用ALMA调查土星外源物质 II. HCN的探测与制图
Surveying exogenous species in Saturn with ALMA. II. Detecting and Mapping HCN
- LIRA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université Paris Cité（巴黎天文台，巴黎文理研究大学，法国国家科学研究中心，索邦大学，巴黎西岱大学）
- Univ. Bordeaux, CNRS, LAB, UMR 5804（波尔多大学，法国国家科学研究中心，实验室）
- LMD/IPSL, CNRS, Sorbonne Université, École Polytechnique, Institut Polytechnique de Paris, École Normale Supérieure, Université PSL（气象动力学实验室/地球物理研究所，法国国家科学研究中心，索邦大学，巴黎综合理工学院，巴黎理工学院，巴黎高等师范学院，巴黎文理研究大学）
机构由 AI 辅助整理，请以论文原文为准。
AI总结：
利用ALMA探测并绘制土星平流层HCN发射线，测量其丰度与分布，探讨外部来源与内部输运对含氮物种的贡献。
AI中文摘要：
多个外部来源（环、卫星、彗星撞击、行星际尘埃粒子）向土星高层大气提供物质。这种外部流入的特征已在电离层电子和离子密度以及平流层含氧分子（CO、CO2、H2O）中得到记录。我们寻找土星平流层中含氮物种中外部来源的化学特征。我们以HCN(4-3)转动谱线为目标，并于2018年使用ALMA的C43-3配置在行星边缘绘制了其发射线，提供了0.61\"×0.50\"的合成波束。HCN在所有可观测纬度（从20°S到60°N）均被探测到。HCN丰度在两个独立的压力水平（0.1和1 hPa）下测量。测得的体积混合比在两个压力水平上在不确定度范围内随纬度均匀分布。在0.1和1 hPa处的平均vmr分别为$(7\pm 2)\times 10^{-11}$和$(4\pm 1)\times 10^{-12}$。这对应于平均柱密度$(4\pm 1)\times 10^{12}$ cm$^{-2}$，以及总氮质量$(3.7 \pm 0.9)\times 10^{10}$ g。陡峭的垂直剖面表明，除了光解离之外，还存在另一种损失机制，可能是气溶胶上的附着或异相化学。卡西尼号在环平面探测到的巨大物质流入在HCN上没有留下可探测的化学特征。探测到的HCN质量似乎与多个来源兼容：中纬度的环雨、土卫二或一次古老的彗星撞击。探测到的HCN也可能是从深层大气输送到平流层的内部N2的化学产物。
英文摘要：
Several external sources (the rings, the satellites, cometary impacts, interplanetary dust particles) supply material to the upper atmosphere of Saturn. The signature of this external influx has been documented on the ionospheric electronic and ionic densities, and in the stratosphere on oxygenated molecules (CO, CO2, H2O). We search for the chemical signature of the external sources in nitrogen-bearing species in Saturn's stratosphere. We have targeted the HCN(4-3) rotational line and mapped its emission line at the planet limb with ALMA in 2018 with configuration C43-3, providing a synthetic beam of 0.61"x0.50". HCN was detected at all observable latitudes from 20°S to 60°N. The HCN abundance was measured at two independent pressure levels, 0.1 and 1 hPa. The measured volume mixing ratios are latitudinally uniform within uncertainties at both pressure levels. The mean vmr is $(7\pm 2)\times 10^{-11}$ and $(4\pm 1)\times 10^{-12}$ at 0.1 and 1 hPa respectively. This corresponds to a mean column-density of $(4\pm 1)\times 10^{12}$ cm$^{-2}$, and a total nitrogen mass of $(3.7 \pm 0.9)\times 10^{10}$ g. The steep vertical profile suggests that another loss mechanism, possibly sticking or heterogeneous chemistry on aerosols, is active besides photodissociation. The huge influx of material detected in the ring plane by Cassini left no detectable chemical signature on HCN. The detected HCN mass seems compatible with several sources: the ring rain at mid-latitudes, Enceladus, or an old cometary impact. The detected HCN could also be a chemical product of internal N2 transported from the deep atmosphere up to the stratosphere.
