解析TW长蛇座一号磁盘中的化学成分。氘代物种
Resolving the chemistry in the disk of TW Hydrae I. Deuterated species
摘要：
利用亚毫米波阵列（SMA）在角秒尺度上观测了金牛座T星长蛇座TW周围盘中的几种氘代物质，包括DCN J=3-2和DCO+ J=3-2谱线，以及HDO 3（1，2）-2（2，1），ortho-H2 D + 1（1，0）-1（1，1）和para-D2 H + 1（1，0）-1（0，1）跃迁的上限。我们还观测了HCN J=3-2，HCO+ J=3-2和H_（13）CO + J=4-3谱线，并与它们的氘代同位素作了比较。我们约束的径向和垂直分布的光盘中的各种物种的拟合数据，使用一个模型，其中的分子发射从一个照射吸积盘采样与二维蒙特卡罗辐射转移代码。我们发现DCO ~+的分布与HCO ~+的分布有很大的不同。D/H比推断的变化至少有一个数量级（0.01至0.1）的半径<30 AU to >70 AU和DCO+的丰度在半径大于90 AU的快速下降。使用一个简单的分析化学模型，我们将存在这些分子的圆盘层的电离度x（e-）=n（e-）/n（H2）限制为~10^-7。假设DCN的分布遵循HCN的分布，则DCN与HCN的比值被确定为1.7 \pm 0.5 \times 10^-2;然而，该比值对HCN的垂直分布约束很差非常敏感。解析的DCO+的径向分布表明，{ \it原位}氘分馏仍然活跃在TW长蛇座盘，必须考虑在环星吸积盘的分子演化。
英文摘要：
We present Submillimeter Array (SMA) observations of several deuterated species in the disk around the classical T Tauri star TW Hydrae at arcsecond scales, including detections of the DCN J=3-2 and DCO+ J=3-2 lines, and upper limits to the HDO 3(1,2)-2(2,1), ortho-H2D+ 1(1,0)-1(1,1) and para-D2H+ 1(1,0)-1(0,1) transitions. We also present observations of the HCN J=3-2, HCO+ J=3-2 and H13CO+ J=4-3 lines for comparison with their deuterated isotopologues. We constrain the radial and vertical distributions of various species in the disk by fitting the data using a model where the molecular emission from an irradiated accretion disk is sampled with a 2D Monte Carlo radiative transfer code. We find that the distribution of DCO+ differs markedly from that of HCO+. The D/H ratios inferred change by at least one order of magnitude (0.01 to 0.1) for radii <30 AU to >70 AU and there is a rapid falloff of the abundance of DCO+ at radii larger than 90 AU. Using a simple analytical chemical model, we constrain the degree of ionization, x(e-)=n(e-)/n(H2), to be ~10^-7 in the disk layer(s) where these molecules are present. Provided the distribution of DCN follows that of HCN, the ratio of DCN to HCN is determined to be 1.7\pm0.5 \times 10^-2; however, this ratio is very sensitive to the poorly constrained vertical distribution of HCN. The resolved radial distribution of DCO+ indicates that {\it in situ} deuterium fractionation remains active within the TW Hydrae disk and must be considered in the molecular evolution of circumstellar accretion disks.
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