Issue |
A&A
Volume 390, Number 3, August II 2002
|
|
---|---|---|
Page(s) | 1089 - 1113 | |
Section | The Sun | |
DOI | https://doi.org/10.1051/0004-6361:20020773 | |
Published online | 14 August 2002 |
The rich 6 to 9
m spectrum of interstellar PAHs*
1
SRON National Institute for Space Research, PO Box 800, 9700 AV Groningen, The Netherlands
2
Kapteyn Institute, PO Box 800, 9700 AV Groningen, The Netherlands
3
Astronomical Institute “Anton Pannekoek”, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
4
Instituut voor Sterrenkunde, K.U.Leuven, Celestijnenlaan 200B, 3100 Heverlee, Belgium
5
NASA-Ames Research Center, Space Science Division, MS: 245-6, Moffett Field, CA 94035-1000, USA
Corresponding author: E. Peeters, peeters@astro.rug.nl
Received:
4
December
2001
Accepted:
16
May
2002
IR spectroscopy provides a valuable tool for the
characterisation and identification of interstellar molecular
species. Here, we present 6–9 spectra of a sample of
reflection nebulae, HII regions, YSOs, evolved stars and galaxies
that show strong unidentified infrared bands, obtained with the SWS
spectrograph on board ISO. The IR emission features in this
wavelength region show pronounced variations. 1) The 6.2
feature shifts from 6.22 to 6.3
and clearly shows profile
variations. 2) The 7.7
complex is comprised of at least two
subpeaks peaking at 7.6 and one longwards of 7.7
. In some cases
the main peak can apparently shift up to 8
. Two sources do not
exhibit a 7.7
complex but instead show a broad emission
feature at 8.22
. 3) The 8.6
feature has a symmetric
profile in all sources and some sources exhibit this band at
slightly longer wavelengths. For the 6.2, 7.7 and 8.6
features, the sources have been classified independently based on
their profile and peak position. The classes derived for these
features are directly linked with each other. Sources with a 6.2
feature peaking at ~6.22
exhibit a 7.7
complex dominated by the 7.6
component. In contrast, sources
with a 6.2
profile peaking longwards of 6.24
show a
7.7
complex with a dominant peak longwards of 7.7
and
a 8.6
feature shifted toward the red. Furthermore, the
observed 6–9
spectrum depends on the type of object. All
ISM-like sources and a few PNe and Post-AGB stars belong to the
first group while isolated Herbig AeBe stars, a few Post-AGB stars
and most PNe belong to the second group. We summarise existing
laboratory data and theoretical quantum chemical calculations of the
modes emitting in this wavelength region of PAH molecules. We
discuss the variations in peak position and profile in view of the
exact nature of the carrier. We attribute the observed 6.2
profile and peak position to the combined effect of a PAH family and
anharmonicity with pure PAHs representing the 6.3
component
and substituted/complexed PAHs representing the 6.2
component. The 7.6
component is well reproduced by both pure
and substituted/complexed PAHs but the 7.8
component remains
an enigma. In addition, the exact identification of the 8.22
feature remains unknown. The observed variations in the
characteristics of the IR emission bands are linked to the local
physical conditions. Possible formation and evolution processes that
may influence the interstellar PAH class are highlighted.
Key words: circumstellar matter / stars: pre-main sequence / HII regions / ISM: molecules / planetary nebulae: general / infrared: ISM: lines and bands
© ESO, 2002
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