英仙座低质量原恒星的Herschel / PACS观测中令人震惊的低水丰度
Shockingly low water abundances in Herschel / PACS observations of low-mass protostars in Perseus
摘要：
原恒星通过喷流和风与周围环境相互作用，撞击外壳并产生冲击，但这些冲击的性质仍然知之甚少。我们的目标是调查远红外分子线发射从一个统一的和重要的样本深埋低质量的年轻恒星物体，以表征冲击和紫外线辐射在直接原恒星环境中的可能作用。作为“William Herschel Line Legacy”调查的一部分，获得了英仙座分子云中22个物体的Herschel/PACS光谱图。针对文献中的冲击模型测试了H $_\mathrm{2}$ O、CO和OH的线发射。观测到的线比非常相似，并没有显示出与源物理参数的变化。观测表明，与冲击模型的线比相同的物种进行比较时，有很好的一致性。各种H-$_\mathrm{2}$-O线的比值提供了一个特别好的激波前气体密度的诊断，$n_\mathrm{H}\sim10^{5}$ cm $^{-3}$，与从激波后气体的观测中获得的典型密度一致。通过与CO谱线比的比较，得到相应的激波速度大于20 km\，s $^{-1}$。然而，观测结果一致表明，H $_\mathrm{2}$ O-CO和H $_\mathrm{2}$ O-OH线比现有激波模型预测的低一到两个数量级。高估的模型H $_\mathrm{2}$ O通量最有可能是由于模型中H $_\mathrm{2}$ O的过量引起的，因为激发被很好地再现。在星盘系统中或在激波中产生的紫外光子照射受冲击的物质，会降低H $_\mathrm{2}$ O的丰度，使模型与观测结果一致。热H $_\mathrm{2}$ O和强OH线的检测支持这种情况。
英文摘要：
Protostars interact with their surroundings through jets and winds impacting on the envelope and creating shocks, but the nature of these shocks is still poorly understood. Our aim is to survey far-infrared molecular line emission from a uniform and significant sample of deeply-embedded low-mass young stellar objects in order to characterize shocks and the possible role of ultraviolet radiation in the immediate protostellar environment. Herschel/PACS spectral maps of 22 objects in the Perseus molecular cloud were obtained as part of the `William Herschel Line Legacy' survey. Line emission from H$_\mathrm{2}$O, CO, and OH is tested against shock models from the literature. Observed line ratios are remarkably similar and do not show variations with source physical parameters. Observations show good agreement with the shock models when line ratios of the same species are compared. Ratios of various H$_\mathrm{2}$O lines provide a particularly good diagnostic of pre-shock gas densities, $n_\mathrm{H}\sim10^{5}$ cm$^{-3}$, in agreement with typical densities obtained from observations of the post-shock gas. The corresponding shock velocities, obtained from comparison with CO line ratios, are above 20 km\,s$^{-1}$. However, the observations consistently show one-to-two orders of magnitude lower H$_\mathrm{2}$O-to-CO and H$_\mathrm{2}$O-to-OH line ratios than predicted by the existing shock models. The overestimated model H$_\mathrm{2}$O fluxes are most likely caused by an overabundance of H$_\mathrm{2}$O in the models since the excitation is well-reproduced. Illumination of the shocked material by ultraviolet photons produced either in the star-disk system or, more locally, in the shock, would decrease the H$_\mathrm{2}$O abundances and reconcile the models with observations. Detections of hot H$_\mathrm{2}$O and strong OH lines support this scenario.
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