![]() |
Figure 4: The emission (the bold line) and absorption (the thin line) spectra of QCC annealed at 723 K. The spectra are displaced in ordinate for clarity |
In Fig. 4 we compare emission and absorption spectra of QCC
annealed at 723 K. Both spectra are almost identical in the respective
profiles. The coincidence suggests that emissions are basically the
transition from upper vibrationally excited states to ground states in
various modes; not being associated with so-called hot bands. We
conclude that the broad and intense emission band at the
m region toward class B sources is surely not due to a hot
band or combined effect of them.
The measured temperatures of the samples in emission are between 310 K
and 370 K in the present experiment. On the other hand the temperature
at the inner edge of the dust shell under a typical PPN condition is
calculated as,
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Figure 5:
The comparison of the spectral profiles between
observations (the open circles) and the annealed QCC samples (the
solid lines) at 700 K for IRAS 05341+0852 (Joblin et al. [1996]),
at 723 K and 700 K for synthetics 3.3 ![]() |
We also compare the profiles of prototypic 3.3 m emission bands
of Type 1 and 2 presented by Tokunaga et al. ([1991]) with the
spectra of QCC annealed at 723 K and 873 K. We can see the excellent
match between the slopes at the shorter wavelength tails of the
observed 3.3
m band and those of the QCC samples. In each case,
the QCC spectrum exhibits significant excess above the observational
data at the longer wavelength side of the 3.3
m band. Although
the exact cause for the discrepancy is currently uncertain, it may be
possibly due to the insufficient subtraction of the continuum spectra
or a band broadening due to the interaction between chemical groups as
a consequence of the measurement in a bulky film (Joblin et al. [1994]).
![]() |
Figure 6:
The relative strength of aromatic C-H bonds to the
whole feature intensity. The 3.2 to 3.6 ![]() |
![]() |
Figure 7: The absorption spectrum of evaporated material when the QCC samples are heated in a evacuated quartz tube. The spectra indicate the material is rich with aliphatic C-H bonds |
It is obvious that the band arise from aromatic C-H bonds begins to
increase relative to the whole feature intensity when heated up over
600 K. The proportion of the band intensity continues to rise up to
annealing at
900 K where the whole band strength is
attributable to purely aromatic C-H bonds. It must be borne in mind
that the increase of the aromatic band ratio is the combined effect of
two independent consequences of thermal annealing process. First it
can be attributed to the chemical structural transformation of
aliphatic carbon skeleton to aromatic or graphitic one. It is
supported by Robertson ([1991]) and Dischler et al.
([1983]) who claim the graphitization of amorphous carbon
material subjected to thermal annealing. In the meantime, when we heat
the QCC sample in an evacuated quartz tube, comparatively volatile
component of QCC evaporates from the film and re-adheres cooler wall
of quartz tube. We collect the evaporated material, and examine the
absorption spectra to find that the adhesion is rich with aliphatic
C-H bonds (Fig. 7). Therefore it is possible the effect shown
in Fig. 6 would be explained not by the net increase of
aromatic C-H bonds but by the relative decrease of aliphatic C-H
bonds. Nevertheless we consider the formation of aromatic C-H bonds
occurs at least with significant contribution because we have
confirmed the existence of newly formed carbon onion-like particles
which consist of concentric spherical graphitic shells in the HRTEM
micrograph of the annealed QCC sample. The HRTEM images of the
as-deposited and annealed QCC are shown in Fig. 8 in the
sequence of the annealing temperature. No periodic structure can be
seen in the as-deposited film and the sample annealed at lower
temperature. However, several stratified layers with curvature appear
in the sample annealed at 760 K. We can clearly see a number of carbon
onion-like spherules in the sample annealed at 823 K. We assume the
growth of sp2 hybridized aromatic rings or formation of carbon
onion-like spherules is the main cause for the spectroscopic
transformation of QCC in the following discussion.
We compare the spectral sequence of the annealed QCC with the various
spectra of PPNe in Fig. 1. We can see a remarkable
similarity between the laboratory spectra and the observational
data. The QCC spectra in each stage of thermal annealing, which
gradually lose 3.4 m band, fairly closely trace the corresponding
PPNe spectra. It is noteworthy that the variety of PPNe spectra can be
systematically reproduced by the QCC samples by themselves; from
IRAS 05341+0852 with predominant 3.4
m emission to
the Egg and the Red Rectangle with slight or no
3.4
m bands.
The thermal annealing process also affects the feature to continuum
ratio significantly. The
m features are by far
prominent in as-deposited QCC sample, in the same time continuum
emission levels uprise after the annealing (Fig. 3). The
strong continuum emission may be directly related with sp2hybridized bond in the sample.
![]() |
Figure 9: The spectra of the extreme class B sources adopted referring to Joblin et al. ([1996]) and Geballe et al. ([1992]). They are obtained by being divided by underlying continuum which are assumed as blackbody. The continuum levels are indicated by dotted lines |
It is especially interesting to note that the UIR band intensity of
PPNe relative to the continuum also drops off as the 3.4 m band
intensity decreases relative to the 3.3
m band. The spectra of
three extreme class B sources observed by Joblin et al. ([1996])
and Geballe et al. ([1992]) are shown in Fig. 9 in the
sequence of 3.4
m band intensity relative to the 3.3
m band.
The spectra are divided by blackbody continuum. It can be seen that
the feature intensity normalized to the continuum diminishes in the
sequence. We suppose the common behavior of the feature to continuum
ratio strongly suggests the common nature between the carrier of the
UIR band toward the class B sources and the thermally annealed QCC.
