[论文解读] Chasing discs around O-type (proto)stars - ALMA evidence for an SiO disc and disc wind from G17.64+0.16

L. T. Maud, R. Cesaroni+29 more|UvA-DARE (University of Amsterdam)|Oct 9, 2018

Astrophysics and Star Formation Studies参考文献 144被引用 18

一句话总结

本文利用ALMA对质量巨大的原恒星G17.64+0.16在0.2角秒分辨率下进行观测，探测到一个紧凑的开普勒型SiO盘（半径约200 au）和一个径向膨胀的盘状喷流。结合H30α复合线发射的数据表明，该系统中存在一颗20–30 M⊙的O型恒星，正处于原恒星收缩的最后阶段，其盘和喷流特征与大质量恒星形成的晚期阶段情景一致。

ABSTRACT

We present high angular resolution 0.2 arcsec continuum and molecular emission line Atacama Large Millimeter/sub-millimeter Array (ALMA) observations of G17.64+0.16 in Band 6 (220GHz) taken as part of a campaign in search of circumstellar discs around (proto)-O-stars. At a resolution of 400au the main continuum core is essentially unresolved and isolated from other strong and compact emission peaks. At a resolution of 400au the main continuum core is essentially unresolved and isolated from other strong and compact emission peaks. We detect SiO (5-4) emission that is marginally resolved and elongated in a direction perpendicular to the large-scale outflow seen in the 13CO (2-1) line using the main ALMA array in conjunction with the Atacama Compact Array (ACA). Morphologically, the SiO appears to represent a disc-like structure. Using parametric models we show that the position-velocity profile of the SiO is consistent with the Keplerian rotation of a disc around an object between 10-30Mo in mass, only if there is also radial expansion from a separate structure. The radial motion component can be interpreted as a disc wind from the disc surface. Models with a central stellar object mass between 20 and 30Mo are the most consistent with the stellar luminosity (100000 Lo) and indicative of an O-type star. The H30a millimetre recombination line (231.9GHz) is also detected, but spatially unresolved, and is indicative of a very compact, hot, ionised region co-spatial with the dust continuum core. Accounting for all observables, we suggest that G17.64 is consistent with a O-type young stellar object in the final stages of protostellar assembly, driving a wind, but that has not yet developed into a compact HII region. The existance and detection of the disc in G17.64 is likely related to its isolated and possibly more evolved nature, traits which may underpin discs in similar sources.

研究动机与目标

• 利用高分辨率ALMA观测搜索（原）O型恒星周围的原行星盘。
• 研究G17.64+0.16中分子发射的运动学与形态结构，以识别盘状特征。
• 通过SiO和H30α线轮廓确定中心天体的质量，并评估原恒星的演化阶段。
• 探讨宽角喷流的起源及其与盘和周围原生云的相互作用。

提出的方法

• 在220–230 GHz频段进行高角分辨率（0.2角秒）ALMA Band 6观测，以成像尘埃连续辐射和分子线发射。
• 对SiO (5–4)发射进行参数化建模，以拟合开普勒旋转和径向膨胀，区分盘和喷流成分。
• 将CH3CN和CH3OH的热辐射与OH脉泽位置进行比较，以追踪内层喷流腔壁。
• 分析H30α复合线的宽度和空间形态，以推断电离核心大小和整体运动。
• 利用13CO (2–1)发射映射大尺度喷流结构和腔壁。
• 结合主ALMA阵列与阿塔卡马紧凑孔径阵列（ACA）进行联合建模，以恢复延伸发射并提高动态范围。

实验结果

研究问题

• RQ1 G17.64+0.16是否在大质量原恒星周围存在原行星盘，如SiO发射的形态和运动学所指示的那样？
• RQ2 根据SiO盘的开普勒旋转推断的中心天体质量是多少，且是否与恒星光度一致？
• RQ3 H30α线的宽线宽和紧凑发射如何与中心区域的电离和整体运动相关？
• RQ4 CH3CN和CH3OH的弧形喷流的起源是什么，它们如何与喷流和脉泽发射相关？
• RQ5 SiO发射中的径向膨胀是否最适宜由盘状喷流解释，其方向如何与大尺度喷流方向关联？

主要发现

• SiO (5–4)发射略微被解析，且沿大尺度CO喷流方向（PA ~135°）垂直方向拉长，位置-速度剖面与盘中开普勒旋转一致。
• 建模结果表明中心天体质量在10至30 M⊙之间，最佳拟合模型为20–30 M⊙，与O型恒星及其1×10⁵ L⊙光度一致。
• SiO发射中的径向速度分量最适宜由一个独立的径向膨胀结构解释，该结构被解读为来自盘面的盘状喷流。
• 紧凑的H30α发射（FWHM = 81.9 km s⁻¹）在空间上未被解析，与一个高温电离核心一致，表明持续的电离和喷流驱动。
• CH3CN和CH3OH发射追踪着从盘面平面弯曲向外的弧形喷流，与OH脉泽位置重合，可能标记了宽角喷流的内侧工作表面。
• 该源孤立存在，且无紧凑H II区，表明其处于原恒星收缩的最后阶段，尚未形成完整的H II区。