CO/H2 ABUNDANCE RATIO ≈ 10−4 IN A PROTOPLANETARY DISK
CO/H2 ABUNDANCE RATIO ≈ 10−4 IN A PROTOPLANETARY DISK
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DOI：
10.1088/0004-637x/794/2/160
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发表时间：
2014-09
期刊：
影响因子：
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通讯作者：
K. France;G. Herczeg;M. McJunkin;S. Penton
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文献类型：
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作者：
K. France;G. Herczeg;M. McJunkin;S. Penton
年轻恒星周围的行星形成盘中原子和分子物质的相对丰度为光化学盘模型提供了重要的约束条件，并为通过测量痕量物质计算盘质量提供了基线。相对于分子氢（H2）的绝对丰度的知识是具有挑战性的，因为H2的旋转振动跃迁阶梯很弱，并且无法在空间上分辨出星周环境中不同的发射成分。为了解决这两个问题，我们通过经典金牛座T星RW Aurigae a周围原行星盘的“温暖分子层”对CO和H2吸收进行了新的同期测量。我们使用哈勃太空望远镜宇宙起源光谱仪的新观测模式首次探测到该区域的温暖H2吸收。RW Aur的发射和吸收光谱分析显示了来自恒星光球附近的吸积区、分子盘和一些流出成分。温暖的H2和CO吸收线与盘状起源一致。我们对RW Aur的1092-1117 Å光谱进行建模，得到在Trot(H2) = 440±39 K时log10 N(H2) = 19.90 cm−2。从1410到1520 Å观测到的CO A - X波段在Trot(CO) = 200 K时的最佳拟合值为log10 N(CO) = 16.1 cm−2。结合对H i， H2和CO柱密度的直接测量，我们在fH2大于或等于0.47的热盘表面发现了分子分数，并得出CO/H2的分子丰度比= 1.6 × 10−4，两者都与典型的星际致密云值一致。
The relative abundances of atomic and molecular species in planet-forming disks around young stars provide important constraints on photochemical disk models and provide a baseline for calculating disk masses from measurements of trace species. A knowledge of absolute abundances, those relative to molecular hydrogen (H2), are challenging because of the weak rovibrational transition ladder of H2 and the inability to spatially resolve different emission components within the circumstellar environment. To address both of these issues, we present new contemporaneous measurements of CO and H2 absorption through the “warm molecular layer” of the protoplanetary disk around the Classical T Tauri Star RW Aurigae A. We use a newly commissioned observing mode of the Hubble Space Telescope Cosmic Origins Spectrograph to detect warm H2 absorption in this region for the first time. An analysis of the emission and absorption spectrum of RW Aur shows components from the accretion region near the stellar photosphere, the molecular disk, and several outflow components. The warm H2 and CO absorption lines are consistent with a disk origin. We model the 1092–1117 Å spectrum of RW Aur to derive log10 N(H2) = 19.90 cm−2 at Trot(H2) = 440 ± 39 K. The CO A – X bands observed from 1410 to 1520 Å are best fit by log10 N(CO) = 16.1 cm−2 at Trot(CO) = 200 K. Combining direct measurements of the H i, H2, and CO column densities, we find a molecular fraction in the warm disk surface of fH2 ⩾ 0.47 and derive a molecular abundance ratio of CO/H2 = 1.6 × 10−4, both consistent with canonical interstellar dense cloud values.