  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). At a wavelength of lambda = 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 lambda = 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).
We found that both the ISO spectrum and the visibility of OH 104.9+2.4 can be
well reproduced by a radiative transfer model with an effective temperature
T_eff = 2500 +/- 500 K of the central source, a dust temperature T_in = 1000
+/- 200 K at the inner shell boundary R_in = 9.1 R_star = 25.4 AU, an optical
depth tau = 6.5 +/- 0.3 at 2.2 micron, and dust grain radii ranging from a_min
= 0.005 +/- 0.003 micron to a_max = 0.2 +/- 0.02 micron with a power law with
index -3.5. It was found that even minor changes in a_max have a major impact
on both the slope and the curvature of the visibility function, while the SED
shows only minor changes. Our detailed analysis demonstrates the potential of
dust shell modeling constrained by both the SED and visibilities.
