The Role of Magnetic Fields in Star Formation: Polarization Observations of Molecular Clouds

理解恒星形成是现代天体物理学中的突出问题之一，磁场是否在调节恒星形成过程中发挥作用是一个重大问题。磁场和湍流在引力束缚的星际云和星核的形成以及它们随后坍塌形成恒星的过程中的相对重要性是什么？只有对致密分子云中磁场的观测才能检验磁主星形成的理论预测，并验证或消除这一模型。这项提议是理查德·克鲁彻教授长期而成功的研究计划的延续，他以测试恒星形成理论为具体重点，观测分子云中的磁场。这些观察将在三年资助期内进行。首先，将用射电天文研究所的30米望远镜观测CN和C2H分子在3毫米波长的光谱中的塞曼分裂。(IRAM)在西班牙。这将给出对致密分子云核的重要样本的视线磁场强度的测量。其次，计划用阿塔卡马大毫米阵列(ALMA)对从IRAM样品中选择的CN和C2H塞曼靶进行合成成像，同时对多个分子物种的线偏振尘埃发射和线偏振谱线发射进行ALMA合成成像，许多ALMA观测可以同时进行。到拟议的授权期结束时，手头将有大量的单盘塞曼观测，可以进行贝叶斯统计分析，从测量的视线强度推断总磁场强度的概率分布函数。这项提议的工作将为采样不同密度区域的两个分子物种的磁场视线分量的高密度分子区域提供灵敏的ALMA图。此外，它还将给出尘埃发射的天空平面图，对整个视线进行采样，并从多个分子跃迁中的线性极化光谱线提供采样，包括不同的云层和两极外流。此外，还将创建一个计算机代码，根据分子云密度和磁场结构的3D模型或模拟生成地图，与使用ALMA观察到的完全相同。然后，将ALMA图与模型图进行比较，可以用来推断磁场的三维结构，并检验恒星形成理论的预测。到资助期结束时，研究人员预计在理解磁场在恒星形成中的作用方面将取得重大进展。更广泛的影响：这里提出的研究在恒星形成研究的多个领域具有更广泛的影响。这种影响的例子有：改进了用于大规模结构形成和星系演化模拟的恒星形成方法；根据维里定理改进了对分子云质量的估计(通常忽略了磁压)；改进了对磁场在年轻恒星甚至活动星系核(AGN)产生双极风过程中的作用的理解。该项目还将有助于加强研究和教育的基础设施，特别是ALMA的设施和仪器。使用合成成像阵列的偏振测量有许多潜在的缺陷，而ALMA的目标是充分发挥其偏振映射的能力。该小组将是ALMA的首批用户之一，并将利用其在干涉偏振测量方面的专业知识和经验，使ALMA能够尽早以最佳方式进行偏振科学研究。这将包括参与ALMA硬件调试和测试ALMA偏振成像软件，并向科学界提供建模代码，以便所有用户都可以充分利用ALMA偏振映射的结果。此外，还将对一名研究生进行观察和分析技术培训。随着ALMA在不久的将来上线，学生接受与ALMA研究相关的领域的培训是至关重要的。首席调查员有培训学生进行无线电偏振测量的历史，并将继续培训能够充分利用ALMA的博士生。

Understanding star formation is one of the outstanding problems in modern astrophysics and whether magnetic fields play a role in regulating the star formation process is a major question. What is the relative importance of magnetic fields and turbulence in the formation of gravitationally bound interstellar clouds and cores and their subsequent collapse to form stars? Only observations of magnetic fields in dense molecular clouds can test the theoretical predictions of magnetically dominated star formation and validate or eliminate this model. This proposal is the continuation of the long-term and successful research program of Professor Richard Crutcher to observe magnetic fields in molecular clouds with the specific focus of testing star formation theory. These observations will be done over the three-year grant period. First, observations of the Zeeman splitting in spectra of the molecules CN and C2H at 3-mm wavelength will be done with the 30-m telescope of the ?Institut de Radio Astronomie Millimétrique? (IRAM) in Spain. This will give measurements of the line-of-sight magnetic field strengths toward a significant sample of dense molecular cloud cores. Second, synthesis imaging of CN and C2H Zeeman targets selected from the IRAM sample with the Atacama Large Millimeter Array (ALMA) is planned, together with ALMA synthesis imaging of linearly polarized dust emission and linearly polarized spectral line emission from multiple molecular species.Many of the ALMA observations can be done simultaneously. By the end of the proposed grant period, a significant number of single-dish Zeeman observations will be at hand that a Bayesian statistical analysis can be carried out to infer the probability distribution function of the total magnetic field strength from the measured line-of-sight strengths. The work from this proposal will give sensitive ALMA maps of high-density molecular regions for the line-of-sight component of the magnetic field in two molecular species that sample different density regimes. In addition it will give plane-of-sky maps from dust emission that samples the entire line of sight and from linearly polarized spectral lines in multiple molecular transitions that sample different layers of clouds as well as bipolar outflows. In addition a computer code will be created that generates maps from 3D models or simulations of molecular cloud density and magnetic field structures exactly as they would be observed with ALMA. Then a comparison of ALMA maps with model maps can be used to infer the 3D structure of magnetic fields and to test the predictions of star formation theories. By the end of the grant period, the researchers expect a significant advance in understanding the role of magnetic fields in star formation.Broader Impacts: The research proposed here has broader impacts in multiple areas of star formation studies. Examples of such impact are: An improved recipe for star formation for use in large-scale structure formation and galaxy evolution simulations; improved mass estimates of molecular clouds based on the virial theorem (where magnetic pressure is generally ignored); and improved understanding of the role of magnetic fields in the generation of bipolar winds from young stars and even Active Galactic Nuclei (AGNs). This project will also help to enhance the infrastructure for research and education, in particular, the ALMA facility and instrumentation. Polarimetry with a synthesis-imaging array has many potential pitfalls, and ALMA has the goal of reaching its full capability for polarization mapping. This group will be among the first users of ALMA and will bring its expertise and experience in interferometric polarimetry to bear such that ALMA will be able to do polarization science optimally as early as possible. This will include involvement with ALMA hardware commissioning and testing the ALMA software for polarization imaging and providing modeling code to the scientific community, so that the results of ALMA polarization mapping may be fully exploited by all users. In addition, a graduate student will be trained in the observational and analytical techniques. With ALMA coming on line in the near future, it is critical that students are trained in areas relevant to ALMA research. The principal investigator has a history of training students in radio polarimetry, and will continue to train PhD students who will be able to make full use of ALMA.