Are the spin axes of stars randomly aligned within a cluster?

作者：Jackson R J^*; Jeffries R D

来源：Monthly Notices of the Royal Astronomical Society, 2010, 402(2): 1380-1390.

DOI：10.1111/j.1365-2966.2009.15983.x

• methods: statistical
• stars: formation
• open clusters and associations: individual: Pleiades
• open clusters and associations: individual: Alpha Per

We investigate to what extent the spin axes of stars in young open clusters are aligned. Assuming that the spin vectors lie uniformly within a conical section, with an opening half-angle between lambda=0 degrees (perfectly aligned) and lambda=90 degrees (completely random), we describe a Monte Carlo modelling technique that returns a probability density for this opening angle given a set of measured sin i values, where i is the unknown inclination angle between a stellar spin vector and the line of sight. Using simulations we demonstrate that although azimuthal information is lost, it is easily possible to discriminate between strongly aligned spin axes and a random distribution, providing that the mean spin-axis inclination lies outside the range 45 degrees-75 degrees. We apply the technique to G- and K-type stars in the young Pleiades and Alpha Per clusters. The sin i values are derived using rotation periods and projected equatorial velocities, combined with radii estimated from the cluster distances and a surface brightness/colour relationship. For both clusters we find no evidence for spin-axis alignment: lambda=90 degrees is the most probable model and lambda>40 degrees with 90 per cent confidence. Assuming a random spin-axis alignment, we redetermine the distances to both clusters, obtaining 133 +/- 7 pc for the Pleiades and 182 +/- 11 pc for Alpha Per. If the assumption of random spin-axis alignment is discarded however, whilst the distance estimate remains unchanged, it has an additional (+18)(-32) per cent uncertainty.

• 出版日期2010-2-21