星际甲醛冰的宇宙射线诱导溅射
Cosmic-ray induced sputtering of interstellar formaldehyde ices
AI总结：
本研究通过重离子辐照实验量化了纯H2CO冰的宇宙射线溅射产额，发现其溅射率过低无法解释前恒星核L1689B中的气相丰度，但假设H2CO在CO或CO2冰中共脱附可重现观测结果。
摘要：
H2CO 是星际介质（ISM）和前恒星核气相中普遍存在的分子，且很可能存在于冰幔中，但其主要脱附机制尚不明确。本文旨在量化 H2CO 在宇宙射线撞击下的脱附效率，以确定宇宙射线诱导的溅射是否能解释在前恒星核中观测到的 H2CO 丰度。我们利用 GANIL 加速器中的重离子束作为宇宙射线模拟源，在高真空条件下对 10 K 的纯 H2CO 冰膜进行辐照，并通过红外光谱监测冰膜的演化，同时利用质谱法分析气相中溅射物质的组成。我们推导了纯 H2CO 冰的有效溅射产额和完整溅射产额。我们发现，在重离子辐照下 H2CO 在冰中极易发生聚合，并被辐射分解为 CO 和 CO2。在气相中，主要的溅射物质是 CO，而完整的 H2CO 仅是次要物质。我们测得纯 H2CO 冰的完整溅射产额为 $2.5\times 10^3$ molecules ion$^{-1}$，对应的电子阻止本领为 $S_e\sim2830$ eV ($10^{15}$ molecules cm$^{-2}$)$^{-1}$。相应的宇宙射线溅射率为 $Γ_\mathrm{CRD}=1.5\times 10^{18}ζ$ molecules cm$^{-2}$ s$^{-1}$，其中 $ζ$ 为 ISM 中分子氢的宇宙射线电离率。在一个简单的冻存与非热脱附稳态化学模型框架下，我们发现该实验宇宙射线溅射率过低（相差一个数量级），无法解释我们在前恒星核 L1689B 中推导出的 H2CO 气相丰度观测值。然而我们发现，如果假设稀释在 CO 或 CO2 冰中的 H2CO 以与纯 CO 或纯 CO2 冰相同的溅射率发生共脱附，则可以重现这一丰度。
英文摘要：
H2CO is a ubiquitous molecule in the ISM and in the gas phase of prestellar cores, and is likely present in ice mantles, but its main desorption mechanism is unknown. In this paper our aim is to quantify the desorption efficiency of H2CO upon cosmic-ray impact in order to determine whether cosmic-ray induced sputtering could account for the H2CO abundance observed in prestellar cores. Using a heavy-ion beam as a cosmic-ray analogue at the GANIL accelerator, we irradiated pure H2CO ice films at 10 K under high vacuum conditions and monitored the ice film evolution with infrared spectroscopy and the composition of the sputtered species in the gas phase using mass spectrometry. We derived both the effective and intact sputtering yield of pure H2CO ices. We find that H2CO easily polymerises under heavy-ion irradiation in the ice, and is also radiolysed into CO and CO2. In the gas phase, the dominant sputtered species is CO and intact H2CO is only a minor species. We determine an intact sputtering yield for pure H2CO ices $2.5\times 10^3$ molecules ion$^{-1}$ for an electronic stopping power of $S_e\sim2830$ eV ($10^{15}$ molecules cm$^{-2}$)$^{-1}$. The corresponding cosmic-ray sputtering rate is $Γ_\mathrm{CRD}=1.5\times 10^{18}ζ$ molecules cm$^{-2}$ s$^{-1}$, where $ζ$ is the rate of cosmic-ray ionisation of molecular hydrogen in the ISM. In the frame of a simple steady-state chemical model of freeze-out and non-thermal desorption, we find that this experimental cosmic-ray sputtering rate is too low (by an order of magnitude) to account for the observed H2CO gas-phase abundance we derived in the prestellar core L1689B. We find however that this abundance can be reproduced if we assume that H2CO diluted in CO or CO2 ices co-desorbs at the same sputtering rate as pure CO or pure CO2 ices.
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