Title:

High-Resolution Near-Infrared Speckle Interferometry
and Radiative Transfer Modeling of the OH/IR star OH~104.9+2.4

Authors:

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


(1)Max Planck Institute for Astronomy, Heidelberg, Germany
(2)Max Planck Institute for Radioastronomy, Bonn, Germany
(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 104.9+2.4
in the K' band obtained with the 6m telescope of the Special Astrophysical
Observatory (SAO) in Oct. 2002 and 2003. At a wavelength of lamda = 2.12 micron
the diffraction-limited resolution of 74 mas was attained. The reconstructed
visibility reveals a spherically symmetric, circumstellar dust shell (CDS)
surrounding the central star. The visibility function shows that the stellar
contribution to the total flux at lamda = 2.12 micron is less than ~50 %, 
indicating a rather large optical depth of the CDS. The azimuthally averaged
1-dimensional Gaussian visibility fit yields a diameter of 47 +/- 3mas (FHWM),
which corresponds to 112+/- 13 AU for an adopted distance of D = 2.38 +/- 0.24 kpc.
To determine the structure and the properties of the CDS of OH 104.9+2.4, radiative
transfer calculations using the code DUSTY were performed to simultaneously model
its visibility and the spectral energy distribution (SED). Since OH 104.9+2.4 is
highly variable, the observational data taken into consideration for the modeling
correspond to different phases of the object's variability cycle. This offers the
possibility to derive several physical parameters of the central star and its CDS
as a function of phase. For instance, according to our final model the effective
temperature of the central star increases from T_eff = 2250 K at minimum phase
(phi=0.5) to T_eff = 3150 K at maximum phase (phi = 0.0), while the stellar radius
decreases from $R = 730 R_sun at phi = 0.5 to 675 R_sun at phi = 0.0.
For the CDS, we found that the inner boundary of the dust shell is located at 8.3
R_sun at minimum phase and approximately a factor of two further away at
maximum phase (R_in/R_star = 17.5), and the optical depth at 2.2 micron
decreases from 8.5 to 3.5 between minimum and maximum phase. Our detailed analysis 
demonstrates the potential of dust shell modeling constrained by both the SED and 
visibilities obtained from interferometric measurements.