Infrared photometry of the symbiotic novae V1016 Cyg and HM Sge in 1978-1999

The photometric JHKLM observations of the symbiotic novae V1016 Cyg and HM Sge in 1978-1999 are presented. Parameters of the cool stars themselves and the dust envelopes are estimated. The periods of 470 +/- 5 days (for V1016 Cyg) and 535 +/- 5 days (for HM Sge) are reliably determined from the enti re set of our photometric J data for V1016 Cyg and HM Sge. In addition, mon otonic light and color variations are observed on a time scale of several t housand days, with the increase in infrared brightness occurring with the s imultaneous decrease in infrared color indices; i.e., the dust envelopes in which both components of the systems were embedded before the outburst of their hot sources in 1964 and 1975, respectively, had continued to disperse until late 1999. The amplitudes of these variations for HM Sge are almost twice those for V1016 Cyg. For HM Sge, the dust envelope reached a maximum density near JD 2447500 and then began to disperse. In the case of V1016 Cy g, a maximum density of the dust envelope was probably reached near JD 2444 800, and its dispersal has been continuing for about 20 years. Thus, in bot h symbiotic novae, their dust envelopes reached a maximum density approxima tely eight years after the outburst of the hot component and then began to disperse. An analysis of the color-magnitude (J-K, J) diagram reveals that grains in the dust envelopes of V1016 Cyg and HM Sge are similar in their o ptical properties to impure silicates. The observed [J-K, K-L] color variat ions for the symbiotic novae under study can be explained in terms of the s imple model we chose by variations in the Mira's photospheric temperature f rom 2400 to 3000 K and in the dust-envelope optical depth from 1 to 3 at a wavelength of 1.25 mu m for a constant grain temperature. The observed J-K and K-L color indices for both symbiotic novae, while decreasing, tend to t he values typical of Miras. The dust envelopes of both symbiotic novae are optically thick. The dust envelope around HM Sge is, on the average, twice as dense as that around V1016 Cyg; the Mira in V1016 Cyg is slightly cooler (similar to 2800-2900 K) than that in HM Sge (similar to 2600-2700 K). The dust-envelope density decreases as the Mira's temperature increases. The a bsolute bolometric magnitudes are -5.(m)1 +/- 0.(m)15 for V1016 Cyg and 5.( m)27 +/- 0.(m)17 for HM Sge. Their distances are 2.8 +/- 0.6 and 1.8 +/- 0. 4 kpc, respectively; the luminosities and radii of their cool components (M iras) are 8.6 x 10(3)L., 1 x 10(4)L., 500R., and 540R.. The radii of their dust envelopes are 1400R. and 1500R.; the masses are (3-3.3) x 10(-5)M. and (4-8) x 10(-5)M.. The dust envelope of V1016 Cyg disperses slower than tha t of HM Sge by almost a factor of 25. (C) 2000 MAIK "Nauka/Interperiodica".