we propose to use the short wavelength spectrometer to observe a sample of highly-obscured asymptotic giant branch stars in both the lmc and the smc. these stars, which have been identified through near-ir follow-up observations of iras sources, are in the process of evolving from the agb to become planetary nebulae. although the classical limit to agb evolution is m(bol)=-7.1, these giants have bolometric magnitudes as low as -6. the aim of our observations is to determine whether these stars have the 11.6 micron sic emission that characterises carbon-rich stars, or the broad 10 and 18 micron emission features of the o-rich m-type stars. this is of particular interest since, although carbon stars are common amongst the lower-luminosity agb stars, very few are found brighter than m(bol)=-6, despite the theoretical expectation that the brighter, more massive stars should be more efficient at synthesising carbon. it is possible that envelope-burning may be responsible for maintaining c < o, but another possibility is that the higher opacity in c-rich envelopes may truncate agb evolution through high mass-loss, thereby accounting for the general scarcity of luminous agb stars. in the latter case, we would expect the highly-obscured, cocoon stars to have c-rich dust shells. since all of our sample are optically invisible, iso represents the only method of determining the chemical composition of the circumstellar dust-shells.
Instrument
SWS01
Temporal Coverage
1996-04-29T03:13:37Z/1997-08-01T09:40:36Z
Version
1.0
Mission Description
The Infrared Space Observatory (ISO) was the world's first true orbiting infrared observatory. Equipped with four highly-sophisticated and versatile scientific instruments, it was launched by Ariane in November 1995 and provided astronomers world-wide with a facility of unprecedented sensitivity and capabilities for a detailed exploration of the Universe at infrared wavelengths.
