Garcia-Lario P. 1, Manchado A. 2, Ulla A. 3, & Manteiga M. 3
1 ISO Science Operations Centre. Estacion de Villafranca del Castillo. Apartado de Correos 50727. E-28080 Madrid, Spain
2 Instituto de Astrofisica de Canarias, E-38200 La Laguna, Tenerife, Spain
3 Departamento de Fisica Aplicada. Facultade de Ciencias. Campus Marcosende-Lagoas. Universidade de Vigo. Apartado de Correos 874. E-36200 Vigo, Pontevedra, Spain
Based on ISO SWS+LWS observations, we identify IRAS 16594-4656 as a new member of the rare class of C-rich proto-planetary nebulae with the 21 microns dust feature in emission. In addition, the major PAHs emission bands at 3.3, 6.2, 7.7, 8.6 and 11.3 microns are detected, together with unusually strong features at 12.6 and 13.4 microns, very rarely observed, attributed to PAHs species with a high degree of hydrogenation.
The shapes and relative intensities of the various features found are consistent with a mixed population of transient decomposition products of large hydrogenated amorphous carbonaceous (HAC) grains, consisting of fullerenes with a different degree of hydrogenation, which we suggest to be the main carriers of the strong 21 microns feature, and a combination of small and large partially dehydrogenated cationic PAHs molecules excited by the visible radiation coming from the central star.
Additional features corresponding to crystalline silicates (mostly pyroxenes) are also detected, together with a possible very broad unidentified emission feature centered around 30 microns, previously reported to appear in combination with the 21 microns feature in other proto-planetary nebulae candidates.
The combination of features corresponding to O-rich and C-rich dust grains found, if real, would suggest a recent change to a C-rich chemistry in the outer envelope of IRAS 16594-4656.
With the help of optical ground-based spectroscopy and HST archive images we identify IRAS 16594-4656 as a new transition object between the AGB and the PN stage. The ground-based optical spectrum shows only the Balmer lines in emission and a very faint, probably shocked excited, [O I] emission over a very red continuum where strong, probably circumstellar, DIBs at 5780 and 6281-6284 angstroms are clearly seen. The HST WFPC2 continuum images reveal the presence of a bright central star surrounded by a bipolar reflection nebula with a multiple-axis morphology and a maximum size of 5'' x 11''.