OH observations of IRS 1 in NGC 7538

The OH masers associated with IRS 1 in NGC 7538 have been observed with the Very Large Array at a spatial resolution of 1.2 arcsec and a velocity resolution of 0.65 km s⁻¹. At 1720 MHz, two components with radial velocities of – 57.4 and – 59.3 km s⁻¹ are found to lie on the southern edge of the ultracompact H II region. They are separated by 0.24 ± 0.07 arcsec in a direction roughly perpendicular to the line from the centre of IRS 1. An upper limit of 0.3 arcsec is obtained for the sizes. These masers are not coincident with the two H₂CO masers with similar velocities. Three spatial components are measured at 1665 MHz: one with $$V\simeq-59.0$$ km s⁻¹ is on the northern edge of IRS 1 close to the H₂CO masers, one with $$V\simeq-60.1$$ km s⁻¹ is on the western edge and the third with $$V\simeq-57.9$$ km s⁻¹ is midway between the centre and the 1720-MHz OH masers to the south. The 1720-MHz and 1665-MHz OH masers, together with the two H₂CO masers, lie roughly on a ring which is centred on IRS 1 and whose outer radius is ∼ 0.4 arcsec (∼ 2 × 10¹⁶ cm for an assumed distance of 3.5 kpc). The positions and velocities of the maser spots are interpreted in terms of a simple shell model. Without knowing the H II systemic velocity, it is not possible to distinguish whether the shell is expanding or contracting.