Proto planetary nebulae
Proto PN ? transition phase between AGB and PN phases, Teff of the central star (Sp type A-K) rises and the dust IR emission excess moves towards long l.
Searches of Proto PN are based on IR excesses but optically visible stars.
ISO observations revealed:
- a wealth of new spectral information, new molecules discovered
- unidentified red stars at low Galactic latitude from surveys
- coexistence of C-rich and O-rich circumstellar material
To be addressed by MID IR imaging:
- morphology of circumstellar shells for different spectral types and dust composition (bipolarity associated with chemical composition?).