VLBA确定到附近恒星形成区的距离IV。巨蛇座地核中Herbig原AeBe星EC95的初步距离
VLBA determination of the distance to nearby star-forming regions IV. A preliminary distance to the proto-Herbig AeBe star EC95 in the Serpens Core
摘要：
使用甚长基阵，我们观察了年轻的恒星对象EC 95在巨蛇座云核心在八个时代，从2007年12月至2009年12月。在我们的领域中检测到两个源，并形成一个紧密的二元系统。第一颗（EC 95 a）是一颗4- 5 $M_\odot$原赫比格AeBe天体（可以说是已知最年轻的此类天体），而第二颗（EC 95 b）很可能是一颗低质量的金牛T星。有趣的是，这两种源都是非热发射体。虽然金牛座T星被认为是日冕的动力来源，因为它们在沿Hayashi轨道行进时是对流的，但中等质量的恒星在辐射轨道上接近主序星。因此，预期它们不具有强的表面磁场，并且不应该是磁活性的。我们回顾了几种机制，可以产生的EC 95 a的非热发射，并认为EC 95 a的观察到的属性可能是最容易解释，如果它拥有一个由旋转驱动的对流层供电的电晕。使用我们的观察，我们表明EC 95的三角视差为$π$ = 2.41 $\pm$ 0.02 mas，对应于414.9 $^{+4.4}_{-4.3}$ pc的距离。我们认为这意味着到巨蛇座核心的距离为415 $\pm$ 5 pc，到巨蛇座云的平均距离为415 $\pm$ 25 pc。这一数值明显大于先前基于巨蛇座方向恒星所遭受的消光测量的估计值（$d$ $\sim$ 260 pc）。对这种差异的一个可能的解释是，这些先前的观测挑选出了与天鹰座裂谷系统相关的前景尘埃云，而不是巨蛇座本身。
英文摘要：
Using the Very Long Base Array, we observed the young stellar object EC 95 in the Serpens cloud core at eight epochs from December 2007 to December 2009. Two sources are detected in our field, and are shown to form a tight binary system. The primary (EC 95a) is a 4--5 $M_\odot$ proto-Herbig AeBe object (arguably the youngest such object known), whereas the secondary (EC 95b) is most likely a low-mass T Tauri star. Interestingly, both sources are non-thermal emitters. While T Tauri stars are expected to power a corona because they are convective while they go down the Hayashi track, intermediate-mass stars approach the main sequence on radiative tracks. Thus, they are not expected to have strong superficial magnetic fields, and should not be magnetically active. We review several mechanisms that could produce the non-thermal emission of EC 95a, and argue that the observed properties of EC 95a might be most readily interpreted if it possessed a corona powered by a rotation-driven convective layer. Using our observations, we show that the trigonometric parallax of EC 95 is $π$ = 2.41 $\pm$ 0.02 mas, corresponding to a distance of 414.9$^{+4.4}_{-4.3}$ pc. We argue that this implies a distance to the Serpens core of 415 $\pm$ 5 pc, and a mean distance to the Serpens cloud of 415 $\pm$ 25 pc. This value is significantly larger than previous estimates ($d$ $\sim$ 260 pc) based on measurements of the extinction suffered by stars in the direction of Serpens. A possible explanation for this discrepancy is that these previous observations picked out foreground dust clouds associated with the Aquila Rift system rather than Serpens itself.
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