首次检测到双氘代甲基乙炔（CHD2CCH和CH2DCCD）
First detection of doubly deuterated methyl acetylene (CHD2CCH and CH2DCCD)
摘要：
我们报告的第一次检测在空间的两个双氘同位素的甲基乙炔。CHD 2CCH和CH 2DCCD这两种物质在L483致密核中分别通过使用IRAM 30米望远镜在72-116 GHz范围内的9条和8条旋转谱线被确认。这里观察到的天文频率与在29-47 GHz范围内测量的文献中的实验室频率相结合，以获得两种同位素的更准确的光谱参数。我们推导出CHD 2CCH和CH 2DCCD的光束平均柱密度分别为（2.7 +/- 0.5）e12 cm-2和（2.2 +/- 0.4）e12 cm-2，这转化为丰度比CH 3CCH/CHD 2CCH = 34 +/- 10和CH 3CCH/CH 2DCCD = 42 +/- 13。甲基乙炔的双氘同位素体的丰度仅比单氘同位素体低几倍，具体地说，比CH 3CCD低约2.4倍。相对于CH 3CCH的不同氘代同位素体的丰度合理占气相化学模型，其中氘代发生从前体离子C3 H6 D+和C3 H5 D+，当分子氢的邻-对位比是足够低。这表明气相化学反应，而不是颗粒表面过程，负责形成和氘分馏的CH 3CCH在L483。在L483中观察到的丰度比CH 2DCCH/CH 3CCD = 3.0 +/- 0.9和CHD 2CCH/CH 2DCCD = 1.25 +/- 0.37分别与统计学预期值3和1一致，其中CHD 2CCH与CH 2DCCD相比略微过量可由化学模型很好地解释。
英文摘要：
We report the first detection in space of the two doubly deuterated isotopologues of methyl acetylene. The species CHD2CCH and CH2DCCD were identified in the dense core L483 through nine and eight, respectively, rotational lines in the 72-116 GHz range using the IRAM 30m telescope. The astronomical frequencies observed here were combined with laboratory frequencies from the literature measured in the 29-47 GHz range to derive more accurate spectroscopic parameters for the two isotopologues. We derive beam-averaged column densities of (2.7 +/- 0.5)e12 cm-2 for CHD2CCH and (2.2 +/- 0.4)e12 cm-2 for CH2DCCD, which translate to abundance ratios CH3CCH/CHD2CCH = 34 +/- 10 and CH3CCH/CH2DCCD = 42 +/- 13. The doubly deuterated isotopologues of methyl acetylene are only a few times less abundant than the singly deuterated ones, concretely around 2.4 times less abundant than CH3CCD. The abundances of the different deuterated isotopologues with respect to CH3CCH are reasonably accounted for by a gas-phase chemical model in which deuteration occurs from the precursor ions C3H6D+ and C3H5D+, when the ortho-to-para ratio of molecular hydrogen is sufficiently low. This points to gas-phase chemical reactions, rather than grain-surface processes, as responsible for the formation and deuterium fractionation of CH3CCH in L483. The abundance ratios CH2DCCH/CH3CCD = 3.0 +/- 0.9 and CHD2CCH/CH2DCCD = 1.25 +/- 0.37 observed in L483 are consistent with the statistically expected values of three and one, respectively, with the slight overabundance of CHD2CCH compared to CH2DCCD being well explained by the chemical model.
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