RUI: Dynamic UC HII regions in Sgr B2: Flickering and Ionized Flows

德·普雷博士和他在艾格尼丝·斯科特学院的团队利用超大阵列（EVLA）进行观测，研究大质量恒星的形成。特别是，他们研究了HII区域（即星际介质中氢主要以电离原子形式存在的区域）大质量恒星形成期间的辐射通量变化。大质量恒星驱动的过程丰富了ISM的化学元素，并显著影响了星系的演化。Sgr B2区域是银河系中资源最丰富的大质量恒星形成区域之一。它包含许多形态多样的HII区域，并且不寻常的宽线源的数量使其成为测试超紧凑HII区域进化理论的理想实验室。最近的高分辨率辐射流体动力学模拟表明，形成大质量恒星所需的密集、旋转的吸积流很快就会变得引力不稳定。这改变了捕获的电离辐射量和相关的超紧凑HII区域的大小。因此，新生大质量恒星附近密集团块和细丝的轨道可能不规则地捕获并暴露它们的电离辐射。这些物体的光学厚度为2厘米波长，因此尺寸变化也表示为通量变化。因此，随着时间的推移，由此产生的HII区域在整个主恒星吸积阶段的大小在超紧和超紧之间“闪烁”，而不是单调地膨胀。吸积流持续的时间比HII区域的自由膨胀时间长10倍，该模型可以解释为什么超紧凑的HII区域不会更快地膨胀和消散。这也可以解释观察到的形态分布。Sgr B2区域包含49个可以在1.3厘米波长成像的区域，其中25个超紧凑的HII区域的物理直径小于5000天文单位。De Pree博士和他的团队将使用EVLA对这个超大样本的超紧凑HII区域进行重新成像，以便：(1)确定近20年时间基线（1989年以来）超紧凑HII区域通量和大小波动的频率和幅度；(2)约束和测试理论模型；(3)利用新的EVLA相关器改进的光谱分辨率和带宽观测复合线，并表征线廓线和速度梯度；(4)研究具有特别宽或多峰线廓线的源的动力学。EVLA具有空间和光谱分辨率，可以正确地检查拥挤的Sgr B2区域的宽线重组物体。无论是否检测到闪烁，这些观测将提供这个原型大质量恒星形成区域的最高分辨率，最敏感的无线电图像，并且减少的数据将公开提供。EVLA观测将为闪烁的UC HII区域模型提供一个明确的测试，并将是第一次尝试对超紧凑HII区域的大样本进行时域（20年基线）观测。阿格尼斯·斯科特学院拥有30%的非裔美国学生，是东南天文学研究协会（SARA）联盟中唯一的一所女子学院，物理和天文系在培养女性研究生和从事科学工作方面有着成功的记录。De Pree博士和他的团队在阿格尼斯·斯科特学院的布拉德利天文台开展了一项完善的外展计划，该计划与地区K12机构相连，每年接待1500多名学生。

Dr. De Pree and his team at Agnes Scott College perform observations with the Expanded Very Large Array (EVLA) to study massive star formation. In particular, they investigate radiation flux variations during massive star formation in HII regions (i.e., regions in the interstellar medium in which hydrogen is mainly present as ionized atoms). Massive stars drive processes that enrich the ISM in chemical elements and significantly impact the evolution of galaxies. The Sgr B2 region is one of the most source-rich massive star forming regions in the Milky Way. It contains many morphologically diverse HII regions, and the number of unusual broad line sources make it an ideal laboratory for testing theories of ultra-compact HII region evolution. Recent high-resolution, radiation-hydrodynamic simulations indicate that dense, rotating, accretion flows that are required to form massive stars quickly become gravitationally unstable. This changes the amount of trapped ionizing radiation and the sizes of the associated ultra-compact HII regions. The orbits of dense clumps and filaments near newly born massive stars thus may irregularly trap and expose their ionizing radiation. These objects are optically thick at a wavelength of 2 cm and thus size variations are also expressed as flux changes. Hence over time, a resulting HII region "flickers" in size between being hyper-compact and ultra-compact throughout the main star accretion phase, rather than monotonically expanding. The accretion flow continues for a period ten times longer than the free expansion timescale for an HII region, and the model can address why ultra-compact HII regions do not more rapidly expand and dissipate. It could also explain the observed distribution of morphologies. The Sgr B2 region contains 49 regions that can be imaged at 1.3 cm wavelength, and 25 of these hyper-compact HII regions have physical diameters less than 5000 Astronomical Units. Dr. De Pree and his team will use the EVLA to reimage this large sample of ultra-compact HII regions to: (1) Determine the frequency and magnitude of ultra-compact HII region flux and size fluctuations over about a 20 year time baseline (since 1989) (2) Constrain and test the theoretical models, (3) Observe recombination lines with the improved spectral resolution and bandwidth of the new EVLA correlator, and characterize line profiles and velocity gradients, and (4) Examine the dynamics of sources with especially broad or multiply peaked line profiles. The EVLA has the spatial and the spectral resolution to properly examine the Broad Line Recombination Objects in the crowded Sgr B2 region. Whether or not flickering is detected, these observations will provide the highest resolution, most sensitive radio image of this prototype massive star forming region, and the reduced data will be made publicly available. The EVLA observations will provide a definitive test of the flickering UC HII region model and will be the first attempt to make time-domain (20 year baseline) observations of a large sample of ultra-compact HII regions.Agnes Scott College, with a 30% African American student body is the only women's college in the Southeastern Association for Research in Astronomy (SARA) consortium, and the Physics and Astronomy Department has a record of success in sending women on to graduate work and careers in the sciences. Dr. De Pree and his group are engaged in the well-established outreach program at Bradley Observatory at Agnes Scott College which is connected to area K12 institutions and hosting over 1500 students each year.