2004A&A...424..165R

We present near-infrared speckle interferometry of the OH/IR star OH104.9+2.4 in the K' band obtained with the 6m telescope of the Special Astrophysical Observatory (SAO). At a wavelength of λ=2.12µm the diffraction-limited resolution of 74mas 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 λ=2.12µm 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±13AU for an adopted distance of D=2.38±0.24kpc. To determine the structure and the properties of the CDS of OH104.9+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 OH104.9+2.4 can be well reproduced by a radiative transfer model with an effective temperature T_eff=2500±500K of the central source, a dust temperature T_in=1000±200K at the inner shell boundary R_in≃9.1R_*=25.4AU, an optical depth τ_2.2µm=6.5±0.3, and dust grain radii ranging from a_min=0.005±0.003µm to a_max=0.2±0.02µm with a power law n(a)∝a^–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.