Photometric observations of R coronae borealis in the optical and infrared

The results of long-term photometric observations of R CrB in the UBV JHKLM bands are presented. The temporal and color characteristics of the emission of the star itself and of its extended dust envelope are analyzed in detail. No stable harmonic has been found in the semiregular variations of the optical brightness of R CrB. Two harmonics with periods P approximate to 3.3 and 11.3 yrs have been detected in the brightness variations of the dust envelope; the minima of these variations coincided in 1999, resulting in a record decrease in the LM brightness of the envelope. This by chance coincided in time with a deep minimum of the visual brightness of the star, resulting in a unique decrease in the total brightness of the star and dust envelope. This enabled estimation of the bolometric flux of the hot dust clouds, which made up only a few per cent of the bolometric flux of the dust envelope. The brightness variations of the dust envelope are not accompanied by appreciable color changes and are associated with variations of its optical depth tau(V) in the range 0.2-0.4. The dust envelope forms at a large and fairly constant distance from the star (r(in) approximate to 110R(*),T-gr approximate to 860 K), from material in its stellar wind, whose intensity (M-gas approximate to 2.1 x 10(-7) M-circle dot/year) obeys a Reimers law. No variations synchronous with those of the optical depth of the dust envelope, in particular, with the period P approximate to 3.3 yrs, have been found in the optical emission of R CrB, suggesting that the stellar wind is not spherically symmetric. The dust envelope consists of small grains (a(gr) less than or equal to 0.01 mum), while the clouds screening the star from the observer are made up of large grains (a(gr) approximate to 0.1 mum). The activity of R CrB, whose nature is unclear, is reflected in variations of the stellar-wind intensity and the appearance of dust clouds in the line of sight: these variations are repeated by corresponding changes in the optical depth of the dust envelope with a delay of similar to4 years (the time for a particle moving at V-env approximate to 45 km/s to move from the star to the boundary of the dust envelope). (C) 2002 MAIK "Nauka/lnterperiodica"