The Circumstellar Environment of R Coronae Borealis: White Dwarf Merger or Final-helium-shell Flash?

Clayton, Geoffrey C., Sugerman, Ben E. K., Stanford, S. Adam, Whitney, B. A., Honor, J., Babler, B., Barlow, M. J., Gordon, K. D., Andrews, J. E., Geballe, T. R., Bond, Howard E., De Marco, O., Lawson, W. A., Sibthorpe, B., Olofsson, G., Polehampton, E., Gomez, H. L., Matsuura, M., Hargrave, P. C., Ivison, R. J., Wesson, R., Leeks, S. J., Swinyard, B. M., & Lim, T. L.
2011, The Astrophysical Journal, 743, 44

In 2007, R Coronae Borealis (R CrB) went into a 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 ЮМm with Gemini/GMOS, Hubble Space Telescope/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 ЮМm. The spectral energy distribution of R CrB can be well fitted 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 2 M т??, 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. Based in part on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, and from the data archive at STScI, which are operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.

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