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Astron. Astrophys. 362, 984-1003 (2000)

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6. Conclusions

In this article we reported further investigations of the IRAS selected sample of PN candidates that was presented in Van de Steene & Pottasch (1993). About 20% of the candidates in that sample have been detected in the radio and/or H[FORMULA] and were later confirmed as PNe. Here we investigated the nature of the non-radio-detected sources.

  • Of sixteen positively identified objects, seven show Br[FORMULA] in absorption. The absorption lines are very narrow in six objects, indicating a low surface gravity. This is a strong indication for the post-AGB nature of these objects. Another six objects show Br[FORMULA] 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 Br[FORMULA] absorption or emission was visible.

  • The objects showing Br[FORMULA] 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 0.55 mJy/beam at 6 cm and 0.7 mJy/beam at 3 cm, while they should have been easily seen if the radio emission was optically thin and Case B recombination was applicable. It is argued that the Br[FORMULA] emission may be due to ionization in the post-AGB wind, present before the star is hot enough to ionize the AGB shell.

  • 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. These post-AGB stars also cover a larger range in color and are generally much redder than the ones known so far.

  • In the near-infrared color-color diagrams our objects cover a very large range of extinction. Near-infrared versus far-infrared color-color diagrams show trends which reflect the expected evolution of the expanding circumstellar shell. No distinction can be made between the objects showing Br[FORMULA] in emission, absorption, or a flat spectrum in the near- and far-infrared color-color diagrams. Whether the positions of the objects in the color-color diagrams can be directly related to the temperature and core mass of the central star needs further investigation.

  • We identified the K -L versus L -[25] diagram as a potentially useful tool to distinguish: 1) extended from unresolved post-AGB stars, and 2) obscured objects of CLASS  II having thick circumstellar shells from the brighter CLASS  IV objects which show a stellar signature in their near-infrared SEDs. However, this result should be confirmed with a larger sample.

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© European Southern Observatory (ESO) 2000

Online publication: October 30, 2000