论文标题
非球形粉尘谷物的自散射
Self-scattering of non-spherical dust grains
论文作者
论文摘要
自从使用ALMA的高分辨率成像可用，对（亚）毫米极化的理解已经向前跃升。在其他效果中，自散射（即，在其他谷物上的热粉尘发射的散射）被认为是毫米极化的起源。这打开了第一个窗口，可以直接测量光学厚连续发射区域中的灰尘尺寸，因为它可以在原行星磁盘和恒星形成区域中找到。但是，晶粒尺寸的新得出值通常约为$ {\ sim} 100 \，μ$ m，因此比从更多间接测量值以及从理论（$ {\ sim} 1 \，$ mm）中获得的数量级小。我们在当今自散射模拟的应用尘埃模型中看到了这种矛盾的起源：一个完美的紧凑型球体。这项研究的目的是通过研究非球形晶粒形状对自散射信号的影响来检验我们的假设。我们应用离散的偶极近似模拟，以在三种情况下研究晶粒形状对自散射极化的影响：在固定的和变化的入射极化角度下的非极化和极化传入波。我们发现，当比较非球形与球形晶粒时，尤其是在瑞利体制之外，即> 100 $ \，μ$ m尺寸的谷物时，观察到$ 870 \，μ$ m wavelength时，我们发现所得的自散射极化的显着偏差。与挑战观察到的毫米极化起源的挑战的球体相比，由颗粒的自散射产生较高的极化程度。需要（几乎）非球形晶粒的（几乎）完美比对，以说明原球磁盘中观察到的毫米极化。我们的发现表明，对（亚）MM极化得出的灰尘尺寸的必要重新评估。
The understanding of (sub-)millimetre polarisation has made a leap forward since high-resolution imaging with ALMA came available. Amongst other effects, self-scattering (i.e., scattering of thermal dust emission on other grains) is thought to be the origin of millimetre polarisation. This opens the first window to a direct measurement of dust grain sizes in regions of optically thick continuum emission as it can be found in protoplanetary disks and star-forming regions. However, the newly derived values of grain sizes are usually around ${\sim}100\,μ$m and thus one order of magnitude smaller than those obtained from more indirect measurements as well as those expected from theory (${\sim}1\,$mm). We see the origin of this contradiction in the applied dust model of today's self-scattering simulations: a perfect compact sphere. The aim of this study is to test our hypothesis by investigating the impact of non-spherical grain shapes on the self-scattering signal. We apply discrete dipole approximation simulations to investigate the influence of the grain shape on self-scattering polarisation in three scenarios: an unpolarised and polarised incoming wave under a fixed as well as a varying incident polarisation angle. We find significant deviations of the resulting self-scattering polarisation when comparing non-spherical to spherical grains, in particular outside the Rayleigh regime, i.e. for >100$\,μ$m size grains observed at $870\,μ$m wavelength. Self-scattering by oblate grains produces higher polarisation degrees compared to spheres which challenges the interpretation of the origin of observed millimetre polarisation. A (nearly) perfect alignment of the non-spherical grains is required to account for the observed millimetre polarisation in protoplanetary disks. Our findings point towards a necessary re-evaluation of the dust grain sizes derived from (sub-)mm polarisation.
