arXiv 0705.39222007-05astro-ph

亚恒星质量天体大气化学平衡的系统研究

A Systematic Study of Departures from Chemical Equilibrium in the Atmospheres of Substellar Mass Objects

Ivan Hubeny, Adam Burrows

备注：: Accepted to the Astrophysical Journal; 21 figures in a total of 26 emulateapj pages
DOI：: 10.1086/522107

摘要：

我们提出了一个系统的研究光谱的后果偏离化学平衡的大气中的L和T矮星，甚至更冷的矮星。非平衡模型的温度/压力分布与非平衡化学完全一致。我们的网格的非平衡模型包括光谱的有效温度从200 K到1800 K，三个表面重力，四个可能的值的涡流扩散系数在辐射区，和三个不同的CO/CH $_4$化学反应处方。我们发现，在600 ~ 1800 K有效温度范围内，CO的非平衡过剩转化为M（$\sim4-$ 5 $μ$ m）带最多$\sim$ 40%的通量供应。在$T_{\rm eff} \approx 1100$ K附近影响最大。由于非平衡化学导致的氨丰度不足转化为不超过$\sim$ 20%的通量增强，对于$T_{\rm eff}$范围从300至1800 K，在$T_{\rm eff}$的最低值处具有最大的影响。偏离化学平衡的幅度随着重力的减小、涡流扩散系数的增大以及CO/CH $_4$反应速度的减小而增大。虽然这些影响是温和的，他们导致更好地配合测量T矮星光谱。此外，由于CO丰度的非平衡增强，M波段的抑制在$\sim$ 500 K以下消失，在$\sim$ 500 K以上仅部分消失，保留了M波段通量作为冷大气的有用诊断，并保持其在寻找比T矮星更冷的棕矮星方面的重要性。

英文摘要：

We present a systematic study of the spectral consequences of departures from chemical equilibrium in the atmospheres of L and T dwarfs, and for even cooler dwarfs. The temperature/pressure profiles of the non-equilibrium models are fully consistent with the non-equilibrium chemistry. Our grid of non-equilibrium models includes spectra for effective temperatures from 200 K to 1800 K, three surface gravities, four possible values of the coefficient of eddy diffusion in the radiative zone, and three different CO/CH$_4$ chemical reaction prescriptions. We find that the non-equilibrium overabundance of CO translates into flux suppressions in the M ($\sim4-$5 $μ$m) band of at most $\sim$40% between effective temperatures of 600 and 1800 K. The effect is largest around $T_{\rm eff} \approx 1100$ K. The underabundance of ammonia due to non-equilibrium chemistry translates into flux enhancements of no more than $\sim$20% for the $T_{\rm eff}$ range from 300 to 1800 K, with the largest effects at the lowest values of $T_{\rm eff}$. The magnitude of the departure from chemical equilibrium increases with decreasing gravity, with increasing eddy diffusion coefficient, and with decreasing speed of the CO/CH$_4$ reaction. Though these effects are modest, they lead to better fits with the measured T dwarf spectra. Furthermore, the suppression in the M band due to non-equilibrium enhancements in the CO abundance disappears below $\sim$500 K, and is only partial above $\sim$500 K, preserving the M band flux as a useful diagnostic of cool atmospheres and maintaining its importance for searches for brown dwarfs cooler than T dwarfs.

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摘要：

英文摘要：

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