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Post Info TOPIC: R Coronae Borealis star
Blobrana


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RE: R Coronae Borealis star
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Blobrana


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Posts: 131433
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HD 141527
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Title: What is the Shell Around R Coronae Borealis?
Author: Edward J. Montiel, Geoffrey C. Clayton, Dominic C. Marcello, Felix J. Lockman

The hydrogen-deficient, carbon-rich R Coronae Borealis (RCB) stars are known for being prolific producers of dust which causes their large iconic declines in brightness. Several RCB stars, including R CrB, itself, have large extended dust shells seen in the far-infrared. The origin of these shells is uncertain but they may give us clues to the evolution of the RCB stars. The shells could form in three possible ways. 1) they are fossil Planetary Nebula (PN) shells, which would exist if RCB stars are the result of a final, helium-shell flash, 2) they are material left over from a white-dwarf merger event which formed the RCB stars, or 3) they are material lost from the star during the RCB phase. Arecibo 21-cm observations establish an upper limit on the column density of H I in the R CrB shell implying a maximum shell mass of \lesssim 0.3 solar masses. A low-mass fossil PN shell is still a possible source of the shell although it may not contain enough dust. The mass of gas lost during a white-dwarf merger event will not condense enough dust to produce the observed shell, assuming a reasonable gas-to-dust ratio. The third scenario where the shell around R CrB has been produced during the star's RCB phase seems most likely to produce the observed mass of dust and the observed size of the shell. But this means that R CrB has been in its RCB phase for ~104 yr.

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Blobrana


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RE: R Coronae Borealis star
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Title: Direct imaging of a massive dust cloud around R Coronae Borealis
Authors: S.V. Jeffers (U Utrecht), M. Min (U Utrecht), L.B.F.M. Waters (SRON, U Amsterdam), H. Canovas (U Utrecht), M. Rodenhuis (U Utrecht), M. De Juan Ovelar (U Utrecht), A.L. Chies-Santos (U Utrecht), C.U. Keller (U Utrecht)

We present recent polarimetric images of the highly variable star R CrB using ExPo and archival WFPC2 images from the HST. We observed R CrB during its current dramatic minimum where it decreased more than 9 mag due to the formation of an obscuring dust cloud. Since the dust cloud is only in the line-of-sight, it mimics a coronograph allowing the imaging of the star's circumstellar environment. Our polarimetric observations surprisingly show another scattering dust cloud at approximately 1.3" or 2000 AU from the star. We find that to obtain a decrease in the stellar light of 9 mag and with 30% of the light being reemitted at infrared wavelengths (from R CrB's SED) the grains in R CrB's circumstellar environment must have a very low albedo of approximately 0.07%. We show that the properties of the dust clouds formed around R CrB are best fitted using a combination of two distinct populations of grains size. The first are the extremely small 5 nm grains, formed in the low density continuous wind, and the second population of large grains (~0.14 {\mu}m) which are found in the ejected dust clouds. The observed scattering cloud, not only contains such large grains, but is exceptionally massive compared to the average cloud.

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Blobrana


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R Corona Borealis
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Title: Nebula around R Corona Borealis
Authors: N. Kameswara Rao (1 and 2), David L. Lambert (1), ((1) The W. J. McDonald Observatory, The University of Texas, Austin, USA, (2) Instituto de Astrofísica de Canarias, IAC. La Laguna, Tenerife, Spain)

The star R Corona Borealis (R CrB) shows forbidden lines of [O II], [N II], and [S II] during the deep minimum when the star is fainter by about 8 to 9 magnitudes from normal brightness, suggesting the presence of nebular material around it. We present low and high spectral resolution observations of these lines during the ongoing deep minimum of R CrB, which started in July 2007. These emission lines show double peaks with a separation of about 170 km/s. The line ratios of [S II] and [O II] suggest an electron density of about 100 cm^{-3}. We discuss the physical conditions and possible origins of this low density gas. These forbidden lines have also been seen in other R Coronae Borealis stars during their deep light minima and this is a general characteristic of these stars, which might have some relevance to their origins.

