對星際絲狀紅外暗雲SDC18的碎斷與磁場的多尺度研究

Abstract

本文使用JCMT和SMA望遠鏡的毫米及次毫米偏振光觀測進行對星際絲狀紅外暗雲SDC18.624-0.070的多尺度磁場研究。我們發現磁場總是垂直於各尺度結構(絲狀、分子雲團塊及分子雲核)的主軸。雖然平均磁場方向改變了不到10度，但磁場位置角的標準差隨著尺度變小而增大。從多解析度N2H+譜線觀測量到的速度梯度也垂直於絲狀結構的主軸。這些特徵符合磁場可以影響重力塌縮方向的強磁場模型的預測。我們根據觀測到的瀰漫SiO輻射以及角動量對轉動半徑的關係提出上述的磁場以及速度梯度可以由大尺度湍流的碰撞及絲狀結構的旋轉產生。經由測量重力，磁場及湍流在多尺度下的能量密度，我們發現重力，磁場及湍流的相對重要性與分子雲團塊的碎斷有關聯。呈現較少碎斷的SDC18-S相較於呈現較多碎斷的SDC18-N有更強的磁場強度，意味著更強的磁場可以抑制分子雲團塊的碎斷。

We present a multiscale study of the magnetic field toward the filamentary infrared dark cloud SDC18.624-0.070 using dust polarization continuum observations at 850 μm and 1.3 mm with various resolutions from the JCMT and the SMA. Our observations cover filament (~10 pc), filament-embedded clump (~1 pc), isolated clump (~0.1 pc), and clump-embedded core (~0.01 pc) scales, which are central to investigate the impact of the magnetic field on fragmentation. We find a magnetic field which is predominantly perpendicular to the major axes of all structures (filament, clumps, and cores). While its mean orientation is preserved within about 10 degree, a systematically increasing field dispersion toward smaller scales indicates a growing impact of gravity. Velocity gradients traced with N2H+ with resolutions similar to the polarization observations are also perpendicular to the filament's major axis. All these features suggest that the magnetic field constrains the direction of accretion and initial contraction as predicted by strong-field models. We argue that the observed magnetic field and velocity gradient can result from a combination of converging flows, based on a detected SiO component along the filament, and rotation, based on the measured N2H+ specific angular momentum profile. A multiscale energy analysis of gravity, magnetic field, and turbulence quantifying their relative importance shows that SDC18-S, displaying less fragmentation, has a larger field strength than SDC18-N which harbors more fragments. A faster (SDC18-N) and slower transition (SDC18-S) to a gravity-dominated regime is found to explain the different fragmentation at clump-embedded core scale, with the stronger magnetic field in SDC18-S suppressing fragmentation more.