arXiv 2406.096832024-06-14astro-ph.GA

星际氮同位素比值：对 C$^{14}$N 和 C$^{15}$N 示踪剂的测量

Interstellar Nitrogen Isotope Ratios: Measurements on tracers of C$^{14}$N and C$^{15}$N

J. L. Chen, J. S. Zhang, C. Henkel, Y. T. Yan, H. Z. Yu, Y. X. Wang, Y. P. Zou, J. Y. Zhao, X. Y. Wang

备注：: 34 pages, 9 figures, 6 tables
发表信息：: The Astrophysical Journal (2004)

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AI总结：

本研究利用 IRAM 30 m 与 ARO 12 m 望远镜直接测量恒星形成区的 C14N/C15N，发现氮同位素比值随银心距先升至约 9 kpc 后在外银河系下降。

摘要：

氮同位素比值 14N/15N 是追踪银河系恒星核合成并约束银河系化学演化的有力工具。以往观测发现，银心处的 14N/15N 比值较低，而银盘中的数值较高。这与我们银河系由内向外形成的情景相一致。然而，以往研究大多使用同时包含 12C/13C 的双同位素比值，这会引入额外的不确定性。因此，我们在此展示对一组恒星形成区方向上 C14N 及其稀有同位素分子 C15N 的观测，这些观测由 IRAM 30 m 和/或 ARO 12 m 望远镜在波长 λ 约 3 mm 处测得。对于两种同位素分子均被探测到的 35 个源，我们确定了其物理参数。此外，我们利用 CN 和 C15N 最强的超精细成分获得了氮同位素比值。对于那些表现出小幅偏离局部热动平衡和/或自吸收的源，我们使用可能不受后一种效应影响的最弱超精细成分来获得可靠的 14N/15N 值。我们通过 C14N 和 C15N 测量得到的 14N/15N 同位素比值，与我们早先对 NH3 和 15NH3 的测量结果（论文 I）相符，即比值在银心距约 9 kpc 以前持续增加。非加权二阶多项式拟合给出 C14N/C15N = (-4.85 ± 1.89) kpc^-2 × R_GC^2 + (82.11 ± 31.93) kpc^-1 × R_GC - (28.12 ± 126.62)。朝向外银河系方向，该同位素比值趋于下降，这支持了早先由 H13CN/HC15N 得到的发现。银河系化学演化模型与我们对 14N/15N 同位素比值的测量相一致，即从银心区域到约 9 kpc 呈上升趋势，随后随着 R_GC 增大、朝向外银河系方向呈下降趋势。

英文摘要：

The nitrogen isotope ratio 14N/15N is a powerful tool to trace Galactic stellar nucleosynthesis and constraining Galactic chemical evolution. Previous observations have found lower 14N/15N ratios in the Galactic center and higher values in the Galactic disk. This is consistent with the inside-out formation scenario of our Milky Way. However, previous studies mostly utilized double isotope ratios also including 12C/13C, which introduces additional uncertainties. Here we therefore present observations of C14N and its rare isotopologue, C15N, toward a sample of star forming regions, measured by the IRAM 30 m and/or the ARO 12 m telescope at $λ$ ~3 mm wavelength. For those 35 sources detected in both isotopologues, physical parameters are determined. Furthermore we have obtained nitrogen isotope ratios using the strongest hyperfine components of CN and C15N. For those sources showing small deviations from Local Thermodynamical Equilibrium and/or self-absorption, the weakest hyperfine component, likely free of the latter effect, was used to obtain reliable 14N/15N values. Our measured 14N/15N isotope ratios from C14N and C15N measurements are compatible with those from our earlier measurements of NH3 and 15NH3 (Paper I), i.e., increasing ratios to a Galacticentric distance of ~9 kpc. The unweighted second order polynomial fit yields $\frac{{\rm C^{14}N}}{{\rm C^{15}N}} = (-4.85 \pm 1.89)\;{\rm kpc^{-2}} \times R_{\rm GC}^{2} + (82.11 \pm 31.93) \;{\rm kpc^{-1}} \times R_{\rm GC} - (28.12 \pm 126.62)$. Toward the outer galaxy, the isotope ratio tends to decrease, supporting an earlier finding by H13CN/HC15N. Galactic chemical evolution models are consistent with our measurements of the 14N/15N isotope ratio, i.e. a rising trend from the Galactic center region to approximately 9 kpc, followed by a decreasing trend with increasing $R_{\rm GC}$ toward the outer Galaxy.

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