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Blobrana


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RE: R Coronae Borealis star
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Title: The Circumstellar Environment of R Coronae Borealis: White Dwarf Merger or Final Helium Shell Flash?
Authors: Geoffrey C. Clayton, Ben E.K. Sugerman, S. Adam Stanford, B. A. Whitney, J. Honor, B. Babler, M.J. Barlow, K.D. Gordon, J.E. Andrews, T.R. Geballe, Howard E. Bond, O. De Marco, W.A. Lawson, B. Sibthorpe, G. Olofsson, E. Polehampton, H. L. Gomez, M. Matsuura, P. C. Hargrave, R. J. Ivison, R. Wesson, S. J. Leeks, B. M. Swinyard, T. L. Lim

In 2007, R Coronae Borealis (R CrB) went into an historically deep and long decline. In this state, the dust acts like a natural coronagraph at visible wavelengths, allowing faint nebulosity around the star to be seen. Imaging has been obtained from 0.5 to 500 micron with Gemini/GMOS, HST/WFPC2, Spitzer/MIPS, and Herschel/SPIRE. Several of the structures around R CrB are cometary globules caused by wind from the star streaming past dense blobs. The estimated dust mass of the knots is consistent with their being responsible for the R CrB declines if they form along the line of sight to the star. In addition, there is a large diffuse shell extending up to 4 pc away from the star containing cool 25 K dust that is detected all the way out to 500 micron. The SED of R CrB can be well fit by a 150 AU disk surrounded by a very large diffuse envelope which corresponds to the size of the observed nebulosity. The total masses of the disk and envelope are 10^-4 and 10^2 M(Sun), respectively, assuming a gas-to-dust ratio of 100. The evidence pointing toward a white-dwarf merger or a final-helium-shell flash origin for R CrB is contradictory. The shell and the cometary knots are consistent with a fossil planetary nebula. Along with the fact that R CrB shows significant Lithium in its atmosphere, this supports the final-helium-shell flash. However, the relatively high inferred mass of R CrB and its high fluorine abundance support a white-dwarf merger.


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Blobrana


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R Coronae Borealis and DY Per type stars
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Title: New Magellanic Cloud R Coronae Borealis and DY Per type stars from the EROS-2 database: the connection between RCBs, DYPers and ordinary carbon stars
Authors: P.Tisserand, P.R.Wood, J.B.Marquette, C.Afonso, J.N.Albert, J.Andersen, R.Ansari, E.Aubourg, P.Bareyre, J.P.Beaulieu, X.Charlot, C.Coutures, R.Ferlet, P.Fouqué, J.F.Glicenstein, B.Goldman, A.Gould, M.Gros, J.de Kat, E.Lesquoy, C.Loup, C.Magneville, E.Maurice, A.Maury, A.Milsztajn, M.Moniez, N.Palanque-Delabrouille, O.Perdereau, J.Rich, P.Schwemling, M.Spiro, A.Vidal-Madjar

R Coronae Borealis stars (RCB) are a rare type of evolved carbon-rich supergiant stars that are increasingly thought to result from the merger of two white dwarfs, called the Double degenerate scenario. This scenario is also studied as a source, at higher mass, of type Ia Supernovae (SnIa) explosions. Therefore a better understanding of RCBs composition would help to constrain simulations of such events. We searched for and studied RCB stars in the EROS Magellanic Clouds database. We also extended our research to DY Per type stars (DYPers) that are expected to be cooler RCBs (T~3500 K) and much more numerous than their hotter counterparts. The light curves of ~70 millions stars have been analysed to search for the main signature of RCBs and DYPers: a large drop in luminosity. Follow-up optical spectroscopy was used to confirm each photometric candidate found. We have discovered and confirmed 6 new Magellanic Cloud RCB stars and 7 new DYPers, but also listed new candidates: 3 RCBs and 14 DYPers. We estimated a range of Magellanic RCB shell temperatures between 360 and 600 K. We confirm the wide range of absolute luminosity known for RCB stars, M_V~-5.2 to -2.6. Our study further shows that mid-infrared surveys are ideal to search for RCB stars, since they have thinner and cooler circumstellar shells than classical post-AGB stars. In addition, by increasing the number of known DYPers by ~400%, we have been able to shed light on the similarities in the spectral energy distribution between DYPers and ordinary carbon stars. We also observed that DYPer circumstellar shells are fainter and hotter than those of RCBs. This suggests that DYPers may simply be ordinary carbon stars with ejection events, but more abundance analysis is necessary to give a status on a possible evolutionary connexion between RCBs and DYPers.

