Optical polarization observations in the Scorpius region: NGC 6124

TL;DR

This study uses optical polarimetry of stars in NGC 6124 to analyze interstellar medium properties, revealing lower polarization efficiency, a non-parallel polarization orientation possibly due to the Lupus Cloud, and multiple absorption layers affecting the cluster.

Contribution

First polarimetric analysis of NGC 6124 that characterizes ISM properties and distinguishes cluster members from non-members using polarization data.

Findings

Polarization efficiency is lower than typical ISM values.

Polarization orientation is about 8.1°, not aligned with the Galactic Disk.

Multiple absorption sheets are present between Sun and cluster.

Abstract

We have obtained optical multicolour (UBVRI) linear polarimetric data for 46 of the brightest stars in the area of the open cluster NGC 6124 in order to investigate the properties of the interstellar medium (ISM) that lies along the line of sight toward the cluster. Our data yield a mean polarization efficiency of $P_V/E_{B-V}=3.1\pm$0.62, i.e., a value lower than the polarization produced by the ISM with normal efficiency for an average color excess of $E_{B-V}=0.80$ as that found for NGC 6124. Besides, the polarization shows an orientation of $\theta \sim 8^\circ$.1 which is not parallel to the Galactic Disk,an effect that we think may be caused by the Lupus Cloud. Our analysis also indicates that the observed visual extinction in NGC 6124 is caused by the presence of three different absorption sheets located between the Sun and NGC 6124. The values of the internal dispersion of the polarization ($\Delta P_V\sim 1.3% $) and of the colour excess ($\Delta E_{B-V}\sim 0.29$ mag) for the members of NGC 6124 seem to be compatible with the presence of an intra-cluster dust component. Only six stars exhibit some evidence of intrinsic polarization.Our work also shows that polarimetry provides an excellent tool to distinguish between member and non-member stars of a cluster.