本研究旨在探討早期恆星形成中圍繞 protoplanetary disk 中心的周圍的吸積結構，特別是區分絲狀（filamentary）和片狀（sheet）結構的演化特徵。我們分析 Lee et al. (2021) 提出的三維 protoplanetary disk 形成模型，採用氣體密度梯度向量定位局部密度峰值（peaks），並利用主成分分析（PCA）進行結構分類。結果顯示，在恆星形成初期，吸積結構主要以 sheet 的形式呈現，filament 結構較稀少且多嵌入於 sheet 結構中。隨著 protoplanetary disk 的形成與演化，filament 結構逐漸明顯，但其吸積行為多受 sheet 結構的影響。我們發現吸積特性由 sheet 結構主導，而 filament 結構的產生可能是因為局部密度不均，導致物質集中於高密度區域，最終形成看似 filament 的結構。因此 disk 中心周圍主導吸積的吸積結構實際上應該是”accretion plane”而不是”accretion flow”。本研究為理解恆星形成過程中的吸積動力學機制提供了新的視角，並揭示了觀測中所見 filament 結構可能不是主導吸積行為的主要角色。

Our study aims to investigate the accretion structures surrounding the center of the protoplanetary disk in early star formation, with a particular focus on distinguishing the evolutionary characteristics of filamentary and sheet-like structures. Using the three-dimensional magnetohydrodynamic (MHD) simulation model carried out by Lee et al. (2021), we identify local density peaks using the gas density gradient vector and perform structural classification and geometric analysis via principal component analysis (PCA). The results show that, during the early stages of star formation, the accretion structures predominantly appear in the form of sheets, with filamentary structures being rare and often embedded within the sheet structures. As the protoplanetary disk forms and evolves, filamentary structures become more prominent, although their accretion behavior is still largely influenced by the sheet structures. We find that the accretion characteristics are dominated by sheet structures, while filamentary structures may be the result of local density inhomogeneities. Therefore, the dominant accretion structure around the disk center should be regarded as an “accretion plane” rather than an “accretion flow.” This study provides new insights into the accretion dynamics during the star formation process and reveals that the filamentary structures observed may merely be a projection effect.

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