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Blobrana


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Posts: 131433
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RE: R Coronae Borealis star
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R Coronae Borealis is at its' minimum, V = 14.5


R Coronae Borealis is a yellow supergiant star, and is the prototype of the RCB class of variable stars, which fade by several magnitudes at irregular intervals. R Coronae Borealis itself normally shines at approximately magnitude 6, just about visible to the naked eye, in the constellation of Corona Borealis, but at intervals of several months to many years fades to as faint as magnitude 14.
Source 

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Blobrana


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R Coronae Borealis stars
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Title: R Coronae Borealis stars in the Galactic Bulge discovered by EROS-2
Authors: P.Tisserand, J.B.Marquette, P.R.Wood, E.Lesquoy, J.P.Beaulieu, A.Milsztajn, C.Hamadache, C.Afonso, J.N.Albert, J.Andersen, R.Ansari, E.Aubourg, P.Bareyre, X.Charlot, C.Coutures, R.Ferlet, P.Fouqué, J.F.Glicenstein, B.Goldman, A.Gould, M.Gros, J.Haissinski, J.de Kat, L.Le Guillou, C.Loup, C.Magneville, E.Maurice, A.Maury, M.Moniez, N.Palanque-Delabrouille, O.Perdereau, Y.Rahal, J.Rich, M.Spiro, A.Vidal-Madjar, S.Zylberajch

Rare types of variable star may give unique insight into short-lived stages of stellar evolution. The systematic monitoring of millions of stars and advanced light curve analysis techniques of microlensing surveys make them ideal for discovering also such rare variable stars. One example is the R Coronae Borealis (RCB) stars, a rare type of evolved carbon-rich supergiant. We have conducted a systematic search of the EROS-2 database for the Galactic catalogue Bulge and spiral arms to find Galactic RCB stars. The light curves of ~100 million stars, monitored for 6.7 years (from July 1996 to February 2003), have been analysed to search for the main signature of RCB stars, large and rapid drops in luminosity. Follow-up spectroscopy has been used to confirm the photometric candidates. We have discovered 14 new RCB stars, all in the direction of the Galactic Bulge, bringing the total number of confirmed Galactic RCB stars to about 51. After reddening correction, the colours and absolute magnitudes of at least 9 of the stars are similar to those of Magellanic RCB stars. This suggests that these stars are in fact located in the Galactic Bulge, making them the first RCB stars discovered in the Bulge. The localisation of the 5 remaining RCBs is more uncertain: 4 are either located behind the Bulge at an estimated maximum distance of 14 kpc or have an unusual thick circumstellar shell; the other is a DY Per RCB which may be located in the Bulge, even if it is fainter than the known Magellanic DY Per. From the small scale height found using the 9 new Bulge RCBs, 61<h^{RCB}_{Bulge}<246 pc (95% C.L.), we conclude that the RCB stars follow a disk-like distribution inside the Bulge.

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Blobrana


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RE: R Coronae Borealis star
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Title: Fluorine in R Coronae Borealis Stars
Authors: Gajendra Pandey (1), David L. Lambert (2), N. Kameswara Rao (1) ((1) Indian Institute of Astrophysics, Bangalore, India, (2) The W.J. McDonald Observatory, University of Texas at Austin, TX, USA)

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. Sakurai's object (V4334 Sgr), a final He-shell flash product, shows no detectable F I lines.

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RY Sagittari.kmz
Google Sky File

-- Edited by Blobrana at 16:32, 2008-01-12

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