O9巨星猎户座ζ A的弱磁场
The weak magnetic field of the O9.7 supergiant zeta Orionis A
摘要：
我们在这里报告检测到一个弱的磁场的O9.7超巨星zeta猎户座A，使用光谱偏振观测获得的2米望远镜伯纳德Lyot顶部Pic du Midi（法国）。猎户座ζ A是第三颗拥有磁场的O型星（还有猎户座θ ^1 C和HD 191612），也是第一颗在“正常”快速旋转的O型星上发现的。猎户座ζ A的磁场是在大质量恒星上探测到的最弱的磁场。测得的磁场低于热均分极限（约100 G）。通过将NLTE模型大气与我们的光谱拟合，我们确定猎户座ζ A是一颗40 Msun的恒星，半径为25 Rsun，年龄约为500 - 600万年，没有表面氮增强，质量损失率约为2x 10 ^（-6）Msol/yr。猎户座ζ A的磁拓扑结构显然比偶极子更复杂，涉及位于同一半球两侧的两个主要磁极;我们的数据还表明，猎户座ζ A旋转约7.0 d，距离地球观测者的极点约40度。尽管它的弱点，检测到的磁场显着影响风结构;相应的阿尔芬半径是非常接近的表面，从而产生不同的旋转调制风线比其他两个已知的磁性O星。猎户座ζ A相对于猎户座θ ^1 C的快速旋转看起来令人惊讶，这两颗恒星具有相似的无符号磁通量（一旦重新换算为相同的半径）;这可能表明猎户座ζ A上检测到的次均分场不是化石遗迹（与猎户座θ ^1 C和HD 191612的次均分场相反），而是由MHD不稳定性产生的奇异发电机作用的结果。
英文摘要：
We report here the detection of a weak magnetic field of 50 - 100 G on the O9.7 supergiant zeta Ori A, using spectropolarimetric observations obtained with NARVAL at the 2m Telescope Bernard Lyot atop Pic du Midi (France). zeta Ori A is the third O star known to host a magnetic field (along with theta^1 Ori C and HD 191612), and the first detection on a 'normal' rapidly-rotating O star. The magnetic field of zeta Ori A is the weakest magnetic field ever detected on a massive star. The measured field is lower than the thermal equipartition limit (about 100 G). By fitting NLTE model atmospheres to our spectra, we determined that zeta Ori A is a 40 Msun star with a radius of 25 Rsun and an age of about 5 - 6 Myr, showing no surface nitrogen enhancement and losing mass at a rate of about 2x10^(-6) Msol/yr. The magnetic topology of zeta Ori A is apparently more complex than a dipole and involves two main magnetic polarities located on both sides of the same hemisphere; our data also suggest that zeta Ori A rotates in about 7.0 d and is about 40 degrees away from pole-on to an Earth-based observer. Despite its weakness, the detected magnetic field significantly affects the wind structure; the corresponding Alfven radius is however very close to the surface, thus generating a different rotational modulation in wind lines than that reported on the two other known magnetic O stars. The rapid rotation of zeta Ori A with respect to theta^1 Ori C appears as a surprise, both stars having similar unsigned magnetic fluxes (once rescaled to the same radius); it may suggest that the sub-equipartition field detected on zeta Ori A is not a fossil remnant (as opposed to that of theta^1 Ori C and HD 191612), but the result of an exotic dynamo action produced through MHD instabilities.
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