[论文解读] Hot ammonia around young O-type stars

C. Goddi, Adam Ginsburg+1 more|arXiv (Cornell University)|Jan 6, 2015

Astrophysics and Star Formation Studies参考文献 28被引用 2

一句话总结

本研究利用高分辨率（0.2–0.3角秒）的JVLA观测，对W51 Main大质量恒星形成区中的高能级NH3亚稳态反转线（J,K = 6,6 至 13,13）进行了测绘，揭示了四个不同的高温核心。由于存在强烈的吸积、外流、脉泽活动以及大量热气体（约20 M⊙），W51e2-E被确定为主要的吸积中心；而W51e2-W则拥有一个超紧凑HII区，其NH3呈现吸收特征，并表现出速度梯度，表明在小于1000 AU的尺度上存在旋转，表明其中心O型星已完成大部分吸积过程。

ABSTRACT

Context. This paper is the second in a series of ammonia (NH3) multilevel imaging studies in high-mass star forming regions. Aims. We want to identify the location of the maser emission from highly excited levels of ammonia within the W51 IRS2 high-mass star forming complex that was previously discovered in a single dish monitoring program. Methods. We have used the Karl Jansky Very Large Array (JVLA) at the 1 cm band to map five highly excited metastable inversion transitions of NH3, (J,K) = (6,6), (7, 7), (9, 9), (10, 10), and (13, 13), in W51 IRS2 with ~0.̋2 angular resolution. Results. We present detections of both thermal (extended) ammonia emission in the five inversion lines, with rotational states ranging in energy from about 400 K to 1700 K, and point-like ammonia maser emission in the (6, 6), (7, 7), and (9, 9) lines. For the point-like emission, we estimate lower limits to the peak brightness temperatures of 1.7 × 105 K, 6 × 103 K, and 1 × 104 K for the (6, 6), (7, 7), and (9, 9) transitions, respectively, confirming their maser nature. The thermal ammonia emits around a local standard of rest velocity of VLSR = 60 km s-1, near the cloud’s systemic velocity, appears elongated in the east-west direction across 4′′ and is confined by the HII regions W51d (to the north), W51d1 (to the east), and W51d2 (to the west). The NH3 masers are observed in the eastern tip of the dense clump traced by thermal NH3, offset by 0.̋65 to the east from its emission peak, and have a peak velocity at ~47.5 km s-1. No maser components are detected near the systemic velocity. The NH3 masers arise close to but separated from (0.̋65 or 3500 AU) the rare vibrationally excited SiO masers, which are excited in a powerful bipolar outflow driven by the deeply embedded high-mass young stellar object (YSO) W51-North. This means that the two maser species cannot be excited by the same object. Interestingly, the NH3 masers originate at the same sky position as a peak in a submm line of SO2 imaged with the Submillimeter Array, tracing a face-on circumstellar disk or ring around W51-North. In addition, the thermal emission from the most highly excited NH3 lines, (10, 10) and (13, 13), shows two main condensations, the dominant one towards W51-North with the SiO and H2O masers, and a weaker peak at the NH3 maser position. Conclusions. We propose a scenario where the ring seen in SO2 emission is a circumbinary disk surrounding (at least) two high-mass YSOs, W51-North (exciting the SiO masers) and a nearby companion (exciting the NH3 masers), separated by 3500 AU. This finding indicates a physical connection (in a binary) between the two rare SiO and NH3 maser species.

研究动机与目标

• 利用高角分辨率NH3谱线发射，表征W51 Main区域大质量原恒星对象（YSOs）的物理与动力学特性。
• 通过分析速度梯度、吸积特征与气体储层，确定W51e2复合体中主导的吸积中心。
• 通过温度、密度与分子气体质量估算，评估多个高温核心（W51e2-W、e2-E、e2-NW、e8）的演化状态。
• 研究旋转、吸积与外流在W51 Main区域大质量恒星形成动力学中的作用。

提出的方法

• 利用Karl Jansky甚大阵列（JVLA）在亚角秒分辨率（~0.2–0.3角秒）下观测了五个亚稳态NH3反转跃迁（J,K）=（6,6）、（7,7）、（9,9）、（10,10）与（13,13）。
• 采用超精细结构（HFS）建模拟谱线，考虑四极分裂及主峰与卫星峰的相对强度。
• 应用光学厚度修正，通过归一化柱密度（Nu/g）与能量（Eu）的关系斜率，准确推导出转动温度与柱密度。
• 分析位置-速度（PV）图以识别速度梯度，并推断旋转或吸积运动。
• 将观测到的谱线不对称性（蓝移偏斜）与辐射转移模型对比，检测光学厚NH3发射中的吸积信号。
• 假设[NH3]/[H2] = 10⁻⁷估算气体质量，并通过转动图分析推导出动力学温度与密度。

实验结果

研究问题

• RQ1 W51 Main区域的高温核心的物理条件（温度、密度、质量）如何？
• RQ2 在W51e2复合体中，哪个核心是当前大质量吸积的主要场所？
• RQ3 在W51e2中心区域，存在哪些动力学特征（旋转、吸积、外流）？
• RQ4 基于气体储层与活动特征，W51e2子核与W51e8的演化状态如何比较？
• RQ5 在HC HII区观测到的速度梯度对理解大质量恒星形成动力学具有何种意义？

主要发现

• 位于超紧凑HII区以东约0.8′′的W51e2-E核心，拥有约20 M⊙的热分子气体（T ~170 K，nH2 ~5 × 10⁷ cm⁻³），是主要的吸积中心，表现出强烈的外流、脉泽活动以及蓝移不对称谱线轮廓，表明存在吸积。
• W51e2-W核心环绕超紧凑HII区，在所有NH3谱线中均表现出清晰的东-西速度梯度，表明在小于1000 AU的尺度上存在旋转，尽管未检测到开普勒型轮廓。
• W51e2-W中心原恒星体可能已基本完成质量吸积，其可用吸积气体（~5 M⊙）远低于其从射电光度估算出的质量（>20 M⊙）。
• W51e2-NW与W51e8分别为温度约140 K与200 K的高温核心，气体质量分别约为30 M⊙与70 M⊙，表明其可能孕育低质量或早期阶段的大质量原恒星体。
• W51e2-W中缺乏脉泽活动与外流，且气体储层较低，支持其为比W51e2-E更演化的系统的结论。
• NH3谱线的转动温度分析显示，W51e2-E与W51e2-NW的T_rot ~170 K，W52e2的T_rot ~167 K，而W51e8的T_rot ~515 K，表明最明亮核心中存在显著的热激发。