[论文解读] ATLASGAL-selected massive clumps in the inner Galaxy: I. CO depletion and isotopic ratios

A. Giannetti, F. Wyrowski+9 more|Repository of the Academy's Library (Library of the Hungarian Academy of Sciences)|Jul 8, 2014

Astrophysics and Star Formation Studies参考文献 74被引用 54

一句话总结

本研究利用CO同位素发射线和尘埃连续谱，基于银河系内区ATLASGAL巡天的102个大质量团块，调查了CO耗尽及同位素比值。研究发现，CO耗尽在温度较低、演化阶段较早的团块中最强，耗尽半径为0.2–0.8 pc，并揭示了$^{12}$C/$^{13}$C随径向增加，而$^{18}$O/$^{17}$O比值在银河系内区基本恒定，且存在显著的内在离散度。

ABSTRACT

In the low-mass regime, it is found that the gas-phase abundances of C-bearing molecules in cold starless cores rapidly decrease with increasing density, as the molecules form mantles on dust grains. We study CO depletion in 102 massive clumps selected from the ATLASGAL 870 micron survey, and investigate its correlation with evolutionary stage and with the physical parameters of the sources. Moreover, we study the gradients in [12C]/[13C] and [18O]/[17O] isotopic ratios across the inner Galaxy, and the virial stability of the clumps. We use low-J emission lines of CO isotopologues and the dust continuum emission to infer the depletion factor fD. RATRAN one-dimensional models were also used to determine fD and to investigate the presence of depletion above a density threshold. The isotopic ratios and optical depth were derived with a Bayesian approach. We find a significant number of clumps with a large fD, up to ~20. Larger values are found for colder clumps, thus for earlier evolutionary phases. For massive clumps in the earliest stages of evolution we estimate the radius of the region where CO depletion is important to be a few tenths of a pc. Clumps are found with total masses derived from dust continuum emission up to ~20 times higher than the virial mass, especially among the less evolved sources. These large values may in part be explained by the presence of depletion: if the CO emission comes mainly from the low-density outer layers, the molecules may be subthermally excited, leading to an overestimate of the dust masses. CO depletion in high-mass clumps seems to behave as in the low-mass regime, with less evolved clumps showing larger values for the depletion than their more evolved counterparts, and increasing for denser sources. The C and O isotopic ratios are consistent with previous determinations, and show a large intrinsic scatter.

研究动机与目标

• 调查银河系内区不同演化阶段大质量团块中的CO耗尽情况。
• 确定CO耗尽对温度、密度和演化阶段的依赖关系。
• 测量银河系内区$^{12}$C/$^{13}$C和$^{18}$O/$^{17}$O同位素比值并评估其梯度。
• 评估CO耗尽对virial质量估计和团块稳定性的影响。
• 检验耗尽是否影响尘埃质量与virial参数计算的可靠性。

提出的方法

• 从ATLASGAL 870 μm巡天中选取102个质量介于~10²至3×10⁴ M☉之间的大质量团块。
• 利用低J CO同位素谱线发射和尘埃连续谱推导CO耗尽因子$f_D$。
• 应用RATRAN一维辐射转移模型模拟CO耗尽并确定临界密度。
• 采用贝叶斯方法从谱线发射推导同位素比值和光学厚度。
• 将尘埃质量与virial质量进行比较，以评估引力稳定性。
• 使用阈值密度模型模拟CO耗尽，其中CO在密度超过临界密度（约几×10⁴ cm⁻³）时冻结。

实验结果

研究问题

• RQ1 CO耗尽可能否随大质量团块的演化阶段和物理条件而变化？
• RQ2 在银河系内区，$^{12}$C/$^{13}$C和$^{18}$O/$^{17}$O同位素比值的径向趋势如何？
• RQ3 CO耗尽可能在多大程度上导致大质量团块中尘埃质量和virial质量估计的偏差？
• RQ4 大质量团块中CO耗尽区域的特征半径是多少，其与团块演化的相关性如何？
• RQ5 观测到的同位素比值是否与银河系化学演化模型一致，还是表明存在内在变化？

主要发现

• CO耗尽因子最高可达~20，且在温度较低、演化阶段较早的团块中显著更高。
• CO耗尽区域的半径估计为~0.2–0.8 pc，且在更远距离的源中倾向于更大，可能由于Beam平均效应导致的团块结构影响。
• $^{12}$C/$^{13}$C比值随旋臂距离增加而上升，在太阳邻近区域达到~66±12，与银河系化学演化模型一致。
• $^{18}$O/$^{17}$O比值在2–8 kpc范围内近似恒定于~4，内在离散度约为1，尽管个别源的值在2至6之间变化。
• 少数位于~4 kpc处的源表现出~5.5的$^{18}$O/$^{17}$O比值，与前太阳云物质值相似。
• 在部分团块中，virial质量估计值显著低于尘埃质量（最高低至20倍），表明CO耗尽可能导致尘埃质量被高估，virial稳定性被低估。