Magnetic fields and chemical peculiarities of the very young intermediate-mass binary system HD 72106
C. P. Folsom, G. A. Wade, O. Kochukhov, E. Alecian, C. Catala, S. Bagnulo, T. Boehm, J. -C. Bouret, J. -F. Donati, J. Grunhut, D. A. Hanes, J. D. Landstreet
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Accepted by Monthly Notices of the Royal Astronomical Society, September 2008. 15 pages, 10 figures
The recently discovered magnetic Herbig Ae and Be stars may provide qualitatively new information about the formation and evolution of magnetic Ap and Bp stars. We have performed a detailed investigation of one particularly interesting binary system with a Herbig Ae secondary and a late B-type primary possessing a strong, globally ordered magnetic field. Twenty high-resolution Stokes V spectra of the system were obtained with the ESPaDOnS instrument mounted on the CFHT. In these observations we see clear evidence for a magnetic field in the primary, but no evidence for a magnetic field in the secondary. A detailed abundance analysis was performed for both stars, revealing strong chemical peculiarities in the primary and normal chemical abundances in the secondary. The primary is strongly overabundant in Si, Cr, and other iron-peak elements, as well as Nd, and underabundant in He. The primary therefore appears to be a very young Bp star. In this context, line profile variations of the primary suggest non-uniform lateral distributions of surface abundances. Interpreting the 0.63995 +/- 0.00009 day variation period of the Stokes I and V profiles as the rotational period of the star, we have modeled the magnetic field geometry and the surface abundance distributions of Si, Ti, Cr and Fe using Magnetic Doppler Imaging. We derive a dipolar geometry of the surface magnetic field, with a polar strength of 1230 G and an obliquity of 57 degrees. The distributions Ti, Cr and Fe are all qualitatively similar, with an elongated patch of enhanced abundance situated near the positive magnetic pole. The Si distribution is somewhat different, and its relationship to the magnetic field geometry less clear.
Rotation and surface abundance peculiarities in A-type stars
为了弄清光球丰度异常与恒星自转之间的联系，探索“正常A”星的本质，利用BOES高色散光谱数据，用光谱拟合方法，测定了46颗A型场星的C、O、Si、Ca、Ti、Fe和Ba 7种元素的丰度和投影自转速度。我们发现，在慢旋子中观察到的特性（C、O和Ca的丰度不足; Ba的丰度过高）随着自转的增加而有效地降低，当v_e sin i > 100 km s^-1时，这些特性几乎消失。这进一步表明，具有足够大的旋转速度的恒星可能会保留表面的原始成分而不会被改变。考虑到这种快速旋转（假定为正常）的A型恒星的元素丰度所表现出的日下趋势（在下面的十分之几），我们怀疑气体金属丰度可能在太阳诞生后有所减少，这与银河系化学演化的普遍看法相反。
