L1544星前核中复杂有机分子的空间分布
The Spatial Distribution of Complex Organic Molecules in the L1544 Pre-stellar Core
摘要：
复杂有机分子（complex organic molecules，COMs）在星前核（T<10 K）等冷源中的探测，对我们理解星际介质中COMs的形成过程提出了挑战。最近的COM化学建模在低温下提供了新的见解，这些过程预测COM的形成强烈依赖于参数，如视觉消光和CO冻结的水平。我们报告了对L1544星前核心两个位置的COMs的深度观测：在2700 au内部的Av>=30等的高密度，高度熄灭的连续峰;以及位于距离核心中心4000 au的低密度壳层，平均Av约为7.5-8等，明亮的CH 3OH。我们的观察表明，CH 3 O，CH 3 OCH 3和CH 3CHO是更丰富的（约2-10倍）朝向低密度壳比朝向连续峰。其他COM如CH 3 OCHO、c-C3 H2O、HCCCHO、CH 2CHCN和HCCNC显示出轻微的增强（因子<=3），但相关的不确定性很大。这表明COM正在活跃地形成，并且已经存在于星前核心的低密度壳层中。L1544中含氧化合物的化学模拟表明，这些物质在这个壳层中得到增强，因为i）CO开始冻结到尘埃颗粒上，从而驱动活性表面化学; ii）视觉消光足够高，以防止外部星际辐射场对COM的紫外光解离; iii）密度仍然适中，以防止COM严重耗尽到颗粒上。
英文摘要：
The detection of complex organic molecules (COMs) toward cold sources such as pre-stellar cores (with T<10 K), has challenged our understanding of the formation processes of COMs in the interstellar medium. Recent modelling on COM chemistry at low temperatures has provided new insight into these processes predicting that COM formation depends strongly on parameters such as visual extinction and the level of CO freeze out. We report deep observations of COMs toward two positions in the L1544 pre-stellar core: the dense, highly-extinguished continuum peak with Av>=30 mag within the inner 2700 au; and a low-density shell with average Av~7.5-8 mag located at 4000 au from the core's center and bright in CH3OH. Our observations show that CH3O, CH3OCH3 and CH3CHO are more abundant (by factors ~2-10) toward the low-density shell than toward the continuum peak. Other COMs such as CH3OCHO, c-C3H2O, HCCCHO, CH2CHCN and HCCNC show slight enhancements (by factors <=3) but the associated uncertainties are large. This suggests that COMs are actively formed and already present in the low-density shells of pre-stellar cores. The modelling of the chemistry of O-bearing COMs in L1544 indicates that these species are enhanced in this shell because i) CO starts freezing out onto dust grains driving an active surface chemistry; ii) the visual extinction is sufficiently high to prevent the UV photo-dissociation of COMs by the external interstellar radiation field; and iii) the density is still moderate to prevent severe depletion of COMs onto grains.
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