Infrared photometry of the unique object FG Sge in 1985–2001

Abstract

Our analysis of many years of infrared photometry of the unique object FG Sge indicates that the dust envelope formed around the supergiant in August 1992 is spherically symmetrical and contains compact, dense dust clouds. The emission from the spherically symmetrical dust envelope is consistent with the observed radiation from the star at 3.5–5 µm, and the presence of the dust clouds can explain the radiation observed at 1.25–2.2 µm. The mean integrated flux from the dust envelope in 1992–2001 was ∼(1.0±0.2)×10⁻⁸ erg s⁻¹cm⁻². The variations of its optical depth in 1992–2001 were within 0.5–1.0. The maximum density of the dust envelope was recorded in the second half of 1993 and corresponded to mean optical depths as high as unity. Several times in the interval from 1992 to 2001, the dusty material of the envelope partially dissipated and was then replenished. For example, the optical depth of the dust cloud at λ=1.25 µm during the last brigthness minimum in the J band was τ_1.25≈4.3, which is much higher than the optical depth of the dust envelope of FG Sge. During maxima of the J brightness, the mean spectral energy distribution at 0.36–5 µm can be represented as a combination of radiation from a G0 supergiant that is attenuated by a dust envelope with a mean optical depth of 0.65±0.15 and emission from the spherically symmetrical dust envelope itself, with the temperature of the graphite grains being 750±150 K. At minima of the J brightness, only radiation from the dust envelope is observed at 1.65–5 µm, with the radiation from the supergiant barely detectable at 1.25 µm. As a result, the integrated flux during J minima is almost half that during J maxima. The mean mass of the spherically symmetrical dust envelope of FG Sge in 1992–2001 was (3 ± 1) × 10⁻⁷ M _⊙. This envelope’s mass varied by nearly a factor of two during 1992–2001, in the range (2 – 4) × 10⁻⁷ M _⊙. In Autumn 1992, the mass-loss rate from the supergiant exceeded 2 × 10⁻⁷ M _⊙/yr. The average rate at which matter was injected into the envelope during 1993–2001 was 10⁻⁸ M _⊙/yr. The mean rate of dissipation of the dust envelope was about 1 × 10⁻⁸ M _⊙/yr. During 1992–2001, the supergiant lost about 8.7 × 10⁻⁷ M _⊙. The parameters of the dust envelope were relatively constant from 1999 until the middle of 2001.