[论文解读] Characterization of Infrared Dark Clouds -- NH$_3$ Observations of an Absorption-contrast Selected IRDC Sample

R.-A. Chira, H. Beuther+5 more|Feb 27, 2013

Astrophysics and Star Formation Studies参考文献 2被引用 29

一句话总结

本研究基于氨（NH₃）(1,1) 和 (2,2) 倒转谱线观测，对218个北部高对比度红外暗云（IRDCs）进行了统计表征。结果表明，IRDCs的平均动能温度为15 K，线宽约为2 km s⁻¹，氨柱密度约为10¹⁴ cm⁻²，自由落体质量在100至3000 M⊙之间，表明其接近自由落体平衡状态，尽管目前稳定，但未来极有可能发生坍缩。

ABSTRACT

Despite increasing research in massive star formation, little is known about its earliest stages. Infrared Dark Clouds (IRDCs) are cold, dense and massive enough to harbour the sites of future high-mass star formation. But up to now, mainly small samples have been observed and analysed. To understand the physical conditions during the early stages of high-mass star formation, it is necessary to learn more about the physical conditions and stability in relatively unevolved IRDCs. Thus, for characterising IRDCs studies of large samples are needed. We investigate a complete sample of 218 northern hemisphere high-contrast IRDCs using the ammonia (1,1)- and (2,2)-inversion transitions. We detected ammonia (1,1)-inversion transition lines in 109 of our IRDC candidates. Using the data we were able to study the physical conditions within the star-forming regions statistically. We compared them with the conditions in more evolved regions which have been observed in the same fashion as our sample sources. Our results show that IRDCs have, on average, rotation temperatures of 15 K, are turbulent (with line width FWHMs around 2 km s$^{-1}$), have ammonia column densities on the order of $10^{14}$ cm$^{-2}$ and molecular hydrogen column densities on the order of $10^{22}$ cm$^{-2}$. Their virial masses are between 100 and a few 1000 M$_\odot$. The comparison of bulk kinetic and potential energies indicate that the sources are close to virial equilibrium. IRDCs are on average cooler and less turbulent than a comparison sample of high-mass protostellar objects, and have lower ammonia column densities. Virial parameters indicate that the majority of IRDCs are currently stable, but are expected to collapse in the future.

研究动机与目标

• 为了理解早期大质量恒星形成中的物理条件，这些条件由于样本规模有限而仍缺乏充分表征。
• 利用一个大规模、完整的样本，研究相对未演化的IRDCs的温度、湍流和稳定性。
• 将IRDCs的物理性质与更演化的高质​​量原恒星对象进行比较。
• 确定IRDCs是否具有引力束缚性，并处于坍缩的临界状态。
• 利用氨作为温度计和动力学示踪剂，推导出动能温度、线宽和自由落体质量。

提出的方法

• 从MSX星表中基于红外吸收对比度，选取了218个北部高对比度IRDCs的完整样本。
• 观测氨(1,1)和(2,2)倒转跃迁，以推导动能温度、线宽（作为湍流的代理）和柱密度。
• 利用(1,1)线的亮度温度比，通过倒转跃迁比方法估算动能温度。
• 测量线宽（FWHM）以推断湍流运动和自由落体参数。
• 利用银河系旋转曲线模型和源径向速度计算动力学距离。
• 基于线宽和柱密度数据计算自由落体质量，并利用自由落体参数评估引力稳定性。

实验结果

研究问题

• RQ1 未演化的IRDCs中的典型动能温度和湍流线宽是多少？
• RQ2 IRDCs中的物理条件与更演化的高质​​量原恒星对象相比如何？
• RQ3 IRDCs是否具有引力束缚性，其自由落体参数是多少？
• RQ4 IRDCs中氨和分子氢柱密度的分布如何？
• RQ5 IRDCs是否处于自由落体平衡状态，这对它们未来的演化意味着什么？

主要发现

• IRDCs的平均动能温度为15 K，由氨(1,1)线的亮度温度比推导得出。
• IRDCs表现出FWHM约为2 km s⁻¹的线宽，表明其内部存在显著湍流。
• 氨柱密度约为10¹⁴ cm⁻²，分子氢柱密度约为10²² cm⁻²。
• 自由落体质量范围为100至数千倍太阳质量，表明其具有大质量潜力。
• 自由落体参数表明IRDCs接近自由落体平衡，暗示其目前稳定但未来极有可能坍缩。
• 与更演化的高质​​量原恒星对象相比，IRDCs温度更低、湍流更弱，且氨柱密度更低。