Dust Formation in R Coronae Borealis Stars
Abstract
In the 1930's, it was deduced that the spectacular declines seen in RCB stars were caused by the condensation of carbon-rich gas into an optically thick dust cloud lying along the line of sight. However, we still don't understand the mechanisms of mass loss or dust formation in these stars. The nature of the emission seen during the declines is also not well understood. In the last 5 years, large numbers of visible and ultraviolet spectroscopic, photometric and polarimetric observations have been obtained of several R CrB stars which provide compelling observational evidence that dust forms in or near the photospheres of these stars. The problem with this conclusion is that the photospheric temperature of RCB stars is 5000-7000 K. Typical dust models require temperatures of < 2000 K to condense. However, recent laboratory results show that amorphous carbons can form at higher temperatures approaching 4000 K. The gas which condenses into dust may not be in thermal equilibrium, so the thermal equilibrium temperature is not particularly relevant. If these results are applicable to RCB star dust formation then the wide range of observations made during declines can be understood. Dust forms in or near the photosphere, accelerates due to radiation pressure to ~ 200 km/s in a few weeks, eclipses the photosphere and chromosphere, and gradually disperses on timescales of months. This work has been supported by NASA grant NAG5-1672.
- Publication:
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American Astronomical Society Meeting Abstracts
- Pub Date:
- December 1992
- Bibcode:
- 1992AAS...181.1005C