ISO Spectra of Planetary Nebulae

1
ISO Spectra of Planetary Nebulae Kevin Volk and
Sun Kwok, University of Calgary, Canada
I. Introduction The ISO spectra of planetary
nebulae (PNs) are characterized by strong
emission lines of atoms and ions and continuum
emission from dust. The chemical composition of
the dust varies from those seen in AGB stars
(amorphous carbon, amorphous silicates and
silicon carbide), to aromatic hydrocarbons formed
during the post-AGB evolution. Also detected in
the ISO spectra are features of crystalline
silicates. While emission features at 20, 26,
and 33 ?m are strong in proto-PN (Hrivnak et al.
2000), they are only weakly present in the
spectra of PNs.
II. Featureless continuum The SWS01 and LWS01
spectra of A77 (IRAS 213065540) show a cool dust
continuum devoid of any spectral
features. III. Line emission The SWS01 and
LWS01 spectra of NGC 2440 are dominated by
emission lines.
IV. Aromatic hydrocarbons Many carbon-rich PNs
show strong emission features at 3.3, 6.2, 7.7,
8.6, and 11.3 ?m characteristic of aromatic
compounds. IRAS 212825050 shows strong aromatic
hydrocarbon bands (AHB) with very few emission
lines.
NGC 2440
V. Aromatic hydrocarbon and crystalline
silicates Cn1-1 (IRAS 15476-4836) shows a strong
continuum due to AHB and crystalline silicate
bands. No emission lines are seen. M1-78 (IRAS
211905140) is a PN with a WC central star. It
shows AHB features as well as crystalline
silicates
VI. Amorphous silicates The 10 and 18 ?m
amorphous silicates features, seen in over 3000
oxygen-rich asymptotic giant branch (AGB) stars,
are present in the spectra of PNs. The spectrum
of H2-1 (IRAS 17013-3355) shows amorphous
silicate features in emission. In addition to
showing the 10 and 18 ?m amorphous silicates
features in absorption, the PN He2-90 (IRAS
13064-6103) also shows emission features due to
crystalline silicates.
VII. Peculiar Mz-3 (IRAS 16133-5151) is a
bipolar reflection nebulae with a cool (32000 K)
central star. With the exception of a couple
crystalline silicate features, the dust continuum
is of unknown origin and has a peculiar shape.
VIII. Discussion While the general infrared
continuum of PNs are due to dust (e.g. amorphous
carbon and amorphous silicates) left over from
the AGB, it is clear that new dust components
(aromatic compounds and crystalline silicates)
are formed beyond the AGB. Since the dynamical
lifetime of the nebulae is only 104 yr, the
chemical processes forming these material must be
extremely efficient (Kwok et al. 1999). The
change in the level of the 8 and 12 ?m plateau
features from PPN to PN also suggests that
photochemistry plays a role in evolution of the
dust component (Kwok et al. 2001).
References Hrivnak, B.J., Volk, K., and Kwok S.
2000, ApJ., 535, 275 Kwok, S., Volk, K., and
Hrivnak, B.J. 1999, Astr. Ap., 350, L35 Kwok,
S., Volk, K., and Bernath, P. 2001, ApJ., 554,
L87