The dust envelope of the symbiotic nova HM Sge

We have studied the brightness and color variations of the symbiotic nova H M Sge based on long-term UBVRJHKLM photometry of the star and data on its e nergy distribution in the middle infrared (7.7-22.7 mum) obtained with the low resolution spectrometers of the IRAS satellite and ISO orbital observat ory. We have also calculated models for the steady-state, spherically symme trical, extended dust envelope of the star for two extreme heating cases: h eating only by radiation from the cool component of the system and by the c ombined radiation from both components. Model fitting to the IRAS and ISO d ata indicates that models with a single, central Mira-type source are more appropriate. This indicates that the radiation of the hot component is larg ely processed by the surrounding gas, and does not substantially affect the infrared spectrum of the symbiotic nova directly. The mean spectral energy distribution based on 1983 IRAS data differs appreciably from the ISO spec trum obtained on October 1, 1996. The observed evolution of the envelope sp ectrum probably reflects an increase of the density and decrease of the tem perature of the dust grains near the inner boundary of the envelope, relate d to a decrease of the luminosity and increase of the temperature: of the h ot component. We estimate the total mass-loss rate, velocity of gas expansi on at the outer envelope boundary, and upper limit for the mass of the cent ral source of radiation. (C) 2001 MAIK "Nauka/Interperiodica".