A Polarimetric Method for Identifying Member Stars in the Open Star Clusters (a case of NGC 1817 and NGC 7380)

Polarimetric observations serve as an effective diagnostic tool for studying the interstellar medium and identifying stellar membership of an open star cluster. Starlight becomes linearly polarized due to the dichroic extinction from aligned asymmetric interstellar dust grains. The resulting polarization signatures differ for member and non-member stars of the cluster. This makes polarimetry a valuable tool for separating member stars from the foreground or background field.

The open star clusters NGC 1817 and NGC 7380, representing intermediate-age and young stellar environments, respectively, offer important laboratories for exploring star formation, stellar evolution, and cluster dynamics. However, uncertainties in membership determination continue to affect the accuracy of their derived properties. In this study, I will present a method based on linear polarization of stars that can be used to evaluate and refine the membership probabilities of stars in open star clusters. Using the polarimetric technique, I have calculated the membership probability of stars of the cluster NGC 1817 and NGC 7380. A good correlation is found between my results and previously estimated probabilities of members from the proper motion technique. From this study, one can infer that the polarization property can be used for deriving the membership probability. Additionally, I will discuss some drawbacks of this technique.