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==> DISCRETIONARY TIME PROPOSAL
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As shown in the A&A ISO special issue, the spectrum of R CrB in the
PHT-S range (2.5um to 5um and 6um to 11.6um) is very unusual, in that
it shows a broad plateau around 6um to 8um. Although the dust shell
is known to be very carbon rich, there was no sign of PAH (Polycyclic
Aromatic Hydrocarbon) emission. R CrB has been found to have no hydrogen
in its stellar atmosphere. A second member of the class, RY Sgr, shows
a very similar spectrum, with a broad plateau and no PAH emission,
and again this star is known to have no hydrogen in its atmosphere.
A second PHT-S spectrum of R CrB has shown the flux density of the
plateau has changed over several months.
We propose to observe two R CrB stars, V854 Cen and FG Sge, which have
a little hydrogen in their atmospheres (1) to see if they show PAH
emission (2) to see if they show this broad plateau and (3) to see if
the spectrum changes over a period of months.
V854 Cen is nearing the end of its visibility to ISO, so the request for
time is urgent (its visibility ends on 6th September 1997).