2002A&A...394..107B

We have carried out sensitive observations of the highly excited ² Π_3/2, J7/2 state of OH (290K above the ground-level) at 13441MHz (F=4-4) and 13435MHz (F=3-3) 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 J5/2 OH maser survey of star-forming regions taken from the IRAS point-source catalog. We have observed weak ² Π_3/2, J7/2, F=4-4 and 3-3 absorption in 7 sources and discovered 3 new weak F=4-4 maser sources at 13441MHz 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 13441MHz peak flux density observed in W3(OH). 21413+5442 and G11.90-0.14 exhibit extremely narrow velocity extents of the order of 0.4km/s. However, in ON1 there are two groups of narrow velocity features separated by about 14km/s; 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 J7/2, F=3-3 emission feature lying on the red wing of the 13435MHz absorption feature in W3(OH) no J7/2, F=3-3 emission was discovered. We conclude that the 13441MHz OH maser emission from W3(OH) remains exceptionally strong but not unique. In contrast with the J5/2, 6035MHz maser emission observed in a large number of star-forming regions, J7/2, 13441MHz emission is not widespread. This is expected from OH maser theory, and because the involved J7/2 levels are high in the OH energy ladder and not directly connected to the ground-state.