Measurements of gas mass in protoplanetary gas disks form the basis for estimating the conditions of planet formation. Among the most important constraints derived from disk diagnostics are the abundances of gas-phase species critical for understanding disk chemistry. Toward this end, we present direct line-of-sight measurements of H₂ and CO, employing UV absorption spectroscopy from Hubble Space Telescope Cosmic Origins Spectrograph to characterize disk composition, molecular excitation temperatures, and spatial distribution in the circumstellar material around the Herbig Ae stars HK Ori and T Ori. We observe strong CO (N(CO) = 10^15.5 cm⁻²; T _rot(CO) = 19 K) and H₂ (N(H₂) = 10^20.34 cm⁻²; T _rot(H₂) = 141 K) absorption toward HK Ori with a CO/H₂ ratio of (≡N(CO)/N(H₂)) = 10⁻⁵. These measurements place direct empirical constraints on the CO-to-H₂ conversion factor in the disk around a Herbig Ae star for the first time, although there is uncertainty concerning the exact viewing geometry of the disk. The spectra of T Ori show CO (N(CO) = 10^14.9 cm⁻²; T _rot(CO) = 124 K) absorption. Interestingly, we do not detect any H₂ absorption toward this star (N(H₂) < 10^15.9 cm⁻²). We discuss a potential scenario for the detection of CO without H₂, which deserves further investigation. The low abundance ratio measured around HK Ori suggests significant depletion of CO in the circumstellar gas, which conforms with the handful of other recent CO abundance measurements in protoplanetary disks.

原行星气体盘中气体质量的测量构成了估计行星形成条件的基础。来自磁盘诊断的最重要限制之一是对理解磁盘化学至关重要的气相物种的丰度。为此，我们提出了 H ₂和 CO 的直接视线测量，利用哈勃太空望远镜宇宙起源光谱仪的紫外吸收光谱来表征 Herbig Ae 周围星周物质的盘组成、分子激发温度和空间分布明星HK Ori和T Ori。我们观察到强 CO (N(CO) = 10 ^15.5 cm ^-2 ; T _rot (CO) = 19 K) 和 H ₂ (N(H ₂) = 10 ^20.34厘米^-2 ; T _rot (H ₂ ) = 141 K) 向HK Ori 吸收，CO/H ₂比率为(≡N(CO)/N(H ₂ )) = 10 ^-5。这些测量首次对 Herbig Ae 星周围圆盘中的 CO 到 H ₂转换因子设置了直接的经验限制，尽管关于圆盘的精确观察几何形状存在不确定性。T Ori 的光谱显示 CO (N(CO) = 10 ^14.9 cm ^-2；T _rot (CO) = 124 K) 吸收。有趣的是，我们没有检测到任何对这颗恒星的H ₂吸收（N(H₂ ) < 10 ^15.9厘米^-2 )。我们讨论了在没有 H _{2 的}情况下检测 CO 的潜在方案，这值得进一步研究。HK Ori 周围测量的低丰度比表明星周气体中 CO 显着消耗，这与原行星盘中其他少数其他近期 CO 丰度测量结果一致。