Infrared Features in Planetary Nebulae

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Infrared Features in Planetary Nebulae
Kevin Volk, University of Calgary
HST image from Ciardullo et al. (1999) AJ, 119,
488
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With help from Sun Kwok (U. of Calgary), Bruce
Hrivnak (Valparaiso University) Albertina Pei,
Ben Warrington (students at U. of Calgary) W. B.
Latter (IPAC) Scott Fisher and the U. of Florida
Infrared Camera Group The ISO SWS and LWS
Instrument Teams
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The Past previous to the last PN Symposium
Excess infrared emission from PNs was discovered
Around 1970, and attributed to dust in/around
the nebula. This dust is now convincingly
attributed to being the remnants of the dust
ejected when the star was on the Asymptotic
Giant Branch. Good reviews of the previous work
are given in two of the last three PN symposia
Barlow (1993) IAU Symp. 155, page 163 Roche
(1989) IAU Symp. 131, page 117
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After Terzian (1989) IAU Symp. 131, page 17
Dust emission
(dust-free ionized region)
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The true magnitude of the dust emission from PNs
was only discovered in the IRAS mission, when
some 60 of known galactic PNs were
detected. Zhang and Kwok (1991) AA, 250, 179
showed that for a sample of 66 PNs the average
proportion of the total energy in the dust
component was 37 14. This sample is probably
somewhat biased towards the dustier objects, but
shows the importance of dust emission in
PNs. Despite this, there is little evidence of
the existence of the dust in the optical spectra
of PNs.
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Unresolved Questions
What effect does the dust have on the structure
of the ionized region? see Baldwin et al.
(1991) ApJ, 374, 580 Dopita and
Sutherland (2000) ApJ, 539, 742 Volk
(2001) Plasma 2000, in press Stasinska
and Szczerba (2001) AA in press The dust may
effect the ionized region sufficiently to solve
the t2 problem but this requires a lot of
dust. Determinations of the dust mass fraction
in PNs tends to produce small values (order 10-3
typically).
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Is the dust destroyed in the ionized region?
see Stasinska and Szczerba (1999) AA, 352,
297 van Hoof et al. (2000) ApJ, 532,
384 How is the dust in the ionized region
distributed? Is it uniformly mixed with the
gas, or in clumps inside the ionized region,
or both? Does dust collect at the
ionization front? Is the dust changed in PN
compared to in AGB stars?
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There is much other work that I cannot review
here Ground-based spectral observations in the
10 ?m windowP. Roche and associates, in
particular. see the Roche (1989) review, for
example Infrared imaging in the 3 to 12 ?m
regionJ. Hora and associates in particular
also others. see Hora, et al. (1993), ApJ,
413, 304, for example
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The Present last 4 years or so
Two of the most obvious important current areas
of work are (1) Infrared Space Observatory
spectroscopy of PNs (some initial results at the
Groningen meeting see the papers by P. Cox and
by D. A. Beintema) (2) Ground-based imaging with
adaptive optics In many ways these build on
previous results from the past 10 years, but they
represent dramatic improvements in quality of
data.
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Note the two spectra have been scaled to match
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11.6 ?m SiC?
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ISO discovered various new features in the longer
wavelengths, especially crystalline
silicates Frank Molster will talk about
these. With ISO spectra of many objects, some
study of the statistics of the various dust
features can be made. I have therefore been
extracting ISO SWS01 and LWS01 spectra from the
archive and looking at various types of dust
features. From a sample of more than 50 objects
I have the following results
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All Sources
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Oxygen-rich dust shells
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Carbon-rich Dust Shells
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Note the following
UIRs and crystalline silicates are found
together, but not other C-based dust grains and
crystalline silicates. Since the UIRs are
seen in the Orion Nebula and elsewhere in the
ISM, I suggest that UIRs do not always require
a carbon-rich gas chemistry.
