Effects of Particle Size Distribution on Opacity Curves of Protoplanetary Disks around T Tauri Stars
Effects of Particle Size Distribution on Opacity Curves of Protoplanetary Disks around T Tauri Stars
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DOI：
10.1006/icar.1993.1156
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发表时间：
1993-11
期刊：
影响因子：
3.2
通讯作者：
K. Miyake;Y. Nakagawa
中科院分区：
文献类型：
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作者：
K. Miyake;Y. Nakagawa
摘要对金牛座T星周围星周盘亚毫米和毫米波长区域的不透明度指数进行了新的解释。Beckwith和萨金特（1991，Astrophys，J. 381，250-258）最近在亚毫米和毫米波段对金牛座T星进行的光度观测表明，金牛座T星周围星周盘中粒子不透明度与频率相关的幂律指数β为0-1;这些β值低于星际β值β = 2。星际尘埃颗粒和星周尘埃颗粒的β值存在明显差异。我们已经发现，颗粒生长可以解释观察到的不透明度指数β的这种差异，而不考虑任何组成变化。也就是说，星际物质是如此的稀疏，以至于粒子很少会相互碰撞，也几乎不会增长，但是在更密集的环境中，比如金牛座T星周围的星周盘，粒子会通过相互碰撞凝聚而增长;这解释了β的明显差异。我们进行了米氏计算，以获得除了亚微米尺寸的尘埃颗粒外，厘米或更大数量级的充分生长的尘埃颗粒的颗粒不透明度曲线，这些尘埃颗粒由硅酸盐和H2O-冰混合物的通常介电晶体材料组成，假设太阳元素丰度;我们发现，低β可以通过小的β_0 ~ 2粒子（瑞利散射区）和大的β_0 ~ 0粒子（几何光学区）的叠加来实现。此外，我们还发现，具有某些粒度分布的尘埃颗粒（幂律粒度分布，指数为3.5或更小，最小和最大截止半径分别为0.01 μm和0.1-10 cm）在亚毫米和毫米波长区域产生比颗粒不透明度标准值更大的不透明度（Hilde-brand，1983，Q. J. R. Astron.Soc.24，267-282），从中我们可以估算出典型的盘质量，其质量低至最小质量太阳星云模型。同时，我们还可以证明，具有这种尺寸分布的尘埃粒子在某些典型金牛座T型星周围的星周盘的观测光谱能量分布中，在λ = 100μm附近产生了与“翻转”相一致的不透明度。
Abstract A new interpretation for observed opacity indices at submillimeter- and millimeter-wavelength regions of circumstellar disks around T Tauri stars is presented. Recent photometric observations of T Tauri stars at submillimeter and millimeter wavelength regions by Beckwith and Sargent (1991, Astrophys, J. 381, 250-258) have revealed that power law indices β of frequency dependence of particle opacities in circumstellar disks around T Tauri stars are 0-1; and these values of β are lower than the interstellar value of β ≅ 2. There is distinct difference in β between interstellar and circumstellar dust particles. We have found that the particle growth can explain this difference in observed opacity indices β without considering any change in composition. That is, the interstellar matter is so sparse that particles will rarely collide with each other and will hardly grow, but in denser environments such as circumstellar disks around T Tauri stars particles will grow through coagulation by mutual collisions; this accounts for the distinct difference in β. We performed the Mie calculations to obtain the particle opacity curves of sufficiently grown dust particles on the order of centimeters or more in addition to submicrometer-sized dust particles, composed of the usual dielectric crystalline materials of silicate and H2O-ice mixture assuming the solar elemental abundance; we found that the low β can be realized as a result of the superposition of small particles of β ≅ 2 (Rayleigh scattering regime) and large particles of β ≅ 0 (geometrical optics regime). Furthermore, we show that dust particles with some size distributions (a power law size distribution with an exponent of 3.5 or less and minimum and maximum cut-off radii of 0.01 μm and 0.1-10 cm, respectively) produce larger opacities at submillimeter- and millimeter-wavelength regions than the standard value of particle opacity (Hilde-brand, 1983, Q. J. R. Astron. Soc. 24, 267-282), from which we can estimate typical disk masses as low as minimum-mass solar nebula models. Also we can show that dust particles with such size distributions give, at the same time, opacities consistent with a "turnover" around λ = 100μm for the observed spectral energy distributions of circumstellar disks around some typical T Tauri stars.