Radio Continuum Emission at OH and H 2 O Maser Sites

Abstract

A search for centimeter radio continuum emission associated with OH and H masers has been 2O made toward 26 Ðelds containing 45 separate maser sites using the Australia Telescope Compact Array (ATCA) at 8.2 and 9.2 GHz. The sensitivity achieved for Ðelds not limited by dynamic range is D0.15 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 2A 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/H maser sites do not contain well-developed ultracompact H II regions (UCHIIs). The 2O 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 preÈmain-sequence contraction stage.