EGOS入侵证据的搜寻I：北方样本
A Search for Infall Evidence in EGOs I: the Northern Sample
摘要：
我们报告了第一个系统的调查分子线（包括HCO+（1-0）和12 CO，13 CO，C18 O（1-0）线在3毫米波段）对88个大质量的年轻恒星对象（MYSO）候选者的新样本与正在进行的外流（称为扩展的绿色物体或EGO）确定从斯皮策GLIMPSE调查在北半球与PMO-13.7米射电望远镜。通过分析72个EGO中69个的光学粗线HCO+的不对称性，我们发现29个具有蓝色不对称轮廓的源和19个具有红色不对称轮廓的源。这导致0.14的蓝色过量，被视为所观察到的EGO样品中的塌缩核心的特征。在我们的完整样本中测量到的相对较小的蓝色过量是由于所观察到的EGO主要由流出主导，并且处于与IRDC和6.7 GHz甲醇脉泽相关的早期演化阶段。由CO谱线导出的EGO周围的云的物理性质类似于大质量团块，其中可能嵌入了与EGO相关的大质量恒星形成核。从20个可能的陨石坑中得出的陨石坑速度和质量陨石坑率也与多年期SO观测中发现的典型值一致。因此，我们的观测结果进一步支持Cyganowski等人（2008）的推测，即从统计的角度来看，EGO追踪了一个具有持续外流活动和大质量原恒星演化的活跃快速吸积阶段的星群，尽管由于大光束的单点测量可能存在局限性。
英文摘要：
We report the first systematic survey of molecular lines (including HCO+ (1-0) and 12CO, 13CO, C18O (1-0) lines at 3 mm band) towards a new sample of 88 massive young stellar object (MYSO) candidates associated with ongoing outflows (known as extended green objects or EGOs) identified from the Spitzer GLIMPSE survey in the northern hemisphere with the PMO-13.7 m radio telescope. By analyzing the asymmetries of the optically thick line HCO+ for 69 of 72 EGOs with HCO+ detection, we found 29 sources with blue asymmetric profiles and 19 sources with red asymmetric profiles. This results in a blue excess of 0.14, seen as a signature of collapsing cores in the observed EGO sample. The relatively small blue excess measured in our full sample due to that the observed EGOs are mostly dominated by outflows and at an earlier evolutionary phase associated with IRDCs and 6.7 GHz methanol masers. The physical properties of clouds surrounding EGOs derived from CO lines are similar to those of massive clumps wherein the massive star forming cores associated with EGOs possibly embedded. The infall velocities and mass infall rates derived for 20 infall candidates are also consistent with the typical values found in MYSOs. Thus our observations further support the speculation of Cyganowski et al. (2008) that EGOs trace a population with ongoing outflow activity and active rapid accretion stage of massive protostellar evolution from a statistical view, although there maybe have limitations due to single-pointing survey with a large beam.
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