标题： 关于年轻恒星周围的星云的反思：对II类盘上晚期物质吸积的系统搜索 显示英文标题

标题： Reflections on nebulae around young stars: A systematic search for late-stage infall of material onto Class II disks

摘要： 上下文。 虽然通常认为II类天体主要在其环境中独立演化，但许多II类天体仍靠近分子云，并可能继续与它们相互作用。 这可能导致物质在后期吸积到盘上，这可能会显著影响盘过程和行星形成。 目标。 为了系统地研究气体向盘的后期流入，我们在反射星云（RN）附近识别可能正在经历这一过程的II类天体候选者。 方法。 首先，我们针对具有已知千天文单位尺度气体结构的II类天体——可能是由于后期物质流入所致——并在光学和近红外图像中搜索附近的RN。 其次，我们编制了一个与RN相关的II类天体目录，并在档案ALMA数据中寻找大尺度CO结构。 利用原恒星和RN的目录，我们还估计了II类天体与周围物质相互作用的概率。 结果。 所有具有大尺度气体结构的天体在其附近也表现出一定程度的反射星云。 同样，至少有五个与明显RN相关的II类天体，且有足够的ALMA观测数据，被发现具有螺旋或流状结构，这可能是由于后期流入所致。 我们报告了在S CrA周围首次检测到这些结构。 结论。 我们的结果表明，邻近恒星形成区域中相当一部分II类盘可能与RN有关，因此可能正在经历气体的后期吸积。 对RN以及II类天体周围的千天文单位尺度气体结构的调查将有助于我们更好地理解后期流入现象的频率和影响。 显示英文摘要

摘要： Context. While it is generally assumed that Class II sources evolve largely in isolation from their environment, many still lie close to molecular clouds and may continue to interact with them. This may result in late accretion of material onto the disk that can significantly influence disk processes and planet formation. Aims. In order to systematically study late infall of gas onto disks, we identify candidate Class II sources in close vicinity to a reflection nebula (RN) that may be undergoing this process. Methods. First we targeted Class II sources with known kilo-au scale gas structures - possibly due to late infall of material - and we searched for RNe in their vicinity in optical and near-infrared images. Second, we compiled a catalogue of Class II sources associated with RNe and looked for the large-scale CO structures in archival ALMA data. Using the catalogues of protostars and RNe, we also estimated the probability of Class II sources interacting with surrounding material. Results. All of the sources with large-scale gas structures also exhibit some reflection nebulosity in their vicinity. Similarly, at least five Class II objects associated with a prominent RNe, and for which adequate ALMA observations are available, were found to have spirals or stream-like structures which may be due to late infall. We report the first detection of these structures around S CrA. Conclusions. Our results suggest that a non-negligible fraction of Class II disks in nearby star-forming regions may be associated with RNe and could therefore be undergoing late accretion of gas. Surveys of RNe and kilo-au scale gas structures around Class II sources will allow us to better understand the frequency and impact of late-infall phenomena.