通过HC $_3$ N的干涉观测研究PDKEEPAAA +10216内部风的异常光化学
Investigating Anomalous Photochemistry in the Inner Wind of IRC+10216 Through Interferometric Observations of HC$_3$N
摘要：
近年来，许多关于演化恒星富碳风中光化学产物化学性质的问题出现了。为了解决这些问题，必须通过高角分辨率的干涉观测，覆盖多个旋转过渡约束这些物种的分布。我们利用ALMA的观测档案，绘制了碳星IRC +10216的内包络（半径<8 "/1000 AU）中含有高能级氰基乙炔（HC $_3$ N）的旋转线。所观测到的谱线包括HC $_3$ N基态的J = 28 - 27、J = 30 - 29和J = 38 - 37跃迁。与先前对AGB星的线性碳链的观测相反，它在15 "-20"半径处显示出延伸的中空发射。（例如C $_4$ H、C $_6$ H、HC $_5$ N），这里HC $_3$ N线的图显示了包括各种弧和密度增强的紧凑形态，在距离中心AGB星约3 "（350天文单位）的角距离处，有大量气体团的辐射。我们比较了可见度采样的非LTE辐射传输模型与观测到的亮度分布，并推导出一个分数丰度相对于H $_2$的$10^{-8}$ HC $_3$ N在半径探测这些线。这些结果是一致的增强光化学发生在温暖的（~200 K）地区的拱星信封。IRC +10216的一个专门的化学模型的应用程序后，我们发现的证据表明，增强HC $_3$ N丰度的内风最有可能是由于太阳型的双同伴在该地区启动光化学。
英文摘要：
In recent years, many questions have arisen regarding the chemistry of photochemical products in the carbon-rich winds of evolved stars. To address them, it is imperative to constrain the distributions of such species through high angular resolution interferometric observations covering multiple rotational transitions. We used archival ALMA observations to map rotational lines involving high energy levels of cyanoacetylene (HC$_3$N) toward the inner envelope (radius <8"/1000 AU) of the carbon star IRC+10216. The observed lines include the J=28-27, J=30-29, and J=38-37, transitions of HC$_3$N in its ground vibrational state. In contrast to previous observations of linear carbon chains toward this AGB star which show extended, hollow emission at 15"-20" radii (e.g. C$_4$H, C$_6$H, HC$_5$N), the maps of the HC$_3$N lines here show compact morphologies comprising various arcs and density enhancements, with significant emission from gas clumps at an angular distance of ~3" (350 AU) from the central AGB star. We compared visibility sampled non-LTE radiative transfer models with the observed brightness distributions, and derive a fractional abundance with respect to H$_2$ of $10^{-8}$ for HC$_3$N at the radii probed by these lines. These results are consistent with enhanced photochemistry occurring in warm (~200 K) regions of the circumstellar envelope. After application of a specialized chemical model for IRC+10216, we find evidence that the enhanced HC$_3$N abundances in the inner wind are most likely due to a solar-type binary companion initiating photochemistry in this region.
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