Astron. Astrophys. 362, 984-1003 (2000)
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 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 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 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 absorption or emission was
The objects showing Br 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 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 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 - 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.
© European Southern Observatory (ESO) 2000
Online publication: October 30, 2000