during a programme to study apparently normal early-type stars at high galactic latitudes, we have identified a small number of targets with common peculiarities. these include very small projected rotational velocities and systematic metal under- abundances coupled with larger carbon depletions. we have identified these targets as at a pagb evolutionary stage intermediate between the agb/cool-pagb and planetary nebula regimes. our prototype is lsiv -12 111 for which we have extensive ultra-violet and optical spectroscopy and infra-red photometry. analysis of these data indicate that the central star has an effective temperature of 24000k, a logarithmic surface gravity of 2.8 and a typical chemical composition with most metals being moderately underabundant but with carbon being more severely depleted by at least 1.0 dex. the stellar temperature is sufficient to excite the surrounding nebula which has a mass of 0.005 solar masses. iso observations will be made of both the nebular emission lines and the stellar/nebular continuum. the former are required to determine the nebular abundances and in particular confirm the carbon abundance deduced from the stellar spectrum. this significantly differs from those deduced from cool pagb stellar spectra and implies that there may be different classes of pagb stars depending on whether they leave the agb prior to or after the third dredge up. the continuum observations will provide information on the rate of dispersal of the agb remnant, while the relative strengths of dust features will provide further constraints on the carbon abundance.
Instrument
LWS01 , SWS01
Temporal Coverage
1996-10-17T23:18:52Z/1996-10-18T01:49:38Z
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.
