利用 SMA 200–400 GHz 勘測約束 Class II 原行星盤的塵埃譜指數

Abstract

我們對 Taurus-Auriga 恆星形成區的 47 個 Class II 原行星盤(protoplanetary disks)進行了新的 SMA 勘測。我們的觀測在 200-400 GHz 頻率範圍內進行了12個獨立的光通量密度測量。我們將大多數的原行星盤光譜指數約束在2.0±0.2的極窄範圍內；只有少數幾個被解析的大原行星盤（例如，直徑>250 AU）呈現較大的光譜指數。對於這個結果最簡單的解釋是，所有觀測目標的（亞）毫米波光度都由極度光學厚(optically thick)（例如，τ > 10）的塵埃熱輻射主導。如果這種解釋屬實，則基於光學薄(optically thin)假設先前的測量可能會系統地低估塵埃質量至少一個數量級。我們基於光學厚(optically thick)、均勻塵埃板模型的分布模擬更偏好最大塵埃顆粒大小(maximum grain size)小於0.1mm。這可能表明，在雪線(water snowline)外的塵埃顆粒生長可能受到反彈/碎裂障壁(bouncing/fragmentation barriers)限制，並且冰包覆的塵埃顆粒可能比我們過去認的更不黏。在 Class II 原行星盤中，雪線外大部分的塵埃質量可能保留，而不是因為行星形成被消耗。雖然 Class II 原行星盤仍具有足夠的塵埃質量得以在之後的時間內供給行星形成，但塵埃凝聚和行星形成是否能在雪線外高效或自然地進行仍有疑問。

We present a new SMA survey towards 47 Class II disks in the Taurus-Auriga region. Our observations made 12 independent samples of flux densities over the 200–400 GHz frequency range. We tightly constrained the spectral indices of most sources to an incredibly narrow range of 2.0 ± 0.2; only a handful of spatially resolved extended (e.g., diameter >250 au) disks present larger spectral indices. The simplest interpretation for this result is that the (sub)millimeter luminosities of all of the observed target sources are dominated by very optically thick (e.g., τ >10) dust thermal emission. This interpretation, if true, may imply that the previous measurements based on an optically thin assumption have systematically underestimated dust mass by at least one order of magnitude. Our population synthesis based on a optically thick, uniform dust slab model prefers a maximum grain size (a max ) smaller than 0.1 mm. This may indicate that the dust grain growth outside the water snowline may be limited by the bouncing/fragmentation barriers and that the ice-coated dust grain may be less sticky than we used to consider. In the Class II disks, the dust mass budget outside of the water snowline may be largely retained instead of being mostly consumed by planet formation. While Class II disks still possess sufficient dust masses to feed planet formation at a later time, whether or not dust coagulation and planet formation can be efficient or natural outside of the water snowline appear questionable.