Astronomy and Astrophysics – Astrophysics – Solar and Stellar Astrophysics
Scientific paper
2009-11-05
Astronomy and Astrophysics
Astrophysics
Solar and Stellar Astrophysics
Accepted for publication in MNRAS
Scientific paper
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 (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--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 re-determine the distances to both clusters, obtaining 133+/-7pc 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 percent uncertainty.
Jackson Russell J.
Jeffries Robin D.
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