Abstract:
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 es-
timated 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 (per cent) and u (per cent) for the
proper-motion member stars depends on the interstellar and intracluster differential reddening
in the open cluster. It is found that this method could be used to estimate the cluster member-
ship 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 (λmax ), the unit weight error of the fit (σ₁ ), the dispersion in the polarimetric
position angles (⁻є ), reddening (E(B − V)) or the differential intracluster reddening ( 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.