后渐近巨星分支星的21微米神秘辐射特征与质量损失率的关系
On the Relation between the Mysterious 21 Micrometer Emission Feature of Post-Asymptotic Giant Branch Stars and Their Mass Loss Rates
摘要：
二十多年前，在四个行星前星云（PPNe;也被称为“原行星星云”）中偶然发现了一个突出的神秘发射带，峰值约为20.1微米。到目前为止，这一光谱特征，指定为“21微米”的功能，已被视为在18个富碳PPNe。虽然已经提出了许多候选材料，但这种特征的载体的性质仍然未知。21微米的光源也表现出同样神秘的，未识别的发射特征，峰值在30微米。虽然21微米的特征只出现在PPNe中，这是渐近巨星分支（AGB）和行星状星云（PN）阶段之间的一个短暂的演化阶段，但30微米的特征通常出现在恒星演化的所有阶段，从AGB阶段到PPN阶段。我们推导出恒星质量损失率（M_{loss}），这些21微米的来源，从他们的尘埃红外（IR）发射，使用“2-尘埃”辐射转移代码的轴对称尘埃系统，它允许一个区分的质量损失率的AGB阶段（\dot{M_{AGB}}）的超风（\dot_{M_{SW}}）阶段。我们研究了\dot{M_{AGB}}或\dot_{M_{SW}}与从21和30微米特征发射的通量之间的相关性。我们发现，这两个功能往往与\dot{M_{AGB}}，这表明他们的运营商可能是在AGB阶段形成。在AGB星中未探测到21微米的特征表明，与30微米的特征不同，21微米特征的载流子的激发可能需要紫外光子，这在PVNe中是可用的，但在AGB星中不是。
英文摘要：
Over two decades ago, a prominent, mysterious emission band peaking at ~20.1 micrometer was serendipitously detected in four preplanetary nebulae (PPNe; also known as "protoplanetary nebulae"). So far, this spectral feature, designated as the "21 micrometer" feature, has been seen in 18 carbon-rich PPNe. The nature of the carriers of this feature remains unknown although many candidate materials have been proposed. The 21 micrometer sources also exhibit an equally mysterious, unidentified emission feature peaking at 30 micrometer. While the 21 micrometer feature is exclusively seen in PPNe, a short-lived evolutionary stage between the end of the asymptotic giant branch (AGB) and planetary nebula (PN) phases, the 30 micrometer feature is commonly observed in all stages of stellar evolution from the AGB through PPN to PNe phases. We derive the stellar mass loss rates (M_{loss}) of these 21 micrometer sources from their dust infrared (IR) emission, using the "2-DUST" radiative transfer code for axisymmetric dusty systems which allows one to distinguish the mass loss rates of the AGB phase (\dot{M_{AGB}}) from that of the superwind (\dot_{M_{SW}}) phase. We examine the correlation between \dot{M_{AGB}} or \dot_{M_{SW}} and the fluxes emitted from the 21 and 30 micrometer features. We find that both features tend to correlate with \dot{M_{AGB}}, suggesting that their carriers are probably formed in the AGB phase. The nondetection of the 21 micrometer feature in AGB stars suggests that, unlike the 30 micrometer feature, the excitation of the carriers of the 21 micrometer feature may require ultraviolet photons which are available in PPNe but not in AGB stars.
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