Near-infrared and Br gamma observations of post-AGB stars

In this article we report further investigations of the IRAS selected sampl e of Planetary Nebula (PN) candidates that was presented in Van de Steene & Pottasch (1993). About 20% of the candidates in that sample have been dete cted in the radio and/or H alpha and later confirmed as PNe. Here we invest igate 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 r esolved, despite adverse weather conditions. We obtained medium resolution Br gamma spectra and we took high resolution JHKL images of these 20 object s. 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 field s. Of sixteen positively identified objects, seven show Br gamma in absorption . The absorption lines are very narrow in six objects, indicating a low sur face gravity. Another six objects show Br gamma in emission. Two of these a lso show photospheric absorption lines. All emission line sources have a st rong underlying continuum, unlike normal PNe. In another three objects, no clear Br gamma absorption or emission was visible. The fact that our object s were mostly selected from the region in the IRAS color-color diagram wher e typically PNe are found, may explain our higher detection rate of emissio n line objects compared to previous studies, which selected their candidate s from a region between AGB and PNe. The: objects showing Br gamma in emission were re-observed in the radio con tinuum with the Australia Telescope Compact Array. None of them were detect ed above a detection limit of 0.55 mJy/beam at 6 cm and 0.7 mJy/beam at 3 c m, while they should have been easily detected if the radio flux was optica lly thin and Case B recombination was applicable. It is suggested that the Br gamma emission originates in the post-Asymptotic Giant Branch (post-AGB) wind, and that the central star is not yet hot enough to ionize the AGE sh ell. We measured the JHKL magnitudes of the objects and present their infrared s pectral energy distributions. They are typical for post-AGE stars according to the scheme of van der Veen et al. (1989). We also constructed Various c olor-color diagrams using the near-infrared and IRAS magnitudes. No distinc tion can be made between the, objects showing Br gamma in emission, absorpt ion, 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 ver sus far-infrared color-color diagrams show trends that are consistent with the expected evolution of the circumstellar shell. This sample of post-AGE stars show a larger range in color and are generally redder and closer to t he galactic plane than the ones known so far. The properties of most of these objects are fully consistent with the assum ption that they are post-AGE stars that have not evolved far enough yet to ionize a significant fraction of their circumstellar material.