Discovery of new highly excited OH sources in regions of high-mass star formation

Abstract

We have carried out sensitive observations of the highly excited state of OH (290 K above the ground-level) at 13 441 MHz () and 13 435 MHz () in both right and left circular polarizations with the Effelsberg 100-m telescope. Our sample included 27 compact or ultra-compact HII regions. Most of them have been selected from our previous OH maser survey of star-forming regions taken from the IRAS point-source catalog. We have observed weak and 3-3 absorption in 7 sources and discovered 3 new weak maser sources at 13 441 MHz in addition to W3(OH). The new sources, 20081+3122 (ON1), 21413+5442 and G11.90-0.14, are 50 to 100 times or more weaker than the 13 441 MHz peak flux density observed in W3(OH). 21413+5442 and G11.90-0.14 exhibit extremely narrow velocity extents of the order of 0.4 km s-1. However, in ON1 there are two groups of narrow velocity features separated by about 14 km s-1; these two groups are also observed in lower rotational levels and suggest complex kinematics in the envelope of the young ultra-compact HII region. Apart from a very weak, polarized, and narrow emission feature lying on the red wing of the 13 435 MHz absorption feature in W3(OH) no emission was discovered. We conclude that the 13 441 MHz OH maser emission from W3(OH) remains exceptionally strong but not unique. In contrast with the , 6035 MHz maser emission observed in a large number of star-forming regions, , 13 441 MHz emission is not widespread. This is expected from OH maser theory, and because the involved levels are high in the OH energy ladder and not directly connected to the ground-state.