年轻星周星云中CO第四次正发射的表征
Characterizing CO Fourth Positive Emission in Young Circumstellar Disks
摘要：
一氧化碳是原行星盘中常用的红外/亚毫米气体示踪剂。本文分析了12颗经典金牛T星的{HST}-COS光谱中的紫外CO发射。CO A^1 <$- X^1 <$^+（第四正）电子跃迁系统的几个振转谱带与其它原子和H_2的发射谱线相分辨。CO A^1 λ v '= 14态是通过吸收Lyα光子而形成的，Ly α光子是在恒星表面的吸积柱中产生的。对于具有强CO发射的目标，我们将Lyα辐射场建模为简单荧光模型的输入，以估计CO旋转激发温度和柱密度。典型的柱密度范围为N_{CO} = 10^{18} - 10^{19} cm^{-2}。我们测得的激发温度大多低于T_{CO} = 600 K，比典型的M带CO发射冷。这些温度和发射谱线宽度意味着UV发射起源于与IR发射不同的CO群体。我们还发现了一个显着的相关性之间的CO排放和磁盘吸积率M_{acc}和年龄。我们的分析表明，紫外CO发射可以是一个有用的诊断CTTS盘气体。
英文摘要：
Carbon Monoxide is a commonly used IR/sub-mm tracer of gas in protoplanetary disks. We present an analysis of ultraviolet CO emission in {HST}-COS spectra for 12 Classical T Tauri stars. Several ro-vibrational bands of the CO A^1Π- X^1Σ^+ (Fourth Positive) electronic transition system are spectrally resolved from emission of other atoms and H_2. The CO A^1Πv'=14 state is populated by absorption of Lyαphotons, created at the accretion column on the stellar surface. For targets with strong CO emission, we model the Lyαradiation field as an input for a simple fluorescence model to estimate CO rotational excitation temperatures and column densities. Typical column densities range from N_{CO} = 10^{18} - 10^{19} cm^{-2}. Our measured excitation temperatures are mostly below T_{CO} = 600 K, cooler than typical M-band CO emission. These temperatures and the emission line widths imply that the UV emission originates in a different population of CO than that which is IR-emitting. We also find a significant correlation between CO emission and the disk accretion rate M_{acc} and age. Our analysis shows that ultraviolet CO emission can be a useful diagnostic of CTTS disk gas.
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