Polarimetry of an intermediate-age open cluster: NGC 5617

ORSATTI, A.M; FEINSTEIN, C.; VERGNE, M.M.; MARTINEZ, R. E.; VEGA, E. I.

ASTRONOMY AND ASTROPHYSICS

EDP SCIENCES S A

Año: 2010 vol. 513 p. 1 - 9

0004-6361

Aims. We present polarimetric observations in the UBVRI bands corresponding to 72 stars located in the direction of the medium age open cluster NGC 5617. Our intention is to use polarimetry as a tool in the task of membership identification, miss considered in past investigations intended mainly to determine the cluster´s general characteristics rather than to provide memberships suitable for studies like stellar content and metallicity, as well as to study the characteristics of the dust lying between the Sun and the cluster. Methods. The obsevations were carried out using the five-channel photopolarimeter of the Torino Astronomical Observatory attached to the 2.15m telescope at the Complejo Astronóomico El Leoncito (CASLEO; Argentina). Results. In this investigation, we were able to add 32 stars to the list of members of NGC 5617, and to review the situation of others listed in the literature as such. In particular, we find that five blue straggler stars in the region of the cluster are located behind the same dust as the member stars are, and we have also confirmed the membership of two red giants. The proposed polarimetric memberships were compared with those coming from photometric and kinematical methods, with excellent results. Among the observed stars, we have identified 10 with intrinsic polarization in their light. NGC 5617 can be polarimetrically characterized with Pmax = 4.40% and theta = 73◦.1. The spread in polarization values for the stars observed in direction of the cluster seems to be due to the uneven distribution of dust in front of the cluster´s face. Finally, we have detected that in direction of the cluster, the interstellar medium is apparently free of dust, from the Sun´s position up to the Carina-Sagittarius arm,  where NGC 5617 seems to be located at its farthest border.