Detection of OH radicals from IRAS sources

Abstract

Natural OH maser emission originates in the dusty circumstellar envelopes of highly evolved long-period variables and super-giant stars undergoing mass-loss. The OH spectra of such stars, at a microwave frequency of 1,612 MHz, are sharp double-peaked emission line profiles in which the two strongest features have a velocity separation of 5–60 km s⁻¹. Their far infrared colours are typical of sources with colour temperatures below 500 K. Most of the 200 known OH/infrared stars were discovered in systematic OH-line surveys, at 1,612 MHz along the galactic plane^1–4, generally using small antennae and having a limiting sensitivity of ∼ 1 Jy. But 40 well-studied sources were readily detected with the IRAS satellite⁵ at 12, 25 and 60 µm and were characterized by a limited range of infrared colours. A more efficient method for detecting new OH/infrared stars is to begin with IRAS source positions, selected for appropriate infrared colours, and using radio-line observations to confirm the OH properties. We demonstrate the validity of this approach here, using the Arecibo 305 m radio-telescope to confirm the 1,612 MHz line observations of sources in IRAS Circulars 8 and 9 (ref. 6); our present observations identify 21 new OH/infrared stars. The new sources have weaker 1,612 MHz fluxes, bluer (60–25) µm colours and a smaller mean separation between the principal emission peaks than previous samples.