Near-infrared and Br_gamma observations of post-AGB stars

We investigate the infrared properties of the IRAS sources not confirmed as PNe in Van de Steene & Pottasch (1993). We observed 28 objects in the N-band of which 20 were detected and 5 were resolved. We obtained medium resolution Br_gamma spectra and we took high resolution JHKL images of these 20 objects. We critically assessed the identification of the IRAS counterpart in the images. Of sixteen positively identified objects, seven show Br_gamma in absorption, another six objects show Br_gamma in emission on top of a continuum. In another three objects, no clear Br_gamma absorption or emission was visible. Our detection rate of emission line objects is higher compared to previous studies. None of the objects with Br_gamma in emission were detected above a detection limit of 0.6 mJy/beam at 6 cm, while they should have been easily detected if the radio flux was optically thin and Case B recombination was applicable. Their SED's are typical for post-AGB stars. We also constructed various color-color diagrams using the near-infrared and IRAS magnitudes. No distinction can be made between the objects showing Br_gamma 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 closer to the galactic plane than the ones known so far