DUST TRANSPORT IN PROTOSTELLAR DISKS THROUGH TURBULENCE AND SETTLING
DUST TRANSPORT IN PROTOSTELLAR DISKS THROUGH TURBULENCE AND SETTLING
复制标题
原恒星盘中的尘埃通过湍流和沉降进行传输
DOI：
--
复制
发表时间：
2009
期刊：
影响因子：
--
通讯作者：
T. Sano
中科院分区：
文献类型：
--
作者：
N. Turner;A. Carballido;A. Carballido;T. Sano
我们应用电离平衡和磁流体动力学（MHD）计算来研究由尘埃颗粒重组缓和的磁活动是否可以解释质量吸积率和原恒星盘的中红外光谱和可变性。MHD计算采用分层剪切箱方法，包括颗粒沉降和粉尘丰度变化对气体电阻率的反馈。在太阳质量的金牛座T星之间，20年的吸积速率差异太大，不可能仅仅是由颗粒大小和恒星x射线亮度的变化造成的，但可以通过改变这些参数和磁盘磁通量来产生。中红外硅酸盐带的不同形状和强度可能来自于颗粒沉降与磁旋湍流分布的耦合，通过以下三种效应。首先，在1 μm或更小的颗粒上的复合产生一个从中间延伸两个尺度高度以上的磁不活跃死区，而磁活跃盘大气中的湍流运动仅超过死区边界约一个尺度高度。其次，死区深处的颗粒在上面的湍流层的驱动下以波浪运动的方式垂直振荡，但平均沉降速度与层流相同，因此死区内部是一个颗粒汇，除非从其他地方补充，否则圆盘大气将变得尘埃耗尽。第三，在充分耗竭的情况下，死区会变薄，混合会将颗粒从中层清除。这些过程中的最后一个使诸如沉降程度之类的进化特征有时会随着年龄的增长而降低。MHD的结果还表明，磁活动间歇性地将小颗粒云提升到大气中。因此，在几个轨道的时间尺度上，光球层的高度变化高达三分之一，而沿着掠过圆盘表面的视线的消光则以2倍的倍数变化，低至轨道的十分之一。我们认为，在许多年轻恒星中发现的每日到每月的中红外变化是由外盘尘埃云投下的变化阴影造成的。
We apply ionization balance and magnetohydrodynamical (MHD) calculations to investigate whether magnetic activity moderated by recombination on dust grains can account for the mass accretion rates and the mid-infrared spectra and variability of protostellar disks. The MHD calculations use the stratified shearing-box approach and include grain settling and the feedback from the changing dust abundance on the resistivity of the gas. The two-decade spread in accretion rates among solar-mass T Tauri stars is too large to result solely from variations in the grain size and stellar X-ray luminosity, but can plausibly be produced by varying these parameters together with the disk magnetic flux. The diverse shapes and strengths of the mid-infrared silicate bands can come from the coupling of grain settling to the distribution of the magnetorotational turbulence, through the following three effects. First, recombination on grains 1 μm or smaller yields a magnetically inactive dead zone extending more than two scale heights from the midplane, while turbulent motions in the magnetically active disk atmosphere overshoot the dead zone boundary by only about one scale height. Second, grains deep in the dead zone oscillate vertically in wave motions driven by the turbulent layer above, but on average settle at the rates found in laminar flow, so that the interior of the dead zone is a particle sink and the disk atmosphere will become dust-depleted unless resupplied from elsewhere. Third, with sufficient depletion, the dead zone is thinner and mixing dredges grains off the midplane. The last of these processes enables evolutionary signatures such as the degree of settling to sometimes decrease with age. The MHD results also show that the magnetic activity intermittently lifts clouds of small grains into the atmosphere. Consequently the photosphere height changes by up to one-third over timescales of a few orbits, while the extinction along lines of sight grazing the disk surface varies by factors of 2 over times down to a tenth of an orbit. We suggest that the changing shadows cast by the dust clouds on the outer disk are a cause of the daily to monthly mid-infrared variability found in many young stars.