Detectability of dirty dust grains in brown dwarf atmospheres
Detectability of dirty dust grains in brown dwarf atmospheres
复制标题
褐矮星大气中肮脏尘埃颗粒的可探测性
DOI：
10.1051/0004-6361:20064944
复制
发表时间：
2006
影响因子：
6.5
通讯作者：
M. Fridlund
中科院分区：
文献类型：
--
作者：
Ch. Helling;W. Thi;W. Thi;P. Woitke;M. Fridlund
语境。尘埃云影响褐矮星的大气结构，它们影响传热并改变气相化学。然而，它们的形成和演化的物理原理尚不清楚。灰尘的成分可以根据热力学平衡或时间依赖性化学来预测，其中考虑了种子颗粒的形成、颗粒生长、蒸发和漂移。目标。在这封信中，我们预测了尘埃特征，并根据尘埃形成理论预测的不同固体成分的光谱特征，提出了对褐矮星大气中尘埃形成物理的潜在观测测试。方法。将脏尘埃颗粒形成（成核、生长、蒸发和漂移）的动量方法应用于静态棕矮星大气结构，以计算尘埃颗粒特性，特别是异质颗粒成分和颗粒尺寸。有效介质和米氏理论用于计算这些球形颗粒的消光。结果。粉尘形成导致颗粒的成分与平衡状态下形成的颗粒的成分不同。我们的动力学模型预测，无定形 SiO 2 [s]（二氧化硅）是最丰富的固体成分之一，其次是无定形 Mg 2 SiO 4 [s] 和 MgSiO 3 [s]，而 SiO 2 [s] 在平衡模型中不存在，因为它是亚稳态固体。固态非晶态 SiO 2 [s] 具有以 8.7 μm 为中心的强宽吸收特征，而非晶态 Mg 2 SiO 4 [s]/MgSiO 3 [s] 除了在较长波长处具有其他吸收特征外，还在 9.7 μm 处吸收。正如我们的模型所提出的，如果高层大气中的颗粒很小（半径<0.2 μm），那么 λ < 15 μm 处的这些特征可以在吸收中检测到。结论。我们认为，在深红外光谱中检测到 8.7 μm 的特征可以为非平衡尘埃形成提供证据，这种尘埃形成在褐矮星大气中产生由亚稳态固体组成的颗粒。如果硅酸盐（例如镁钙石）更加丰富，则该特征将向 10μm 移动并变宽。
Context. Dust clouds influence the atmospheric structure of brown dwarfs, and they affect the heat transfer and change the gas-phase chemistry. However, the physics of their formation and evolution is not well understood. The dust composition can be predicted from thermodynamical equilibrium or time-dependent chemistry that takes into account seed particle formation, grain growth, evaporation, and drift. Aims. In this Letter, we predict dust signatures and propose a potential observational test of the physics of dust formation in brown dwarf atmospheres based on the spectral features of the different solid components predicted by dust formation theory. Methods. A momentum method for the formation of dirty dust grains (nucleation, growth, evaporation, and drift) is applied to a static brown dwarf atmosphere structure to compute the dust grain properties, and in particular, the heterogeneous grain composition and the grain size. The effective medium and Mie theories are used to compute the extinction of these spherical grains. Results. Dust formation results in grains whose composition differs from that of grains formed at equilibrium. Our kinetic model predicts that amorphous SiO 2 [s] (silica) is one of the most abundant solid components, followed by amorphous Mg 2 SiO 4 [s] and MgSiO 3 [s], while SiO 2 [s] is absent in equilibrium models because it is a metastable solid. Solid amorphous SiO 2 [s] possesses a strong broad absorption feature centered at 8.7 μm, while amorphous Mg 2 SiO 4 [s]/MgSiO 3 [s] absorbs at 9.7 μm in addition to other absorption features at longer wavelengths. Those features at λ < 15 μm are detectable in absorption if the grains are small (radius <0.2 μm) in the upper atmosphere, as proposed by our model. Conclusions. We suggest that the detection of a feature at 8.7 μm in deep infrared spectra could provide evidence for non-equilibrium dust formation that yields grains composed of metastable solids in brown dwarf atmospheres. This feature will shift towards 10μm and broaden if silicates (e.g. fosterite) are much more abundant.