原恆星盤 MWC 758 裡的塵埃懸臂

行星是在原恆星盤面裡面形成，並在盤面產生非對稱性的構造。我們可以使用高解析力和高靈敏度的阿塔卡瑪大型毫米及次毫米波陣列干涉儀 (ALMA) 來觀測原恆星盤系統並研究行星形成。盤面的非對稱性結構能夠提供間接的證據有關隱藏在盤面裡的物體。我們使用 0.9 毫米的波段來分析原恆星盤 MWC 758 的塵埃分佈。我們將盤面每隔 5 度分割成 72 個小扇形來分析結構。對於每一個扇形，我們畫出強度對半徑的分佈圖，並使用兩個高斯函數來描述這個散佈圖。我們使用得到的高斯參數來代表盤面上的峰值位置。我們把高斯的強度與位置與對角度作圖來做進一步分析得到細部構造。我們找到了 3 個非對稱性的特徵。有兩個是塵埃團在不同的半徑但都帶有懸臂構造的尾巴。第三個是一個懸臂的構造在盤面的東南方。我們使用懸臂密度波理論來分析我們看到的懸臂特徵。隱藏在盤面裡的天體能夠透過重力擾動盤面而形成懸臂的特徵。根據數學模型推導的解析解可以得出理論的懸臂形狀。我們使用上述的方程式來分析觀測到的三個懸臂。觀測到的三個特徵可以用被行星引起的懸臂密度波理論解釋，並推測三個行星的可能位置。我們比較在毫米波段看到的懸臂與紅外線偏振光觀測到的懸臂做比較。紅外線的懸臂座落在毫米懸臂的內側，這樣的結構可以用散射來解釋。紅外線所觀測到的是打在毫米懸臂結構上的散射光。此外，兩波段的懸臂距離在西北方 (近) 較大而東南方 (遠) 較小，這樣的現象可以用傾斜盤面的遠近來解釋。我們比較我們觀測的結果並與更高解析度的新圖片比較，我們得出不一樣的結構運用不一樣的分析方法，而我們觀測到的結構也出現在高解析度的新圖片裡。

關鍵字

原恆星盤；恆星 (MWC 758) ；行星與盤的互動；塵埃懸臂

並列摘要

Planets formed within disks such as the Solar System and expected to induced substructures within disks. With Atacama Large Millimeter/submillimeter Array (ALMA), we can probe into protoplanetary systems to study planets formation. Asymmetrical features within disks might provide the indirect evidences of embedded objects. We analyze the disk structures of MWC 758 traced with the continuum emission at wavelengths of 0.9 millimeter (mm). We split the dust ring into segments in azimuth to study the disk structure. For each segment, we fit two Gaussian functions to the intensity versus radius profile. We analyze the best-fit parameters as a function of azimuth. We describe the disk geometry with peak locations. We find asymmetries and identify three structures. Among the three, two (arm 2 and arm 3) are dust clumps at different radius with similar intensity profiles and a spiral-like tail. The third feature (arm 1) is a spiral-like structure located at southeast. We compare our spirals with the spirals seen in the near-infrared (NIR) polarized intensity. The counterparts of arm 1 and arm 2 in NIR appear at smaller radius than our spirals. By comparing the spirals seen in 0.9 mm and NIR, we found that the spatial offset can be explained by the scattering from the inner edge of the sub-mm spirals. Comparing the spirals in sub-mm and in NIR, there is a larger offset in arm 2 (near side) than in arm 1 (far side) due to the projection effect of the actual disk geometry. We fit our features with the spiral density wave theory using the WKB approximation and results in two sets of disk aspect ratio. One is unphysical low value (∼0.03), the other is the upper bound of the parameter (∼0.2). The planet locations predicted by the upper bounded result are similar to the one determined by Benisty et al. (2015). The spiral patterns can be explained by the density wave induced by the planets. We compare our spirals with a higher-resolution ALMA image by Dong et al. (2018). We identify different structures in the West of the disk due to the resolution of the image and the analysis method.