[论文解读] Linear polarization of submillimetre masers. Tracing magnetic fields with ALMA

A. F. Pérez-Sánchez, W. H. T. Vlemmings|Chalmers Publication Library (Chalmers University of Technology)|Jan 4, 2013

Astrophysics and Star Formation Studies参考文献 57被引用 12

一句话总结

本文研究了利用ALMA的全偏振能力探测SiO、H₂O和HCN的亚毫米波线性偏振脉泽的可能性。通过辐射转移建模，表明在Zeeman效应主导的条件下，线性偏振度最高可达13–19%，从而实现对恒星形成区及（后）AGB恒星包层中磁场的追踪。

ABSTRACT

(Abridged) The goal of this work is to investigate if SiO, H2O and HCN maser emission within the ALMA frequency range can be detected with observable levels of fractional linear polarization in the regime where the Zeeman frequency (gOmega) is greater than the stimulated emission rate (R) and the decay rate of the molecular state (Gamma). We have used a radiative transfer code to calculate the fractional linear polarization as a function of the emerging brightness temperature for a number of rotational transition of SiO, H2O and HCN which have been observed to display maser emission at submillimetre wavelengths. We assume typical magnetic field strengths measured towards galactic star forming regions and circumstellar envelopes of late-type stars from previous VLBI observations. Since the Landé g-factors have not been reported for the different rotational transitions we have modeled, we performed our calculations assuming conservative values of the Zeeman frequency for the different molecular species. Setting a lower limit for the Zeeman frequency which still satisfies the criteria gOmega>R and gOmega>Gamma, we find fractional polarization levels of up to 13%, 14% and 19% for the higher J transitions analysed for SiO, H2O and HCN, respectively, without considering anisotropic pumping or any other non-Zeeman effect. Such upper limits were calculated assuming a magnetic field oriented perpendicular to the direction of propagation of the maser radiation. According to our results SiO, H2O and HCN maser emission within the ALMA frequency range can be detected with suitable linear polarization to trace the magnetic field structure towards star forming regions and late-type stars, even if the detected polarization has been enhanced by non-Zeeman effects.

研究动机与目标

• 评估ALMA频段内SiO、H₂O和HCN脉泽在Zeeman效应主导条件下是否可表现出可观测的线性偏振度。
• 确定当Zeeman频率大于受激辐射速率和自发衰变速率时，可实现的最大线性偏振度。
• 评估利用偏振脉泽辐射作为恒星形成区和包层中磁场结构可靠示踪器的可行性。
• 提供不同脉泽跃迁的可探测偏振度的定量估计，考虑亮度温度和束流效应。

提出的方法

• 基于Nedoluha & Watson (1992)的辐射转移代码，计算了线性偏振度分数随亮度温度与束流立体角乘积（TbΔΩ）的变化关系。
• 分析假设了保守的Landé g因子，并采用银河系恒星形成区及（后）AGB包层中观测到的典型磁场强度。
• 模型评估了条件gΩ > R和gΩ > Γ，以确保偏振方向反映真实的磁场方向。
• 受激辐射速率R由TbΔΩ推导得出，其中Tb由观测通量密度和波束尺寸计算得到。
• 研究考虑了SiO、H₂O和HCN的振动激发态至v=3的跃迁，重点关注高-J转动能级跃迁。
• 利用ALMA灵敏度计算器推导出检测灵敏度估计，以确定实现1%偏振度检测所需的源上积分时间。

实验结果

研究问题

• RQ1 在ALMA频段内，SiO、H₂O和HCN脉泽在Zeeman效应主导条件下是否可表现出可探测的线性偏振度？
• RQ2 当gΩ > R且gΩ > Γ时，这些脉泽可实现的最大线性偏振度是多少？
• RQ3 亮度温度和束流立体角如何影响偏振作为磁场示踪器的可靠性？
• RQ4 ALMA需要多长积分时间才能探测到低通量密度脉泽特征中1%的线性偏振度？
• RQ5 非Zeeman效应在多大程度上可增强偏振度而不破坏磁场解释的可靠性？

主要发现

• 在Zeeman效应主导条件下，SiO脉泽在ALMA频段内可实现最高达13%的线性偏振度分数。
• 当gΩ > R且gΩ > Γ时，H₂O脉泽的线性偏振度分数最高可达14%。
• HCN脉泽表现出最高潜力，其在线性偏振度分数最高可达19%。
• 对于峰值通量密度为几十Jy/beam的SiO脉泽，1%线性偏振度的5σ检测可在1小时以内的源上积分时间内实现。
• 对于183 GHz的H₂O脉泽，若束流立体角超过1.6×10⁻³ sr，则可在短积分时间内探测到1%的偏振度水平。
• 即使对于强脉泽，仍需仔细分析TbΔΩ，以确认其保持在gΩ > R区域，从而确保磁场追踪的可靠性。