FAUST XVII：行星形成系统IRS 63中吸积流撞击盘处的超级氘化现象
FAUST XVII: Super deuteration in the planet forming system IRS 63 where the streamer strikes the disk
AI总结：
本研究依托FAUST项目观测IRS 63原恒星盘，首次在行星形成盘及吸积流中探测到氘代甲醛，发现D₂CO分布不对称，揭示吸积流对盘质量和化学组成的关键作用。
摘要：
近期观测表明，行星形成始于年龄≤10^5年的原恒星盘阶段，这类盘的特征是会与周围环境发生强烈相互作用，例如通过吸积流（accretion streamers）和分子外流的形式。要研究这类现象对盘物理与化学性质的影响，核心是理解行星从其诞生环境中继承了怎样的化学组成。在ALMA大型项目“类太阳原恒星盘/包层系统化学的五十天文单位研究”（Fifty AU STudy of the chemistry in the disk/envelope system of Solar-like protostars, FAUST）的框架下，我们呈现了针对年轻行星形成盘IRS 63的H₂CO、HDCO和D₂CO观测结果，观测空间尺度覆盖约1500 au至约60 au。H₂CO可示踪盘内气体，以及撞击盘东南（SE）侧的大尺度（~1500 au）吸积流中的气体。我们首次在行星形成盘中探测到氘代甲醛HDCO和D₂CO，同时也在为盘提供物质的吸积流中探测到了HDCO。基于此我们估算了盘中H₂CO的氘化率：[HDCO]/[H₂CO]约为0.1-0.3，[D₂CO]/[H₂CO]约为0.1。有意思的是，HDCO的分布与H₂CO在盘和吸积流中的分布一致，但D₂CO的分布呈现高度不对称性，其发射峰（以及[D]/[H]比值峰）位于吸积流撞击盘的东南侧。此外，我们在蓝移和红移外流沿线的两个位置也探测到了D₂CO。这表明：（i）在盘中，HDCO的形成与H₂CO类似，以气相反应为主；而（ii）D₂CO主要形成于前恒星阶段的冰幔表面，和/或在盘内部形成，目前在吸积流与外流驱动的激波中被释放到气相中。这些发现证实了吸积流在盘的构建过程中发挥关键作用，既影响可供行星形成的最终质量，也影响盘的化学组成。
英文摘要：
Recent observations suggest that planets formation starts early, in protostellar disks of $\le10^5$ yrs, which are characterized by strong interactions with the environment, e.g., through accretion streamers and molecular outflows. To investigate the impact of such phenomena on disk physical and chemical properties it is key to understand what chemistry planets inherit from their natal environment. In the context of the ALMA Large Program Fifty AU STudy of the chemistry in the disk/envelope system of Solar-like protostars (FAUST), we present observations on scales from ~1500 au to ~60 au of H$_2$CO, HDCO, and D$_2$CO towards the young planet-forming disk IRS~63. H$_2$CO probes the gas in the disk as well as in a large scale streamer (~1500 au) impacting onto the South-East (SE) disk side. We detect for the first time deuterated formaldehyde, HDCO and D$_2$CO, in a planet-forming disk, and HDCO in the streamer that is feeding it. This allows us to estimate the deuterium fractionation of H$_2$CO in the disk: [HDCO]/[H$_2$CO]$\sim0.1-0.3$ and [D$_2$CO]/[H$_2$CO]$\sim0.1$. Interestingly, while HDCO follows the H$_2$CO distribution in the disk and in the streamer, the distribution of D$_2$CO is highly asymmetric, with a peak of the emission (and [D]/[H] ratio) in the SE disk side, where the streamer crashes onto the disk. In addition, D$_2$CO is detected in two spots along the blue- and red-shifted outflow. This suggests that: (i) in the disk, HDCO formation is dominated by gas-phase reactions similarly to H$_2$CO, while (ii) D$_2$CO was mainly formed on the grain mantles during the prestellar phase and/or in the disk itself, and is at present released in the gas-phase in the shocks driven by the streamer and the outflow. These findings testify on the key role of streamers in the build-up of the disk both concerning the final mass available for planet formation and its chemical composition.
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