MERLIN observations of the asymmetrical OH envelope around NML Cygnus

Abstract

Observations of the 1612-MHz OH maser emission from the star NML Cyg have been made with the Jodrell Bank MERLIN. These observations show that the circumstellar envelope surrounding NML Cyg has a complex structure. The envelope as a whole shows a large-scale spatial asymmetry and in addition the inner regions are expanding faster than the outer. It is suggested that an increase in the stellar luminosity by a factor of 2 or 3 over a 500-year period could result in the observed acceleration. The maps demonstrate that the structure of the maser emission consists of two components: (i) an outer ring of angular radius 2.5 arcsec concentrated in the west and north-west; (ii) an inner component of angular radius 1.5 arcsec concentrated in the south-east and centre. It is suggested that this spatial asymmetry is the result of the photodissociation of H₂O to form OH on the north-west side of the envelope by the intense UV photon flux from the Cyg OB2 association. The data also imply that the radial velocity of the central star of NML Cyg is + 2 ± 2 km s⁻¹ in approximate agreement with the value determined from thermal SiO data. The discrepancy between this value and the midpoint of the OH profile (− 3 ± 1 km s⁻¹) is attributed to the lack of redshifted emission along the line-of-sight.