Observations of variable stars with the R Coronae Borealis phenomenon and other unique objects

Abstract

The results of photometry and polarimetry of the R Coronae Borealis type stars and other interesting objects are given. The observation of the former objects are obtained at the light maximum or at a brightness lower by 2-3 mag. IntroductionThe unique behaviour of visual brightness of R Coronae Borealis and several other related stars formed the basis to distinguish the RCB variable star type. The reason for attributing a variable to the RCB group is the combination of the following features: 1) irregular brightness decreases by 1-8 mag during several tens of days with a longer recovery to the normal light; 2) peculiar abundances: a large deficit of hydrogen and a carbon excess; 3) infrared excess due to circumstellar dust (a permanent envelope). The carbon excess according to the Loreta-O'Keefe hypothesis, which has been developed by many authors, facilitates the formation of carbon-rich dust in the upper layers in the stellar atmosphere which obscures the star. The dilution of this dust in a surrounding space by the radiation pressure leads to the recovery of normal brightness. A growth of the polarization up to 10% and more is observed during the active state. The permanent circumstellar envelope is not a consequence of these rare enough events, but it is the result of permanent intense mass loss from the star. The permanent mass loss results from the pulsations of these stars (Rosenbush 1981(Rosenbush , 1984Fadeyev 1984). The luminosity of the circumstellar envelope (i.e. its mass) is related in some way to the amplitude of stellar pulsations. Variable stars with the R Coronae Borealis phenomenon are few and this is due to the fact that they are connected with a short stage of stellar evolution. Their peculiar abundance is associated with the loss of upper hydrogen-rich stellar layers and with the exposed deeper carbon-rich ones. The existence of a very cold (the effective temperature is several tens of degrees) envelope around R CrB is related just to this process (Gillett et al. 1986). Such an envelope exists around SU Tau and V348 Sgr, too (Rao and Nandy 1986).Unfortunately, an insufficiency of the information about intrinsic fundamental properties, which are not distorted by observational selection, compels some conclusions to be spread on the whole group of stars. For example, the high luminosity of RY Sgr and some members of the Large Magellanic Cloud (W Men, HV 5637 and HV 12842) are attributed to the rest of the group. But this conclusion was not confirmed by our studies (Rosenbush 1981(Rosenbush , 1989a. The RCB stars occupy a large region in the HR diagram, which may testify that they belong to different evolutionary tracks. This circumstance together with the fact known long ago that RCB photometric behaviour is observed in some novae (Rosenbush 1988) and in PU Vul (Men'shchikov et al. 1985) has served the basis for our conclusion that there is no compact group of variable stars of RCB type with respect to their physical properties, but there are stars which are at different late stages of ste...

Polarimetry of evolved stars

“…Note, however, that our data were obtained in different filters (I C and "C"), so a direct comparison is almost certainly not justified. The UBVR polarimetry of RY Sgr published by Rosenbush (1995) show a higher level of polarization (∼0.5−0.8%, compared to our 0.2% in I C and 0.37% in "C"), with PA θ = 178…”

“…Some objects show relatively stable IR fluxes during several decline phases (Feast 1990), or incoherent variations of IR and optical fluxes on a time-scale a few hundred days (see Fig. 1 of Rosenbush 1995), while for others IR flux and decline activity are correlated (Glass 1994).…”

Finding the most variable stars in the Orion Belt with the All Sky Automated Survey

“…It has been investigated in detail, especially after Kovalchuk (1984) observed a flare-like event on its light curve at the U BV R bands (Wouterloot & Walmsley 1986;Sterken et al 1993;Rosenbush 1995;Marconi et al 2000;Balona et al 2002;Vieria et al 2003). It has been proposed that a temporarily strong accretion of matter onto the star or extinction by circumstellar dust clouds takes place to explain the remarkable V351 Ori change of behaviour from "that of a [HAeBe] star with strong photometric variations [...] to that of an almost non-variable star" (van den Ancker et al 1996).…”

Dust around R Coronae Borealis stars: I. Spitzer/IRS observations

“…Photometry at maximum light is assembled from the literature: BVRI (Rosenbush 1995), JHK (2MASS), JHKL (Gaustad et al 1988). An interstellar reddening of E(B-V)=0.5 is adopted .…”