FG SGE: Evolution of the circumstellar dust shell (JHKLM Photometry in 1985–2013)

Long-term (1985–2013) JHKLM photometry for FG Sge is analyzed. Parameters of the supergiant and optical properties of the dust in the dust shell (DS) have been refined. Given the optical photometry, the spectral energy distribution of FG Sge at J brightness maxima in the range 1.25–5 µm is consistent with the classification of the supergiant as a K3 star. It is shown that the observed changes in J and H magnitudes in 1993–2013 can be explained by the joint action of two factors: the absorption of emission from the supergiant in the DS with dust grains similar to interstellar ones and the changes in parameters (spectral type and radius) of the supergiant itself. The DS has a clumpy structure, its temperature changed within the range 450–800 K, and its radius changed by a factor of 3. The optical depth of the DS estimated from its emission changed within the range τ em(J) ≈ 0.8–3; its changes on the line of sight were τ ext(J) ≈ 0–5. On average, the rate of dust supply to the DS from 1998 to 2013 was 10−6 M ⊙ yr−1.