Title:

High-Resolution Near-Infrared Speckle Interferometry
and Radiative Transfer Modeling of the OH/IR star OH~26.5+0.6

Authors:

T. Driebe(1), D. Riechers(2), Y. Balega(3), K.-H. Hofmann(1),
A.B.\ Men'shchikov(4), and G. Weigelt(1)

(1)Max Planck Institute for Radioastronomy, Bonn,
(2)Max Planck Institute for Astronomy, Heidelberg,
(3)Special Astrophysical Observatory, Nizhnij Arkhyz, Russia,
(4)Institute for Computational Astrophysics, Halifax, Canada




We present near-infrared speckle interferometry of the OH/IR star OH 26.5+0.6
in the K' band obtained with the 6m telescope of the Special Astrophysical
Observatory (SAO) in Oct. 2003. At a wavelength of lambda = 2.12 micron
the diffraction-limited resolution of 74 mas was attained. The reconstructed
visibility function shows that the stellar contribution to the total flux at
lambda = 2.12 micron is less than ~50 %, indicating a rather large optical depth
of the circumstellar dust shell (CDS) surrounding this highly reddened object,
which is in accordance with the strong silicate absorption feature seen in the
spectral energy distribution (SED). With respect to the asymmetry found from the
recent VLTI/MIDI observations in the mid-infrared (Chesneau et al. 2004,
A&A 435, 563), and from L-band observations (Starck et al. 1994, A&A 283, 349), 
we carefully checked our K-band data for signs of asymmetry, but given the 
accuracy of our measurements, no such asymmetry could be detected.

Our modeling approach follows a similar strategy as it was recently successfully
applied to interpret observations of the OH/IR star OH 104.9+2.4 (Riechers
et al. 2005, A&A 436, 925). 
For the radiative transfer modeling of the CDS of OH 26.5+0.6
with the code DUSTY, we used our K-band vibility data from 2003 as well as the
ISO spectrum as observational input. Since OH 26.5+0.6 is a LPV, both observations
are associated with different phases of the object's variability cycle. While the ISO
observations were carried out close to minimum phase (phi=0.5), our K-band visibility
data correspond to phi=0.13. From our analysis, we derived several physical parameters
of the central star and the CDS for these two phases and found a phase dependence
similar to the results for OH 104.9+2.4. Since OH 26.5+0.6 was recently also observed
with VLTI/MIDI, we finally discuss the implications of the MIDI results with
respect to our model.