The following is a proposed working hypothesis of carbon dust
formation and evolution. (1) Carbon dust is solidified as QCC-like
amorphous material with rich sp3 hybridized CH bonds in the
vicinity of the evolved carbon star. The material shows complex
3.4 m emission which is similar to that of
IRAS 05341+0852 or a QCC film immediately after the
deposition. (2) The solidified carbon dust is then suffered from
annealing. The intensity of 3.4
m emission decreases because of
transformation of aliphatic C-H bonds to the aromatic one. The
emission band at 3.4
m is diminished before the carbon dust
leaves the star to diffuse interstellar medium. (3) The aliphatic C-H
bonds that survive through the destructive annealing would contribute
m plateau emission of typical class A UIR spectra
observed toward H II regions, reflection nebulae, and planetary
nebulae.
The first point of the scenario is that it describes a fresh carbon dust as an amorphous material which contains rich aliphatic C-H bonds. The second point is that it refers the destruction of sp3hybridized CH bonds which separately occurs from the dust formation.
Frenklach & Feigelson ([1989]) and Cherchneff et al. ([1992]) have claimed that it requires exceptional physical and chemical conditions such as high density of seed C2H2 molecules and slow mass loss velocity to produce sufficient amount of PAH directly out of a stellar atmosphere. We infer the condensation and subsequent chemical reaction to aromatization may provide a round way to avoid the difficulty of direct formation of aromatic material.
The size distribution of the particles is also affected by the thermal annealing. By heating, carbon onion-like spherules of certain diameter are produced out of a QCC film which has no specific partitional scale immediately after the deposition. When a dust model is given, the size of the grains is one of a few parameters that are determinable in fairly stringent way from observations. The size distribution of dust grains is considered to be controlled by (1) nucleation and growth rate, and (2) grain destruction by collision and shattering. The latter is known to provide a secure ground of the power law size distribution required for standard comprehensive dust models (Mathis et al. [1977]). On the other hand as concerns well-known 2175 Å bump in UV interstellar extinction curve, the constant band center independent of the line of sight together with the narrow bump profiles prefers graphite grains to be highly uniform in size and shape (Draine [1989]). The fragmentation into carbon onion-like spherules from amorphous material might be a process disregarded so far which controls the shape and the size of dust grains.
The compositional change of carbon dust from circumstellar to
interstellar environment is previously pointed out by Buss et al.
([1993]) who argued the systematic spectral variations through
AGB, PPN, to planetary nebula in the mid infrared region. Duley
([1995]) proposed an evolutional model characterized by the
formation of featureless amorphous carbon in AGB phase, subsequent
photochemical rehydrogenation to HAC in the presence of UV radiation,
and possible dissociation to PAH molecules in a planetary nebula shock.
Schnaiter et al. ([1999]) gave a support to the idea by
demonstrating the production of nano-sized carbon particles with laser
pyrolysis of C2H2 that have only weak C-H features. On the other
hand a lot of carbon rich PPNe do have 3.3 m emission due to
aromatic C-H bonds and in several cases additional broad 3.4
m
emission due to aliphatic C-H bonds. The spectral types of the most of
the central stars of PPNe are found to be F-G by spectroscopic
observations in visible (Hrivnak et al. [1995]). It is not
likely that those PPNe are able to provide sufficient UV photons
required to promote the active hydrogenation of amorphous carbon. The
lack of plausible hydrogenation mechanism of amorphous carbon would be
alleviated by our result that reveals the carbonaceous material rich
in aliphatic C-H bonds can be directly produced by hydrocarbon plasma
deposition. Chiar et al. ([1998]) proposed the condensation of
aliphatic hydrocarbon around PPNe and postprocessing to aromatic dust
based on the infrared spectroscopy of AFGL 618 which exhibits
an absorption feature at 3.4
m together with the weak 3.3
m
emission feature. The transformation of aliphatic hydrocarbon to
aromatic material is exactly what is observed in our thermal annealing.
Although Chiar et al. ([1998]) suggested the photochemical
reaction driven by FUV radiation as the promoter of the conversion,
we would like to imply that the alternative thermal process also
enables the spectroscopic transition.
What is not explained in our scenario is the lack of hydrocarbon features toward carbon rich AGB stars. The dust emission observed in the far infrared region clearly ensures the existence of large amount of dust around those progenitors of PPNe. The previous evolutional models attribute the featureless continuum toward AGB stars to amorphous carbon grains with little C-H bonds. We speculate if the dust formation mechanism is possibly not continuous between the AGB and PPN phase. A certain mode-switching synchronized with the end of the main mass loss phase could accompany with the changes in the physical and chemical circumstances.
The thermal evolution proposed above requires heating of dust up to
800 K after the solidification. Here we conclude by proposing
three possibilities such heating could occur. That is, (1) stochastic
heating of a dust grain impinged by a single energetic photon radiated
either from a natal star or unknown binary companions, (2) heating by
gas-grain collision, or (3) gradual thermal annealing when a dust
transverse a warm dust shell. We imply gradual annealing may be the
most promising. However, it is also accompanied with serious question
whether the observable amount of aliphatic C-H material, which is
easily affected by thermal annealing, can survive through the inner
warmer region of a dust shell. The hypothesis is highly speculative
and needs to be confirmed by the extensive laboratory works which more
precisely express the physical environment around PPNe as well as
observations on thermal and dynamical structure of each PPN.
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