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Register a new userUBVJHKLM photometry and modeling of R Coronae Borealis
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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 ~0.8 mag and ~0.6 mag, 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 ~ 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 ~ 110 R_*, temperature T_dust(r_in) ~ 860 K, dust density rho_dust(r_in) ~ 1.1x10^{-20} g cm^-3, optical depth tau(V) ~ 0.32 at 0.55 micron, mean dust formation rate [dM/dt]_dust ~ 3.1x10^-9 M_sun / yr, mass-loss rate [dM/dt]_gas ~ 2.1x10^-7 M_sun / yr, size of the amorphous carbon grains <(~) 0.01 micron, and B-V ~ -0.28.
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Neutral fluorine (F I) lines are identified in the optical spectra of several R Coronae Borealis stars (RCBs) at maximum light. These lines provide the first measurement of the fluorine abundance in these stars. Fluorine is enriched in some RCBs by factors of 800 to 8000 relative to its likely initial abundance. The overabundances of fluorine are evidence for the synthesis of fluorine. These results are discussed in the light of the scenario that RCBs are formed by accretion of an He white dwarf by a C-O white dwarf. Sakurais object (V4334 Sgr), a final He-shell flash product, shows no detectable F I lines.
EROS 2 (Experience de Recherche dObjets Sombres) conducted a survey of the SMC between July 1996 and February 2003 in two EROS broad-band colours, V_E and R_E. The photometric data of 4.2 million stars have been searched for behaviour typical of R Coronae Borealis (RCB) candidates such as drastic changes in magnitude. Five objects have been found, four of them being catalogued in the Simbad database as RAW 21, RAW 233, RAW 476, and [MH95] 431 with confirmed carbon-rich atmospheres, characteristic of RCB. From the EROS 2 light curve of RAW 21 and its spectrum reported by Morgan et al. (2003), we confirm that it is the first RCB to be found in the SMC. The other objects are new RCB candidates with absolute luminosity and colour close to those found for RCBs in the LMC. We propose that 2 of them are DY Per-like RCBs.
The R Coronae Borealis (RCB) stars are rare hydrogen-deficient, carbon-rich, supergiants, best known for their spectacular declines in brightness at irregular intervals. Efforts to discover more RCB stars have more than doubled the number known in the last few years and they appear to be members of an old, bulge population. Two evolutionary scenarios have been suggested for producing an RCB star, a double degenerate merger of two white dwarfs, or a final helium shell flash in a planetary nebula central star. The evidence pointing toward one or the other is somewhat contradictory, but the discovery that RCB stars have large amounts of 18O has tilted the scales towards the merger scenario. If the RCB stars are the product of white dwarf mergers, this would be a very exciting result since RCB stars would then be low-mass analogs of type Ia supernovae. The predicted number of RCB stars in the Galaxy is consistent with the predicted number of He/CO WD mergers. But, so far, only about 65 of the predicted 5000 RCB stars in the Galaxy have been discovered. The mystery has yet to be solved.
The R Coronae Borealis (RCB) stars are rare hydrogen--deficient, carbon--rich supergiants. They undergo extreme, irregular declines in brightness of many magnitudes due to the formation of thick clouds of carbon dust. It is thought that RCB stars result from the mergers of CO/He white dwarf (WD) binaries. We constructed post--merger spherically asymmetric models computed with the MESA code, and then followed the evolution into the region of the HR diagram where the RCB stars are located. We also investigated nucleosynthesis in the dynamically accreting material of CO/He WD mergers which may provide a suitable environment for significant production of 18O and the very low 16O/18O values observed. We have also discovered that the N abundance depends sensitively on the peak temperature in the He--burning shell. Our MESA modeling consists of engineering the star by adding He--WD material to an initial CO--WD model, and then following the post--merger evolution using a nuclear--reaction network to match the observed RCB abundances as it expands and cools to become an RCB star. These new models are more physical because they include rotation, mixing, mass-loss, and nucleosynthesis within MESA. We follow the later evolution beyond the RCB phase to determine the stars likely lifetimes. The relative numbers of known RCB and Extreme Helium (EHe) stars correspond well to the lifetimes predicted from the MESA models. In addition, most of computed abundances agree very well with the observed range of abundances for the RCB class.
Some of the observational aspects related to the evolutionary status and dust production in R Cor Bor stars are discussed. Recent work regarding the surface abundances, stellar winds and evidence for dust production in these high luminosty hydrogen deficient stars are also reviewed. Possibility of the stellar winds being maintained by surface magnetic fields is also considered.
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