OH masers associated with compact H II regions in the G45.1+0.1 complex

The mainline OH masers in the G45.1+0.1 complex have been mapped at high resolution using MERLIN. Relative positions of the masers have been measured to an accuracy of ∼10 milliarcsec. The maser emission is found to come from two sources OH45.07+0.13 and OH45.12+0.13 separated by 3 arcmin, each of which is associated with a compact H II region. The masers occur near the edges of the H II regions at projected distances of $$\sim2\times10^{17}$$ cm from the centre. It is suggested that the masers are in expanding envelopes of gas and dust which are being driven outwards at a velocity of ∼6km s⁻¹. The magnetic field direction determined from Zeeman splitting shows a surprising reversal between the two sources. The association between type I OH masers and compact H II regions is briefly reviewed, and it is shown that there is a general tendency for OH masers to lie at greater distances from more luminous stars, following a power law $$r_\text{OH} \propto L_\star^{1/2}$$.