[论文解读] The Circumstellar Environment of High-Mass Protostellar Objects: IV. C17O Observations and Depletion

H. Thomas, G. A. Fuller|ArXiv.org|Dec 10, 2007

Astrophysics and Star Formation Studies参考文献 31被引用 11

一句话总结

本研究通过针对84个候选源的C17O和C18O J=2→1观测，调查了大质量原恒星中的CO耗尽现象。研究发现C17O丰度范围广泛，与尘埃或温度变化无关，表明源之间CO冻结程度存在本质差异；CS谱线宽度较窄的源具有更高的C17O丰度，表明存在一个演化序列，即在持续2–4×10⁵年的冷、高密度条件下，CO得以长期冻结。

ABSTRACT

We observe 84 candidate young high-mass sources in the rare isotopologues C17O and C18O to investigate whether there is evidence for depletion (freeze-out) towards these objects. Observations of the J=2-1 transitions of C18O and C17O are used to derive the column densities of gas towards the sources and these are compared with those derived from submillimetre continuum observations. The derived fractional abundance suggests that the CO species show a range of degrees of depletion towards the objects. We then use the radiative transfer code RATRAN to model a selection of the sources to confirm that the spread of abundances is not a result of assumptions made when calculating the column densities. We find a range of abundances of C17O that cannot be accounted for by global variations in either the temperature or dust properties and so must reflect source to source variations. The most likely explanation is that different sources show different degrees of depletion of the CO. Comparison of the C17O linewidths of our sources with those of CS presented by other authors reveal a division of the sources into two groups. Sources with a CS linewidth >3 km/s have low abundances of C17O while sources with narrower CS lines have typically higher C17O abundances. We suggest that this represents an evolutionary trend. Depletion towards these objects shows that the gas remains cold and dense for long enough for the trace species to deplete. The range of depletion measured suggests that these objects have lifetimes of 2-4x10^5 years.

研究动机与目标

• 调查大质量原恒星中是否发生类似低质量原恒星中观测到的CO耗尽现象。
• 利用稀有同位素分子C17O和C18O作为气相丰度的示踪剂，确定CO耗尽的程度与分布。
• 评估观测到的丰度变化是否由物理条件（温度、尘埃性质）引起，或源于源之间的内在耗尽差异。
• 通过将C17O丰度与CS谱线宽度关联，探索大质量原恒星的演化状态及其与年龄和演化阶段的关系。

提出的方法

• 在JCMT上利用RxA3和RxB3接收机，通过DAS互相关相关器以位置切换模式观测了C17O和C18O的J=2→1跃迁。
• 通过主波束效率修正（220 GHz时ηmb = 0.69，337 GHz时ηmb = 0.63），从射电望远镜温度推导出主波束温度。
• 假设发射光学薄，结合已知的同位素丰度比，从积分射电温度计算C17O的柱密度。
• 利用WFS04的亚毫米波连续成分估计尘埃柱密度，并计算气尘质量比以供比较。
• 通过C17O柱密度与H₂柱密度的比值计算C17O的相对丰度，其中H₂柱密度由尘埃连续成分推断。
• 对部分源使用RATRAN进行辐射转移建模，以验证丰度差异并非建模假设导致的伪影。

实验结果

研究问题

• RQ1 大质量原恒星是否表现出与低质量原恒星类似的CO耗尽现象，表现为C17O丰度降低？

主要发现

• 观测到C17O相对H₂的相对丰度范围广泛，从~10⁻⁹到~10⁻⁸不等，表明部分源中存在显著的CO耗尽。
• 观测到的丰度变化无法用尘埃温度或光学厚度的全局变化来解释，表明源之间耗尽水平存在内在差异。
• CS谱线宽度>3 km s⁻¹的源表现出较低的C17O丰度，而CS谱线更窄的源则具有更高的C17O丰度，表明与演化阶段存在相关性。
• 观测到的趋势暗示一个演化序列：早期动态活跃阶段（谱线宽）之后是平静、冷、高密度阶段（谱线窄），后者有利于CO在2–4×10⁵年内长期冻结。
• 推断的耗尽 timescale 为2–4×10⁵年，与冷、高密度核心中CO冻结所需时间一致。
• RATRAN建模证实，观测到的丰度分布是物理真实的，而非柱密度推导或波束填充假设导致的伪影。