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arXiv 1901.02689astro-ph.SRastro-ph.GA
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HD 163296原行星盘中CH $_3$ OH的上限：低气相CH $_3$ OH/H $_2$ CO比率的证据

Upper limits on CH$_3$OH in the HD 163296 protoplanetary disk: evidence for a low gas-phase CH$_3$OH/H$_2$CO ratio

M. T. Carney, M. R. Hogerheijde, V. V. Guzmán, C. Walsh, K. I. Öberg, E. C. Fayolle, L. I. Cleeves, J. M. Carpenter, C. Qi

备注：
Accepted to Astronomy & Astrophysics, 11 pages, 3 figures
发表信息：
A&A 623, A124 (2019)
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摘要：

甲醇（CH $_3$ OH）是原行星盘中有机冰化学的根源。然而，其微弱的发射使检测变得困难。迄今为止，气相CH $_3$ OH仅在一个II类磁盘TW Hya中检测到。我们使用阿塔卡马大型毫米/亚毫米阵列（ALMA），以寻找总共四个CH $_3$ OH发射线在频带6和7对周围的年轻赫比格Ae星HD 163296磁盘。甲醇及其相关物种甲醛（H $_2$ CO）的磁盘平均柱密度估计假设光学薄排放在当地的热力学平衡。我们比较这些结果的气相柱密度的TW Hya磁盘。单独或在线叠加后均未检测到目标甲醇线。3个$σ$磁盘积分强度上限为$< 51$ mJy km s $^{-1}$（对于频带6线）和$< 26$ mJy km s $^{-1}$（对于频带7线）。带7线提供了对具有$N_{\mathrm{avg}} < 5.0 \times 10^{11}$ cm $^{-2}$的磁盘平均柱密度的最严格的3 $σ$上限。与TW Hya盘中的$1.27$比率相比，HD 163296盘中的甲醇与甲醛比率为CH $_3$ OH/H $_2$ CO $< 0.24$。赫比格磁盘恒星辐射的差异相比，金牛座T磁盘可能会影响气态甲醇和甲醛含量。HD 163296甲醇/甲醛比较低的可能原因包括：HD 163296盘中H $_2$ CO的气相形成高于预期，CH $_3$ OH和H $_2$ CO的颗粒表面形成效率的不确定性，以及磁盘结构和/或CH $_3$ OH和H $_2$ CO解吸过程，将分子从冰幔释放回气相。这些结果提供的观测证据表明，在磁盘中发现的气相化学复杂性可能受到宿主恒星光谱类型的强烈影响。

英文摘要：

Methanol (CH$_3$OH) is at the root of organic ice chemistry in protoplanetary disks. However, its weak emission has made detections difficult. To date, gas-phase CH$_3$OH has been detected in only one Class II disk, TW Hya. We use the Atacama Large Millimeter/submillimeter Array (ALMA) to search for a total of four CH$_3$OH emission lines in bands 6 and 7 toward the disk around the young Herbig Ae star HD 163296. The disk-averaged column density of methanol and its related species formaldehyde (H$_2$CO) are estimated assuming optically thin emission in local thermodynamic equilibrium. We compare these results to the gas-phase column densities of the TW Hya disk. No targeted methanol lines were detected individually nor after line stacking. The 3$σ$ disk-integrated intensity upper limits are $< 51$ mJy km s$^{-1}$ for the band 6 lines and $< 26$ mJy km s$^{-1}$ for the band 7 lines. The band 7 lines provide the strictest 3$σ$ upper limit on disk-averaged column density with $N_{\mathrm{avg}} < 5.0 \times 10^{11}$ cm$^{-2}$. The methanol-to-formaldehyde ratio is CH$_3$OH/H$_2$CO $< 0.24$ in the HD 163296 disk compared to a ratio of $1.27$ in the TW Hya disk. Differences in the stellar irradiation of Herbig disks compared to T Tauri disks likely influence the gaseous methanol and formaldehyde content. Possible reasons for the lower HD 163296 methanol-to-formaldehyde ratio include: a higher than expected gas-phase formation of H$_2$CO in the HD 163296 disk, uncertainties in the grain surface formation efficiency of CH$_3$OH and H$_2$CO, and differences in the disk structure and/or CH$_3$OH and H$_2$CO desorption processes that release the molecules from ice mantles back into the gas phase. These results provide observational evidence that the gas-phase chemical complexity found in disks may be strongly influenced by the spectral type of the host star.

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