First polarimetric images of NML Cyg at 1612 and 1665 MHz

Abstract

We present the first view of the magnetic field structure in the OH shell of the supergiant NML Cygni (NML Cyg). MERLIN interferometric observations of this object were obtained in 1993 October in full polarization, at 1612 and 1665 MHz. They reveal a complex structure in both total intensity and polarization. At 1612 MHz, the majority of the components lie along a north-west-south-east (NW-SE) geometrical axis along which a velocity gradient is clearly evident. The polarization vectors associated with these maser spots are roughly aligned on that same NW-SE axis. The distribution of the external and fainter components is spherical and the associated polarization vectors have a tangential distribution. The total extent of the maser emission at 1612 MHz is about 5.3 arcsec. At 1665 MHz, the maser spot distribution reveals an incomplete spherical shell of total extent about 2.5 arcsec. As observed for the 1612-MHz spherical emission, the polarization vectors associated with the 1665-MHz spherically distributed maser spots show a tangential distribution. This dichotomy strongly suggests the existence of two distinct OH shells; an older shell, exhibiting the spherical maser spot distribution and a tangential distribution of the polarization vectors, and a more recent shell showing a NW-SE maser spot distribution with associated polarization vectors along that same axis.