Optical polarization observations of NGC 6231: evidence for a past SN fingerprint

We present the first linear multicolor polarization observations for a sample of 35 stars in the direction of the Galactic cluster NGC 6231. We have found a complex pattern in the angles of the polarimetric vectors. Near the core of this cluster the structure shows a semi-circular pattern that we have interpreted as a re-orientation of the dust particles, showing the morphology of the magnetic field. We propose that a supernova event has occurred some time ago and produced a shock on the local ISM. We discuss in this paper independent confirmations othis event, both from the studies on the diffuse interstellar absorptions Crawford (2001) and the results of the pre-main sequence stars (PMS) given by Sung et al. (1998). We also show that a supernova is supported by the evolutive status of the cluster.Comment: 24 pages, 4 figures, ApJ Accepte