2000A&A...362..984V

In this article we report further investigations of the IRAS selected sample of Planetary Nebula (PN) candidates that was presented in Van de Steene & Pottasch (1993A&A...274..895V). About 20% of the candidates in that sample have been detected in the radio and/or Hα and later confirmed as PNe. Here we investigate the infrared properties of the IRAS sources not confirmed as PNe. We observed 28 objects in the N-band of which 20 were detected and 5 were resolved, despite adverse weather conditions. We obtained medium resolution Brγ spectra and we took high resolution JHKL images of these 20 objects. We critically assessed the identification of the IRAS counterpart in the images and compared our identification with others in the literature. High spatial resolution and a telescope with very accurate pointing are crucial for correct identification of the IRAS counterparts in these crowded fields. Of sixteen positively identified objects, seven show Brγ in absorption. The absorption lines are very narrow in six objects, indicating a low surface gravity. Another six objects show Brγ in emission. Two of these also show photospheric absorption lines. All emission line sources have a strong underlying continuum, unlike normal PNe. In another three objects, no clear Brγ absorption or emission was visible. The fact that our objects were mostly selected from the region in the IRAS color-color diagram where typically PNe are found, may explain our higher detection rate of emission line objects compared to previous studies, which selected their candidates from a region between AGB and PNe. The objects showing Brγ in emission were re-observed in the radio continuum with the Australia Telescope Compact Array. None of them were detected above a detection limit of 0.55mJy/beam at 6cm and 0.7mJy/beam at 3cm, while they should have been easily detected if the radio flux was optically thin and Case B recombination was applicable. It is suggested that the Brγ emission originates in the post-Asymptotic Giant Branch (post-AGB) wind, and that the central star is not yet hot enough to ionize the AGB shell. We measured the JHKL magnitudes of the objects and present their infrared spectral energy distributions. They are typical for post-AGB stars according to the scheme of van der Veen et al. (1989A&A...226..108V). We also constructed various color-color diagrams using the near-infrared and IRAS magnitudes. No distinction can be made between the objects showing Brγ in emission, absorption, or a flat spectrum in the near and far-infrared color-color diagrams. The near-infrared color-color diagrams show evidence for a very large range of extinction, which in part is of circumstellar origin. Near-infrared versus far-infrared color-color diagrams show trends that are consistent with the expected evolution of the circumstellar shell. This sample of post-AGB stars show a larger range in color and are generally redder and closer to the galactic plane than the ones known so far. The properties of most of these objects are fully consistent with the assumption that they are post-AGB stars that have not evolved far enough yet to ionize a significant fraction of their circumstellar material.