Star Formation Initiated by Decay of Turbulence

大振幅的磁流体动力学（MHD）波在分子云中传播，在那里恒星正在形成。这些MHD波被认为可以防止分子云坍缩形成恒星。换句话说，如果MHD波衰减，恒星形成将变得活跃。这一思想促进了旨在推导MHD波衰减率的数值模拟。然而，迄今为止的数值模拟受到了MHD波的严重数值衰减的影响。此外，波长的动态范围也受到可用计算机资源的限制。我们利用圆极化Alfven波的解析解计算了数值黏度引起的衰减。我们已经证明，只有当使用超过32个网格点来解析波长时，数值衰减才足够小。这意味着即使在每个空间方向上使用256个网格点，动态范围也只能是波长的8倍。我们还证明了阿尔芬波的物理衰减不仅与波长有关，而且与偏振有关。在丝状云中行进的圆偏振波以与波长无关的电子折叠时间标度呈指数衰减。一个线偏振的衰减与时间的平方成反比，并且衰减率在较短的波长下较大。这些结果已在两次国际会议上发表，并将在不久的将来在一份评审期刊上发表。

Magnetohydrodynamical (MHD) waves of large amplitudes propagate in molecular clouds where star formation is now ongoing. These MHD waves are thought to prevent the molecura clouds collapsing to form stars. In other words, star formation will become active if the MHD waves decay. This idea have promoted numerical simulations which aim to derive the decay rate of the MHD waves. However, the numerical simulations so far have suffered from *erious numerical decays of the MHD waves. Also the dynamic range in the wavelength was also limited by computer resources available. We evaluated the decay due to the numerical viscosity using the analytical solution of the circularly polarized Alfven wave. We have shown that the numerical decay is small enough only when more than 32 mesh points are used to resolve a wavelength. This means that the dynamic range can be only by a factor 8 in the wavelength even when 256 mesh points are used in each spatial direction. We have also shown that the physical decay of the Alfven wave depends not only on the wavelength but on the polarization. A circularly polarized wave traveling in a filamentary cloud decays exponentially with a wavelength independent e-folding timescale. A linearly polarized one decays inversely proportional to the square of the time and the decay rate is larger for a shorter wavelength. These results were shown at two international conferences and will be shown in a refereed journal in near future.