Coma Abundances of Volatiles at Small Heliocentric Distances: Compositional Measurements of Long-period Comet C/2020 S3 (Erasmus)

C. Ejeta, E. Gibb, N. Roth, M. DiSanti, N. Dello Russo, M. Saki, A. McKay, H. Kawakita, Y. Khan, B. Bonev, R. Vervack, M. Combi
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Abstract

We report production rates of H2O and nine trace molecules (C2H6, CH4, H2CO, CH3OH, HCN, NH3, C2H2, OCS, and CO) in long-period comet C/2020 S3 (Erasmus) using the high-resolution, cross-dispersed infrared spectrograph (iSHELL) at the NASA Infrared Telescope Facility, on two pre-perihelion dates at heliocentric distances R h = 0.49 and 0.52 au. Our molecular abundances with respect to simultaneously or contemporaneously measured H2O indicate that S3 is depleted in CH3OH compared to its mean abundance relative to H2O among the overall comet population (Oort Cloud comets and Jupiter-family comets combined), whereas the eight other measured species have near-average abundances relative to H2O. In addition, compared to comets observed at R h < 0.80 au at near-infrared wavelengths, S3 showed enhancement in the abundances of volatile species H2CO, NH3, and C2H2, indicating possible additional (distributed) sources in the coma for these volatile species. The spatial profiles of volatile species in S3 in different instrumental settings are dramatically different, which might suggest temporal variability in comet outgassing behavior between the nonsimultaneous measurements. The spatial distributions of simultaneously measured volatile species C2H6 and CH4 are nearly symmetric and closely track each other, while those of CO and HCN co-measured with H2O (using different instrument settings) are similar to each other and are asymmetric in the antisunward direction.
小日心距离上的彗发丰度:长周期彗星 C/2020 S3 的成分测量(伊拉斯谟)
我们报告了长周期彗星C/2020 S3 (Erasmus)中H2O和九种痕量分子(C2H6、CH4、H2CO、CH3OH、HCN、NH3、C2H2、OCS和CO)的生成率,报告是在日心距离R h = 0.49和0.52 au的两个近日点前日期,利用美国宇航局红外望远镜设施的高分辨率交叉分散红外光谱仪(iSHELL)进行的。与同时或同期测量到的 H2O 相比,我们的分子丰度表明,在整个彗星群(奥尔特云彗星和木星家族彗星的总和)中,相对于 H2O 的平均丰度,S3 的 CH3OH 含量较低,而测量到的其他八种物质相对于 H2O 的丰度接近平均值。此外,与在近红外波长下 R h < 0.80 au 处观测到的彗星相比,S3 的挥发性物质 H2CO、NH3 和 C2H2 的丰度有所提高,表明这些挥发性物质可能在彗星彗尾中有额外的(分布式)来源。在不同的仪器设置下,S3 中挥发性物质的空间分布有很大不同,这可能表明非同步测量之间彗星排气行为的时间变化。同时测量到的挥发性物质 C2H6 和 CH4 的空间分布几乎是对称的,并且彼此紧密跟踪,而与 H2O 同时测量到的 CO 和 HCN(使用不同的仪器设置)的空间分布则彼此相似,并且在反向不对称。
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