论文标题
$ \ rm N_2H^+$ nitrogen同位素比率测量的第一个样本
First sample of $\rm N_2H^+$ nitrogen isotopic ratio measurements in low-mass protostars
论文作者
论文摘要
语境。氮同位素比被认为是恒星形成过程的重要诊断工具，$ n_2h^+$尤其重要，因为它与分子氮$ n_2 $直接相关。但是，理论模型仍然缺乏为观察到的$^{14} n/^{15} n $值提供详尽的解释。 目标。最近的理论作品表明，$^{14} n/^{15} n $行为由两种竞争反应主导，这些反应破坏了$ n_2h^+$：与CO的分离重组和反应，当CO从气相耗尽CO时，与$ N_2H^+$重组率相对较低，如果较低的是$ N^^$ n^^$ n^$ n^^^^^^^{15} 快点。这意味着原恒星中的$ N_2H^+$同位素比应低于Prestellar核的$ N_2H^+$，并且与〜440的元素值一致。我们旨在检验这一假设，并在低质量质体中产生$ n_2h^+ / n^{15} nh^+ $ $测量的第一个样本。 方法。我们观察到$ n_2h^+$和$ n^{15} nh^+$最低的旋转过渡向Perseus和Taurus Molecular Clouds中的六个年轻恒星对象。我们使用常数$ t_ {ex} $方法使用自定义的Python代码对光谱进行建模，以适合观测值。我们在附录中讨论了该假设的有效性。派生的色谱柱密度用于计算氮同位素比。 结果。我们的分析平均得出的$ \ rm ^{14} n/ ^{15} n | _ {pro} = 420 \ pm 15 $在Protostellar样本中。这与440的原始极值一致，并且显着低于先前在Prestellar对象样本中获得的平均值。结论。我们的结果与以下假设一致：当CO从气相耗尽时，与游离电子的解离重组破坏了$ N^{15} NH^+$比$ n_2h^+$快的速度，从而导致prestellar核的高同位素比率高，从而使CO在粉尘颗粒上冷冻。
Context. The nitrogen isotopic ratio is considered an important diagnostic tool of the star formation process, and $N_2H^+$ is particularly important because it is directly linked to molecular nitrogen $N_2$. However, theoretical models still lack to provide an exhaustive explanation for the observed $^{14}N/^{15}N$ values. Aims. Recent theoretical works suggest that the $^{14}N/^{15}N$ behaviour is dominated by two competing reactions that destroy $ N_2H^+$: dissociative recombination and reaction with CO. When CO is depleted from the gas phase, if $N_2H^+$ recombination rate is lower with respect to the $N^{15}NH^+$ one, the rarer isotopologue is destroyed faster. This implies that the $N_2H^+$ isotopic ratio in protostars should be lower than the one in prestellar cores, and consistent with the elemental value of ~440. We aim to test this hypothesis, producing the first sample of $N_2H^+ / N^{15}NH^+$ measurements in low mass protostars. Methods. We observe the $N_2H^+$ and $N^{15}NH^+$ lowest rotational transition towards six young stellar objects in Perseus and Taurus molecular clouds. We model the spectra with a custom python code using a constant $T_{ex}$ approach to fit the observations. We discuss in appendix the validity of this hypothesis. The derived column densities are used to compute the nitrogen isotopic ratio. Results. Our analysis yields an average of $\rm ^{14}N/^{15}N|_{pro} = 420 \pm 15$ in the protostellar sample. This is consistent with the protosolar value of 440, and significantly lower than the average value previously obtained in a sample of prestellar objects. Conclusions. Our results are in agreement with the hypothesis that, when CO is depleted from the gas-phase, dissociative recombinations with free electrons destroy $N^{15}NH^+$ faster than $N_2H^+$, leading to high isotopic ratios in prestellar cores, where CO is frozen on dust grains.
