AI 中文总结
该研究通过观测与模拟分析年轻原核中HCN同位素的空间变化，发现其与H$_2$柱密度相关，源于$^{12}$C/$^{13}$C比变化，提醒需评估该比变化后才可采用双同位素法推导$^{14}$N/$^{15}$N比。
AI 中文摘要
背景：同位素分馏可作为恒星与行星形成过程中化学演化的有力示踪剂，为准确解读观测结果，明确不同演化阶段氮、碳分馏的主导途径至关重要。目的：研究恒星形成初始阶段的一个年轻原核样本中的氮、碳分馏情况。方法：对一个无恒星原核和三个前恒星原核周围的H$^{13}$CN与HC$^{15}$N进行成图，计算原核范围内$N$(H$^{13}$CN)/$N$(HC$^{15}$N)柱密度比，并与Herschel/SPIRE的$N$(H$_2$)图对比；此外，采用双同位素法计算$^{14}$N/$^{15}$N图，与早期研究结果比较；将结果与一维前恒星原核模型的碳、氮分馏天体化学模拟结果对比。结果：计算得到的$N$(H$^{13}$CN)/$N$(HC$^{15}$N)比在图中呈现出明显的空间变化，四个原核中有三个的该变化与$N$(H$_2$)相关。结论：分析揭示H$^{13}$CN/HC$^{15}$N比与$N$(H$_2$)图存在相关性；根据天体化学模型，该相关性主要源于$^{12}$C/$^{13}$C比的变化；因此，研究提醒在未独立评估$^{12}$C/$^{13}$C比可能存在的空间变化时，不可采用双同位素法推导$^{14}$N/$^{15}$N比；此外，模型中同位素变化的主要原因并非同位素选择光解离，而是原核致密区域低温下交换反应导致的更高效分馏。
英文摘要
Context. Isotopic fractionation can serve as a powerful tracer of the chemical evolution during star and planet formation. To accurately interpret observations, it is crucial to identify the dominant pathways of nitrogen and carbon fractionation at different evolutionary stages. Aims. We aim to study nitrogen and carbon fractionation in a sample of young cores at the onset of star formation. Methods. We map H$^{13}$CN and HC$^{15}$N around one starless and three pre-stellar cores. We compute the $N$(H$^{13}$CN)/$N$(HC$^{15}$N) column density ratio across the cores and compare the distribution with $N$(H$_2$) maps from $\textit{Herschel}$/SPIRE. In addition, we calculate $^{14}$N/$^{15}$N maps using the double isotope method for comparison with earlier studies. The results are compared with astrochemical modeling of carbon and nitrogen fractionation for a one-dimensional pre-stellar core model. Results. The computed $N$(H$^{13}$CN)/$N$(HC$^{15}$N) ratio exhibit clear spatial variation across the maps. This variation is correlated with $N$(H$_2$) in three out of four cores. Conclusions. Our analysis reveals a correlation between the H$^{13}$CN/HC$^{15}$N ratios and the $N$(H$_2$) maps. According to the astrochemical model, the correlation is mainly due to variations in the $^{12}$C/$^{13}$C ratio. Consequently, the results caution against applying the double-isotope method to derive $^{14}$N/$^{15}$N ratios without independently assessing possible spatial variations in the $^{12}$C/$^{13}$C ratio. Furthermore, the leading cause of the isotopic variation in the model is not isotope-selective photodissociation, but rather more efficient fractionation through exchange reactions at lower temperatures in the denser regions of the cores.
CommentsAccepted for publication in A&A
