Infrared Space Observatory observations of IRAS 16594-4656: A new proto-planetary nebula with a strong 21 micron dust feature

Based on ISO short-wavelength spectrograph (SWS)+ long-wavelength spectrogr aph (LWS) observations, IRAS 16594-4656 is identified as a new member of th e rare class of C-rich proto-planetary nebulae with the 21 mu m dust featur e in emission. In addition, the major polycyclic aromatic hydrocarbon (PAH) emission bands at 3.3, 6.2, 7.7, 8.6, and 11.3 mu m are detected, together with unusually strong features at 12.6 and 13.4 mu m, very rarely observed , attributed to PAH 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 hydrogenat ed amorphous carbonaceous (HAC) grains, consisting of fullerenes with diffe rent degrees of hydrogenation, which we suggest to be the main carriers of the strong 21 mu m feature, and a combination of small and large partially dehydrogenated cationic PAH molecules excited by the visible radiation comi ng from the central star. Additional features corresponding to crystalline silicates (mostly pyroxenes) are also tentatively detected, together with a possible very broad unidentified emission feature centered around 30 mu m, previously reported as appearing in combination with the 21 mu m feature i n other proto-planetary nebulae candidates. The combination of features cor responding to O-rich and C-rich dust grains, if real, would suggest a recen t change to a C-rich chemistry in the outer envelope of IRAS 16594-4656. Wi th the help of optical ground-based spectroscopy and recently obtained Hubb le Space Telescope (HST) images, we identify IRAS 16594-4656 as a new trans ition object between the asymptotic giant branch (post-AGB) and the planeta ry nebula (PN) stage. The ground-based optical spectrum shows only the Balm er lines in emission and a faint, probably shock-excited, [O I] emission ov er a very red continuum where strong, probably circumstellar, diffuse inter stellar absorption bands (DIBs) at 5780 and 6281-6284 Angstrom are clearly seen. HST WFPC2 continuum images reveal the presence of a bright central st ar surrounded by a bipolar reflection nebula with a multiple-axis morpholog y and a maximum size of similar to 5" x 11".