UBVJHKLM photometry and modeling of R Coronae Borealis^*

¹ Sternberg Astronomical Institute, Universitetskii prospect 13, 119899 Moscow, Russia
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ Isaac Newton Institute of Chile, Moscow Branch, Russia
⁴ David Dunlap Observatory, University of Toronto, Box 360, Richmond Hill, Ontario L4C 4Y6, Canada
⁵ Central Astronomical Observatory RAS at Pulkovo, Pulkovo 65/1, 196140 St. Petersburg, Russia

Corresponding author: B. Yudin, yudin@sai.msu.ru

Received: 26 April 2002
Accepted: 7 August 2002

Abstract

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 ~3.3 yr and ~11.9 yr and the full amplitude of ~ and ~, respectively. The colors of the warm dust shell (resolved by Ohnaka et al. [CITE]) 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 -0.4. Anticorrelated changes of the optical brightness (in particular with  yr) have not been found. Their absence suggests that the stellar wind of R CrBdeviates 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 , temperature  K, dust density , optical depth at 0.55 μm, mean dust formation rate , mass-loss rate , size of the amorphous carbon grains m, and .