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arXiv 1901.00657astro-ph.EP
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富$\mathrm{N_2}$彗星C/2016 R2（PanSTARRS）的高分辨率光谱学

High resolution optical spectroscopy of the $\mathrm{N_2}$-rich comet C/2016 R2 (PanSTARRS)

C. Opitom, D. Hutsemékers, E. Jehin, P. Rousselot, F. J. Pozuelos, J. Manfroid, Y. Moulane, M. Gillon, Z. Benkhaldoun

备注：
Submitted to Astronomy and Astrophysics, pending minor revisions
发表信息：
A&A 624, A64 (2019)
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摘要：

对彗星C/2016 R2（PanSTARRS）的早期观测表明，这颗彗星的组成非常奇特。我们获得了高分辨率光谱的彗星在2月时，它是在2.8 au的太阳。我们使用了ESO VLT的UVES光谱仪，并辅以TRAPPIST望远镜获得的窄带图像。我们检测到离子$\mathrm{N_2^+}$和$\mathrm{CO^+}$的强烈发射，但也有$\mathrm{CO_2^+}$，CH自由基的发射，以及CN，$\mathrm{C_2}$和$\mathrm{C_3}$自由基的微弱发射，这些在以前的彗星观测中没有检测到。我们没有检测到OH或$\mathrm{H_2O^+}$，并推导出$\mathrm{H_2O^+/CO^+}$比的上限为0.4，这意味着彗星有一个低的水丰度。我们测量$0.06\pm0.01$的$\mathrm{N_2^+/CO^+}$比率。没有探测到$\mathrm{NH_2}$表明彗星的大部分氮含量位于$\mathrm{N_2}$内。加上高的$\mathrm{N_2^+/CO^+}$比率，这可能表明彗星的形成温度较低，或者彗星是一个大的差异柯伊伯带天体的碎片。$\mathrm{CO_2^+/CO^+}$比例为$1.1\pm0.3$。我们没有探测到$\mathrm{^{14}N^{15}N^+}$谱线，只能把从$\mathrm{N_2^+}$测得的$\mathrm{^{14}N/^{15}N}$比的下限定为100左右，这与在其他彗星中对$\mathrm{NH_2}$和CN的相同同位素比的测量结果是一致的。最后，除了[OI]和[CI]禁戒线外，我们还首次在彗星彗发中探测到了519.79和520.04 nm的氮禁戒线[NI]双峰。

英文摘要：

Early observations of comet C/2016 R2 (PanSTARRS) have shown that the composition of this comet is very peculiar. We obtained high resolution spectra of the comet in February when it was at 2.8 au from the Sun. We used the UVES spectrograph of the ESO VLT, complemented with narrow-band images obtained with the TRAPPIST telescopes. We detect strong emissions from the ions $\mathrm{N_2^+}$ and $\mathrm{CO^+}$, but also $\mathrm{CO_2^+}$, emission from the CH radical, and much fainter emissions of the CN, $\mathrm{C_2}$, and $\mathrm{C_3}$ radicals which were not detected in previous observations of this comet. We do not detect OH or $\mathrm{H_2O^+}$, and derive an upper limit of the $\mathrm{H_2O^+/CO^+}$ ratio of 0.4, implying that the comet has a low water abundance. We measure a $\mathrm{N_2^+/CO^+}$ ratio of $0.06\pm0.01$. The non-detection of $\mathrm{NH_2}$ indicates that most of the nitrogen content of the comet lies within $\mathrm{N_2}$. Together with the high $\mathrm{N_2^+/CO^+}$ ratio, this could indicate a low formation temperature of the comet, or that the comet is a fragment of a large differentiated Kuiper Belt object. The $\mathrm{CO_2^+/CO^+}$ ratio is $1.1\pm0.3$. We do not detect $\mathrm{^{14}N^{15}N^+}$ lines, and can only put a lower limit on the $\mathrm{^{14}N/^{15}N}$ ratio measured from $\mathrm{N_2^+}$ of about 100, compatible with measurements of the same isotopic ratio for $\mathrm{NH_2}$ and CN in other comets. Finally, in addition to the [OI] and [CI] forbidden lines, we detect for the first time the forbidden nitrogen lines [NI] doublet at 519.79 and 520.04 nm in the coma of a comet.

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