The main component of protoplanetary nebulae (PPNe) is thought to be
neutral or slightly ionized atomic gas. In fact, such a composition is
characteristic of PPNe, evolutionarily placed between the (commonly)
molecule-rich AGB envelopes and the strongly ionized planetary nebulae.
Since the excitation of the PPN gas is still low, the observation of such
atomic component should be done by means of the fine-structure lines. For
well known technical reasons, however, these lines were difficult to detect
before ISO. The fact that we were observationally missing the main nebular
component suggests that our ideas on the nebular conditions and structure
during this evolutionary phase may be severely incomplete.
Our ISO project 'Atomic gas in protoplanetary nebulae' (PROPID: PPNA, OBSID:
VBUJARRA) is already yielding data on atomic line emission in PPNe. The
first results by ISO are very encouraging at this respect and show quite
rich atomic spectra in PPNe. But these first results, together with some
other recent data, also indicate that the present ISO observations need a
follow up. ISO is not properly studying the atomic emission from important
PPN subclasses, mainly young molecule-rich and IR-bright PPNe and objects
showing strong shocks. We argue that data with high spectral resolution are
in particular needed.
We therefore propose to complete the ongoing observations with new data of
fine-structure atomic lines in selected PPNe. The line list, that includes
transitions of different atoms and with different excitation requirements,
is similar to that of our previous work. The source sample, on the contrary,
consists of objects selected in the sense explained above, i.e. young
molecule-rich and IR-identified PPNe, as well as PPNe showing strong shocks.