论文标题
星际介质中的氘化多环芳烃：单授予物种的C-D带强度
Deuterated Polycyclic Aromatic Hydrocarbons in the Interstellar Medium: The C--D Band Strengths of Mono-Deuterated Species
论文作者
论文摘要
氘（D）是在大爆炸中产生的光元素之一。随着银河系的发展，星际介质（ISM）中的D/H丰度从由于“挤压”引起的原始值降低。但是，某些星际视线的观察到的气相D/H的丰度大大低于仅通过挤压而预期的减少。丢失的D可能已耗尽到多环芳烃（PAH）分子上，这些分子在星际区域中无处不在且丰富。为了定量地探索PAH作为星际D的储层的假设，我们从化学上计算了单置PAHS（及其阳离子）的各种大小的量子振动光谱。我们发现，正如预期的那样，当PAHS中的H取代D时，C-H的伸展和弯曲模式分别为3.3、8.6和11.3 $ $ m m，以$ \ sim $ 4.4、11.4和15.4 $ $ m的较长波长转移到更长的波长，乘以$ \ sim $ \ sim $ \ sim \ sqrt ^ 13/7} $ ress and-s.-sideres，sirstand and-n. sirstand ress，n. We derive from the computed spectra the mean intrinsic strengths of the 3.3 $μ$m C-H stretch and 4.4 $μ$m C--D stretch to be $\langle A_{3.3} \rangle \sim 13.4$ km/mol and $\langle A_{4.4} \rangle \sim 7.4$ km/mol for neutral deuterated PAHs which would dominate the星际3.3和4.4 $μ$ m排放。通过比较$ \ langle a_ {4.4}/a_ {3.3} \ rangle \ sim 0.56 $的计算衍生的平均比率，用于中性pahs的平均值，其平均值的平均比率是$ \ langle（i_ {4.4}/im sim {3.3}）的强度的平均值。我们估计属性的程度（即，以d的形式连接到C的外围原子的比例为〜2.4 \％，相对于星际d/h丰度相对于〜1200的d-含量〜1200。
Deuterium (D) is one of the light elements created in the big bang. As the Galaxy evolves, the D/H abundance in the interstellar medium (ISM) decreases from its primordial value due to "astration". However, the observed gas-phase D/H abundances of some interstellar sightlines are substantially lower than the expected reduction by astration alone. The missing D could have been depleted onto polycyclic aromatic hydrocarbon (PAH) molecules which are ubiquitous and abundant in interstellar regions. To quantitatively explore the hypothesis of PAHs as a possible reservoir of interstellar D, we compute quantum-chemically the infrared vibrational spectra of mono-deuterated PAHs (and their cations) of various sizes. We find that, as expected, when H in PAHs is replaced by D, the C-H stretching and bending modes at 3.3, 8.6 and 11.3 $μ$m respectively shift to longer wavelengths at $\sim$4.4, 11.4 and 15.4 $μ$m by a factor of $\sim$$\sqrt{13/7}$, the difference in reduced mass between the C-H and C-D oscillators. We derive from the computed spectra the mean intrinsic strengths of the 3.3 $μ$m C-H stretch and 4.4 $μ$m C--D stretch to be $\langle A_{3.3} \rangle \sim 13.4$ km/mol and $\langle A_{4.4} \rangle \sim 7.4$ km/mol for neutral deuterated PAHs which would dominate the interstellar 3.3 and 4.4 $μ$m emission. By comparing the computationally-derived mean ratio of $\langle A_{4.4}/A_{3.3} \rangle \sim 0.56$ for neutral PAHs with the mean ratio of the observed intensities of $\langle (I_{4.4}/I_{3.3})_{\rm obs} \rangle \sim 0.019$, we estimate the degree of deuteration (i.e., the fraction of peripheral atoms attached to C in the form of D) to be ~2.4\%, corresponding to a D-enrichment of a factor of ~1200 with respect to the interstellar D/H abundance.
