Near-infrared and Brγ observations of post-AGB stars
G.C. Van de Steene, P.A.M. van Hoof, P.R. Wood
published in: A&A 362, 984 (2000)
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 (1993). 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 0.55 mJy/beam at 6 cm and
0.7 mJy/beam at 3 cm, 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. (1989). 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.
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Peter van Hoof
Royal Observatory of Belgium
Ringlaan 3
1180 Brussel
Belgium
email: p DOT vanhoof AT oma DOT be
