Cluster Membership Probability: Polarimetric Approach
  Interstellar polarimetric data of the six open clusters Hogg 15, NGC 6611,
NGC 5606, NGC 6231, NGC 5749 and NGC 6250 have been used to estimate the
membership probability for the stars within them. For proper-motion member
stars, the membership probability estimated using the polarimetric data is in
good agreement with the proper-motion cluster membership probability. However,
for proper-motion non-member stars, the membership probability estimated by the
polarimetric method is in total disagreement with the proper-motion cluster
membership probability. The inconsistencies in the determined memberships may
be because of the fundamental differences between the two methods of
determination: one is based on stellar proper-motion in space and the other is
based on selective extinction of the stellar output by the asymmetric aligned
dust grains present in the interstellar medium. The results and analysis
suggest that the scatter of the Stokes vectors q(%) and u(%) for the
proper-motion member stars depends on the interstellar and intra-cluster
differential reddening in the open cluster. It is found that this method could
be used to estimate the cluster membership probability if we have additional
polarimetric and photometric information for a star to identify it as a
probable member/non-member of a particular cluster, such as the maximum
wavelength value (\lambda_max), the unit weight error of the fit (\sigma_1),
the dispersion in the polarimetric position angles (\epsilon), reddening
(E(B-V)) or the differential intra-cluster reddening (\Delta E(B-V)). This
method could also be used to estimate the membership probability of known
member stars having no membership probability as well as to resolve
disagreements about membership among different proper-motion surveys.
