[论文解读] The Far-Ultraviolet

Eric Schindhelm, Eric B. Burgh+21 more|arXiv (Cornell University)|Dec 30, 2010

Astrophysics and Star Formation Studies被引用 4

一句话总结

利用哈勃空间望远镜的宇宙起源光谱仪，本研究首次在经典T Tauri星HN Tau、RECX-11和V4046 Sgr周围的原行星盘中探测到远紫外波段的CO吸收与发射特征。研究揭示了吸收态中CO柱密度约为~2 × 10¹⁷ cm⁻²，转动温度约为~500 K；在Lyα泵浦激发的发射态中，柱密度更高（10¹⁸–10¹⁹ cm⁻²），温度超过300 K，表明内盘中CO/H₂比例约为1。

ABSTRACT

We exploit the high sensitivity and moderate spectral resolution of the $HST$-Cosmic Origins Spectrograph to detect far-ultraviolet spectral features of carbon monoxide (CO) present in the inner regions of protoplanetary disks for the first time. We present spectra of the classical T Tauri stars HN Tau, RECX-11, and V4046 Sgr, representative of a range of CO radiative processes. HN Tau shows CO bands in absorption against the accretion continuum. We measure a CO column density and rotational excitation temperature of N(CO) = 2 +/- 1 $ imes$ 10$^{17}$ cm$^{-2}$ and T_rot(CO) 500 +/- 200 K for the absorbing gas. We also detect CO A-X band emission in RECX-11 and V4046 Sgr, excited by ultraviolet line photons, predominantly HI LyA. All three objects show emission from CO bands at $\lambda$ $>$ 1560 \AA, which may be excited by a combination of UV photons and collisions with non-thermal electrons. In previous observations these emission processes were not accounted for due to blending with emission from the accretion shock, collisionally excited H$_{2}$, and photo-excited H2; all of which appeared as a continuum whose components could not be separated. The CO emission spectrum is strongly dependent upon the shape of the incident stellar LyA emission profile. We find CO parameters in the range: N(CO) 10$^{18-19}$ cm$^{-2}$, T_{rot}(CO) > 300 K for the LyA-pumped emission. We combine these results with recent work on photo- and collisionally-excited H$_{2}$ emission, concluding that the observations of ultraviolet-emitting CO and H2 are consistent with a common spatial origin. We suggest that the CO/H2 ratio in the inner disk is ~1, a transition between the much lower interstellar value and the higher value observed in solar system comets today, a result that will require future observational and theoretical study to confirm.

研究动机与目标

• 探测并表征原行星盘内行星形成区域的远紫外CO光谱特征。
• 解决先前观测中CO发射与吸积激波、H₂及连续谱发射混合造成的模糊性。
• 确定这些盘中CO的激发机制，区分紫外泵浦（Lyα）与碰撞激发。
• 推断内盘中CO/H₂丰度比，并与星际及彗星值进行比较。

提出的方法

• 利用哈勃空间望远镜宇宙起源光谱仪（HST/COS）获取的高灵敏度、中等分辨率远紫外光谱。
• 分析波长>1560 Å处的CO A-X电子跃迁在吸收（HN Tau）和发射（RECX-11、V4046 Sgr）状态下的特征。
• 通过吸收谱带深度测量的转动温度诊断与柱密度，对CO激发态进行建模。
• 通过将观测到的CO发射轮廓与恒星Lyα发射轮廓形状进行比较，推断激发机制。
• 结合近期的H₂发射数据，约束紫外激发物种的空间与物理起源。
• 使用辐射转移与激发态建模，区分Lyα泵浦与碰撞激发的CO发射。

实验结果

研究问题

• RQ1 原行星盘内区CO气体的物理条件（温度、柱密度）如何？
• RQ2 远紫外波段CO的主要激发机制是什么？其与恒星Lyα轮廓的关系如何？
• RQ3 能否将远紫外波段的CO发射与吸积激波、H₂等混合发射源区分开？
• RQ4 内盘中CO/H₂丰度比是多少？与星际及彗星值相比有何差异？
• RQ5 紫外激发的CO与H₂在盘中是否存在共同的空间起源？

主要发现

• 在HN Tau中探测到CO吸收特征，柱密度为N(CO) = 2 ± 1 × 10¹⁷ cm⁻²，转动温度为T_rot(CO) = 500 ± 200 K。
• 在RECX-11和V4046 Sgr中探测到CO A-X带发射，柱密度范围为10¹⁸至10¹⁹ cm⁻²，转动温度超过300 K。
• CO发射主要由恒星Lyα光子激发，另有来自非热电子碰撞的额外贡献。
• 入射Lyα发射轮廓的形状显著影响观测到的CO发射谱。
• 观测到的CO与H₂发射与内盘中的共同起源一致，表明激发机制具有共位性。
• 推断出内盘中CO/H₂比例约为1，表明其从低星际值向太阳系彗星中观测到的较高值过渡。