Contents of: VI/111/./abstract/JHARRING_HPOORPNE.abs

The following document lists the file abstract/JHARRING_HPOORPNE.abs from catalogue VI/111.
A plain copy of the file (without headers/trailers) may be downloaded.

  Hydrogen-poor  planetary nebulae offer an exceptional opportunity to advance
our  knowledge of the physical nature,  size  distribution,  and  evolution of
carbonaceous  dust  grains.   The  inner  ejecta  of  these  objects  has   an
extraordinarily high dust to gas ratio ( approaching 1/5) with the consequence
that the thermal dust continuum is not confused by line emission from the gas.
This is true even in the 3 - 10 micron region,  where the dust emission is due
to  the  stochastic  heating of small carbon  clusters.  Using  the  available
ground-based  and  IRAS data,  we have already demonstrated for one  of  these
objects  - Abell 30 - that is is feasible to model the entire IR spectrum from
2  - 200 microns and in the process derive the (dust/gas) ratio,  the  minimum
grain size,  and the slope of the size distribution,  i.e., the ratio of small
to large grains. With ISO data we will be able determine these parameters more
precisely and for all the known members of this class.  Since the known H-poor
planetaries form an evolutionary sequence,  we will thus be able to  determine
whether and how dust evolves with time in these nebulae.
  Another important objective of this study is to look for emission features,
such  as  the  C-C stretch,  which  the small amorphous  carbon  clusters  may
produce.  The smallest grains in the H-poor planetaries are expected to differ
from  those  in  normal  planetaries because they formed  in  the  absence  of
hydrogen,  and we would not find PAHs,  for example.  The high sensitivity and
spectral resolution of ISO will allow us to search for any such features.
  Finally,  the imaging capabilities of ISO will allow us,  for the largest of
these  objects,  to map the grain properties as a function of position in  the
nebula.  This  will  allow  us  to test models of heating  and  to  study  the
transport of dusty ejecta by mass-loading of the stellar wind from the central
star.  This  later  application  will  complement our  ongoing  study  of  the
morphology  of  the  H-poor  planetaries  with  the  Hubble  Space  Telescope.

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