Journal Article

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

R. J. Jackson and R. D. Jeffries

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 402, issue 2, pages 1380-1390
Published in print February 2010 | ISSN: 0035-8711
Published online February 2010 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2009.15983.x
Are the spin axes of stars randomly aligned within a cluster?

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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 λ= 0° (perfectly aligned) and λ= 90° (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°. 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: λ= 90° is the most probable model and λ >40° 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.

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

Journal Article.  8576 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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