[论文解读] Radio Jets from Young Stellar Objects

Guillem Anglada, Rodriguez, Luis F.+1 more|arXiv (Cornell University)|Jun 17, 2018

Astrophysics and Star Formation Studies被引用 8

一句话总结

本文综述了年轻恒星物体（YSOs）的热射电喷流，表明厘米波长的自由-自由辐射可追踪原恒星附近电离喷流的基底，从而通过射电复合线实现三维速度场研究。主要发现显示，从大质量恒星到棕矮星，射电光度、总辐射光度与喷流动量输出率之间存在强烈相关性，表明存在一种由激波驱动、与盘-喷流模型一致的普遍喷流-吸积机制。

ABSTRACT

Jets and outflows are ubiquitous in the process of formation of stars since outflow is intimately associated with accretion. Free-free (thermal) radio continuum emission is associated with these jets. This emission is relatively weak and compact, and sensitive radio interferometers are required to study it. Observations in the cm range are most useful to trace the base of the ionized jets, close to the central protostar, where optical or near-IR imaging is made difficult by the high extinction present. Radio recombination lines in jets (in combination with proper motions) should provide their 3D kinematics at very small scale. Future instruments such as the Square Kilometre Array (SKA) and the Next Generation Very Large Array (ngVLA) will be crucial to perform this kind of sensitive observations. Thermal jets are associated with both high and low mass protostars and possibly even with substellar objects. The ionizing mechanism of these radio jets appears to be related to shocks in the associated outflows, as suggested by the observed correlation between the cm luminosity and the outflow momentum rate. Some protostellar jets show indications of non-thermal emission in their lobes. Linearly polarized synchrotron emission has been found in the jet of HH 80-81, allowing one to map the jet magnetic field, a key ingredient to determine the collimation and ejection mechanisms. As only a fraction of the emission is polarized, very sensitive observations such as those that will be feasible with the interferometers previously mentioned are required to perform studies in a large sample of sources. Jets are present in many kinds of astrophysical scenarios. Characterizing radio jets in young stars, where thermal emission allows one to determine their physical conditions, would also be useful in understanding acceleration and collimation mechanisms in all kinds of astrophysical jets.

研究动机与目标

• 理解年轻恒星物体（YSOs）中射电喷流的物理起源与加速机制，特别是在光学与红外观测受限的高消光区域。
• 通过光谱与形态学诊断，区分热射电喷流与其他致密射电源（如超紧凑HII区与光致电离盘）之间的差异。
• 评估激波与磁场在电离与准直化喷流中的作用，特别是通过非热辐射与偏振同步辐射进行探测。
• 评估未来望远镜（如SKA与ngVLA）在实现射电复合线与偏振辐射高灵敏度、高分辨率研究方面的潜力。
• 建立射电光度、总辐射光度与喷流动量输出率之间的经验相关性，作为恒星与亚恒星质量范围内统一喷流形成机制的证据。

提出的方法

• 利用高分辨率厘米波长射电干涉测量（如VLA、Jansky VLA）探测并解析与YSOs相关的紧凑、拉长的射电连续源。
• 通过定义为Sν ∝ ν^α的谱指数（α）分析，区分热发射（自由-自由，α ≈ 0.6）与非热发射（磁化同步辐射，α < 0）成分。
• 应用射电复合线光谱学探测喷流的三维速度场，预期喷流中的谱线宽度比光致电离盘大一个数量级。
• 利用光度图（射电光度与总辐射光度对比）区分热喷流、光致电离盘与超紧凑HII区。
• 通过多波长成像（如3 cm与7 mm）与光谱能量分布（SED）拟合，分离尘埃辐射与自由-自由及电离气体成分。
• 基于厘米波光度与喷流动量输出率之间的观测相关性，将电离机制建模为激波驱动。

实验结果

研究问题

• RQ1 年轻恒星物体中热射电连续辐射的起源是什么？它与吸积和喷流过程有何关联？
• RQ2 如何利用射电复合线与自行运动在亚角秒尺度上确定原恒星附近喷流的三维速度场？
• RQ3 在光谱、形态与光度特性方面，热射电喷流与光致电离盘或超紧凑HII区有何区别？
• RQ4 射电光度、总辐射光度与喷流动量输出率之间的相关性在多大程度上支持跨恒星质量范围的统一喷流形成机制？
• RQ5 磁场与激波电离在喷流加速与准直化中扮演何种角色？如何通过偏振辐射进行探测？

主要发现

• 在大质量与低质量原恒星（包括棕矮星）中均探测到热射电喷流，其厘米波光度范围从大质量恒星的~100 mJy kpc²至年轻棕矮星的~3×10⁻³ mJy kpc²。
• 喷流的射电光度与关联的大尺度喷流的总辐射光度及动量输出率之间存在强烈相关性，表明其起源与吸积及激波电离密切相关。
• 喷流中的射电复合线预期具有比光致电离盘大一个数量级的线宽（数十km s⁻¹ vs. ~100 km s⁻¹），是区分两者的关键诊断工具。
• 具有负谱指数的非热辐射与线性偏振同步辐射（如HH 80-81中所示）表明存在磁场，可能在喷流准直化中发挥作用。
• 在GM Aur等系统中，高分辨率成像同时解析出自由-自由射电喷流与来自盘面的光致蒸发风，通过SED拟合与多波长去卷积分离出尘埃辐射。
• SKA与ngVLA等未来望远镜对实现大样本喷流中射电复合线的探测及偏振辐射的详细研究至关重要。