用稀有的CO同位素探测行星形成带
Probing Planet Forming Zones with Rare CO Isotopologues
摘要：
在形成行星的原恒星盘的中平面附近的气体，由于通常观测到的分子（如CO和H $_2$ O）具有很高的光学深度，因此在很大程度上未被观测到。然而，来自稀有分子的旋转发射线在垂直方向上可能具有接近1的光学深度，使得这些线足够强以被检测到，但仍保持足够透明以追踪盘中平面。在这里，我们提出了一个演化的T-金牛座盘的化学模型，并预测了旋转跃迁的光学深度的$^{12}$ C $^{16}$ O，$^{13}$ C $^{16}$ O，$^{12}$ C $^{17}$ O和$^{12}$ C $^{18}$ O。由于死区的形成，MRI活跃盘主要由中心恒星加热。在我们模拟的类太阳恒星周围的$70$ AU区域内，CO不会冻结。然而，由于复杂有机分子（COM）的形成，CO的丰度降低，产生了一种可能被误解为“雪线”的效果。这些结果是强大的变化，在我们的假设的气体尘埃比的演变。C $^{17}$ O的低阶旋转线的光学深度约为1，使得可以使用C $^{17}$ O看到磁盘中平面。将观测结果与我们提供的模型C $^{17}$ O $/$ H $_2$比率相结合，可以得到原行星盘气体质量的估计。
英文摘要：
The gas near the midplanes of planet-forming protostellar disks remains largely unprobed by observations due to the high optical depth of commonly observed molecules such as CO and H$_2$O. However, rotational emission lines from rare molecules may have optical depths near unity in the vertical direction, so that the lines are strong enough to be detected, yet remain transparent enough to trace the disk midplane. Here we present a chemical model of an evolving T-Tauri disk and predict the optical depths of rotational transitions of $^{12}$C$^{16}$O, $^{13}$C$^{16}$O, $^{12}$C$^{17}$O and $^{12}$C$^{18}$O. The MRI-active disk is primarily heated by the central star due to the formation of the dead zone. CO does not freeze out in our modeled region within $70$AU around a sunlike star. However, the abundance of CO decreases because of the formation of complex organic molecules (COM), producing an effect that can be misinterpreted as the "snow line". These results are robust to variations in our assumptions about the evolution of the gas to dust ratio. The optical depths of low-order rotational lines of C$^{17}$O are around unity, making it possible to see into the disk midplane using C$^{17}$O. Combining observations with modeled C$^{17}$O$/$H$_2$ ratios, like those we provide, can yield estimates of protoplanetary disks' gas masses.
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