Radio Continuum Emission at OH and H2O Maser Sites

Abstract

A search for centimeter radio continuum emission associated with OH and H2O masers has been made toward 26 fields containing 45 separate maser sites using the Australia Telescope Compact Array (ATCA) at 8.2 and 9.2 GHz. The sensitivity achieved for fields not limited by dynamic range is ~0.15 mJy rms. This is the deepest radio search for embedded stellar sources at sites containing OH and H2O masers made to date. Only 17 compact continuum sources were detected within 2'' of the 45 maser sites searched. The absence of 3 cm continuum emission at the mJy level in 62% of the maser sites shows that most OH/H2O maser sites do not contain well-developed ultracompact H II regions (UCHIIs). The implications of this result on the nature of the source pumping the maser emission are discussed. While other possibilities cannot be ruled out, we argue that many maser sites without detected UCHIIs are most likely massive stars still in the pre-main-sequence contraction stage.