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arXiv 1902.07060astro-ph.SRastro-ph.GA
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热芯NGC 6334 I MM1-3中的甲胺和其它简单含氮物质

Methylamine and other simple N-bearing species in the hot cores NGC 6334I MM1-3

E. G. Bøgelund, B. A. McGuire, M. R. Hogerheijde, E. F. van Dishoeck, N. F. W. Ligterink

摘要:

在寻找生命的基石时,含氮分子特别令人感兴趣,因为含氮键对于氨基酸的连接以及最终形成更大的生物结构至关重要。难以捉摸的分子甲胺(CH $_3$ NH $_2$)被认为是一种关键的前生物物种,但迄今为止只在巨大的分子云Sgr B2中被安全地检测到。我们确定CH $_3$ NH $_2$和其他简单的含氮物种对NGC 6334 I的三个热核心。柱密度比的推导,以调查作为生物分子的前体的各个物种的相关性。用ALMA获得的观测结果用于研究CH $_3$ NH $_2$、CH $_2$ NH、NH $_2$ CHO以及$^{13}$ C-和$^{15}$ N-甲基氰化物(CH $_3$ CN)同位素体的跃迁。假设LTE和激发温度范围为CH $_3$ NH $_2$ 220-340 K,CH $_3$ CN同位素体70-110 K,NH $_2$ CHO和CH $_2$ NH 120-215 K,推导出每种物质的柱密度。我们报告了对NGC 6334 I的CH $_3$ NH $_2$的首次检测,对于三个热核MM 1、MM 2和MM 3,CH $_3$ OH的柱密度比分别为5.9 $\times$ 10 $^{-3}$、1.5 $\times$ 10 $^{-3}$和5.4 $\times$ 10 $^{-4}$。这些值略低于Sgr B2的值,但比低质量原恒星IRAS 16293- 2422 B的值高出一个数量级以上。在NGC 6334 I的热核心中探测到CH $_3$ NH $_2$,暗示CH $_3$ NH $_2$在星际介质中普遍存在,尽管高灵敏度的观测对它的探测至关重要。CH $_3$ NH $_2$比值的模型预测值与对NGC 6334 I的观测值之间的良好一致性表明,主要的形成途径是通过晶粒表面上的自由基复合。

英文摘要:

In the search for the building blocks of life, nitrogen-bearing molecules are of particular interest since nitrogen-containing bonds are essential for the linking of amino acids and ultimately the formation of larger biological structures. The elusive molecule methylamine (CH$_3$NH$_2$) is thought to be a key pre-biotic species but has so far only been securely detected in the giant molecular cloud Sgr B2. We identify CH$_3$NH$_2$ and other simple nitrogen-bearing species towards three hot cores in NGC 6334I. Column density ratios are derived in order to investigate the relevance of the individual species as precursors of biotic molecules. Observations obtained with ALMA were used to study transitions of CH$_3$NH$_2$, CH$_2$NH, NH$_2$CHO, and the $^{13}$C- and $^{15}$N-methyl cyanide (CH$_3$CN) isotopologues. Column densities are derived for each species assuming LTE and excitation temperatures in the range 220-340 K for CH$_3$NH$_2$, 70-110 K for the CH$_3$CN isotopologues, and 120-215 K for NH$_2$CHO and CH$_2$NH. We report the first detections of CH$_3$NH$_2$ towards NGC 6334I with column density ratios with respect to CH$_3$OH of 5.9$\times$10$^{-3}$, 1.5$\times$10$^{-3}$, and 5.4$\times$10$^{-4}$ for the three hot cores MM1, MM2, and MM3, respectively. These values are slightly lower than the values derived for Sgr B2 but higher by more than order of magnitude as compared with the values derived for the low-mass protostar IRAS 16293-2422B. The detections of CH$_3$NH$_2$ in the hot cores of NGC 6334I hint that CH$_3$NH$_2$ is generally common in the interstellar medium, albeit high-sensitivity observations are essential for its detection. The good agreement between model predictions of CH$_3$NH$_2$ ratios and the observations towards NGC 6334I indicate a main formation pathway via radical recombination on grain surfaces.

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