红超巨星NML的距离和大小的VLBA和VLA天体测量
The Distance and Size of the Red Hypergiant NML Cyg from VLBA and VLA Astrometry
摘要：
我们利用圆星H_2O和SiO脉泽的VLBA多历元观测，测量了NML天鹅座的年视差和自行。NML Cyg的测量视差为0.620+/-0.047 mas，对应的距离为1.61+/-0.12 kpc。NML天鹅座自行的测量值为：向东mu_x = -1.55+/-0.42mas/yr，向北mu_y= -4.59+/-0.41mas/yr。距离和自行都与天鹅座OB 2一致，在它们的联合不确定性范围内，证实了它们的关联。考虑到对NML Cyg的分子吸收特征，我们认为NML Cyg位于Cyg OB 2关联的远侧。根据我们的距离修正的恒星光度使天鹅座NML明显低于红超巨星的经验光度极限。利用射电光球的VLA观测和SiO脉泽作为相位参考，我们部分解析了NML Cyg 43 GHz的射电光球，发现其直径约为44质量，如果射电光球的尺寸是光学光球的2倍，则光学恒星直径为22质量。根据星周SiO脉泽相对于射电光球的位置，我们估计了NML Cyg在2008.868历元的绝对位置为R.A. = 20h46m25.5382s +/- 0.0010s，Decl. = 40d06'59.379”+/- 0.015”。NML天鹅座的特殊运动，天鹅座OB 2中恒星的平均值，以及其他四个恒星形成区域排除了以天鹅座OB 2为中心的膨胀的“斯特罗姆格伦球”是天鹅座X区域运动学的原因。
英文摘要：
We have measured the annual parallax and proper motion of NML Cyg from multiple epoch VLBA observations of the circulstellar H2O and SiO masers. The measured parallax of NML Cyg is 0.620+/-0.047 mas, corresponding to a distance of 1.61+/-0.12 kpc. The measured proper motion of NML Cyg is mu_x = -1.55+/-0.42 mas/yr eastward and mu_y= -4.59+/-0.41 mas/yr northward. Both Both the distance and proper motion are consistent with that of Cyg OB2, within their joint uncertainty, confirming their association. Taking into consideration molecular absorption signatures seen toward NML Cyg, we suggest that NML Cyg lies on the far side of the Cyg OB2 association. The stellar luminosity revised with our distance brings NML Cyg significantly below the empirical luminosity limit for a red supergiant. Using the VLA observation the radio photosphere and the SiO maser as a phase reference, we partially resolve the radio photosphere of NML Cyg at 43 GHz and find its diameter is about 44 mas, suggesting an optical stellar diameter of 22 mas, if the size of radio photosphere is 2 times the optical photosphere. Based on the position of circumstellar SiO masers relative to the radio photosphere, we estimate the absolute position of NML Cyg at epoch 2008.868 to be R.A. = 20h46m25.5382s +/- 0.0010s, Decl. = 40d06'59.379" +/- 0.015". The peculiar motions of NML Cyg, the average of stars in Cyg OB2, and four other star-forming regions rules out that an expanding "Stroemgren sphere" centered on Cyg OB2 is responsible for the kinematics of the Cygnus X region.
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