Collisional Pumping of OH Masers in the 2 Pi 1/2 Ladder

The collisional pumping of OH masers in star-forming regions has been studied, and the first detailed calculations of such pumping since collision rates with the correct parities have become available are reported. It is found that collisions of OH with H2 pump not only the J = 1/2 Lambda-doublet but also the J = 3/2 and J = 5/2 Lambda-doublets of 2Pi(1/2). Using parameters inferred by VLBI for W3(OH), it is found that collisional pumping can easily explain the observed brightness temperatures and luminosities of the J = 1/2 Lambda-doublet masers. Furthermore, what the brightness temperature and luminosity should be for the J = 3/2 and J = 5/2 Lambda-doublets in the same region is predicted.