Amorphous silicate features may be in the
minority in the spectra of oxygen-rich objects
The 11.6 ?m feature is rare in (carbon-rich)
PNs, and is different than what is seen in
carbon stars where it is attributed to SiC
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The 11.6 ?m Feature
V Cyg continuum divided feature
NGC 6790 continuum divided feature
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11.3 ?m Feature Properties
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Is the change in the feature width is due to an
additional dust component that forms after the
AGB phase? (See the poster by Pei and Volk.)
This is not UIRs, since if so we would see the
feature structure in the ISO spectra. Is this an
indication of some chemical change in the SiC
grain? It might be due to grain destruction,
producing nano-sized SiC grains see Hofmeister,
Rosen Speck (2000), in Thermal Emission
Spectroscopy of Dust, Disks, and Regoliths, ASP
Conf. Ser. 196, 291.
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The 21 ?m Feature
The 21 ?m feature seems to be present in the
spectrum of IC 418 but it is rather weak. Hony,
Waters and Tielens (2001), AA, 378, 41 state
that NGC 40 and NGC 6369 have the 21 ?m feature.
I am rather skeptical of this result, but if so
the feature is again very weak. So, why is the
21 ?m feature weaker in PNs than in PPNs which
presumably are their progenitors? Is this
related to the 11.6 ?m/UIR feature changes?
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See the poster by Pei and Volk
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High Resolution Infrared Imaging
The newer telescopes (i.e.Gemini North/South) are
being built to allow high-resolution infrared
imaging using adaptive optics. At K-band these
telescopes have resolutions better than that for
HST in the V-band. At N-band, the resolution can
be as good as 0.4 arc-seconds FWHM. This allows
us to trace the feature emission regions using
narrow-band filters. The general idea is not new
but the resolution and sensitivity are much
better than they were even 4 years ago.
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BD30º3639 11.7 ?m OSCIR camera July 10,
2001 Gemini North
Field of view 11 square
Colour-coded intensity map
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HST NICMOS 1.87 ?m (thanks to W. B. Latter.)
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NGC 7027 N-band mosaic OSCIR camera Gemini
North July 10, 2001
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HST NICMOS K-short filter (W. B. Latter)
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BD30º3639
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NGC 7027
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The result is that the UIR source region has
been mapped, and is located just outside the main
ionized region. This general result has been
known for some time, but maps of the UIR source
region have not been made at this resolution
previously (i.e. see Ueta et al. (2001) in
Post-AGB Objects as a Phase of Stellar Evolution,
Szczerba Gorny (eds.), 339). For the future,
it is the prospect of doing this for other
features (crystalline silicates, 21 ?m, 11.6 ?m,
and amorphous silicate features) in combination
with spectral fitting which will help us get a
much better idea of the grain properties grain
size and physical distribution in the PN.
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The Future speculation
In the immediate future, it would seem that
ground based imaging is going to give us the most
new information. In a few years SOFIA will
become operational and allow mid-infrared
spectroscopy (and imaging?) of PNs outside the
normal atmospheric windows. The AstroBiology
Explorer is another possibility, for the longer
term, to get high resolution spectroscopy of the
IR features.
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With high resolution spectroscopic instruments
(such as on SOFIA or ground-based telescopes) we
may be able to resolve structure in some of these
dust features. This is particularly important
for the UIR features that are thought to come
from small grains/large molecules. We may be
able to detect size or temperature effects in the
solid grain feature shapes as well, given good
spectral resolution.
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Understanding the images and spectroscopy will
require new modelling techniques. Understanding
the mineralogy of the various features this will
shed light on the physical/chemical processes in
the gas. For me the biggest general question
about PN structure is why do the dust images
(and sometimes the H2 images) often look so
similar to the optical forbidden line images?
How can molecular gas and dust be well mixed
with the ionized gas, as seems to be the case?
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Copies of this presentation and the associated
write-up (when it is finished) can be obtained
from http//www.iras.ucalgary.ca/volk/iau209.htm
l This work was supported in part by the Natural
Sciences and Engineering Research Council of
Canada.
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The End