A New Approach to Identifying Massive Young Stellar Objects (MYSOs)

最近斯皮策太空望远镜红外阵列相机（IRAC）对银道面的调查揭示了一类新的物体，其特征是在IRAC 4.5微米波段上有明亮的扩展发射。超过300个这样的物体（被称为ego =延伸的绿色物体，因为4.5微米波段在3色IRAC图像中通常被编码为绿色）已经被爱德华·丘吉尔博士（威斯康星大学麦迪逊分校）和他的团队编目。对ego的中红外光谱能量分布（SEDs）分析表明，它们是大质量年轻恒星天体（myso）。激发4.5微米辐射的机制，被认为是超音速流出的H2或CO气体的冲击，表明ego是处于活跃吸积状态的myso。研究人员将进行一套全面的多波长研究，旨在描述新发现的ego的特性，并对大质量恒星形成理论施加限制；特别是吸积、流出和聚类是如何同时发生的。除了从斯皮策数据中得出的中红外特性外，目前对ego知之甚少。在这个项目下进行的研究将通过近红外到厘米波长的高角分辨率研究，对精心挑选的30个ego样本的流出特性、光度、质量和聚类特性进行表征。其中包括一个扩展甚大阵列（EVLA）搜索44 GHz的I类CH3OH微波激射器，已知与分子外流有关，它将与低分辨率分子线调查（HCO+和SiO）一起使用，以测试IRAC 4.5微米发射的流出源假设，并测量流出特性。EVLA搜索与myso相关的6.7 GHz II类CH3OH脉振，将与同时获得的高分辨率3.6 cm和7 mm连续数据一起使用，以确定ego的质量范围。将确定样品中所有源的运动距离，允许使用原恒星模型网格从它们的SEDs中提取光度，恒星质量和包膜吸积率。对这类新天体的后续观测将为迄今为止隐藏的大质量原恒星演化阶段提供急需的信息。除了更好地理解MYSOs的本质这一主要研究目标外，该项目还将在其持续时间内支持一名全日制研究生和几名本科生的培训，并通过几个当地组织为公众宣传做出贡献，包括威斯康星大学空间中心、天文系校外宣传中心和每周的天文广播节目。

Recent Spitzer Space Telescope Infrared Array Camera (IRAC) surveys of the Galactic Plane have revealed a new class of objects that are characterized by bright extended emission in the IRAC 4.5 micron band. Over 300 of these objects (designated EGOs=extended green objects, for the common coding of the 4.5 micron band as green in 3-color IRAC images) have been catalogued by Dr. Edward Churchwell (University of Wisconsin - Madison) and his team. Analysis of the mid-infrared spectral energy distributions (SEDs) of EGOs indicates that they are massive young stellar objects (MYSOs). The mechanism for exciting the 4.5 micron emission, thought to be shocked H2 or CO gas in supersonic outflows, suggests that EGOs are MYSOs in a state of active accretion. The investigators will carry out a comprehensive set of multiwavelength studies designed to characterize the properties of the newly discovered EGOs and to place constraints on theories of massive star formation; in particular how accretion, outflows and clustering operate simultaneously.Very little is currently known about EGOs other than the mid-Infrared properties derived from Spitzer data. The studies that will be carried out under this project will characterize the outflow properties, luminosities, masses, and clustering properties of a well-chosen sample of 30 EGOs, via high angular resolution studies at near-Infrared through centimeter wavelengths. These include an Extended-Very Large Array (EVLA) search for 44 GHz Class I CH3OH masers, known to be associated with molecular outflows, which will be used in conjunction with lower-resolution molecular line surveys (HCO+ and SiO) to test the hypothesis of an outflow origin for the IRAC 4.5 micron emission and measure outflow properties. An EVLA search for 6.7 GHz Class II CH3OH masers, known to be associated with MYSOs, will be used along with simultaneously obtained high-resolution 3.6 cm and 7 mm continuum data to establish the mass range of EGOs. Kinematic distances will be determined for all the sources in the sample, permitting the luminosity, stellar mass, and envelope accretion rate to be extracted from their SEDs using a grid of protostar models. These follow-up observations on this new class of objects will add much needed information on this heretofore hidden stage of high-mass protostellar evolution.In addition to the primary research goals of better understanding the nature of MYSOs, this project will support the training of one full time graduate student and several undergraduate students over its duration as well as contributing to public outreach through several local organizations, including the University of Wisconsin Space Place, the off-campus outreach center of the Department of Astronomy, and a weekly astronomy radio program.