Spatial and velocity coincidence of 4765- and 1720-MHz OH masers in two star-forming regions Cep A and W75N

Abstract

We present the first maps of 4765-MHz OH masers in two star-forming regions Cepheus A and W75N, made with Multi-Element Radio Linked Interferometer Network. In Cep A the emission has an arc-like structure of size 40 mas with a clear velocity gradient, and lies at the edge of H II region 3b Over a period of 8 weeks the maser diminished in intensity by a factor of 7 but its structure remained stable. This structure coincides with a newly mapped 1720-MHz maser in Cep A within the positional error, and matches it in velocity. The 1720-MHz line exhibits Zeeman splitting that corresponds to a magnetic field strength of -17.3 mG. In W75N the excited 4765-MHz OH maser has a linear structure of size 45 mas with a well defined velocity gradient, and lies at the edge of H II region VLA 1. This structure coincides in position and velocity with the 1720-MHz masers. We conclude that in both sources the 4765-MHz emission coexists with 1720-MHz emission in the same volume of gas. In such a case the physical conditions in these regions are tightly constrained by the maser-pumping models.