AI 中文总结
研究银河系半透明云中氘分馏，通过IRAM NOrthern Extended Millimeter Array（NOEMA）观测，发现其DCN/HCN和DNC/HNC丰度比远高于元素丰度[D]/[H]，挑战现有化学模型，或可用分散致密核心情景解释。
AI 中文摘要
长期以来，氘化（D-）物种被提议用于诊断冷致密分子云的物理条件和化学演化。虽然在致密核心中对氘分馏进行了广泛测量，但在弥漫和半透明云中的观测仍然很少。我们报告通过使用IRAM北半球扩展毫米波阵列（NOEMA）的灵敏吸收观测，在一个半透明云（$A_{\rm V}=1.2\pm0.2$星等，$n_{\rm H_2}=3.9\pm0.2\times10^2$ cm$^{-3}$）中检测到DCN和DNC。此次检测达到了迄今为止观测到氘化的最低柱密度和体积密度范围。有趣的是，观测到的DCN/HCN和DNC/HNC丰度比（分别为$3.3\pm0.6\times10^{-3}$和$3.6\pm1.2\times10^{-3}$）比元素丰度[D]/[H]（$1.5\times10^{-5}$）高出两个多数量级，表明半透明云中氘分馏意外增强。这些结果与考虑氘分馏的现有化学模型有显著差异，现有模型预测在此类环境中D分子形成可忽略不计。虽然尚不清楚D分子如何在半透明气体中积累丰度，但分散致密核心情景可能解释观测到的高氘分馏。这种解释与Price等人在二十多年前提出的观点一致：半透明云可能是通过致密分子云消散形成的瞬态、动态演化结构。
英文摘要
Deuterated (D-) species have long been proposed to diagnose the physical conditions and chemical evolution of cold dense molecular clouds. While deuterium fractionation has been extensively measured in dense cores, observations in diffuse and translucent clouds remain rare. We report here the detection of DCN and DNC toward a translucent cloud ($A_{\rm V} =1.2\pm0.2$ mag, $n_{\rm H_2}$ = $3.9\pm0.2\times10^2$ cm$^{-3}$) through sensitive absorption observations with the IRAM NOrthern Extended Millimeter Array (NOEMA). This detection reaches the lowest column-density and volume-density regime in which deuteration has been observed so far. Interestingly, the observed DCN/HCN and DNC/HNC abundance ratios ($3.3\pm0.6\times10^{-3}$ and $3.6\pm1.2\times10^{-3}$, respectively), which are more than two orders of magnitude higher than the element abundance [D]/[H] (1.5$\times$10$^{-5}$), suggest an unexpected enhancement of deuterium fractionation in the translucent cloud. These results represent a significant departure from established chemical models considering deuterium fractionation, which predict negligible formation of D-molecules in such environments. Although it remains unclear how D-molecules built up their abundances in translucent gas, a dispersed dense core scenario could potentially explain the observed high deuterium fraction. This interpretation is consistent with the idea proposed by Price et al. (2003) more than two decades ago: a translucent cloud may be a transient, dynamically evolving structure formed through the dissipation of a dense molecular cloud.
Comments7 pages, 6 figures, A&A Letters accepted
