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1 Introduction

The unidentified infrared bands (UIR bands) considered ubiquitous where the interstellar carbonaceous dusts are exposed to UV radiation fields have a variety of spectroscopic appearances. Based on the 3 $\mu $m spectra of various sources, Geballe ([1997]) and Tokunaga ([1997]) proposed a classification of UIR sources made of several groups. The class A is the most common type of spectra characterized by intense emission at 3.3 $\mu $m with weaker subsidiary peaks seen in the $3.4-3.6~\mu$m. The class B and class C have relatively stronger emission in the $3.4-3.6~\mu$m, showing a broad 3.4 $\mu $m feature which defined as class B, and sharp emission peaks at 3.43 $\mu $m and 3.53 $\mu $m defined as class C.

All the class B sources are carbon rich proto-planetary nebulae (PPNe). A PPN is an evolved transitional object that ceased violent mass loss in the AGB phase, but the central star has not become hot enough to ionize the circumstellar material to form a planetary nebula (Kwok [1993]). The class B sources are certainly a minor population of the UIR objects. However, we consider the class B sources as the key to understand the open questions on the creation and evolution of UIR carrier because a PPN is one of the main sites of dust formation. It is also interesting to note that there is a sample of PPNe that has isolated 3.3 $\mu $m band without 3.4 $\mu $m emission as The Red Rectangle (HD 44179) (Tokunaga et al. [1991]). There is also large scattering in the intensity of 3.4 $\mu $m band relative to 3.3 $\mu $m band in those PPNe classified in class B.


  \begin{figure}\par\resizebox{11cm}{!}{\includegraphics{ds9205f1.ps}}\end{figure} Figure 1: The spectral sequence of QCC along with temperatures of thermal annealing. The emission and absorption spectra are shown in the left and the middle panels, respectively, in the order of annealing temperature; as-deposited, 600, 700, 723, 740, 760, 823, and 873 K from the top to the bottom. The spectra of several PPNe are shown in the right panel for comparison adapted from Joblin et al. ([1996]), Geballe et al. ([1992]), and Tokunaga et al. ([1991])

We infer the spectral diversity of PPNe is the link to connect class B spectral features with class A. We searched for a laboratory analog of carbon material which has spectral flexibility required to reproduce the variation of PPNe spectra. Sakata et al. ([1990]), Dischler et al. ([1983]), and Bounouh et al. ([1995]) demonstrated that the intensity of the 3.4 $\mu $m absorption band decreases relative to the 3.3 $\mu $m band in amorphous carbon material by heating. Scott & Duley ([1996]), and Scott et al. ([1997]) also reported the similar thermal alteration in the absorption and emission features of hydrogenated amorphous carbon (HAC) prepared by laser ablation of graphite. We produced QCC films with a plasma vapor deposition technique developed by Sakata et al. ([1984]) which intends to mimic the carbon dust solidification in expanding carbon rich stellar atmosphere. In this paper, we investigated the effect of thermal annealing on QCC in terms of emission and absorption spectra. We compare the spectral sequence of thermally annealed QCC with the spectral variation of PPNe in the 3 $\mu $m region to discuss the circumstellar evolution of carbon dust.


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