Two large H I expanding shells in the field of NGC 7538

Observations are presented of two large 'filamentary' H I emission features which appeared, by chance, in an aperture synthesis survey of 21-cm emission towards NGC 7538. One feature, with velocity near 0 km s-1 (LSR), is part of the large, roughly spherical expanding shell of gas related to the Cep OB3 association. The new results show the shell to be less than 0.5 pc thick in places, much thinner than has been previously assumed. The feature is consistent with a type II supernova remnant as suggested by Assousa, Herbst and Turner, with an age approximately 4.3 x 105 yr and an initial energy approximately 1.2 x 1051 erg, but could also represent a 'bubble' produced by stellar winds from the stars in Cep OB3. The other feature has a velocity of about -35 km s-1, and also appears to be an expanding spherical shell. This object is more distant (2.6 to 3 kpc) and larger in extent than the Cep OB3 shell. It contains approximately 104 solar masses of hydrogen expanding at approximately 15 km s-1, and may be an old (t approximately 1.6 x 106 yr) supernova remnant. A stellar wind origin is also plausible energetically, but no correlation is observed between the shell and any known OB association. (author)