Dust Growth and Settling in Protoplanetary Disks and Disk Spectral Energy Distributions. I. Laminar Disks
Dust Growth and Settling in Protoplanetary Disks and Disk Spectral Energy Distributions. I. Laminar Disks
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DOI：
10.1086/429658
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发表时间：
2005-02
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影响因子：
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通讯作者：
Hidekazu Tanaka;Youhei Himeno;S. Ida
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文献类型：
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作者：
Hidekazu Tanaka;Youhei Himeno;S. Ida
尘埃的生长和沉降极大地影响了原行星盘的光谱能量分布（SED）。我们通过数值模拟研究了原行星盘中的尘埃生长和沉降，以检查盘光学厚度和 SED 的时间演化。在本文中，我们将层流盘视为一系列论文的第一步。由于灰尘生长和沉降，在气态盘的中平面周围形成灰尘层。灰尘层形成后，小灰尘颗粒仍漂浮在灰尘层上方。尽管漂浮小颗粒的表面密度远小于灰尘层的表面密度，但它们控制着盘的光学厚度和发射。数值模拟获得的漂浮颗粒的尺寸分布可以用通用幂律分布很好地描述，该分布与盘温度、盘表面密度、盘中的径向位置等无关。与灰尘层的形成相比，漂浮的小颗粒在较长的时间尺度内沉降到灰尘层上。通常，微米级颗粒需要 106 年。圆盘内部的快速晶粒生长使得圆盘光学厚度的径向分布不如圆盘表面密度 Σ 的陡峭。对于 Σ ∝ R-3/2 的圆盘，在 t ≲ 106 年时，对于所有波长，光学厚度的径向分布几乎是平坦的。在 t > 106 年时，内盘的光学厚度（≲几个 AU）几乎消失，这可能对应于斯皮策太空望远镜观测到的盘内孔。此外，我们利用数值结果和两层模型研究了磁盘 SED 的时间演化。晶粒生长和沉降降低了 SED 的强度，尤其是在 λ ≥ 100 μm 时。我们的结果表明，晶粒生长和沉降可以解释在 106-107 年的时间尺度内观察到的毫米/亚毫米波长能量通量的减少，而没有盘的耗尽。
Dust growth and settling considerably affect the spectral energy distributions (SEDs) of protoplanetary disks. We investigated dust growth and settling in protoplanetary disks through numerical simulations to examine time evolution of the disk optical thickness and SEDs. In this paper we considered laminar disks as the first step in a series of papers. As a result of dust growth and settling, a dust layer forms around the midplane of a gaseous disk. After the formation of the dust layer, small dust grains remain floating above the layer. Although the surface density of the floating small grains is much less than that of the dust layer, they govern the disk optical thickness and the emission. Size distributions of the floating grains obtained from numerical simulations are well described by a universal power-law distribution, which is independent of the disk temperature, the disk surface density, the radial position in the disk, etc. The floating small grains settle onto the dust layer in a long timescale compared with the formation of the dust layer. Typically, it takes 106 yr for micron-sized grains. Rapid grain growth in the inner part of disks makes the radial distribution of the disk optical thickness less steep than that of the disk surface density, Σ. For disks with Σ ∝ R-3/2, the radial distribution of the optical thickness is almost flat for all wavelengths at t ≲ 106 yr. At t > 106 yr, the optical thickness of the inner disk (≲a few AU) almost vanishes, which may correspond to disk inner holes observed by Spitzer Space Telescope. Furthermore, we examined time evolution of disk SEDs, using our numerical results and the two-layer model. The grain growth and settling decrease the magnitude of the SEDs, especially at λ ≥ 100 μm. Our results indicate that grain growth and settling can explain the decrease in observed energy fluxes at millimeter/submillimeter wavelengths with timescales of 106-107 yr without depletion of the disks.