UBV JHKLM photometry and modeling of R Coronae Borealis

We present the results of UBVJHKLM photometry of R CrB spanning the period from 1976 to 2001. Studies of the optical light curve have shown no evidence of any stable harmonics in the variations of the stellar emission. In the L band we found semi-regular oscillations with the two main periods of similar to3.3 yr and similar to11.9 yr and the full amplitude of similar to0(.)(m)8 and similar to0(.)(m)6, respectively. The colors of the warm dust shell (resolved by Ohnaka et al. 2001) are found to be remarkably stable in contrast to its brightness. This indicates that the inner radius is a constant, time-independent characteristic of the dust shell. The observed behavior of the IR light curve is mainly caused by the variation of the optical thickness of the dust shell within the interval tau(V) = 0.2-0.4. Anticorrelated changes of the optical brightness (in particular with P approximate to 3.3 yr) have not been found. Their absence suggests that the stellar wind of R CrB deviates from spherical symmetry. The light curves suggest that the stellar wind is variable. The variability of the stellar wind and the creation of dust clouds may be caused by some kind of activity on the stellar surface. With some time lag, periods of increased mass-loss cause an increase in the dust formation rate at the inner boundary of the extended dust shell and an increase in its IR brightness. We have derived the following parameters of the dust shell (at mean brightness) by radiative transfer modeling: inner dust shell radius r(in) approximate to 110 R-*, temperature T- dust(r(in)) approximate to 860 K, dust density rho(dust) (r(in)) approximate to 1.1 x 10(-20) g cm(-3), optical depth tau(V) approximate to 0.32 at 0.55 mum, mean dust formation rate (M) over dot (dust) approximate to 3.1 x 10(-9) M. yr(-1), mass-loss rate (M) over dot (gas) approximate to 2.1 x 10(-7) M. yr(-1), size of the amorphous carbon grains less than or similar to0.01 mum, and B - V approximate to 0.28