellier, France
Abstract.
We present the first diffraction-limited optical and infrared speckle
masking
observations of Mira stars with the 6m SAO telescope.
A resolution of 24 mas was achieved at 700 nm wavelength.
The speckle interferograms were recorded
through various narrow-band interference filters between 673nm and 1043
nm
covering strong and weak TiO absorption bands and the continuum. Mira
stars
show substantial variations of their angular size with wavelength
caused by
the wavelength dependence of TiO opacity. Theoretical models show that
monochromatic radii observed in suitably chosen filter bandpasses are
extremely
sensitive diagnostic tools for studying the structure of Mira
photospheres
(Bessell et al. 1996).
Our reconstructions show that the disk of R Cas
is non-uniform and elongated. The size (elliptical uniform disk fit)
of the elongated disk is 35 mas x 40 mas in the 700 nm pseudo-continuum
filter and 42 mas x 56 mas in the 714 nm TiO absorption band filter.
In the 1045 nm continuum image the disk of R Cas shows no asymmetry and
has
a uniform disk diameter of 30 mas.
The goal of our Mira star project is to provide the basic observations
for a
quantitaive analysis of the photospheric structure of Mira variables
and thus
to test Mira star models.
We compare our observations of R Cas and R Leo with the models of
Bessell et al. (1996).
We have derived wavelength-dependent tau_lambda = 1 radii from the
observed
visibilities by applying model-predicted center-to-limb intensity
variations.
Using the E model series of Bessell et al. we find, for instance,
for R Cas a photospheric (Rosseland) radius of 380 +/- 70 Rsol
(based on the HIPPARCOS distance 107 pc) and a low effective
temperature of
approximately 1900 K near minimum phase.
References:
Bessell M.S., Scholz M., Wood P.R., 1996, A&A 307, 481
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