Publications of the MPIfR
Optical & Infrared
Interferometry Group
V.I. Shenavrin, J.D. Fernie, G.
Weigelt, B.F. Yudin
Photometric Observations of R Coronae
Borealis in the Optical and Infrared
Astronomy Reports, Vol. 46, Issue 10, pp.
805-818
Abstract:
The results of long-term photometric observations of R CrB in the
UBVJHKLM 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] 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 (rin [approximate] 110R*, Tgr [approximate] 860 K), from material
in its stellar wind, whose intensity (M-dot gas [approximate] 2.1 ×
10–7M[sun]/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] 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 (agr <= 0.01
µm), while the clouds screening the star from the observer are made up
of large grains (agr [approximate] 0.1 µm) . 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 ~4 years (the time for a particle
moving at Venv [approximate] 45 km/s to move from the star to the
boundary of the dust envelope).©2002 MAIK "Nauka / Interperiodica".
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