Water maser variability in a high-mass YSO outburst
Water maser variability in a high-mass YSO outburst
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DOI：
10.1051/0004-6361/202039798
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发表时间：
2020-12
影响因子：
6.5
通讯作者：
T. Hirota;R. Cesaroni;L. Moscadelli;K. Sugiyama;R. Burns;Jungha Kim;K. Sunada;Y. Yonekura
中科院分区：
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作者：
T. Hirota;R. Cesaroni;L. Moscadelli;K. Sugiyama;R. Burns;Jungha Kim;K. Sunada;Y. Yonekura
上下文。弄清质量吸积和抛射历史之间的关系是理解大质量恒星形成过程的关键问题之一。目标。我们的目标是研究2015年6月中旬的质量吸积爆事件与大质量原恒星S255 NIRS 3中的喷流抛射之间的可能关系。利用甚长基线干涉仪(VLBI)对22 GHz的H2O脉泽进行了VLBI观测，揭示了S255 NIRS3中H2O脉泽追踪到的流出/急流的三维速度和空间结构。此外，我们还利用7波段的阿塔卡马大毫米/亚毫米阵列(ALMA)对亚毫米连续谱和321 GHz的H2O脉泽进行了跟踪观测。我们成功地测量了与双极外流相关的22 GHz H2O脉泽的自行运动。这一结构与2005年和2010年观察到的结构几乎相同。用22GHzH2O脉泽测得蓝移弓激波的膨胀速度为28公里S−1，对应的动力学时间尺度为60年。与射电喷流相比，脉泽流出的方向略有倾斜，这可能表明在吸积爆事件期间发生了更新的抛射事件。自2017年初开始VLBI监测以来，22 GHz H2O脉泽的总通量密度逐渐增加，并在2018年随后的单盘监测中几乎保持不变。H2O脉泽的增亮作用在东北出流叶更为突出。我们第一次发现了恒星形成区域在22 GHz处扩展的H2O脉泽辐射，这部分被VERA甚至是最广泛的甚大阵列(VLA)配置所解决。我们发现，这种扩展分量的通量变化类似于未分辨脉泽发射的通量变化。与五个月前的观测相比，ALMA波段7的连续辐射没有明显的变化。我们在S255 NIRS3中绘制了321 GHz的H2O脉泽图，为这个脉泽提供了高质量恒星形成区的空间分布的第四个例子。结论。我们的结论是，22 GHz H2O脉泽特征追踪到的弓激波结构不太可能起源于由最近的吸积爆发提供动力的射电急流与周围介质之间的界面。22 GHz H2O脉泽的增亮可能是由于新喷出的物质产生的(衰减的)红外(IR)爆发沿空腔逃逸的光子辐射激发的结果。在蓝移弓形激波中，22 GHz/321 GHz脉泽光度的较低比率意味着温度(>；1000K)高于该区域的其他脉泽特征。
Context. Clarifying the relationship between mass accretion and ejection history is one of the key issues in understanding high-mass star formation processes. Aims. We aim to investigate the possible relationship between the mass accretion burst event in mid-June 2015 and the jet ejection in the high-mass protostar S255 NIRS 3. Methods. The Very Long Baseline Interferometer (VLBI) monitoring observations of the 22 GHz H2O masers were carried out using VLBI Exploration of Radio Astrometry (VERA) to reveal the 3D velocity and spatial structure of the outflow/jet traced by the H2O masers in S255 NIRS 3. In addition, we conducted follow-up observations of the submillimeter continuum and the 321 GHz H2O masers with the Atacama Large Millimeter/submillimeter Array (ALMA) at Band 7. Results. We successfully measured the proper motions of the 22 GHz H2O masers associated with a bipolar outflow. The structure is almost the same as was observed in 2005 and 2010. The expansion velocity of the blueshifted bow shock traced by the 22 GHz H2O masers was measured to be 28 km s−1, corresponding to a dynamical timescale of 60 yr. The direction of the maser outflow is slightly tilted compared with the radio jet, which could suggest a more recent ejection episode during the accretion burst event. The total flux density of the 22 GHz H2O masers gradually increases from the beginning of the VLBI monitoring in early 2017 and becomes almost constant in subsequent single-dish monitoring in 2018. The brightening of the H2O masers is more prominent in the northeast outflow lobe. For the first time, we revealed extended H2O maser emission at 22 GHz in a star-forming region, which is partly resolved out by VERA and even by the most extended Very Large Array (VLA) configurations. We find that the flux variation of such an extended component is similar to that of the unresolved maser emission. The ALMA Band 7 continuum emission did not show significant variations compared with the previous observations performed five months before. We mapped the 321 GHz H2O masers in S255 NIRS 3 providing the fourth example, for this maser, of the spatial distribution in a high-mass star-forming region. Conclusions. We conclude that the bow shock structure traced by the 22 GHz H2O maser features is unlikely to originate at the interface between the radio jet powered by the recent accretion outburst and the surrounding medium. The brightening of the 22 GHz H2O masers could be due to radiative excitation by photons form the (declining) infrared (IR) outburst escaping along the cavity created by the newly ejected material. The lower ratio of the 22 GHz/321 GHz maser luminosity in the blueshifted bow shock suggests a temperature ( >1000 K), higher than for the other maser features in this region.